EXAMINATION OF THE CORNEA

-Dr. Akshay Nayak

ANATOMY IN BRIEF

• Dimensions of cornea-• 1] Anterior surface is elliptical with horizontal diameter 11.75mm and vertical

diameter 11mm• 2]Posterior surface is circular with average diameter 11.5mm• 3]Thickness of cornea in the centre is 0.52mm and at periphery is 0.67mm• 4]Anterior radius of curvature is 7.8mm and Posterior is 6.5mm• 5]Refractive power is 43+ D• 6]Refractive index is 1.37

HISTOLOGY• Epithelium- 1. 50-90microns, 5-6 layers2. Entire epithelium replaced in 6-8 days.3. i]Basal layer-tall columnar,germinal layer; ii]wing cells;iii] flattened cells-zonulae occludentes,microvilii• Bowmans Layer1. 8-14microns2. Condensed superficial part of stroma3. Does not regenerate.• Stroma(substantia propria)1. 0.5mm2. Collagen fibrils(lamellae)3. Lamellae are parallel to each other.4. Cells- keratocytes, macrophages,histiocytes etc

• Descemet’s membrane(posterior elastic lamina)1. Represents basement membrane of the endothelium from which it is produced. 2. 10-12microns3. Can regenerate4. In the periphery, ends at the anterior limit of trabecular meshwork as Schwalbe’s line.• Endothelium1. Flat polygonal.2. 6000 cells/mm2 at birth---2400-3000cells/mm2 in young adults3. Corneal decompensation occurs when counts <500cells/mm2 4. Pump mechanism.

BLOOD SUPPLY-Anterior ciliary vesselsNERVE SUPPLY- Long ciliary nerve

EXAMINATION OF CORNEA-POINTS TO NOTE

• VISUAL ACUITY!!• EXAMINATION OF CORNEA PROPER UNDER FOLLOWING HEADINGS-1. SHAPE2. SIZE3. CURVATURE4. THICKNESS5. TRANSPARENCY6. SENSATION7. VASCULARISATION8. DEPOSITS9. ENCROACHMENT FROM LIMBUS

EXAMINATION OF CORNEAMethods of examination of cornea• Examination under torch light• Examination under focal illumination- 1. Listers lamp2. Handheld slit lamp• Focal illumination with magnification1. Corneal loupe

2. SLIT LAMP BIOMICROSCOPY• Direct illumination- 1]diffuse, 2]narrow beam ,3]optical section-narrow beam

gives linear cut of the cornea without illuminating surrounding. In this mode various levels of cornea available for inspection.

• Direct illumination…..4]Specular Reflection-uses principle of reflection of light. Specular microscopy is a specialised biomicroscopy to examine corneal endothelium.• Indirect illumination- enhances contrast during visualisation of defects at

various levels of cornea. Retro illumination and Sclerotic scatter

• WHAT TO SEE ON SLIT LAMP?-• EPITHELIUM- 1]EROSIONS(GREY WHITE)-• superior—tarsal fb, concretions,Superior limbal keratopathy etc ; inferior—

lagophthalmos, trichiasis entropion etc; interpalpebral– uv light,welding exposure etc• 2]ULCER-• breach in continuity, grey white, surrounded by infiltration• 3]OEDEMA- • bullae– acute congestive glaucoma,trauma, after surgery, keratoconus• 4]FILAMENTS-• peeled off epithelium attached to cornea– recurrent idiopathic corneal erosion,

keratoconjunctivitis sicca etc• 5]PUNCTATE EPITHELIAL KERATITIS

• STAINING OF CORNEA1. Fluorescein and rose Bengal resemble each other in structure, available as

dark crystals, soluble in water and used as 1pc drops2. Fluoroscein stains the damaged epithelium bright green. Rose Bengal

stains devitalised cornea and conjunctival epithelium.

• Opacities

Fully healed opacities do not stain.

• Bowman’s membrane- deposits-1. Calcium2. Hemosiderin3. Copper4. Iron • Stroma1. Oedema- may be associated with infiltrate and seen in disciform keratitis,fuch’s

dystrophy etc2. Infiltrates-leucocytes in stroma through either limbal blood vessels or via epithelial

break3. Vascularisation- a. Superficial(pannus)-trachoma, leprosy, spring catarrh, contact lens

use,lagopthalmos , trichiasis, ectropion, chemical burns etcb. Deep (due to entry of anterior ciliary vessels)- interstitial keratitis, chemical burns

etc

• Descemets membrane- 1. Breaks –congenital glaucoma,trauma, keratoconus. 2. Split- trauma, surgical complication3. Folds- soft eye• Endothelium- KP,vascularisations, fibrin deposits

• KERATOMETRY1. Principle of image formation by a convex mirror.2. Anterior surface of cornea acts as convex mirror.3. Curvature is measured in millimetres or dioptre4. Formula --- D=[n-1]/r ; D is refractive power of cornea, n is refractive index

and r is radius of curvature

• PACHYMETER• Optical pachymeter is attached to slit lamp• Ultrasonic pachymeter is most commonly used device.

• KERATOSCOPY AND CORNEAL TOPOGRAPHY1. Cornea acts as a convex mirror and any change in its surface will produce a

change in shape of image.2. Window reflex3. First purkinje image- small erect virtual, anterior surface of cornea, moves

with movement of light—principle of any keratoscope

• Keratoscopy……• 4.PLACIDO DISK• 10INCHES DIAMETER• Central peephole has a +2D sphere lens attached to relax accommodation of

observer.• Normal cornea will have concentric rings spaced at regular intervals

• Klein keratoscope• Photokeratoscope1. Examine 55pc , whereas keratometers only examine 8pc of corneal surface2. Gives a a visual display photo or a video display3. It is used to follow progressive keratoconus,radial keratometry result,

postsurgical astigmatism

OTHER PROCEDURES

• CORNEAL SCRAPING• EXAMINATION OF TEAR FILM• OCULAR ADNEXA• IOP• SYSTEMIC EXAMINATION

INTERPRETATION

• Shape- elliptical in hypotony, quadrilateral in phthisis• Size- horizontal diameter more than 13mm –megalocornea• Curvature- cornea is more curved vertically due to pressure of lids. increased- keratoconus decreased- cornea plana,perforation of globe irregular- pterygium,keratoconus• Thickness –increased—disciform keratitis, corneal leucoma decreased-keratoconus, buphthalmos, pellucid degeneration

• Transparency—check for any opacity-position,number,shape,staining, vascularisation,iris incarceration and deposits

•

Etiology of opacities-CongenitalTraumaInfectionInflamation-interstitial keratitis and discifrom keratitisAllergy- phlycten, spring catarrhDegenerationsDystrophyEncroachment Deposits- Hudson stahli line,fleischers ring, kayser Fleischer ring, tattoo,foreign body

• Staining• Vascularisation• Incarceration

• Testing sensation• Should be done prior to instilling an anesthetic agent and before schirmer

test.• If anesthetic is used , postpone by 24hrs• Diminished sensations in- herpes simplex and zoster,

leprosy,scar,degenerations, dystrophies, involvement of 5th nerve, C-P angle tumor etc

• Bilateral loss is rare – leprosy, herpes simplex, opacity

• Vascularisation

Causes of superficial vascualrisation-trachoma, leprosy,phlycten,riboflavin deficiency, mooren ulcerDeep- interstitial keratitis ,disciform keratitisIntercorneal –superficial vessels encroaching under lamellar keratoplastyRetrocorneal- epithelial down growth, neo vascularisation of iris, intraocular tumor

• Corneal fistulaIf perforation is in central cornea, iris fails to plug it, wound remains open, acqueos leaks, cornea flattens, absence of AC and soft eye resulting in corneal fistula.Lined by corneal epitheliumPredisposing factors- corneal perforation in case of widely dilated pupil, which does not constrict eg use of atropine for long,large iridectomy,coloboma, aniridia etcFistula is surrounded by a zone of infiltrates which appears white.

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