Ординатура / Офтальмология / Английские материалы / Pediatric Opthalmology_Mukherjee_2005

54.Shukla IM, Mukherjee P.K., Sushma Verma : Primary rhinosporidiosis of the eye. Int. Congress of Ophthalmology. ACTA XXVI Vol. 2, 864 : 1982.

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56.Mukherjee P.K., Agrawal S. : Subconjunctival twin cysticercosis. Ind. Jr. Oph. 23 : 2829, 1975.

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many micro-organism. Thus most of the micro-organism can not invade the cornea so long as the epithelium is intact. The organisms that can bypass the epithelial barrier are gonococcus, meningococcus and C. diptheria. The corneal epithelium is most mitotically active, thus healing of the epithelium is very fast. The corneal epithelium can be rubbed or pealed off with ease.

2. The Bowman’s membrane is not a true membrane. It is formed by condensation of anterior layer of stroma. It is one millimetre shorter than rest of the cornea in all directions. The shortness of the Bowman’s membrane is because its development is completed before full development of rest of the cornea.

Bowman’s membrane is easily destroyed and does not degenerate.

3.The stroma forms 90% bulk of the cornea and is continuous with the sclera.

4.The Descemets membrane is the basal membrane of corneal endothelium. Its thickness increases with age. It is tough and can withstand considerable amount of raised intraocular pressure and offers resistance to infection.

5.Corneal endothelium is single layered structure that is metabolically very active. It is continuous with endothelium of the iris. The endothelium maintains deturgescence of the cornea, keeping it transparent and bright. Damage to endothelium leads to edema of the cornea. The damage can be brought about by trauma, inflamation, toxin and dystrophy.

Whole of the cornea has high metabolism for which it requires constant supply of oxygen from the atmosphere that is dissolved in tear film, when the lids are open. During sleep, the oxygen permeates through the vessels of tarsal conjunctiva. The other source of nutrition to the avascular cornea is from limbal blood vessels which are present only in the peripheral 1 mm all round. The aqueous supplies nutrition to the endothelium.

The nerve supply of the cornea is by trigeminal nerve via ciliary nerve. The nerve fibers enter the stroma from sclera, and conjunctiva. The cornea is very sensitive to pain, all sensation i.e. touch temperature and pressure are converted to pain sensation.

Physiology of The Cornea11, 12, 13, 14

The two functions of the cornea are—1. Optical, and 2. Protective.

1. Optical. For light to travel uninterrupted with out scatter, the cornea must have (a) An optically regular surface and (b) A transparent medium.

Both these are met with due to a smooth epithelial surface, constant supply of nutrient, relative dehydration. The cornea transmit cent percent of white light i.e. 380 nm to 760 nm.

Nutrition of Cornea11, 12, 14

Cornea is highly metabolic tissues. It requires constant supply of oxygen and energy. The oxygen is derived (1) mainly from the atmospheric air dissolved in tear film.(2) from the perilimbal blood vessels (3) from the tarsal conjunctival vessels when the lids are closed.

Other nutrients are supplied by the aqueous from behind. The energy to the cornea is supplied by metabolism of glucose in the form of adenosine triphosphate (A.T.P.). The epithelium can metabolise glucose both aerobically and anaerobically. Most of the metabolism takes place in the epithelium and endothelium. There are three metabolic pathways

involved in metabolism of cornea. They are 1. Glycolysis 65% (2) Kerb-cycle (3) Hexose monophosphate shunt.

Corneal transparency depends upon following factors :

1.Anatomical

2.Deturgscence of cornea

3.Intraocular pressure

4.Uniform refractive index of various layers.

The anatomical factors are :

Absence of vessels in the centre of the cornea. There are no pigments in the cornea, the epithelium is multi-layered but the cells are arranged in a regular fashion. The epithelium is ten times thicker than endothelium yet metabolic requirement of endothelium is more than epithelium. The single layered endothelium is arranged in a regular fashion so are the stromal collagen fibrils. The epithelial cells which are exposed to atmosphere for long time remain non keratanised in normal eye. The tear film obliterates whatever irregularity the epithelial surfaces may have in normal eye to give a better optical surface.

Deturgscence of cornea. The normal cornea should remain in a state of relative dehydration. Accumulation of fluid causes the stroma to lose its transparency. The state of relative dehydration is done by active transference of fluid from stroma towards epithelium and by way of an endothelial pump system from stroma. Loss of endothelial cells is result of embibement of fluid in stroma and epithelial edema. Intraocular Pressure - Acute rise of intraocular pressure causes derangement of fluid transport across the stroma and causes stromal edema and loss of transparency.

Refractive index. The cornea is most important refractive surface of the eye. The anterior surface has more important role as refractive surface than posterior surface. The refractive index of cornea is 1.38.

The cornea protects the eye from most of organism and cuts off ultra violet rays as well. It helps to maintain the structural integrity of the globe.

Development of cornea6, 7, 8, 9, 10. Cornea develops from two sources, surface ectoderm and mesoderm. The former gives rise to corneal epithelium while rest of cornea is mesodermal in origin. As the lens vescicle separates from the surface ectoderm a space develops. This space is invaded by mesoderm that gives rise to all corneal layers except the epithelium. Endothelium cells are first to be differentiated at 12 mm (5 weeks) stage. The Descemet’s membrane is visible at 12 weeks. The Bowman’s membrane is recognisable at 100 mm (4 months). This is the stage when corneo scleral junction is also well defined.

Congenital anomalies of the cornea. Congenital anomalies of the cornea may be localised to the cornea only or associated with other anomalies of the globe. They may be unilateral or bilateral, need not be symmetrical in bilateral cases.

Common congenital anomalies of cornea are : Microcornea

Megalocornea Cornea plana

Keratoglobus Keratoconus26,27,28,29 Anterior embryotoxon Sclero cornea Corneal dystrophies

Microcornea5, 16, 17. Term microcornea is used to denote a condition where the corneal diameter is less than 10 mm in otherwise normal eye. In these cases whole of the anterior segment is small, hence the term anterior microphthalmia is also used to denote the same condition. The recti are inserted more anteriorly than normal eyes. Corneal curvature is flat, resulting into hypermetropia. Other types of errors of refraction are also possible secondary to variation in curvature of cornea and shape of the lens in relation to the cornea. The inter palpebral fissure is narrow. All microphthalmos eyes have small cornea but all eyes with micro cornea need not be microphthalmic. Sometimes it is difficult to differentiate between pure micro-cornea and microphthalmos especially if there are other associated anomalies of the globe like coloboma, microphakia, mesodermal tissue in angle of anterior chamber. In about one fifth of cases there may be glaucoma due to anomalies of the angle. It should not be confused with another congenital anomaly of the globe i.e. nanophtalmos where the eye is small in all meridian but is normal in function.

Megalocornea. This condition is rarer than microcornea and microphthalmos, condition is bilateral, non progressive where anterior segment of the eye is larger than normal. The corneal diameter varies from 13 mm to 16 mm. As the diameter is more there is increase in circumference of the cornea. Corneal curvature is generally normal but may be increased with myopic astigmatism, which is “astigmatism with the rule”. In absence of myopia vision is unaffected. Anterior chamber is deep. The lens may be large. There may be both phacodonesis as well as iridodonesis. Two common complications met with the condition are glaucoma and cataract.

The condition must be differentiated from more serious, progressive vision threatening buphthalmos and keratoglobus that is almost symptomless.

Cornea plana. In this rare condition corneal curvature is reduced. In extreme cases cornea may be flat generally associated with decreased scleral curvature. Sclera may encroach over the cornea giving an impression of small cornea. Sometimes cornea itself is small. There may be diffuse opacities in the cornea. Due to flattening of cornea there is impression of ptosis. The eyes are generally hypermetropic. Anterior chamber is shallow. There may be associated glaucoma. Causes of poor vision is curvature hypermetropia and corneal opacity.

Differential diagnosis consists of micro-cornea, microphthalmos, phthisis, atrophic bulbae and nanophtalmos.

Keratoglobus. This is a relatively rare bilateral condition where corneal diameter is normal but the corneal curvature is increased. The cornea becomes globular in contrast to keratoconus where cornea is conical. In keratoglobus the cornea is thinner than normal almost throughout. The sclera may also be thinned. The condition is stationary and symptomless cases are detected on routine examination. Tension and angle remain normal throughout. Differential diagnosis include all cases of enlarged cornea and corneal causes of deep anterior chamber. Other members of the family may show keratoconus or irregular myopic astigmatism. It should also be differentiated from pellucid degeneration14,15 of cornea that is generally seen after 30 years of age and has a toric bulge in the lower part of the cornea.

Keratoconus (Conical cornea)18, 20, 21, 22, 23, 24, 25. Keratoconus is a non inflammatory ectasia of central cornea due to obscure causes. Some authors classify it as congenital disorder presuming that there is failure of mesoderm from periphery to centre, resulting into weakness. It is worth noting that most of the pathological changes are seen in corneal structure that are mesodermal in origin. Others have put keratoconus under degeneration27 while most put it under dystrophy. Keratoconus is suspected to be due to a kerato softening enzyme14. Keratoconus has world-wide distribution, no race is immune. Proponents of theory of congenital anomaly feel that subtle changes are present years before the disorder becomes manifest.

The disease is bilateral and progressive. One eye is generally more involved than the other. In rare instance the disease has been described as unilateral, in such cases the other eye shows irregular myopic astigmatism without corneal thinning and ectasia. The disease is said to be more common in girls. Generally symptoms start round ten years and progress for eight to ten years, then either the progress comes to standstill or is very slow.7 Increase in keratoconus has been noted up to fourth and fifth decade. In few cases there is a steady increase in keratoconus followed by a phase of non progression to be followed by a period of progression again. Generally puberty heralds a year or two of rapid progression. Though contact lenses are prescribed as treatment of keratoconus they may occasionally worsen it. Repeated rubbing of eye has also been thought to be a contributory factor.27 Keratoconus is non hereditary in 95% of cases. In remaining it can either be autosomal dominant or recessive. Keratoconus generally presents as isolated ocular disorder, however, some ocular disorders are more frequently associated with keratoconus than others. These are—Spring catarrah, retinitis pigmentosa, ectopia lentis, anirida, anterior polar cataract, blue sclera.

Systemic conditions that are frequently associated - atopic dermatitis, Down syndrome Marfan’s syndrome, Lebers familial amaurosis, Ehler-Danols syndrome, neurofibromatosis, mitral valve prolapse.

Clinical features consist of diminished distant vision which is progressive, glare, photophobia, uniocular diplopia.

Signs. Vary according to severity of the case :

In early stages the corneal bulge is minimal and can not be detected on routine examination. Retinoscopy reveals compound myopic astigmatism with oblique axis. Sometimes astigmatism is irregular which is confirmed by Placido disc, Klein keratoscope, and keratometery. Computerised corneal topography is more informative than keratometery. Corneal mapping through photo keratoscope detect very early and subtle changes.

Slit lamp biomicroscopy shows fine vertical lines at the level of deep stroma and Descemet’s membrane, the lines disappear on pressure over the globe. Sometimes prominent corneal nerves are also visible.

Late cases do not present any difficulty in diagnosis. In all sporadic cases of increasing myopic astigmatism, keratoconus should always be kept in mind. On oblique illumination, the cornea looks conical on profile with deep anterior chamber. Generally the distance of corneal vertex to plane of limbus is 2.5 mm. In keratoconus this may increase to 4.5 mm to 5 mm.

In normal cornea, if a narrow beam of light is thrown at the corneoscleral junction from lateral side, the opposite limbus lightsup in a circular fashion. In case of keratoconus, the light reflex on the other side is conical.28

If the patient is asked to look down, the apex of the cornea produces a notch in the lower lid (Munson’s sign). The apex of the cone is not opposite the centre of pupil, it is shifted downwards and medially. The apex may show some opacity.

On keratoscopy and keratometry the findings are exaggerated. Keratometer may fail to measure the cone fully and may require additional plus lenses in the keratometer to measure the curvature completely.

Retinoscopy with plane mirror shows a central dark spot, corresponding to apex of the cone, surrounded by a zone of pink retinoscopic glow, surrounded by a dark rim and then a pink ring up to periphery. The intermediate dark rim represents the base of the cone where there is near total internal reflection. The retinoscopy shows scissors movements and the axis is generally oblique.

On slit lamp biomicroscopy with a bright narrow beam, the central part looks thin while peripheral cornea has uniform, normal thickness. In 50% of eyes an incomplete yellow green ring is visible at the base of the cone. This is due to deposition of hemosiderin superficial to Bowman’s membrane. In advanced cases there may be rupture in Descemet’s membrane, leading to photophobia and lacrimation. Sometimes there may be rupture in Bowman’s membrane as well.

Hydrops of keratoconus denotes acute edema of the cornea due to rupture of Descemet’s membrane resulting into imbibement of aqueous through endothelium. The whole of the cornea becomes opaque, associated with pain, lacrimation and rapid fall of vision. The stage of corneal hydrop lasts for 6-10 weeks and clears with healing of the rupture, however, it leaves some degree of stromal scarring.

Though perforation in high degree of keratoconus has been reported, it is otherwise very rare.

Management of keratoconus29

1.Initially fairly good vision is attained by prescription of spectacles which require frequent changes.

2.In moderate cases where spectacles do not give satisfactory visual improvement contact lenses are prescribed initially, later on special piggy back contact lenses which are a combination of hard and soft contact lenses are prescribed where a hard lens is fitted over soft lens. However, sometimes contact lenses have been reported to worsen keratoconus.

3.Penetrating keratoplasty with a large donar button have very good prognosis, however, induced astigmatism may require correction by contact lens. Thermo keratoplasty, lamelar keratoplasty and epikeratoplasty have not proved better than penetrating keratoplasty, which is a definitive surgical treatment for keratoconus.

Acute hydrops may require lowering of tension by systemic acetazolamide and betablocker. Local instillation of hypertonic solutions, patching or use of a bandage lens may reduce discomfort. Local non-steroidal anti-inflammation drugs along with cycloplegic give relief from pain and photophobia. Generally steroids are not required.

Common Corneal Disorders Seen in Children

Children are prone to suffer from various corneal disorders. Some are very mild and may not reduce vision. Some may reduce vision marginally that pass off with treatment. Other cause extensive visual damage that may be permanent even with best treatment. Many of the corneal disorders are preventable.

Common corneal disorders seen in children are :

Malignancies of cornea are not known, however a growth of conjunctiva at limbus may invade the cornea. Generally such growths are seen only in adults except the limbal dermoid that may be present at birth or may manifest later.

Some Pathological Signs of Corneal Disorder Seen in Children are

Disorders of cornea produce following changes in cornea :

1.Morphology—Shape, size, curvature, surface, thickness

2.Loss of transparency—Opacity, edema, vascularisation, deposits.

3.Diminished sensation

4.Ulceration—its sequel and complication.

5.Xerosis

6.Degeneration

7.Dystrophies

Morphological changes in cornea :

Shape. Cornea looks circular on casual look, however, it is elliptical in shape when seen from front. Its vertical diameter is less than horizontal diameter. The difference becomes obvious when measured by a corneal calliper. In fact the measurement in children should be taken under general anaesthesia along with intraocular tension. In older children local anaesthesia is sufficient. The cornea becomes horizontally oval in soft eyes i.e. phthisis, perforated globe due to pressure of lids. In extreme degree of softness it becomes quadrilateral. Rarely the cornea may be oval vertically or horizontally as a congenital anomaly. There may be an apparent change in corneal shape with encroachment of dermoid or limbal papillae of spring catarrah.

Size of cornea :

1. Small cornea. A cornea smaller than 10 mm is called microcornea, while those larger than 13 mm are known as megalocornea.

Small corneas are seen as congenital anomalies like microcornea, microphthalmos and nanophtalmos. In micro-cornea it may be as small as 3-4 mm. Other causes of small cornea are perforated globe, phthisis bulbae and atrophic bulbae.

2.Large cornea. Cornea larger than 13 mm is seen in buphthalmos, megalocornea and keratoglobus. Myopic eyes have larger cornea than hypermetropic eyes.

3.Curvature. Curvature of cornea—In normal life, corneal curvature can be appreci-

ated by

1.Looking from temporal side when the patient looks at a distant object.

2.Looking at the depth of AC.

3.Asking the patient to look down.

Cornea with normal curvature does not produce any localised dent in the lower lid. In case of increased curvature, the apex of the cornea causes a notch in the middle of the lower lid when patient looks down (Munson’s sign).

Accurate measurement of cornea is done by—keratoscope, keratometer and photo keratoscope.

Corneal curvature is increased in—Keratoconus, buphthalmos and keratoglobus. In case of partial corneal staphyloma, the curvature may be increased irregularly. In pellucid degeneration of cornea—there is a localised bulge in the lower part. It develops in third decade.

Corneal surface. Normal cornea has a smooth, shinning surface that acts as a convex mirror forming miniature, virtual erect image of the object in front of the cornea without any distortion or haze. Irregularity of the corneal surface can be observed by window reflex keratoscope, keratometer, computerised corneal topography.

Causes of irregular corneal surface are—Corneal ulcer, corneal scar, pterygium, keratoconus, deposits on cornea i.e. foreign bodies, band keratopathy, corneal plaque. Other causes are—Keratectasia, adherent leucoma, corneal staphyloma, keratoplasty, growth at the limbus (Dermoid), bulbus keratopathy.

Corneal brightness is diminished in :

1.loss of corneal epithelium i.e. corneal ulcer, trauma.

2.Corneal opacities

3.Deposits on the cornea

4.Epithelial edema, and stromal edema.

5.Vascularisation of cornea both superficial and deep

Changes in corneal thickness. Cornea is not uniform in its thickness. It is thicker at the periphery and thinnest in the centre. Clinically thickness is measured by pachymeter.

Increase in corneal thickness is seen in :

1.Corneal edema.

2.It is more marked if endothelium is damaged than epithelium.

3.The other causes of increased corneal thickness iscongenital hereditary endothelial dystrophy.

Reduced thickness is seen in keratoconus, buphthalmos, Desmetocele corneal ulcer.

Changes in Corneal Transparency

Main function of cornea is optical. For this it has to remain in a state of utmost transparency that is maintained by multiple factors.2,5

They are :

1.Uniform and regular arrangement of epithelial cells.

2.Uniform size of stromal cells, their uniform arrangement and compactness.

3.Virtual absence of vascularisation.

4.Transparent corneal nerves and a

5.Constant state of deturgescence.

Deturgescence of cornea is a state of relative hydration of corneal tissue that is maintained by Na+ – K+ cell pump of endothelium and epithelium. Though endothelium and epithelium both contribute in maintaining corneal deturgescence, former has more pronounced effect than the later. Injury to endothelium by trauma, infection or toxin has more serious effect than epithelial damage. The epithelium regenerates quickly thus damage to epithelium is short, it causes localised swelling of epithelium and is transient causing diminished vision for short time. Integrity of epithelial cells depend on (1) Tear film and (2) Available oxygen.

Derangement of any of the above factor alone or in combination can cause loss of corneal transparency that may be short lived or permanent.

Commonest cause of loss of transparency is replacement of transparent keratocytes by irregularly arranged opaque fibrocyte, due to trauma, infection, inflammation or infiltration. Other causes are superficial and deep vascularisation, incarceration of uvea, raised intraocular tension, degeneration, dystrophies, deposits on the cornea, epithelial down growth, growth of conjunctiva or conjunctival tumour spreading over cornea.

Loss of transparency of cornea is called corneal opacity that can be unilateral or bilateral. Shape, size, number and depth varies according to causative factors and duration. Some