Cornea & External Eye Disease
The cornea is the most powerful lens in the eye, supplying roughly two-thirds of its refractive power, and simultaneously its outermost barrier against a world of dust, bacteria and fingers. Those two jobs are in permanent tension, because a good barrier is opaque and a good lens is not.
Corneal disease is where that tension is lost. The subject becomes manageable once you stop learning conditions by name and start asking which of three layers has failed.
1. Three Layers, Three Jobs
| Layer | Job | Failure produces |
|---|---|---|
| Epithelium | Barrier against organisms and water | Ulceration, infection, erosion |
| Stroma | Regular lattice arrangement of collagen | Scarring, ectasia, opacity |
| Endothelium | Pumping water out of the stroma | Oedema, bullous keratopathy |
The organising tool is to name the failed layer before naming the disease. A red eye with a fluorescein-staining defect is an epithelial problem. A cornea that has changed shape is a stromal problem. A cornea that is uniformly cloudy with a normal surface is an endothelial problem.
One further fact governs prognosis throughout. The epithelium regenerates rapidly and completely; the endothelium does not regenerate at all. Endothelial cells lost to surgery, inflammation or dystrophy are replaced only by the spreading and thinning of their neighbours, and once density falls below a critical level the cornea decompensates permanently.
2. Why the Cornea Is Transparent
Three arrangements produce transparency, and each corresponds to one of the three layers.
The cornea is avascular, taking oxygen from the atmosphere through the tear film, glucose from the aqueous humour, and further supply from the limbal vessels at its edge. Blood vessels within the cornea scatter light, which is why neovascularisation is itself a cause of visual loss.
Stromal collagen fibrils are uniform in diameter and regularly spaced, closer together than the wavelength of light, so scattered rays cancel by destructive interference. Disturb that spacing and the cornea becomes hazy.
The endothelium pumps fluid out of the stroma continuously, maintaining relative dehydration, which is called deturgescence. The stroma's proteoglycans draw water in constantly, so transparency depends on a pump that never stops.
That last point explains a great deal. Anything that raises intraocular pressure above the pump's capacity, or destroys endothelial cells, causes corneal oedema, and this is why acute angle closure presents with a hazy cornea.
3. The Tear Film and the Ocular Surface
The tear film has three components: an outer lipid layer from the meibomian glands that retards evaporation, an aqueous layer from the lacrimal gland, and an inner mucin layer from goblet cells that allows the tears to wet the epithelium.
Dry eye divides by which component has failed. Aqueous deficient dry eye means inadequate lacrimal secretion, of which Sjögren syndrome is the classical cause. Evaporative dry eye means meibomian gland dysfunction, and it is far commoner.
The paradox that confuses students is that dry eye frequently causes watering, because surface irritation triggers reflex tearing that is poor in quality and evaporates immediately.
Vitamin A deficiency destroys goblet cells, producing conjunctival xerosis, Bitot spots and eventually keratomalacia, which is the ocular surface consequence of a nutritional deficiency rather than a primary eye disease.
Pterygium
A pterygium is a triangular fibrovascular growth of degenerated conjunctival tissue encroaching onto the cornea, almost always from the nasal side.
It is a disease of chronic ultraviolet exposure, which makes it common in outdoor workers across India, and dust and wind contribute.
Why the nasal side predominates is worth knowing: light entering temporally is refracted by the cornea and focused onto the nasal limbus, so the stem cell population there receives a concentrated ultraviolet dose.
An iron line at the advancing head, the Stocker line, indicates a slowly growing lesion, whereas a fleshy vascular pterygium without one is progressing.
Excision is indicated for encroachment on the visual axis, induced astigmatism or recurrent inflammation. Recurrence after bare sclera excision is high, and conjunctival autografting substantially reduces it, which is why simple excision is no longer acceptable practice.
A pinguecula is the same degenerative process confined to the conjunctiva without corneal encroachment, and needs no treatment.
4. Conjunctivitis: Reading the Discharge
| Type | Discharge | Distinguishing feature |
|---|---|---|
| Bacterial | Purulent, sticky lids in the morning | Papillae, usually bilateral by spread |
| Viral | Watery | Follicles, preauricular node, often preceded by upper respiratory infection |
| Allergic | Stringy, mucoid | Itching is the dominant symptom, papillae |
| Chlamydial | Mucopurulent, chronic | Follicles, chronicity over weeks |
Itching means allergy, a preauricular node means viral, and pus means bacterial. These three shortcuts resolve the majority of stems.
Vernal keratoconjunctivitis is the Indian variant that matters. It affects boys in hot dry climates, is seasonal, and produces giant cobblestone papillae on the upper tarsus, limbal Horner-Trantas dots, and in severe cases a shield ulcer that threatens vision.
Ophthalmia neonatorum
Neonatal conjunctivitis is separated almost entirely by the day of onset.
| Onset | Cause |
|---|---|
| Day 1 | Chemical, from prophylactic agents |
| Day 2 to 5 | Gonococcal |
| Day 5 to 14 | Chlamydial |
| 1 to 2 weeks | Herpes simplex |
Gonococcal ophthalmia is the emergency, because gonococcus can penetrate an intact corneal epithelium, and perforation can occur within days. It is hyperacute, with profuse purulent discharge and marked lid oedema, and requires systemic ceftriaxone rather than drops alone.
Chlamydial disease is less dramatic in the eye but demands systemic treatment because the organism also colonises the nasopharynx and can cause pneumonitis.
5. Trachoma and Its Elimination in India
Trachoma is chronic keratoconjunctivitis caused by Chlamydia trachomatis serotypes A to C, spread by fingers, fomites and flies in conditions of crowding and poor facial hygiene.
The WHO simplified grading has five stages: follicular inflammation, intense inflammation, conjunctival scarring, trichiasis, and corneal opacity. The sequence matters because blindness comes at the end of it, decades after infection.
The mechanism of blindness is mechanical, not infective. Repeated infection scars the tarsal conjunctiva, the scar contracts and turns the lid margin inward, and the lashes then abrade the cornea with every blink until it opacifies.
Signs to recognise are the Arlt line, a horizontal scar across the upper tarsal conjunctiva, and Herbert pits, depressions at the limbus where follicles have resolved.
The control strategy is remembered as SAFE: surgery for trichiasis, antibiotics with azithromycin, facial cleanliness, and environmental improvement. Only the first two are medical, and the last two are what actually interrupt transmission.
In October 2024 the World Health Organization validated India as having eliminated trachoma as a public health problem, the third country in the South-East Asia Region after Nepal and Myanmar. Blindness from trachoma fell from 5 percent in 1971 to under 1 percent, India was declared free of infective trachoma in 2017, and a trichiasis survey across 200 formerly endemic districts between 2021 and 2024 completed the requirement.
6. Corneal Ulcer
Suppurative keratitis is the central problem of Indian ophthalmology outside cataract, because agricultural injury with vegetable matter is common and because fungal disease is disproportionately frequent here.
| Cause | Suggestive features |
|---|---|
| Bacterial | Rapid onset, well-defined ulcer, dense infiltrate, hypopyon |
| Fungal | Slower, dry raised edge, feathery margins, satellite lesions, vegetative trauma |
| Herpes simplex | Dendritic ulcer, reduced corneal sensation |
| Acanthamoeba | Severe pain out of proportion, ring infiltrate, contact lens or water exposure |
Pain disproportionate to the signs in a contact lens wearer means Acanthamoeba until proved otherwise, and delay is what costs the eye, since these ulcers are routinely treated as herpetic for weeks first.
Feathery margins with satellite lesions after injury with a plant or grain means fungal keratitis, and corneal scraping for microscopy and culture must precede treatment.
The natamycin evidence
Topical natamycin is the treatment of choice for filamentous fungal keratitis, and this rests on Indian trial evidence rather than convention.
The Mycotic Ulcer Treatment Trial compared natamycin with voriconazole and found significantly better three-month visual acuity with natamycin, together with fewer perforations and fewer therapeutic keratoplasties. Enrolment into the voriconazole arm was stopped early because of that excess.
The benefit was concentrated in Fusarium infections, where natamycin was clearly superior, while non-Fusarium cases fared similarly. A second trial found that adding oral voriconazole to topical natamycin did not produce a significant overall benefit in severe ulcers.
7. Herpetic Keratitis
Herpes simplex is the commonest infectious cause of corneal blindness in the developed world and matters here for one rule that reverses the usual instinct.
Epithelial herpetic keratitis produces a dendritic ulcer with terminal bulbs, stains with fluorescein, and is accompanied by reduced corneal sensation, which is the sign most often forgotten.
Steroids are contraindicated in epithelial herpetic disease, because they permit viral replication and convert a dendritic ulcer into a geographic one.
Steroids are required in stromal herpetic keratitis, because that disease is immune-mediated rather than actively infective, and they are given under antiviral cover.
The same organism therefore demands opposite treatment depending on which layer is involved, which is exactly the reason the layer-based framework is worth holding.
8. The Dystrophies and Endothelial Failure
Corneal dystrophies are bilateral, inherited, non-inflammatory opacities, and the three classical stromal ones are separated by deposit, stain and inheritance.
| Dystrophy | Deposit | Stain | Inheritance |
|---|---|---|---|
| Granular | Hyaline | Masson trichrome | Autosomal dominant |
| Lattice | Amyloid | Congo red | Autosomal dominant |
| Macular | Mucopolysaccharide | Alcian blue | Autosomal recessive |
Macular dystrophy is the odd one out on every axis. It is the only recessive of the three, the only one in which the intervening stroma is also hazy rather than clear, and consequently the one that impairs vision earliest and most severely.
Map-dot-fingerprint dystrophy affects the epithelial basement membrane and presents not with opacity but with recurrent corneal erosion, in which the patient wakes with sudden severe pain as the lids peel away poorly anchored epithelium.
Because the endothelium does not regenerate, endothelial disease is progressive and irreversible.
Fuchs endothelial dystrophy is a bilateral age-related loss of endothelial cells with guttae on the posterior surface. Vision is worst on waking and improves through the day, because the closed lids prevent evaporation overnight and the cornea is at its most swollen in the morning.
Pseudophakic bullous keratopathy is endothelial failure following cataract surgery, and it is the commonest indication for corneal transplantation in many series.
When decompensation is advanced, epithelial bullae form and rupture, which is intensely painful because corneal nerve endings are exposed.
9. Keratoconus
Keratoconus is progressive stromal thinning and conical protrusion, usually bilateral but asymmetric, beginning around puberty and associated with eye rubbing, atopy and Down syndrome.
The signs are worth listing because they are examined directly: Munson sign, a V-shaped indentation of the lower lid on downgaze; Vogt striae, fine vertical stromal lines that disappear on pressure; a Fleischer ring of iron at the base of the cone; and a scissoring reflex on retinoscopy.
Acute hydrops occurs when Descemet membrane ruptures, allowing aqueous to flood the stroma, with sudden pain, watering and dramatic corneal oedema. It settles over weeks, usually leaving a scar.
Corneal collagen cross-linking halts progression by using riboflavin and ultraviolet A light to create additional bonds between stromal collagen fibres. It stiffens the cornea and does not restore vision, which is why it is offered early to a progressing young patient rather than late to a scarred one.
Optical correction proceeds through spectacles, then rigid gas permeable contact lenses which vault the cone and create a smooth refracting surface, before transplantation is considered.
10. Corneal Transplantation
| Procedure | What is replaced |
|---|---|
| Penetrating keratoplasty | Full thickness |
| Deep anterior lamellar keratoplasty | Stroma, retaining host endothelium |
| Descemet stripping endothelial keratoplasty | Endothelium with a thin stromal layer |
| Descemet membrane endothelial keratoplasty | Endothelium and Descemet membrane alone |
The principle behind lamellar surgery is to replace only the failed layer. Retaining the patient's own endothelium in deep anterior lamellar keratoplasty removes the main target of rejection, and replacing only the endothelium in the posterior procedures gives faster recovery and a structurally stronger eye.
Endothelial rejection is recognised by the Khodadoust line, a line of keratic precipitates advancing across the graft endothelium, together with graft oedema and anterior chamber activity. It is treated urgently with intensive steroids.
The cornea enjoys relative immune privilege because it is avascular and lacks lymphatics, which is why corneal grafting succeeds without systemic immunosuppression. Vascularisation of the host bed removes that privilege and is the strongest predictor of rejection.
11. Chemical Injury and Limbal Stem Cell Failure
Alkali injuries are worse than acid injuries, and the reason is chemical. Alkali saponifies membrane lipids and penetrates rapidly into the stroma and anterior chamber, while acid coagulates surface proteins and that coagulum forms a barrier limiting further penetration.
Immediate copious irrigation is the entire emergency treatment, continued until the pH of the conjunctival fornix normalises. Nothing done later matters as much, and time spent taking a history before irrigating is time the chemical spends penetrating.
Prognosis is judged by limbal ischaemia, assessed as the extent of perilimbal blanching, because the limbus houses the stem cells that renew the corneal epithelium.
Loss of limbal stem cells causes conjunctival epithelium to grow across the cornea, bringing blood vessels and goblet cells with it. The result is a vascularised, opaque, unstable surface, and a conventional graft placed on it will fail, because the graft has no source of epithelium.
This is why limbal stem cell deficiency must be corrected first, by limbal transplantation or cultivated epithelial techniques, before any attempt at restoring clarity.
12. Worked Examples
Example 1. A farmer has a corneal ulcer three weeks after being struck in the eye by a paddy stalk. The ulcer has a dry raised edge with feathery margins and small satellite lesions. Treatment?
Filamentous fungal keratitis, most likely Fusarium or Aspergillus. Corneal scraping for microscopy and culture is taken first, and topical natamycin is the treatment of choice. The Mycotic Ulcer Treatment Trial found better visual acuity, fewer perforations and fewer therapeutic keratoplasties with natamycin than with voriconazole, with the advantage concentrated in Fusarium.
Example 2. A newborn develops profuse purulent discharge with marked lid oedema on day 3. What is the diagnosis and why is it urgent?
Gonococcal ophthalmia neonatorum. The urgency is that gonococcus penetrates an intact corneal epithelium, so corneal ulceration and perforation can follow within days. Systemic ceftriaxone is required rather than topical treatment alone. Onset on day 1 would suggest chemical conjunctivitis and day 5 to 14 chlamydial infection, which needs systemic treatment for nasopharyngeal colonisation and pneumonitis.
Example 3. A patient with a dendritic corneal ulcer is given topical steroid and returns with a large geographic ulcer. Explain.
Epithelial herpes simplex keratitis was treated with steroid, which suppressed the local immune response and permitted unrestrained viral replication, enlarging the dendrite into a geographic ulcer. Steroids are contraindicated in epithelial herpetic disease. They are, however, required in stromal herpetic keratitis, which is immune-mediated, and are given there under antiviral cover.
Example 4. A patient with a chemical burn has 6 clock hours of perilimbal blanching. Why does this matter more than the corneal appearance?
Because the limbus houses the stem cells that renew corneal epithelium, and the extent of limbal ischaemia predicts whether the surface can ever heal normally. With extensive loss, conjunctival epithelium migrates onto the cornea with vessels and goblet cells, producing a vascularised unstable surface. A conventional graft on such a bed fails for lack of an epithelial source, so limbal stem cell deficiency must be addressed before any attempt at optical restoration.
Summary
Three layers, three jobs. The epithelium is a barrier, the stroma is an ordered lattice, and the endothelium is a pump.
The epithelium regenerates fully; the endothelium not at all, which is why endothelial disease is progressive and irreversible.
Transparency requires avascularity, uniform collagen spacing and continuous deturgescence.
Itching means allergy, a preauricular node means viral, pus means bacterial.
Ophthalmia neonatorum is separated by day of onset, and gonococcal disease at day 2 to 5 is the emergency because the organism penetrates intact epithelium.
Trachoma blinds mechanically through cicatricial entropion, and SAFE is the control strategy. India was validated as having eliminated it in October 2024.
Feathery margins with satellite lesions after vegetative trauma means fungal keratitis, and natamycin beats voriconazole on Indian trial evidence, particularly for Fusarium.
Pain out of proportion in a contact lens wearer means Acanthamoeba.
Steroids are forbidden in epithelial herpes and required in stromal herpes, which is the clearest demonstration of why the layer matters.
Keratoconus is treated by cross-linking to halt progression, not to restore vision.
Lamellar keratoplasty replaces only the failed layer, and the Khodadoust line signals endothelial rejection.
Alkali burns beat acid burns because alkali saponifies and penetrates, and irrigation before anything else is the whole of the emergency treatment.
