Fig. 3.17 Cylindrical lenses: (A) convex; and (B) concave Fig. 3.18 Refraction through a convex cylindrical lens Uses of cylindrical lenses • Prescribed to correct astigmatism. • As a cross cylinder used to check the refraction subjectively. • Maddox rod consist of powerful convex cylindrical lenses mounted together. Types of cylindrical lenses Cylindrical or astigmatic lens may be of three type: • Simple (curved in one meridian only, either convex or concave), compound (curved unequally in both the meridians, either convex or concave). • Compoundcylindricallens is also called spherical cylinder. • Mixed cylinder, in which one meridian is convex and the other is concave. Images formed by a cylindrical lens The images formed by different types of cylindrical lenses are best understood by the study of Sturm’s conoid (see page 42). PhYSIOLOGICAL OPTICS (OPTICS Of ThE EYE) eye as aN OPticaL iNstrumeNt Eye as an optical instrument can be compared to a camera where: • Eyelids acts as shutter of the camera. • Corneaand crystallinelens act as focussing system (lens) of the camera. In fact the cornea (1.32) and crystalline lens (1.47) along with aqueous humour (1.33), and vitreous humour (1.33) constitute a homocentric system of lenses, which when combined in action from a very strong refracting system of short focal length. • Iris acts as a diaphragm which regulates the size of aperture (pupil) and, therefore, the amount of light entering the eye. • Choroidhelps in forming the darkened interior of the camera. • Retinaacts as light sensitive plate or film on which image is formed. Note. To be more precise, the functioning of the eye can be considered to be analogous to a closed circuit colour TV system as depicted in Fig. 3.19. The optic nerve and its connections convey the details of the image to the occipital region of the cerebral cortex, where they are processed before reaching consciousness. schematic eye: cardiNaL POiNts The complex optics of eye, forming a homocentric lens system, has been simplified by Gullstrand by resolving it into six cardinal points (schematic eye) as follows (Fig. 3.20A): • Two principal foci F1 and F2 which lie 15.7 mm in front and 24.4 mm behind the cornea, respectively. • Two principal points P1 and P2 which lie in the anterior chamber, 1.35 mm and 1.60 mm behind the anterior surface of cornea, respectively. • Two nodal points N1 and N2 which lie in the posterior part of lens, 7.08 mm and 7.33 mm behind the anterior surface of cornea, respectively. • Totaldioptricpower of this schematic eye is 58.64 D. reduced eye Understanding of the optics eye has been further simplified by Listing and Donder. From schematic eye, they have given the concept of reduced eye by choosing single principal point and single nodal point lying midway between two principal points and two nodal points, respectively. The simplified data of the Listing’s reduced eye are as follows (Fig. 3.20B): • Principalpoint(P) lies 1.5 mm behind the anterior surface of cornea. • Nodal point (N) is situated 7.2 mm behind the anterior surface of cornea. • Anterior focal point (F1) is 15.7 mm in front of the anterior surface of cornea. Chapter 3 Elementary and Physiological optics 31 A B Fig. 3.19 Functioning of eye as an optical system (A) is in many ways similar to a closed circuit colour TV system A axes aNd VisuaL aNgLes Of the eye The eye has three principal axes and three visual angles (Fig. 3.21). axes of the eye 1.Opticaxisis the line passing through the centre of the cornea (P), centre of the lens (N) and meets the retina (R) on the nasal side of the fovea. In practice it is impossible to determine accurately the optic axis, since we cannot know the exact centre of cornea. However, it is much easier to estimate centre of the pupil, for example by an image of light on the cornea. Therefore, in practice we substitute the optic axis by a line perpendicular to the cornea at the point coinciding to the centre of pupil. This line is called pupillary line. 2.Visualaxisis the line joining the fixation point (O), nodal point (N), and the fovea (F). 3.Fixation axis is the line joining the fixation point (O) and the centre of rotation (C). B Fig. 3.20 Cardinal points of schematic eye (A); and reduced eye (B) • Posteriorfocalpoint (F2) (on the retina) is 24.40 mm behind the anterior surface of cornea. • Anterior focal length (f1) is 17.2 mm (15.7 + 1.5) • Posteriorfocallength(f2) is 22.90 mm (24.40 – 1.5). • Total dioptric power of reduced eye is about + 60 dioptre. Out of which about + 43 D is contributed by the cornea and + 17 D by the crystalline lens. Fig. 3.21 Axes of the eye: optic axis (AR); visual axis (OF); fixation axis (OC) and visual angles: angle alpha (ONA, between optical axis and visual axis at nodal point N); angle kappa (OPA, between optical axis and pupillary line–OP); angle gamma (OCA, between optical axis and fixation axis) 32 Section ii optics and Refraction Visual angles Visual angles eye are (Fig. 3.21): 1.Anglealpha. It is the angle (ONA) formed between the optical axis (AR) and visual axis (OF) at the nodal point (N). 2.Angle gamma. It is the angle (OCA) between the optical axis (AR) and fixation axis (OC) at the centre of rotation of the eyeball (C). 3.Anglekappa. It is the angle (OPA) formed between the visual axis (OF) and pupillary line (AP). The point P on the centre of cornea is considered equivalent to the centre of pupil. Practically, only the angle kappa can be measured and is of clinical significance. A positive angle kappa results in pseudo-exotropia and a negative angle kappa in pseudo-esotropia. OPticaL aberratiONs Of the NOrmaL eye The eye, in common with many optical systems in practical use, is by no means optically perfect. The lapses from perfection are called aberrations. Fortunately, the eyes possess these defects to such a small degree that for functional purposes their presence is negligible. Natural mechanisms to decrease aberrations in the human eye include: • Cutting off of the peripheral rays by iris, • High refractive index of the core of nucleus of the lens than that of the peripheral cortex, • Low sensitivity of the peripheral retina, and • Stiles-Crawford effect, i.e., more sensitivity of the retina to perpendicular rays than the oblique rays. It has been said that despite imperfections the overall performance of the eye is little short of astonishing. Physiological optical defects in a normal eye include the following: 1. Diffraction of light. Diffraction refers to the bending of light rays caused by the edge of an aperture or the rim of a lens. The actual pattern of a diffracted image point produced by a lens with a circular aperture or pupil is a series of concentric bright and dark rings with a bright spot in the centre. Such a pattern is termed as the Airydisc (Fig. 3.22). 2. Spherical aberrations. Spherical aberrations occur owing to the fact that spherical lens refracts peripheral rays more strongly than paraxial rays which in the case of a convex lens brings the more peripheral rays to focus closer to the lens (Fig. 3.23). The human eye, having a power of about +60 D, was long thought to suffer from various amounts of spherical aberrations. However, results from aberroscopy have revealed the fact that the dominant Fig. 3.22 The diffraction of light. Light brought to a focus does not come to a point, but gives rise to a blurred disc of light surrounded by several dark and light bands (the ‘Airy disc’) aberration of human eye is not spherical aberration but rather a coma-like aberration. 3. Chromatic aberrations. Chromatic aberrations result owing to the fact that the index of refraction of any transparent medium varies with the wavelength of incident light. In human eye, which optically acts as a convex lens, blue light is focussed slightly in front of the red (Fig. 3.24). In other words, the emmetropic eye is in fact slightly hypermetropic for red rays and myopic for blue and green rays. This in fact forms the basis of bichrome test used in subjective refraction. Fig. 3.23 Spherical aberration. Because there is greater refraction at periphery of spherical lens than near centre, incoming rays of light do not truly come to a point focus Chapter 3 Elementary and Physiological optics 33 Fig. 3.24 Chromatic aberration. The dioptric system of the eye is represented by a simple lens. The yellow light is focussed on the retina, and the eye is myopic for blue, and hypermetropic for red 4. Decentring. The cornea and lens surfaces alter the direction of incident light rays causing them to focus on the retina. Actually these surfaces are not centred on a common axis. The crystalline lens is usually slightly decentred and tipped with respect to the axis of the cornea and the visual axis of the eye. It has been reported that the centre of curvature of cornea is situated about 0.25 mm below the axis of the lens. However, the effects of deviation are usually so small that they are functionally neglected. 5. Oblique aberration. Objects in the peripheral field are seen by virtue of obliquely incident narrow pencil of rays which are limited by the pupil. Because of this, the refracted pencil shows oblique astigmatism 6. Coma. Different areas of the lens will form foci in planes other than the chief focus. This produces in the image plane a ‘coma effect’ from a point source of light. 4 Errors of refraction and Accommodation Chapter Outline ERRORS OF REFRACTION Emmetropia and ametropia Hypermetropia Myopia Astigmatism Anisometropia Aniseikonia • • • • • • ACCOMMODATION AND ITS ANOMALIES Accommodation • • • Definition Mechanism Range and amplitude Anomalies of Accommodation • • Presbyopia Insufficiency of accommodation ERRORS OF REFRACTION EmmEtropia and amEtropia Emmetropia (optically normal eye) can be defined as a state of refraction, where in the parallel rays of light coming from infinity are focused at the sensitive layer of retina with the accommodation being at rest (Fig. 4.1). Ametropia (a condition of refractive error), is defined as a state of refraction, when the parallel rays of light coming from infinity (with accommodation at rest), are focused either in front or behind the sensitive layer of retina, in one or both the meridians. The ametropia includes: • Myopia, • Hypermetropia, and • Astigmatism. Note. The related conditions aphakia and pseud­ ophakia are also discussed here. HYPERMETROPIA Hypermetropia (hyperopia) or long­sightedness is the refractive state of the eye wherein parallel rays of light coming from infinity are focused behind the • Paralysis of accommodation Spasm of accommodation DETERMINATION OF REFRACTION ERRORS • • Objective refraction • Subjective refraction CORRECTION OF REFRACTIVE ERRORS Spectacles Contact lenses Refractive surgery • Refractive surgery for myopia • Refractive surgery for hyperopia • Refractive surgery for astigmatism • Management of post-keratoplasty astigmatism • Refractive surgery for presbyopia retina with accommodation being at rest (Fig. 4.2). Thus, the posterior focal point is behind the retina, which, therefore, receives a blurred image. Etiology Hypermetropia may be axial, curvatural, index, positional and due to absence of crystalline lens. Fig. 4.1 Refraction in an emmetropic eye Fig. 4.2 Refraction in a hypermetropic eye Chapter 4 Errors of refraction and Accommodation 35 1. Axial hypermetropia is by far the commonest form. In this condition, the total refractive power of eye is normal but there is an axial shortening of eyeball. About 1 mm shortening of the anteroposterior diameter of the eye results in 3 dioptres of hypermetropia. Axial hypermetropia may be developmental or pathological. High hypermetropia occurs in microophthalmos and nanophthalmos due to markedly short axial length (usually less than 20 mm). 2. Curvatural hypermetropia is the condition in which the curvature of cornea, lens or both is flatter than the normal resulting in a decrease in the refractive power of eye. About 1 mm increase in radius of curvature results in 6 dioptres of hypermetropia. It may be developmental or rarely pathological. 3. Index hypermetropia occurs due to decrease in refractive index of the lens in old age due to cortical sclerois. It may also occur in diabetics under treatment. 4. Positional hypermetropia results from posteriorly placed crystalline lens. 5. Absence of crystalline lens either congenitally or acquired (following surgical removal or posterior dislocation) leads to aphakia—a condition of high hypermetropia. Clinical types There are three clinical types of hypermetropia: 1. Simple or developmental or physiological hyper­ metropia is the commonest form. It results from normal biological variations in the development of eyeball. It includes: • Developmental axial hypermetropia, and • Developmental curvatural hypermetropia. 2. Non­physiological hypermetropia results due to either conditions of the eyeball which are outside the normal biological variations of the development. It includes: Congenital non-physiological hypermetropia is seen in following conditions: • Microphthalmos, • Nanophthalmos, • Microcornea, • Congenital posterior subluxation of lens, and • Congenital aphakia. Acquired non-physiological hypermetropia includes: a. Senile hypermetropia or frequently designated as acquired hypermetropia occurs in old age due to two causes: • Index hypermetropia due to acquired cortical sclerosis in old age, and • Curvatural hypermetropia due to decreased curvature of the outer lens fibers developing later in life. b. Positional hypermetropia due to posterior subluxation of lens. c. Aphakia, i.e., congenital or acquired, absence of lens. d. Consecutive hypermetropia due to surgically over­ corrected myopia. e. Acquired axial hypermetropia due to forward displacement of the retina as seen in retinal detachment, central serous retinopathy and orbital tumours. f. Acquired curvatural hypermetropia may occur due to post­traumatic or post­inflammatory corneal flattening. g. Pseudophakic hypermetropia occurs due to implantation of an underpowered intraocular lens. 3. Functional hypermetropia results from paralysis of accommodation as seen in patients with third nerve paralysis and internal ophthalmoplegia. Components of hypermetropia (effect of accommodation) Nomenclature for various components of the hypermetropia vis­a­vis accommodation is as follows: Total hypermetropia is the total amount of refractive error, which is estimated after complete cycloplegia with atropine. It consists of latent and manifest hypermetropia. 1. Latent hypermetropia implies the amount of hypermetropia (about 1D) which is normally corrected by the inherent tone of ciliary muscle. The degree of latent hypermetropia is high in children and gradually decreases with age. The latent hypermetropia is disclosed when refraction is carried after abolishing the tone with atropine. 2. Manifest hypermetropia is the remaining portion of total hypermetropia, which is not corrected by the ciliary tone. It consists of two components, the facultative and the absolute hypermetropia. a. Facultative hypermetropia constitutes that part which can be corrected by the patient’s accommodative effort. b. Absolute hypermetropia is the residual part of manifest hypermetropia which cannot be corrected by the patient’s accommodative efforts. Thus, briefly: Total hypermetropia = latent + manifest (facultative + congenital or acquired) 36 Section ii Optics and refraction age and hypermetropia At birth, the eyeball is relatively short, having +2 to +3 hypermetropia. This is gradually reduced until by the age of 5–7 years, the eye is emmetropic and remains so till the age of about 50 years. After this, there is a tendency to develop hypermetropia again, which gradually increases until the extreme of life by which the eye has the same +2 to +3 with which it started. This senile hypermetropia is due to changes in the crystalline lens. Clinical features Symptoms In patients with hypermetropia, the symptoms vary depending upon the age of patient and the degree of refractive error. These can be grouped as under: 1. Asymptomatic. A small amount of refractive error in young patients is usually corrected by mild accommodative effort without producing any symptom. 2. Asthenopic symptoms. At times the hypermetropia is fully corrected by the use of accommodation. Thus vision is normal, but due to sustained accommodative efforts patient develops asthenopic symptoms. These include: tiredness of eyes, frontal or frontotemporal headache, watering and mild photophobia. These asthenopic symptoms are especially associated with near work and increase towards evening. 3. Defective vision with asthenopic symptoms. When the amount of hypermetropia is such that it is not fully corrected by the voluntary accommodative efforts, then the patients complain of defective vision which is more for near than distance and is associated with asthenopic symptoms due to sustained accommodative efforts. 4. Defective vision only. When the amount of hypermetropia is very high, the patients usually do not accommodate (especially adults) and there occurs marked defective vision for near and distance. • Effect of ageing on vision. Typically patients with low hypermetropia have good vision in young age. However, with ageing, due to decrease in accommodative power, the hypermetropia becomes manifest and patients complain of progressive decrease in vision. To begin with blurring occurs for near vision and then for distant vision also. Signs 1. Size of eyeball may appear small as a whole especially in high hypermetropia. 2. Cornea may be slightly smaller than the normal. 3. Anterior chamber is comparatively shallow. 4. Retinoscopy and autorefractometry reveals hypermetropic refractive error. 5. Fundus examination reveals a small optic disc which may look more vascular with ill­defined margins and even may simulate papillitis (though there is no swelling of the disc, and so it is called pseudopapillitis). The retina as a whole may shine due to greater brilliance of light reflections (shot silk appearance). 6. A-scan ultrasonography (biometry) may reveal a short anteroposterior length of the eyeball in axial hypermetropia. Grading of hypermetropia American Optometric Association (AOA) has defined three grades of hypermetropia as below: • Low hypermetropia, when the error is < + 2D. • Moderate hypermetropia, when the error is between +2 to + 5D. • High hypermetropia, when the error is > + 5D. Complications If hypermetropia is not corrected for a long time the following complications may occur: 1. Recurrent styes, blepharitis or chalazia may occur, probably due to infection introduced by repeated rubbing of the eyes, which is often done to get relief from fatigue and tiredness. 2. Accommodative convergent squint may develop in children (usually by the age of 2–3 years) due to excessive use of accommodation. 3. Amblyopia may develop in some cases. It may be anisometropic (in unilateral hypermetropia), strabismic (in children developing accom­ modative squint) or ametropic (seen in children with uncorrected bilateral high hypermetropia). 4. Predisposition to develop primary narrow angle glaucoma. The eye in hypermetropes is small with a comparatively shallow anterior chamber. Due to regular increase in the size of the lens with increasing age, these eyes become prone to an attack of narrow angle glaucoma. This point should be kept in mind while instilling mydriatics in elderly hypermetropes. treatment A. Optical treatment. Basic principle of treatment is to prescribe convex (plus) lenses, so that the light rays are brought to focus on the retina (Fig. 4.3). ■Fundamental rules for prescribing glasses in hypermetropia include: 1. Total amount of hypermetropia should always be discovered by performing refraction under complete cycloplegia. 2. The spherical correction given should be com­ fortably acceptable to the patient. However, the astigmatism should be fully corrected. Chapter 4 Errors of refraction and Accommodation 37 Fig. 4.3 Refraction in a hypermetropic eye corrected with convex lens 3. Gradually increase the spherical correction at 6 months interval till the patient accepts manifest hypermetropia. 4. In the presence of accommodative convergent squint, full cycloplegic correction should be given at the first sitting. 5. If there is associated amblyopia, full correction with occlusion therapy should be started. Modes of prescription of convex lenses 1. Spectacles are most comfortable, safe and easy method of correcting hypermetropia. 2. Contact lenses are indicated in unilateral hypermetropia (anisometropia). For cosmetic reasons, contact lenses should be prescribed once the prescription has stabilised, otherwise, they may have to be changed many a times. B. Surgical treatment (see page 54). apHaKia Aphakia literally means ‘absence of crystalline lens’ from the eye. However, from the optical point of view, it may be considered a condition in which the lens is absent from the pupillary area. Aphakia produces a high degree of hypermetropia. Causes 1. Congenital absence of lens. It is a rare condition. 2. Surgical aphakia occurring after removal of lens is the commonest presentation. 3. Aphakia due to absorption oflens matter isnoticed rarely after trauma in children. 4. Traumatic extrusion of lens from the eye also constitutes a rare cause of aphakia. 5. Posterior dislocation of lens in vitreous causes optical aphakia. optics of aphakic eye Following optical changes occur after removal of crystalline lens: • Hypermetropia of high degree. • Total power of eye is reduced to about +44 D from +60 D. • Anterior focal point becomes 23.2 mm in front of the cornea (Normal: 15.7 mm). • Posterior focal point is about 31 mm behind the cornea i.e., about 7 mm behind the eyeball (The anteroposterior length of eyeball is about 24 mm). • Accommodation is lost fully. Clinical features Symptoms • Defective vision. Main symptom in aphakia is marked defective vision for both far and near due to high hypermetropia and absence of accommodation. • Erythropsia and cyanopsia i.e., seeing red and blue images. This occurs due to excessive entry of ultraviolet and infrared rays in the absence of crystalline lens. Signs of aphakia • Limbal scar may be seen in surgical aphakia. • Anterior chamber is deeper than normal. • Iridodonesis i.e., tremulousness of iris can be demonstrated. • Pupil is jet black in colour. • Purkinje’s image test shows only two images (nor­ mally four images are seen­ see page 506 and Fig. 23.10). • Fundus examination shows hypermetropic small disc. • Retinoscopy and autorefractometry reveals high hypermetropia. treatment Optical principle is to correct the refractive error by convex lenses of appropriate power so that image is formed on the retina (Fig. 4.3 ). Modalities for correcting aphakia include: (1) spectacles, (2) contact lens, (3) intraocular lens, and (4) refractive corneal surgery. 1. Spectacles prescription has been the most commonly employed method of correcting aphakia in the past, especially in developing countries. Presently, use of aphakic spectacles has decreased markedly. Roughly, about+10D with cylindrical lenses for surgically induced astigmatism are required to correct aphakia in previously emmetropic patients. However, exact number of glasses will differ in individual case and should be estimated by refraction. An addition of +3 to +4 D is required for near vision to compensate the loss of accommodation. 38 Section ii Optics and refraction ■Disadvantages of spectacles. (i) Image is magnified by 30%, so not useful in unilateral aphakia (produce diplopia). (ii) Problem of spherical and chromatic aberrations of thick lenses. (iii) Field of vision is limited. (iv) Prismatic effect of thick glasses. (v) ‘Roving ring Scotoma’ (Jack in the box phenomenon). (vi) Cosmetic blemish especially in young aphakes. 2. Contact lenses. Advantages of contact lenses over spectacles include: (i) Less magnification of image. (ii) Elimination of aberrations and prismatic effect of thick glasses. (iii) Wider and better field of vision. (iv) Cosmetically more acceptable. (v) Better suited for uniocular aphakia. ■Disadvantages of contact lenses are: (i) more cost; (ii) cumbersome to wear, especially in old age and in childhood; and (iii) corneal complications may be associated. 3. Intraocular lens implantation is the best available method of correcting aphakia. Therefore, it is the commonest modality being employed nowadays. • Primary intraocular lens implantation is done during cataract surgery. • Secondary intraocular lens implantation is done in already aphakic patients. (For detail see page 207). 4. Refractive corneal surgery includes: • Keratophakia and Epikeratophakia have been tried without much success. In keratophakia a lenticule prepared from the donor cornea is placed between the lamellae of patient’s cornea and in epikeratophakia, the lenticule prepared from the donor cornea is stitched over the surface of cornea after removing the epithelium. • Hyperopic Lasik may be tried in cases where secondary IOL cannot be implanted (see page 54). pSEUdopHaKia The condition of aphakia when corrected with an intraocular lens implant (IOL) is referred to as pseudophakia or artephakia. For types of IOLs and details of implantation techniques and complications (see page 207). Refractive status of a pseudophakic eye depends upon the power of the IOL implanted as follows: 1. Emmetropia is produced when the power of the IOL implanted is exact. It is the most ideal situation. Such patients need plus glasses for near vision only. 2.Consecutive myopiaoccurs when the IOL implanted overcorrects the refraction of eye. Such patients require glasses to correct the myopia for distance vision and may or may not need glasses for near vision depending upon the degree of myopia. 3. Consecutive hypermetropia develops when the under power IOL is implanted. Such patients require plus glasses for distance vision and additional +2 to +3 D for near vision. Note. Varying degree of surgically induced astigmatism is also present in pseudophakia. Signs of pseudophakia(with posterior chamber IOL). • Surgical scar may be seen near the limbus. • Anterior chamber is slightly deeper than normal. • Mild iridodonesis (tremulousness) of iris may be demonstrated. • Purkinje image test shows four images. • Pupil is blackish in colour but when light is thrown in pupillary area shining reflexes are observed. • Presence of IOL is confirmed (see Fig. 9.32) on slit­ lamp examination after dilating the pupil. • Visual status and refraction will vary depending upon the power of IOL implanted as described above. Management of pseudophakia includes: 1. Spectacles for near vision alone (in pseudophakia with emmetropia) or as bifocal/progressive glasses for both distance and near vision (in pseudophakia with consecutive refractive error) are required. 2. LASIK or Advanced surface ablation (ASA) may be required in moderate consecutive refractive error. 3. Intraocular lens (IOL) exchange or pigiback IOL is required when the consecutive refractive error is large. MYOPIA Myopia or shortsightedness is a type of refractive error in which parallel rays of light coming from infinity are focused in front of the retina when accommodation is at rest (Fig. 4.4). Etiological classification 1. Axial myopia results from increase in antero­ posteriorlength of the eyeball. It is the commonest form. 2. Curvatural myopia occurs due to increased curvature of the cornea, lens or both. Fig. 4.4 Refraction in a myopic eye Chapter 4 Errors of refraction and Accommodation 39 3. Positional myopia is produced by anterior placement of crystalline lens in the eye. 4. Index myopia results from increase in the refractive index of crystalline lens associated with nuclear sclerosis. 5. Myopia due to excessive accommodation occurs in patients with spasm of accommodation. Grading of myopia American Optometric Association (AOA) has defined three grades of myopia: • Low myopia, when the error is ≤–3D. • Moderate myopia, when the error is between­3D to –6D. • High myopia, when the error is ≥–6D. Clinical varieties of myopia 1. Congenital myopia. 2. Simple or developmental myopia. 3. Pathological or degenerative myopia. 4. Acquired or secondray myopia which occurs secon­ dary to some other disease/factors are as follows: • post­traumatic, • post­keratitic, • drug­induced, • pseudomyopia, • space myopia, • night myopia, and • consecutive myopia. ConGEnital myopia • Present since birth, the congenital myopia, is usually diagnosed by the age of 2–3 years. • Anisometropia is usually present, hence most of the time the error is unilateral. Rarely, it may be bilateral. • High degree of error, about 8 to 10D, is usually present, which mostly remains constant. • Convergent squint may develop in order to preferentially see clear at its far point (which is about 10–12 cm). • Associations may include other congenital anom­ alies such as cataract, microphthalmos, aniridia, megalocornea, and congenital separation of retina. • Early correction of congenital myopia is desirable. SimplE myopia Simple or developmental myopia is the commonest variety. It is considered as a physiological error not associated with any disease of the eye. Overall reported prevalence is 20–40% of population. Since, the sharpest rise occurs at school going age i.e., between 8 years to 12 years so, it is also called school myopia. Etiology It results from normal biological variation in the development of eye which may or may not be genetically determined. Some factors associated with simple myopia are as follows: ■Axial type of simple myopia may signify just a physiological variation in the length of the eyeball or it may be associated with precocious neurological growth during childhood. ■Curvatural type of simple myopia is considered to be due to underdevelopment of the eyeball. ■Role of diet in early childhood has also been reported without any conclusive results. ■Role of genetics. Genetics plays some role in the biological variation of the development of eye, as prevalence of myopia is more in children with both parents myopic (20%) than the children with one parent myopic (10%) and children with no parent myopic (5%). ■Theory of excessive near work in childhood was also put forward, but did not gain much importance. In fact, there is no truth in the folklore that myopia is aggravated by close work, watching television and by not using glasses. Clinical features Symptoms ■Poor vision for distance (short­sightedness) is the main symptom of myopia. ■Asthenopic symptoms may occur in patients with small degree of myopia. ■Half shutting of the eyes may be complained by parents of the child. The child does so to achieve the greater clarity of stenopaeic vision. Signs ■Prominent eyeballs. The myopic eyes typically are large and somewhat prominent. ■Anterior chamber is slightly deeper than normal. ■Pupils are somewhat large and a bit sluggishly reacting. ■Fundus is normal; rarely temporal myopic crescent may be seen. ■Magnitude of refractive error. Simple myopia usually occurs between 5 and 10 years of age and it keeps on increasing till about 18–20 years of age at a rate of about –0.5 ± 0.30 every year. In simple myopia, usually the error is low to moderate that usually does not exceed –6D. diagnosis Diagnosis is confirmed by performing refraction (see page 570). patHoloGiCal myopia Pathological/degenerative/progressive myopia, as the name indicates, is a rapidly progressive error which starts in childhood at 5–10 years of age and 40 Section ii Optics and refraction results in high myopia (>–6D) during early adult life which is usually associated with degenerative changes in the eye. It is less common (about 2% of population). Etiology It is unequivocal that the pathological myopia results from a rapid axial growth of the eyeball which is outside the normal biological variations of development. To explain this spurt in axial growth various theories have been put forward. So far no satisfactory hypothesis has emerged to explain the etiology of pathological myopia. However, it is definitely linked with (i) heredity and (ii) general growth process. 1. Role of heredity. It is now confirmed that genetic factors play a major role in the etiology, as the progressive myopia is: (i) familial; (ii) more common in certain races like Chinese, Japanese, Arabs and Jews, (iii) uncommon among Negroes, Nubians and Sudanese and (iv) more common in women than men. Autosomal dominant pathological myopia has been linked to genes 18p11.31 and 12q21.23. It is presumed that heredity­linked growth of retina is the determinant in the development of myopia. The sclera due to its distensibility follows the retinal growth but the choroid undergoes degeneration due to stretching, which in turn causes degeneration of retina. 2. Role of general growth process, though minor, cannot be denied on the progress of myopia. Lengthening of the posterior segment of the globe commences only during the period of active growth and probably ends with the termination of the active growth. Therefore, the factors (such as nutritional deficiency, debilitating diseases, endocrine disturbances and indifferent general health) which affect the general growth process will also influence the progress of myopia. Etiological hypothesis for pathological myopia is summarised in Fig. 4.5. Fig. 4.5 Etiological hypothesis for pathological myopia Clinical features Symptoms 1. Defective vision. There is considerable failure in visual function as the error is usually high. Further, due to progressive degenerative changes, an uncorrectable loss of vision may occur. 2. Muscae volitantes i.e., floating black opacities in front of the eyes are also complained by many patients. These occur due to degenerated liquefied vitreous. 3. Night blindness may be complained by very high myopes having marked degenerative changes. Signs 1. Prominent eyeballs. The eyes are often prominent, appearing elongated and even simulating an exophthalmos, especially in unilateral cases. The elongation of the eyeball mainly affects the posterior pole and surrounding area; the part of the eye anterior to the equator may be normal (Fig. 4.6). 2. Cornea is large. 3. Anterior chamber is deep. 4.Pupils are slightly large and react sluggishly to light. 5. Fundus examination reveals following characteristic signs: a. Optic disc appears large and pale and at its temporal edge a characteristic myopic crescent is present (Fig. 4.7). Sometimes peripapillary crescent encircling the disc may be present, where the choroid and retina is distracted away from the disc margin. A super­traction crescent (where the retina is pulled over the disc margin) may be present on the nasal side. Fig. 4.6 Elongation of the eyeball posterior to equator in pathological myopia Chapter 4 Errors of refraction and Accommodation 41 Fig. 4.7 Myopic crescent b. Degenerative changes in retina and choroid are common in progressive myopia (Fig. 4.8). These are characterized by: • Chorioretinal atrophic patches at the macula with a little heaping up of pigment around them. • Foster-Fuchs’ spot (dark red circular patch due to sub­retinal neovascularization and choroidal haemorrhage) may be present at the macula. • Cystoid degeneration may be seen at the periphery. • Lattice degeneration and or snail track lesions with or without retinal holes/tears may be present; which later may be complicated by retinal detachment. • Total retinal atrophy, particularly in the central area may occur in an advanced case. c. Posterior staphyloma due to ectasia of sclera at posterior pole may be apparent as an excavation with the vessels bending backward over its margins. d. Degenerative changes in vitreous include: liquefaction, vitreous opacities, and posterior vitreous detachment (PVD) appearing as Weiss’ reflex. Fig. 4.8 Fundus changes in pathological myopia 6. Visual fields may show contraction and in some cases ring scotoma may be seen. 7. ERG may reveal subnormal electroretinogram due to chorioretinal atrophy. Complications • Retinal detachment, • Complicated cataract, • Vitreous haemorrhage, • Choroidal haemorrhage, • Strabismus fixus convergence, and • Primary open angle glaucoma, not a complication, but is a reported association. treatment of myopia 1. Optical treatment of myopiaconstitutes prescription of appropriate concave lenses, so that clear image is formed on the retina (Fig. 4.9). ■Basic rule of correcting myopia is converse of that in hypermetropia, i.e., the minimum acceptance providing maximum vision should be prescribed. In very high myopia undercorrection is always better to avoid the problem of near vision and that of minification of images. ■Modes of prescribing concave lenses are spectacles and contact lenses. Their advantages and disadvantages over each other are the same as described for hypermetropia. Contact lenses are particularly justified in cases of high myopia as they avoid peripheral distortion and minification produced by strong concave spectacle lens. 2. Surgical treatment of myopia has become very popular now­a­days (for details see page 52). 3. General measures empirically believed to affect the progress of myopia (unproven usefulness) include: • Balanced diet rich in vitamins and proteins. • Early management of associated debilitating disease. • Visual hygiene is very important to avoid asthenopic symptoms. Care should be taken for proper posture and adequate illumination, especially for near work. Clarity of the print should be good Fig. 4.9 Refraction in a myopic eye corrected with concave lens 42 Section ii Optics and refraction and continuous reading especially, at night hours, should be avoided to prevent undue ocular fatigue. • Avoidance of outdoor sports and strenuous activities is advised to reduce the risk of ocular trauma and consequent complication of retinal detachment. 4.Low visionaids(LVA) are indicated in patients with progressive myopia having advanced degenerative changes, where useful vision cannot be obtained with spectacles and contact lenses. 5. Prophylaxis (genetic counselling). As the pathological myopia has a strong genetic basis, the hereditary transfer of disease may be decreased by advising against marriage between two individuals with progressive myopia. However, if they do marry, they should not produce children. ASTIGMATISM Astigmatism is a type of refractive error wherein the refraction varies in different meridia of the eye. Consequently, the rays of light entering the eye cannot converge to a point focus but form focal lines. Broadly, there are two types of astigmatism: regular and irregular. rEGUlar aStiGmatiSm The astigmatism is regular when the refractive power changes uniformly from one meridian to another (i.e., there are two principal meridians). Etiology 1.Corneal astigmatism is the result of abnormalities of curvature of cornea. It constitutes the most common cause of astigmatism. 2. Lenticular astigmatism is rare. It may be: • Curvatural due to abnormalities of curvature of lens as seen in lenticonus. • Positional due to tilting or oblique placement of lens as seen in subluxation. • Index astigmatismmay occur rarely due to variable refractive index of lens in different meridian. 3. Retinal astigmatism due to oblique placement of macula may also be seen occasionally. types of regular astigmatism Depending upon the axis and the angle between the two principal meridians, regular astigmatism can be classified into the following types: 1.With-the-rule astigmatism (WTR). In this type the two principal meridians are placed at right angles to one another but the vertical meridian is more curved than the horizontal. Thus, correction of this astigmatism will require the concave cylinder at 180° ± 20° or convex cylindrical lens at 90° ± 20°. This is called ‘with­the­ rule’ astigmatism, because similar astigmatic condition exists normally (the vertical meridian is normally rendered 0.25 D more convex than the horizontal meridian probably by the pressure of eyelids). 2. Against-the-rule astigmatism (ATR) refers to an astigmatic condition in which the horizontal meridians is more curved than the vertical meridian. Therefore, correction of this astigmatism will require the prescription of convex cylindrical lens at 180° ± 20° or concave cylindrical lens at 90° ± 20° axis. 3.Oblique astigmatism is a type of regular astigmatism where the two principal meridians are not the horizontal and vertical though these are at right angle to one another e.g., 45° and 135°. 4. Bioblique astigmatism. In this type of regular astigmatism the two principal meridians are not at right angle to each other e.g., one may be at 30° and other at 100°. optics of regular astigmatism As already mentioned, in regular astigmatism the parallel rays of light are not focused on a point but form two focal lines. Sturm’s conoid The configuration of rays refracted through a toric surface is called the Sturm’s conoid. The shape of bundle of the light rays at different levels in Sturm’s conoid (Fig. 4.10) is as follows: • At point A, the vertical rays (V) are converging more than the horizontal rays (H); so the section here is a horizontal oval or an oblate ellipse. • At point B (first focus), the vertical rays have come to a focus while the horizontal rays are still converging and so they form a horizontal line. • At point C, the vertical rays are diverging and their divergence is less than the convergence of the horizontal rays; so a horizontal oval is formed here. Fig. 4.10 Sturm’s conoid Chapter 4 Errors of refraction and Accommodation 43 • At point D, the divergence of vertical rays is exactly equal to the convergence of the horizontal rays from the axis. So, here the section is a circle, which is called the circle of least diffusion. • At point E, the divergence of vertical rays is more than the convergence of horizontal rays; so, the section here is a vertical oval. • At point F (second focus), the horizontal rays have come to a focus while the vertical rays are divergent and so a vertical line is formed here. • Beyond F (as at point G), both horizontal and vertical rays are diverging and so the section will always be a vertical oval or prolate ellipse. • Focal interval of Sturm refers to the distance between the two foci (B and F). Refractive types of regular astigmatism Depending upon the position of the two focal lines in relation to retina, the regular astigmatism is further classified into three types: 1.Simple astigmatism, wherein the rays are focused on the retina in one meridian and either in front (simple myopic astigmatism, Fig. 4.11A) or behind (simple hypermetropic astigmatism, Fig. 4.11B) the retina in the other meridian. 2. Compound astigmatism. In this type, the rays of light in both the meridians are focused either in front or behind the retina and the condition is labelled as compound myopic or compound hypermetropic astigmatism, respectively (Figs. 4.11C and D). 3. Mixed astigmatism refers to a condition wherein the light rays in one meridian are focused in front and in other meridian behind the retina (Fig. 4.11E). Thus, in one meridian eye is myopic and in another hypermetropic. Such patients have comparatively less symptoms as ‘circle of least diffusion’ is formed on the retina (see Fig. 4.10). Clinical features Symptoms 1. Asthenopia (tiredness of eyes relieved by closing the eyes) characterised by difficulty in focussing, transient blurred vision, dull ache in eyes, frontal headache and sometimes nausea and even drowsiness is especially marked in low astigmatism <1D. 2. Blurred vision and defective vision is reported when the astigmatism is > 1D. 3. Elongation of objects proportionate to the degree and type of astigmatism may be noticed in high astigmatism. 4. Keeping the reading material close to the eyes may be needed to achieve large but blurred retinal image. A B C D E Fig. 4.11 Types of astigmatism : (A) simple myopic; (B) simple hypermetropic; (C) compound myopic; (D) compound hypermetropic; and (E) mixed Signs 1. Half closure of the lid. Like myopes, the astigmatic patients may half shut the eyes to achieve the greater clarity of stenopaeic vision. 2. Head tilt. The astigmatic patients may (very exceptionally) develop a torticollis in an attempt to bring their axes nearer to the horizontal or vertical meridians. 3. Oval or tilted optic disc may be seen on ophth­ almoscopy in patients with high degree of astigmatism. 44 Section ii Optics and refraction 4. Different power in two meridians is revealed on retinoscopy or autorefractometry. investigations 1. Retinoscopy reveals different power in two different axes. 2. Keratometry. Keratometry and computerized corneal topography reveal different corneal curvature in two different meridians in corneal astigmatism. 3. Astigmatic fan test (see page 577) and 4. Jackson’s cross cylinder test (see page 577). These tests are useful in confirming the power and axis of cylindrical lenses. treatment 1.Optical treatmentof regular astigmatism comprises the prescription of appropriate cylindrical lens, discovered after accurate refraction. • Spectacles with full correction of cylindrical power and appropriate axis should be used for distance and near vision. • Contact lenses. Rigid contact lenses may correct upto 2–3D of regular astigmatism, while soft contact lenses can correct only little astigmatism. For higher degrees of astigmatism toric contact lenses are needed. In order to maintain the correct axis of toric lenses, ballasting or truncation is required. 2. Surgical correction of astigmatism is quite effective (For details see page 54). irrEGUlar aStiGmatiSm It is characterized by an irregular change of refractive power in different meridians. There are multiple meridians which admit no geometrical analysis. Etiological types 1. Curvatural irregular astigmatism is found in patients with extensive corneal scars or keratoconus. 2. Index irregular astigmatism due to variable refractive index in different parts of the crystalline lens may occur rarely in patients with cataract. Clinical features Symptoms of irregular astigmatism include: • Defective vision, • Distortion of objects, and • Polyopia (seeing multiple images). Signs depicted on investigations are as below: • Retinoscopy reveals irregular pupillary reflex. • Slit-lamp examination may reveal corneal irregularity or Keratoconus. • Placido’s disc test reveals distorted circles (see page 501). • Photokeratoscopy and computerized corneal topography give photographic record of irregular corneal curvature. treatment 1. Optical treatment of irregular astigmatism consists of contact lens which replaces the anterior surface of the cornea for refraction. 2. Phototherapeutic keratectomy (PTK) performed with excimer laser may be helpful in patients with superficial corneal scar responsible for irregular astigmatism. 3. Surgical treatment is indicated in extensive corneal scarring (when vision does not improve with contact lenses) and consists of penetrating keratoplasty or deep anterior lamellar keratoplasty (DALK). ANISOMETROPIA The optical state with equal refraction in the two eyes is termed isometropia. When the total refraction of the two eyes is unequal the condition is called anisometropia. Small degree of anisometropia is of no concern. A difference of 1D in two eyes causes a 2% difference in the size of the two retinal images. A difference up to 5% in retinal images of two eyes is well tolerated. In other words, an anisometropia up to 2.5D is well tolerated and that between 2.5 and 4D can be tolerated depending upon the individual sensitivity. However, if it is more than 4D, it is not tolerated and is a matter of concern. Etiology 1. Congenital and developmental anisometropia occurs due to differential growth of the two eyeballs. 2. Acquired anisometropia may occur due to asymmetric age change, uniocular aphakia after removal of cataractous lens or due to implantation of IOL of wrong power. Clinical types 1. Simple anisometropia. In this, one eye is normal (emmetropic) and the other either myopic (simple myopic anisometropia) or hypermetropic (simple hypermetropic anisometropia). 2. Compound anisometropia. Wherein both eyes are either hypermetropic (compound hypermetropic anisometropia) or myopic (compound myopic anisometropia), but one eye is having higher refractive error than the other. Chapter 4 Errors of refraction and Accommodation 45 3. Mixed anisometropia. In this, one eye is myopic and the other is hypermetropic. This is also called antimetropia. 4. Simple astigmatic anisometropia. When one eye is normal and the other has either simple myopic or hypermetropic astigmatism. 5. Compound astigmatic anisometropia. When both eyes are astigmatic but of unequal degree. Status of binocular vision in anisometropia Three possibilities are as follows: 1. Binocular single vision is present in small degree of anisometropia (less than 3D). 2. Uniocular vision. When refractive error in one eye is of high degree, that eye is suppressed and develops anisometropic amblyopia. Thus, the patient has only uniocular vision. 3. Alternate vision occurs when one eye is hyperme­ tropic and the other myopic. The hypermetropic eye is used for distant vision and myopic for near vision. diagnosis Diagnosis of anisometropia is made after retinoscopic examination and/or autorefractometry in patients with defective vision. treatment 1. Spectacles. The corrective spectacles can be tolerated up to a maximum difference of 4D. After that there occurs diplopia. 2. Contact lenses are advised for higher degrees of anisometropia. 3. Aniseikonic glasses are also available, but their clinical results are often disappointing. 4. Other modalities of treatment include: • Intraocular lens–implantation for uniocular aphakia. • Refractive corneal surgery for unilateral high myopia, astigmatism and hypermetropia. • Phakic Refractive Lenses (PRL) and Refractive Lens Exchange (RLE) are quite useful in very high degree anisometropia. Note. Efforts should be made to fully correct the anisometropia in children to prevent anisometropic amblyopia. In adults with amblyopia under correction of more ametropic eye may be required to avoid ocular discomfort. ANISEIkONIA Aniseikonia is defined as a condition wherein the images projected to the visual cortex from the two retinae are abnormally unequal in size and/or shape. Up to 5% aniseikonia is well tolerated. Etiological types 1. Optical aniseikonia may occur due to either inherent or acquired anisometropia of high degree. 2. Retinal aniseikonia may develop due to: displacement of retinal elements towards the nodal point in one eye due to stretching or oedema of the retina. 3. Cortical aniseikonia implies asymmetrical simultaneous perception inspite of equal size of images formed on the two retinae. Clinical types Clinically, aniseikonia may be of different types (Fig. 4.12): 1. Symmetrical aniseikonia a.Spherical, image may be magnified or minified equally in both meridians (Fig. 4.12A). b.Cylindrical, image is magnified or minified symmetrically in one meridian (Fig. 4.12B). 2. Asymmetrical aniseikonia a.Prismatic. In this image, difference increases progressively in one direction (Fig. 4.12C). b.Pincushion. In this image, distortion increases progressively in both directions, as seen with high plus correction in aphakia (Fig. 4.12D). c. Barrel distortion. In this image, distortion decreases progressively in both directions, as seen with high minus correction (Fig. 4.12E). d.Oblique distortion. In this, the size of image is same, but there occurs an oblique distortion of shape (Fig. 4.12F). Symptoms 1. Asthenopia, i.e., eyeache, browache and tiredness of eyes. 2. Diplopia due to difficult binocular vision when the difference in images of two eyes is more than 5%. 3. Difficulty in depth perception is often noticed. treatment 1. Optical aniseikonia may be corrected by aniseikonic glasses, contact lenses or intraocular lenses or other refractive surgery depending upon the situation. 2. Retinal aniseikonia may be corrected by treating the cause. 3. Cortical aniseikonia is very difficult to treat. ACCOMMODATION AND ITS ANOMALIES ACCOMMODATION definition As we know that in an emmetropic eye, parallel rays of light coming from infinity are brought to focus on the retina, with accommodation being at rest. 46 Section ii Optics and refraction Fig. 4.12 Types of aniseikonia: A, spherical; B, cylindrical; C, prismatic; D, pin cushion; E, barrel distortion; F, oblique distortion However, our eyes have been provided with a unique A B mechanism by which we can even focus the diverging rays coming from a near object on the retina in a bid to see clearly (Fig. 4.13). This mechanism is called accommodation. In this increase in the power of crystalline lens occurs due to increase in curvature of its surfaces (Fig. 4.14). At rest the radius of curvature of anterior surface of the lens is 10 mm and that of posterior surface is 6 mm (Fig. 4.14A). During accommodation, curvature of the posterior surface remains almost the same but that of anterior surface changes. In strong accommodation radius of curvature of anterior surface also becomes 6 mm (Fig. 4.14B). mechanism of accommodation According to von Helmholtz’s capsular theory the process of accommodation is achieved by a change in the shape of lens as below: When the eye is at rest (unaccommodated), the ciliary ring is large and keeps the zonules tense. Because of zonular tension the lens is kept compressed (flat) by the capsule (Fig. 4.14A). Fig. 4.13 Effect of accommodation on divergent rays entering the eye Fig. 4.14 Changes in the crystalline lens during accommodation Contraction of the ciliary muscle causes the ciliary ring to shorten and thus releases zonular tension on the lens capsule. This allows the elastic capsule to act unrestrained to deform the lens substance. The lens then alters its shape to become more convex or conoidal (to be more precise) (Fig. 4.14B). The lens assumes conoidal shape due to configuration of the anterior lens capsule which is thinner at the centre and thicker at the periphery. Chapter 4 Errors of refraction and Accommodation 47 Far point and near point The nearest point at which small objects can be seen clearly is called near point or punctum proximum and the distant (farthest) point is called far point or punctum remotum. Far point and near point of the eye. These vary with the static refraction of the eye as shown below (Fig. 4.15): • In an emmetropic eye far point is infinity (Fig. 4.15A) and near point varies with age. • In hypermetropic eye far point is virtual and lies behind the eye (Fig. 4.15B). • In myopic eye, it is real and lies in front of the eye (Fig. 4.15C). range and amplitude of accommodation Range of accommodation. The distance between the near point and the far point is called the range of accommodation. Amplitude of accommodation. The difference between the dioptric power needed to focus at near point (P) and far point (R) is called amplitude of accommodation (A). Thus A = P–R. ■Amplitude of accommodation and thus the near point of vision (punctum proximum) vary with age. Rough estimate is depicted in Table 4.1. A B C Fig. 4.15 Far point in (A) emmetropic eye; (B) hypermetropic eye; (C) myopic eye Table 4.1 Rough estimate of amplitude of accommodation and near point at different ages Age (in years) Amplitude of Near point in accommodation cms 10 14D 7 20 10D 10 30 07D 14 40 04D 25 50 02D 50 60 01D 100 ANOMALIES OF ACCOMMODATION Anomalies of accommodation are not uncommon. These include: • Presbyopia, • Insufficiency of accommodation, • Paralysis of accommodation, and • Spasm of accommodation. prESByopia pathophysiology and causes Presbyopia (eye sight of old age) is not an error of refraction but a condition of physiological insufficiency of accommodation leading to a progressive fall in near vision. Pathophysiology To understand the pathophysiology of presbyopia a working knowledge about accommodation (as described above) is mandatory. As we know, in an emmetropic eye far point is infinity (∞) and near point varies with age (being about 7 cm at the age of 10 years, 25 cm at the age of 40 years and 33 cm at the age of 45 years). Therefore, at the age of 10 years, amplitude of accommodation (A) =100/7 (dioptric power needed to see clearly at near point)–1/∞ (dioptric power needed to see clearly at far point) i.e., A (at age 10) = 14 dioptres; Similarly A (at age 40) = 100 1 = 4 dioptres. − ¥ 25 Since, we usually keep the book at about 25 cm, so we can read comfortably up to the age of 40 years. After the age of 40 years, near point of accommodation recedes beyond the normal reading or working range. This condition of failing near vision due to age-related decrease in the amplitude of accommodation or increase in punctum proximum is called presbyopia. 48 Section ii Optics and refraction Causes Decrease in the accommodative power of crystalline lens with increasing age, leading to presbyopia, occurs due to: 1. Age-related changes in the lens which include: • Decrease in the elasticity of lens capsule, and • Progressive increase in size and hardness (sclerosis) of lens substance which is less easily moulded. 2. Age-related decline in ciliary muscle power may also contribute in causation of presbyopia. Causes of premature presbyopia are: • Uncorrected hypermetropia. • Premature sclerosis of the crystalline lens. • General debility causing presenile weakness of ciliary muscle. • Chronic simple glaucoma. Symptoms 1. Difficulty in near vision. Patients usually complaint of difficulty in reading small prints (to start with in the evening and in dim light and later even in good light). Another important complaint of the patient is difficulty in threading a needle, etc. 2. Asthenopic symptoms due to fatigue of the ciliary muscle are also complained after reading or doing any near work. 3. Intermittent diplopia, occurring due to disturbed relationship between accommodation and convergence, may be experienced by few patients. treatment 1. Optical treatment. The treatment of presbyopia is the prescription of appropriate convex glasses for near work. Rough guide for providing presbyopic glasses in an emmetrope can be made from the age of the patient. • 45 years : + 1 to + 1.25D • 50 years : +1.5 to 1.75D • 55 years : +2 to + 2.25D • 60 years : +2.5 to + 3D Exact presbyopic addition required, should however, be estimated individually in each eye in order to determine how much is necessary to provide a comfortable range. Basic principles for presbyopic correction are: • Always find out refractive error for distance and first correct it. • Find out the presbyopic correction needed in each eye separately and add it to the distant correction. • Near point should be fixed by taking due consi­ deration for profession of the patient. • The weakest convex lens with which an individual can see clearly at the near point should be prescribed, since overcorrection will also result in asthenopic symptoms. Presbyopic spectacles may be unifocal, bifocal or varifocal, i.e. progressive (see page 55). 2. Surgical treatment of presbyopia is also being considered (see page 55). inSUFFiCiEnCy oF aCCommodation The term insufficiency of accommodation is used when the accommodative power is significantly less than the normal physiological limits for the patient’s age. Therefore, it should not be confused with presbyopia in which the physiological insufficiency of accommodation is normal for the patient’s age. Causes 1. Premature sclerosis of lens. 2. Weakness of ciliary muscle due to systemic causes of muscle fatigue such as debilitating illness, anaemia, toxaemia, malnutrition, diabetes mellitus, pregnancy, stress and so on. 3. Weakness of ciliary muscle associated with primary open­angle glaucoma. Clinical features All the symptoms of presbyopia are present, but those of asthenopia are more prominent than the blurring of vision. treatment 1. Treatment of underlying cause is essential. 2. Near vision spectacles in the form of weakest convex lens which allows adequate vision should be given till the power of accommodation improves. 3. Accommodation exercises help in recovery, if the underlying debility has passed. paralySiS oF aCCommodation Paralysis of accommodation also known as cycloplegia refers to complete absence of accommodation. Causes 1. Drug induced cycloplegia results due to the effect of atropine, homatropine or other parasympatholytic drugs. 2. Paralytic internal ophthalmoplegia (paralysis of ciliary muscle and sphincter pupillae) may result from neuritis associated with diphtheria, syphilis, diabetes, alcoholism, cerebral or meningeal diseases. 3. Paralysis of accommodation as a component of complete third nerve paralysis may occur due to intracranial or orbital causes. The lesions may be traumatic, inflammatory or neoplastic in nature. Clinical features 1. Blurring of near vision is the main complaint in previously emmetropic or hypermetropic Chapter 4 Errors of refraction and Accommodation 49 patients. Blurring of near vision may not be marked in myopic patients. 2. Photophobia (glare) due to accompanying dilatation of pupil (mydriasis) is usually associated with blurring of near vision. 3. Abnormal receding of near point and markedly decreased range of accommodation may be required on assessment. treatment 1. Self-recovery occurs in drug­induced cycloplegia and in diphtheric cases (once the systemic disease is treated). 2. Dark glasses are effective in reducing the glare. 3. Convex lenses for near vision may be prescribed if the paralysis is permanent. SpaSm oF aCCommodation Spasm of accommodation refers to exertion of abnormally excessive accommodation. Causes 1. Drug-induced spasm of accommodation is known to occur after use of strong miotics such as echothiophate and DFP. 2. Spontaneous spasm of accommodation is occasionally found in children who attempt to compensate for a refractive anomaly that impairs their vision. It usually occurs when the eyes are used for excessive near work in unfavourable circumstances such as bad illumination, bad reading position, lowered vitality, state of neurosis, mental stress or anxiety. Clinical features 1. Defective vision due to induced myopia. 2. Asthenopic symptoms are more marked than the visual symptoms. diagnosis It is made with refraction under atropine cycloplegia. treatment 1. Relaxation of ciliary muscle by atropine for few weeks and prohibition of near work allow prompt recovery from spasm of accommodation. 2. Correction of associated causative factors prevent recurrence. 3. Assurance and if necessary psychotherapy should be given. DETERMINATION OF REFRACTIVE ERRORS The procedure of determining refractive errors is termed as clinical refraction. methods of refraction 1. Objective refraction methods includes: • Retinoscopy, • Autorefractometry, and • Photorefraction. 2. Subjective refraction steps include: • Monocular subjective refraction, • Binocular balancing, and • Correction for near vision. (for details see chapter 25 page 576) CORRECTION OF REFRACTIVE ERRORS Modes of correcting refractive errors include: • Spectacles, • Contact lenses and • Refractive surgery. SPECTACLES The lenses fitted in a frame constitute the spectacles. It is a common, cheap and easy method of prescribing corrective lenses in patients with refractive errors and presbyopia. Some important aspects of the spectacles are described here: lens materials Broadly the lens material can be either glass or plastic. 1. Glass lenses Glass lenses are in use for spectacles since long. Mostly crown glass, with refractive index of 1.5223 is used. Problems with glass lenses include: • Shattering on impact, and • Thicker and heavy, especially in high powered lenses, due to lower refractive index. 2. Plastic lenses Plastic lenses being break resistance and light weight have become more popular. Various materials used are: Resin lenses or CR­39 plastic lenses made of an allye resin with refractive index of 1.49 are good alternative to crown glass lenses. These are light, unbreakable but less scratch resistant and thus need protective coating. Further, because of low refractive index, these lenses are a bit thicker. High index plastic lenses (refractive index 1.55 to 1.74) being thinner are preferred especially for high powered spectacles. Various plastic materials with high refractive index include polyurethane (Hyperindex), co­polymer (RLX­light) or allye base (True light). Polycarbonate, with a refractive index of 1.58, is another synthetic material used for making lenses. 50 Section ii Optics and refraction Such lenses are thinner, light weight, impact resistant and also have property of ultraviolet prot­ ection. lens shapes 1. Meniscus lenses are used for making spectacles in small or moderate degree of refractive errors. The standard curved lenses are ground with a concave posterior surface (–1.25D in the periscopic type or –6.0D in the deep meniscus type) and the spherical correction is then added to the anterior surface. 2. Lenticular form lenses are used for high plus and high minus lenses. In this type, the central portion is corrective and the peripheral surfaces are parallel to one another. 3. Aspheric lenses are also used to make high plus aphakic lenses by modifying the lens curvature peripherally to reduce aberrations and provide better peripheral vision. Single versus multiple power lenses 1. Single vision lens refers to a lens having the same corrective power over the entire surface. These are used to correct myopia, hypermetropia, astigmatism or presbyopia. 2. Bifocal lenses have different powers to upper (for distant vision) and lower (for near vision) segments. Different styles of bifocal lenses shown in Fig. 4.16 are: • Two piece bifocals, • Cemented supplementary wafer, • Inserted wafer, • Fused bifocals, and • Solid bifocals. 3. Trifocal lenses have three portions, upper (for distant vision), middle (for intermediate range vision) and lower (for near vision). 4. Multifocal (varifocal) or progressive lenses having many portions of different powers are also available. Fig. 4.16 Bifocal lenses. (A) two-piece; (B) cemented supplementary wafer; (C) inserted wafer; (D) fused; (E) solid tinted lenses Tinted glassesreduce the amount of light they transmit and provide comfort, safety and cosmetic effect. They are particularly prescribed in patients with albinism, high myopia and glare prone patients. Good tinted glasses should be dark enough to absorb 60–80% of the incident light in the visible part of the spectrum and almost all of the ultraviolet and infrared rays. Photochromatic lenses alter their colour according to the amount of ultraviolet exposure. These lenses do not function efficiently indoors and in automobiles. Centring and decentring • For proper centring, the visual axis of the patient and the optical centre of the spectacle lens should correspond, otherwise prismatic effect will be introduced. The distance between the visual axes is measured as interpupillary distance (IPD). • Decentring of the lens is indicated where prismatic effect is required. One prism dioptre effect is produced by 1 cm decentring of a lD lens. • Reading glasses should be decentred by about 2.5 mm medially and about 6.5 mm downward as the eyes are directed down and in during reading. Frames The spectacle frame selected should be comfortable, i.e. neither tight nor loose, light in weight and should not put pressure on the nose or temples of the patient, and should be of optimum size. In children large glasses are recommended to prevent viewing over the spectacles. Ideally, the lenses should be worn 15.3 mm from the cornea (the anterior focal plane of eye), as at this distance the images formed on the retina are of the same size as in emmetropia. CONTACT LENSES Contact lens is an artificial device whose front surface substitutes the anterior surface of the cornea. Therefore, in addition to correction of refractive error, the irregularities of the front surface of cornea can also be corrected by the contact lenses. parts, curves, and nomenclature for contact lens To understand the contact lens specifications following standard nomenclature has been recommended (Fig. 4.17). 1. Diameters of the contact lens are as follows: a. Overall diameter (OD) of the lens is the linear measurement of the greatest distance across the physical boundaries of the lens. It is expressed in millimetres (It should not be confused as being twice the radius of curvature). Chapter 4 Errors of refraction and Accommodation 51 Fig. 4.17 A contact lens b.Optic zone diameter (OZ) is the dimension of the central optic zone of lens which is meant to focus rays on the retina. 2. Curves of the lens are as follows: a.Base curve (BC) or central posterior curve (CPC) is a curve on the back surface of the lens to fit the front surface of cornea. b.Peripheral curves. These are concentric to base curve and include intermediate posterior curve (IPC) and peripheral posterior curve (PPC). These are meant to serve as reservoir of tears and to form a ski for lens movements. c. Central anterior curve(CAC) or front curve (FC) is the curve on the anterior surface of the optical zone of the lens. Its curvature determines the power of contact lens. d.Peripheral anterior curve (PAC) is a slope on the periphery of anterior surface which goes up to the edge. e. Intermediate anterior curve (IAC) is fabricated only in the high power minus and plus lenses. It lies between the CAC and PAC. 3.Edge of the lens. It is the polished and blended union of the peripheral posterior and anterior curves of the lens. 4.Power of the lens. It is measured in terms of posterior vertex power in dioptres. 5. Thickness of the lens. It is usually measured in the centre of the lens and varies depending upon the posterior vertex power of the lens. 6. Tint. It is the colour of the lens. types of contact lenses Depending upon the nature of the material used in their manufacturing, the contact lenses can be divided into following three types: • Hard lenses, • Rigid gas permeable lenses, and • Soft lenses. 1. Hard lenses are manufactured from PMMA (polymethylmethacrylate). The PMMA has a high optical quality, stability and is light in weight, nontoxic, durable and cheap. The hard corneal lenses have a diameter of 8.5–10 mm. Presently, these are not used commonly. ■Disadvantages of PMMA hard contact lenses. (i) PMMA is practically impermeable to O2 thus restricting the tolerance. (ii) Being hard, it can cause corneal abrasions. (iii) Being hydrophobic in nature, resists wetting but a stable tear film can be formed over it. Note. PMMA contact lenses are sparingly used in clinical practice because of poor patient acceptance. 2. Rigid gas permeable (RGP) lenses are made up of materials which are permeable to oxygen. Basically these are also hard, but somehow due to their O2 permeability they have become popular by the name of semisoft lenses. Gas permeable lenses are commonly manufactured from: • Silicone acrylate a copolymer of PMMA and silicone containing vinyl monomer. • Cellulose acetate butyrate (CAB), a class of thermoplastic material derived from special grade wood cellulose has also been used, but is not popular. 3. Soft lenses are made up of HEMA (hydroxye­ thylmethacrylate). These are made about 1–2 mm larger than the corneal diameter. ■Advantages: Being soft and oxygen permeable, they are most comfortable and so well tolerated. ■Disadvantages include problem of wettability, proteinaceous deposits, getting cracked, limited life, inferior optical quality, more chances of corneal infections and inability to correct astigmatism of more than 2 dioptres. Note. In clinical practice soft lenses are most frequently prescribed. indications of contact lens use 1. Optical indications. Optically contact lenses can be used by every patient having refractive error, if so required, for cosmetic purposes. However, the absolute indications include anisometropia, unilateral aphakia, high myopia, keratoconus and irregular astigmatism. Advantages of contact lenses over spectacles: • Irregular corneal astigmatism which is not possible to correct with glasses can be corrected with contact lenses. 52 Section ii Optics and refraction • Contact lenses provide normal field of vision. • Aberrations associated with spectacles (such as peripheral aberrations and prismatic distortions) are eliminated. • Binocular vision can be retained in high anisometropia (e.g., unilateral aphakia) owing to less magnification of the retinal image. • Rain and fog do not condense upon contact lenses as they do on spectacles. • Cosmetically more acceptable especially by females and all patients with thick glasses in high refractive errors. 2. Therapeutic indications are as follows: • Corneal diseases e.g., non­healing corneal ulcers, bullous keratopathy, filamentary keratitis and recurrent corneal erosion syndrome. • Diseases of iris such as aniridia, coloboma and albinism to avoid glare. • In glaucoma as vehicle for drug delivery. • In amblyopia, opaque contact lenses are used for occlusion. • Bandage soft contact lenses are used following keratoplasty and in microcorneal perforation. 3. Preventive indications include: • Prevention of symblepharon and restoration of fornices in chemical burns. • Exposure keratitis. • Trichiasis. 4. Diagnostic indications use during (i) gonioscopy; (ii) electroretinography; (iii) examination of fundus in the presence of irregular corneal astigmatism; (iv) fundus photography; (v) Goldmann’s 3 mirror examination. 5. Operative indications. Contact lenses are used during (i) goniotomy operation for congenital glaucoma; (ii) vitrectomy; and (iii) endocular photocoagulation. 6. Cosmetic indications include (i) unsightly corneal scars (colour contact lenses); (ii) ptosis (haptic contact lens); and (iii) cosmetic scleral lenses in phthisis bulbi. 7. Occupational indications include use by (i) sportsmen; (ii) pilots; and (iii) actors. Contraindications for contact lens use (i) Mental incompetence, and poor motivation; (ii) chronic dacryocystitis; (iii) chronic blepharitis and recurrent styes; (iv) chronic conjunctivitis; (v) dry eye syndromes; (vi) corneal dystrophies and degenerations; and (vii) recurrent diseases like episcleritis, scleritis and iridocyclitis. principles of fitting and care of lenses It is beyond the scope of this chapter. Interested readers are advised to consult some textbooks on contact lenses. REFRACTIVE SURGERY Surgery to correct refractive errors has become very popular. It should be performed after the refractive error has stabilized; preferably after 20 years of age. Various surgical techniques in vogue are described below: rEFraCtivE SUrGEry For myopia a. Cornea based procedures 1. Radial keratotomy (RK) Radial keratotomy (RK) refers to making deep (90% of corneal thickness) radial incisions in the peripheral part of cornea leaving the central 4 mm optical zone (Fig. 4.18). These incisions on healing; flatten the central cornea thereby reducing its refractive power. This procedure gives good correction in low to moderate myopia (2 to 6 D). However, because of its disadvantages and advent of safe techniques (LASIK and PRK) RK is not recommended presently. ■Disadvantages of RK: (i) Cornea is weakened, so chances of globe rupture following trauma are more after RK than after PRK. This point is particularly important for patients who are at high risk of blunt trauma, e.g., sports persons, athletes and military personnel. (ii) Rarely, uneven healing may lead to irregular astigmatism. (iii) Patients may feel glare at night. 2. Laser ablation corneal procedures a. Photorefractive keratectomy (PRK). In this technique, to correct myopia a central optical zone of anterior corneal stroma is photoablated using excimer laser (193­nm UV flash) to cause flattening of the central cornea (Fig. 4.19). Like RK, the PRK also gives very good correction for –2 to –6 D of myopia. A B Fig. 4.18 Radial keratotomy. (A) configuration of radial incisions; (B) depth of incision Chapter 4 Errors of refraction and Accommodation 53 A B Fig. 4.19 Photorefractive keratectomy (PRK) for myopia as seen (A) from front; (B) in cross section b. Laser in­situ keratomileusis (LASIK). In this technique first a flap of 130–160 micron thickness of anterior corneal tissue is raised with the help of an automated microkeratome. Recently, femtosecond laser is being used for more accurate and smooth flaps. After creating a corneal flap midstromal tissue is ablated directly with an excimer laser beam, ultimately flattening the cornea (Fig. 4.20). Currently, this procedure is being considered the refractive surgery of choice for myopia of up to –8 D. Patient selection criteria are: • Patients above 20 years of age. • Stable refraction for at least 12 months. • Motivated patient. • Absence of corneal pathology. • Presence of ectasia or any other corneal pathology and a corneal thickness less than 450 mm is an absolute contraindication for LASIK.. Fig. 4.20 Procedure of laser in-situ keratomileusis (LASIK) Advances in LASIK. Recently many advances have been made in LASIK surgery. Some of the important advances are: • Customized (C) LASIK. C­LASIK is based on the corneal topography and wave front technology. This technique, in addition to spherical and cylindrical correction, also corrects the aberrations present in the eye and gives vision beyond 6/6 i.e., 6/5 or 6/4. • Femto–LASIK also known as ‘All laser LASIK’ or ‘No blade LASIK’ refers to the technique in which the corneal flap is made with the help of femtosecond laser (rather than the micro keratome) for greater precision and consistency. • Custom Femto–LASIK (C F – LASIK ) refers to corneal topography and wave front guided LASIK in which corneal flap is made with femtosecond laser. • Epi-(E) LASIK.In this technique instead of corneal stromal flap only the epithelial sheet is separated mechanically with the use of a customized device (Epiedge Epikeratome). Being an advanced surface ablation procedure, it is devoid of complications related to corneal stromal flap. Advantages of LASIK. (i) Minimal or no postoperative pain. (ii) Recovery of vision is very early as compared to PRK. (iii) No risk of perforation during surgery and later rupture of globe due to trauma unlike RK. (iv) No residual haze unlike PRK where subepithelial scarring may occur. (v) LASIK is effective in correcting myopia of –8 D. Disadvantages. 1. LASIK is much more expensive. 2. It requires greater surgical skill than RK and PRK. 3. There is potential risk of flap related complications which include (i) intraoperative flap amputation, (ii) wrinkling of the flap on repositioning, (iii) postoperative flap dislocation/subluxation, (iv) epithelization of flap­bed interface, and (v) irregular astigmatism. 4. Other possible complications include infection, diffuse lamellar keratitis, dry eye, corneal ectasia, glare and regression. 3. Refractive lenticule extraction (ReLEx) Refractive lenticule extraction (ReLEx), also called as ‘All–Femtolaser­Vision­Correction’, is a technique in which a lenticule of corneal stroma is extracted with the help of femtosecond laser. The technique is now named SMILE (small incision lenticule extraction) (Fig. 4.21). This technique can correct myopia, with and without astigmatism, upto 10D. 4. Intercorneal ring (ICR) implantation Intercorneal ring (ICR) implantation into the peripheral cornea at approximately 2/3 stromal depth 54 Section ii Optics and refraction A B Fig. 4.21 Surgical technique of small incision lenticule extraction (SMILE): A, Creation of lenticule and anterior side-cut; and B, Removal of lenticule is being considered. It results in a vaulting effect that flattens the central cornea, decreasing myopia. The ICR procedure has the advantage of being reversible. Disadvantages of ICR include unpredictable results and keratitis. 5. Orthokeratology Orthokeratology is a non­surgical reversible method of molding the cornea with overnight wear of unique rigid gas permeable contact lenses. It is being considered for correction of myopia upto –5D. It can be used even in patients below 18 years of age. B. lens based procedures 1. Refractive lens exchange. Extraction of clear crystalline lens (Fucala’s operation) has been advocated long back for myopia of –16 to –18D, especially in unilateral cases. Recently, clear lens extraction with intraocular lens (IOL) implantation of appropriate power, i.e., refractive lens exchange (RLE) is being recommended as the refractive surgery for myopia of more than 12D. Possible Complications include endophthalmitis, after cataract and retinal detachment. 2.Phakic refractive lens(PRL) or implantable contact lens (ICL) is also being considered for correction of myopia of >8D. In this technique, a special type of intraocular lens (IOL) is implanted in the anterior chamber or posterior chamber anterior to the natural crystalline lens. Possible Complications include endophthalmitis, Iridocyclitis, cataract formation and secondary glaucoma. rEFraCtivE SUrGEry For HypEropia In general, refractive surgery for hyperopia is not as effective or reliable as for myopia. However, following procedures are used: i. Cornea based procedures 1. Thermal laser keratoplasty (TLK) has been used for low degree of hyperopia. In this technique, 8 laser spots are applied in a ring at the periphery to produce central steepening with mid­infrared energy from Thallium­Holmium­Chromium (THC):YAG laser. Regression effect and induced astigmatism are the main problems. 2. Hyperopic PRK using excimer laser has also been tried. Regression effect and prolonged epithelial healing are the main problems encountered. 3. Hyperopic LASIK is effective in correcting hypermetropia upto +4D. 4. Conductive keratoplasty (CK) is nonablative and nonincisional procedure in which cornea is steepened by collagen shrinkage through the radiofrequency energy applied through a fine tip inserted into the peripheral corneal stroma in a ring pattern. This technique is effective for correcting hyperopia of upto 3D. ii. lens based procedures 1. Phakic refractive lens (PRL) or implantable contact lens (ICL) is being considered a surgical option for hyperopia of more than + 4D. 2. Refractive lens exchange (RLE) is a good option for high hyperopia especially in presbyopic age. rEFraCtivE SUrGEry For aStiGmatiSm Refractivesurgical techniques employed for myopia can be adapted to correct astigmatism alone or simultaneously with myopia as follows: 1. Astigmatic keratotomy (AK) refers to making transverse cuts in the mid­periphery of the steep corneal meridian (Fig. 4.22). AK can be performed Chapter 4 Errors of refraction and Accommodation 55 Fig. 4.22 Astigmatic keratotomy. (A) showing flat and deep meridians of cornea; (B) paired transverse incisions to flatten the steep meridian; (C) showing correction of astigmatism after astigmatic keratotomy alone (for astigmatism only) or along with RK (for associated myopia). 2. Photo-astigmatic refractive keratotomy (PARK) is performed using excimer laser. 3. LASIK procedure can also be adopted to correct astigmatism upto 5 D. 4. SMILE procedure can also be adopted to correct astigmatism. manaGEmEnt oF poSt-KEratoplaSty aStiGmatiSm 1. Selective removal of sutures in steep meridians may improve varying degrees of astigmatism and should be tried first of all. Note. Other procedures mentioned below should be performed only after all the sutures are out and refraction is stable. 2. Arcuate relaxing incisions in the donor cornea along the steep meridian may correct astigmatism up to 4–6D. 3. Relaxing incisions combined with compression sutures may correct astigmatism up to 10D. 4. Corneal wedge resection with suture closure of the wound may be performed in the flat meridian to correct astigmatism greater than 10D. 5. LASIK procedure can also be adopted to correct post­keratoplasty astigmatism. rEFraCtivE SUrGEry For prESByopia Refractive surgery for presbyopia, still under trial, includes: I. Cornea based procedures 1. Monovision LASIK, i.e., one eye is corrected for distance and other is made slightly near sighted. 2. Monovision conductive keratoplasty (CK) is being considered increasingly to correct presbyopia in one eye (non­dominant). Principle is same as for correction of hypermetropia (see page 54). 3. Presbyopic bifocal LASIK or LASIK-PARM, i.e., LASIK by Presbyopia Avalos Rozakis Method is a technique undertrial in which the shape of the cornea is altered to have two concentric vision zones that help the presbyopic patient to focus on near and distant objects. II. Lens based procedures Multifocal or accommodating IOL implantation after lens extraction especially in patients with cataract or high refractive errors (refractive lens exchange) correct far as well as near vision. Monovision with intraocular lenses, i.e., correction of one eye for distant vision and other for near vision with IOL implantation after bilateral cataract extraction also serves as a solution for far and near correction. Similarly, monovision with clear lens extraction and intraocular lens implantation (refractive lens exchange i.e., RLE) is also being considered increasingly in presbyopic patients with associated higher refractive errors. III. Sclera based procedures Following scleral expansion procedures are being tried, but results are controversial. 1. Anterior ciliary sclerotomy (ACS), with tissue barriers is currently under trial. With initial encouraging results, multi­site clinical studies are planned for US and Europe to evaluate this technique. 2. Scleral spacing procedures and scleral ablation with erbium: YAG laser are other sclera­based procedures still under trial. 3. Scleral expansion with insertion of intrascleral segments of collagen or silicone expansion plugs may help by improving accommodation. Section III Diseases of Eye Section Outline 5. Diseases of Conjunctiva 6. Diseases of Cornea 7. Diseases of Sclera 8. Diseases of Uveal Tract 9. Diseases of Lens 10. Glaucoma 11. Diseases of Vitreous 12. Diseases of Retina 13. Neuro-Ophthalmology 14. Disorders of Ocular Motility 15. Disorders of Eyelids 16. Diseases of Lacrimal Apparatus 17. Diseases of Orbit 18. Ocular Injuries 5 Diseases of Conjunctiva CHAPTER OUTLINE APPLIED ANATOMY Parts Structure Glands • • • INFLAMMATIONS OF CONJUNCTIVA Infective conjunctivitis • – – – – Bacterial Chlamydial Viral Granulomatous • • • Allergic conjunctivitis Cicatricial conjunctivitis Toxic conjunctivitis APPLIED ANATOMY Conjunctiva is a translucent mucous membrane which lines the posterior surface of the eyelids and anterior aspect of the eyeball. The name conjunctiva (conjoin: to join) has been given to this mucous membrane owing to the fact that it joins the eyeball to the lids. It stretches from the lid margin to the limbus, and encloses a complex space called conjunctival sac which is open in front at the palpebral fissure. PARTS OF CONJUNCTIVA Conjunctiva can be divided into three parts (Fig. 5.1): 1. Palpebral conjunctiva. It lines the lids and can be subdivided into marginal, tarsal and orbital conjunctiva. i. Marginal conjunctiva extends from the lid margin to about 2 mm on the back of lid up to a shallow groove, the sulcus subtarsalis. It is actually a transitional zone between skin and the conjunctiva proper. ii. Tarsal conjunctiva is thin, transparent and highly vascular. It is firmly adherent to the whole tarsal plate in the upper lid. In the lower lid, it is adherent only to half width of the tarsus. The tarsal glands are seen through it as yellow streaks. DEGENERATIVE CONDITIONS Pinguecula Pterygium Concretions • • • • Amyloid degeneration SYMPTOMATIC CONDITIONS OF CONJUNCTIVA Hyperaemia • • • • • Chemosis Ecchymosis Xerosis Discoloration CYSTS AND TUMOURS Cysts of conjunctiva Tumours of conjunctiva • • iii. Orbital part of palpebral conjunctiva lies loose between the tarsal plate and fornix. 2. Bulbar conjunctiva. It is thin, transparent and lies loose over the underlying structures and thus can be moved easily. It is separated from the anterior sclera by episcleral tissue and Tenon’s Fig. 5.1 Parts of conjunctiva and conjunctival glands 60 Section III Diseases of Eye capsule. A 3 mm ridge of bulbar conjunctiva around the cornea is called limbal conjunctiva. In the area of limbus, the conjunctiva, Tenon’s capsule and the episcleral tissue are fused into a dense tissue which is strongly adherent to the underlying corneoscleral junction. At the limbus, the epithelium of conjunctiva becomes continuous with that of cornea. 3. Conjunctival fornix. It is a continuous circular cul-de-sac which is broken only on the medial side by caruncle and the plica semilunaris. Conjunctival fornix joins the bulbar conjunctiva with the palpebral conjunctiva. It can be subdivided into superior, inferior, medial and lateral fornices. Structure of conjunctiva Histologically, conjunctiva consists of three layers namely, epithelium, adenoid layer, and fibrous layer (Fig. 5.2). 1. Epithelium. This is a 2–5 layered, non-keratinized epithilium. It also contains goblet cells which constitute about 10% of epithelium. The layer of epithelial cells in conjunctiva varies from region to region and in its different parts as follows: A B Fig. 5.2 Microscopic structure of conjunctiva showing three layers (A), and arrangement of epithelial cells in different regions of conjunctiva (B) • Marginal conjunctiva has 5-layered stratified squamous type of epithelium. • Tarsal conjunctiva has 2-layered epithelium: superficial layer of cylindrical cells and a deep layer of flat cells. • Fornix and bulbar conjunctiva have 3-layered epithelium: a superficial layer of cylindrical cells, middle layer of polyhedral cells and a deep layer of cuboidal cells. • Limbal conjunctiva has again many layered (5 to 6) stratified squamous epithelium. Limbal stem cells are present in basal layer of this part. 2. Adenoid layer. It is also called lymphoid layer and consists of fine connective tissue reticulum in the meshes of which lie lymphocytes. This layer is most developed in the fornices. It is not present since birth but develops after 3–4 months of life. For this reason, conjunctival inflammation in an infant does not produce follicular reaction. 3. Fibrous layer. It consists of a meshwork of collagenous and elastic fibres. It is thicker than the adenoid layer, except in the region of tarsal conjunctiva, where it is very thin. This layer contains vessels and nerves of conjunctiva. It blends with the underlying Tenon’s capsule in the region of bulbar conjunctiva. Glands of conjunctiva The conjunctiva contains two types of glands (Fig. 5.1) 1. Mucin secretory glands. These include: • Goblet cells (the unicellular glands located within the epithelium), • Crypts of Henle (present in the tarsal conjunctiva), and • Glands of Manz (found in limbal conjunctiva). These glands secrete mucus which is essential for wetting the cornea and conjunctiva. 2. Accessory lacrimal glands. These are: • Glands of Krause, present in subconjunctival connective tissue of fornices, about 42 in the upper fornix and 8 in the lower fornix, and • Glands of Wolfring, present along the upper border of superior tarsus and along the lower border of inferior tarsus. Plica semilunaris It is a pinkish crescentric fold of conjunctiva, present in the medial canthus. Its lateral free border is concave. It is a vestigeal structure in human beings and represents the nictitating membrane (or third eyelid) of lower animals. Chapter 5 Diseases of Conjunctiva 61 Caruncle The caruncle is a small, ovoid, pinkish mass, situated in the inner canthus, just medial to the plica semilunaris. In reality, it is a piece of modified skin and so is covered with stratified squamous epithelium and contains sweat glands, sebaceous glands and hair follicles. Blood supply of conjunctiva Arteries supplying the conjunctiva are derived from three sources (Fig. 5.3): (1) peripheral arterial arcade of the eyelid; (2) marginal arcade of the eyelid; and (3) anterior ciliary arteries. ■Palpebral conjunctiva and fornices are supplied by branches from the peripheral and marginal arterial arcades of the eyelids. ■Bulbar conjunctiva is supplied by two sets of vessels: • Posterior conjunctival arteries which are branches from the arterial arcades of the eyelids; and • Anterior conjunctival arteries which are the branches of anterior ciliary arteries. Terminal branches of the posterior conjunctival arteries anastomose with the anterior conjunctival arteries to form the pericorneal plexus. Veins from the conjunctiva drain into the venous plexus of eyelids and some around the cornea into the anterior ciliary veins. Lymphatics of the conjunctiva are arranged in two layers: a superficial and a deep. Lymphatics from the lateral side drain into preauricular lymph nodes and those from the medial side into the submandibular lymph nodes. Nerve supply of conjunctiva • A circumcorneal zone of conjunctiva is supplied by the branches from long ciliary nerves which supply the cornea. Fig. 5.3 Blood supply of conjunctiva • Rest of the conjunctiva is supplied by the branches from lacrimal, infratrochlear, supratrochlear, supraorbital and frontal nerves. INFLAMMATIONS OF CONJUNCTIVA Inflammation of the conjunctiva (conjunctivitis) is classically defined as conjunctival hyperaemia associated with a discharge which may be watery, mucoid, mucopurulent or purulent. Types of Conjunctivitis Common types of conjunctivitis include: A. Infective conjunctivitis 1. Bacterial conjunctivitis • Acute bacterial conjunctivitis • Hyperacute bacterial conjunctivitis • Chronic bacterial conjunctivitis • Angular bacterial conjunctivitis 2. Chlamydial conjunctivitis • Trachoma • Adult inclusion conjunctivitis • Neonatal chlamydial conjunctivitis 3. Viral conjunctivitis • Adenovirus conjunctivitis – Epidemic keratoconjunctivitis – Pharyngoconjunctival fever • Enterovirus conjunctivitis • Molluscum contagiosum conjunctivitis • Herpes simplex conjunctivitis 4. Ophthalmia neonatorum (A separate entity) 5. Granulomatous conjunctivitis • Parinaud oculoglandular syndrome B. Allergic Conjunctivitis 1. Simplex allergic conjunctivitis • Hay fever conjunctivitis (rhino conjunctivitis) • Seasonal allergic conjunctivitis (SAC) • Perennial allergic conjunctivitis (PAC) 2. Vernal keratoconjunctivitis (VKC) 3. Atopic keratoconjunctivitis 4. Giant papillary conjunctivitis (GPC) 5. Phlyctenular conjunctivitis (PKC) 6. Contact dermoconjunctivitis (drop conjunctivitis) C. Cicatricial conjunctivitis • Ocular mucous membrane pemphigoid (OMMP), • Stevens Johnson syndrome (SJS), • Toxic epidermal necrolysis (TeN), and • Secondary cicatricial conjunctivitis. D. Toxic conjunctivitis A. INFECTIVE CONJUNCTIVITIS Infective conjunctivitis, i.e., inflammation of the conjunctiva caused by micro-organisms is the 62 Section III Diseases of Eye commonest variety. This is in spite of the fact that the conjunctiva has been provided with natural protective mechanisms in the form of: • Low temperature due to exposure to air, • Physical protection by lids, • Flushing action of tears, • Antibacterial activity of lysozymes, and • Humoral protection by the tear immunoglobulins. BACTERIAL CONJUNCTIVITIS There has occurred a relative decrease in the incidence of bacterial conjunctivitis in general and those caused by Gonococcus and Corynebacterium Diphtheriae in particular. However, in developing countries it still continues to be the commonest type of conjunctivitis. It can occur as sporadic and epidemics cases. Outbreaks of bacterial conjunctivitis, epidemics are quite frequent during monsoon season. Etiology A. Predisposing factors for bacterial conjunctivitis, especially epidemic forms, are flies, poor hygienic conditions, hot dry climate, poor sanitation and dirty habits. These factors help the infection to establish, as the disease is highly contagious. B. Causative organisms. It may be caused by a wide range of organisms in the following approximate order of frequency: • Staphylococcus aureus is the most common cause of bacterial conjunctivitis and blepharoconjunctivitis. • Staphylococcus epidermidis is an innocuous flora of lid and conjunctiva. It can also produce blepharoconjunctivitis. • Streptococcus pneumoniae (pneumococcus) produces acute conjunctivitis usually associated with petechial subconjunctival haemorrhages. The disease has a self-limiting course of 9–10 days. • Streptococcus pyogenes (haemolyticus) is virulent and usually produces pseudomembranous conjunctivitis. • Haemophilus influenzae (aegyptius, Koch-Weeks bacillus). It classically causes epidemics of mucopurulent conjunctivitis, known as ‘red-eye’ especially in semitropical countries. • Moraxella lacunata (Moraxella Axenfeld Bacillus) is most common cause of angular conjunctivitis and angular blepharoconjunctivitis. • Pseudomonas pyocyanea is a virulent organism, which readily invades the cornea. • Neisseria gonorrhoeae typically produces acute purulent conjunctivitis in adults and ophthalmia neonatorum in newborn. It is capable of invading intact corneal epithelium. • Neisseria meningitidis (meningococcus) may produce mucopurulent conjunctivitis. • Corynebacterium diphtheriae causes acute membranous conjunctivitis. Such infections are not known nowadays.