Pathology of sloughing corneal ulcer and formation of anterior staphyloma

When the infecting agent is highly virulent and/or body resistance is very low, the whole cornea sloughs with the exception of a narrow rim at the margin and
total prolapse of iris occurs. The iris becomes inflamed and exudates block the pupil and cover the iris surface; thus a false cornea is formed. Ultimately these exudates organize and form a thin fibrous layer over which the conjunctival or corneal epithelium rapidly grows and thus a pseudocornea is
formed. Since the pseudocornea is thin and cannot withstand the intraocular pressure, so it usually bulges forward along with the plastered iris tissue. This ectatic cicatrix
is called anterior staphyloma which, depending upon its extent, may be either partial or total. The bands of scar tissue on the staphyloma vary in breadth and
thickness, producing a lobulated surface often blackened with iris tissue which resembles a bunch of black grapes (hence the name staphyloma).

Clinical picture
In bacterial infections the outcome depends upon the virulence of organism, its toxins and enzymes, and the response of host tissue.
Broadly bacterial corneal ulcers may manifest as:
i. Purulent corneal ulcer without hypopyon; or
ii. Hypopyon corneal ulcer.

In general, following symptoms and signs may be
present :
Symptoms

1. Pain and foreign body sensation occurs due to mechanical effects of lids and chemical effects of toxins on the exposed nerve endings.
2. Watering from the eye occurs due to reflex hyperlacrimation.
3. Photophobia, i.e., intolerance to light results from stimulation of nerve endings.
4. Blurred vision results from corneal haze.
5. Redness of eyes occurs due to congestion of circumcorneal vessels.
Signs
1. Lids are swollen.
2. Marked blepharospasm may be there.
3. Conjunctiva is chemosed and shows conjunctival hyperaemia and ciliary congestion.
4. Corneal ulcer usually starts as an epithelial defect associated with greyish-white circumscribed infiltrate (seen in early stage). Soon the epithelial
defect and infiltrate enlarges and stromal oedema develops. A well established bacterial ulcer is characterized by (Fig. 1):

Fig. 1. Bacterial corneal ulcer without hypopyon.

• Yellowish-white area of ulcer which may be oval or irregular in shape. Margins of the ulcer are swollen and over hanging.
• Floor of the ulcer is covered by necrotic material.
• Stromal oedema is present surrounding the ulcer area.

Characteristic features produced by some of the causative bacteria are as follows:

• Staphylococal aureus and streptococcus pneumoniae usually produce an oval, yellowish white densely opaque ulcer which is surrounded by relatively clear cornea.
• Pseudomonas species usually produce an irregular sharp ulcer with thick greenish mucopurulent exudate, diffuse liquefactive necrosis and semiopaque (ground glass) surrounding cornea. Such ulcers spread very rapidly and may even perforate within 48 to 72 hours.
• Enterobacteriae (E. coli, Proteus sp., and Klebsiella sp.) usually produce a shallow ulcer with greyish white pleomorphic suppuration and diffuse stromal opalescence. The endotoxins produced by these Gram –ve bacilli may produce ring-shaped corneal infilterate.

5. Anterior chamber may or may not show pus (hypopyon). In bacterial corneal ulcers the hypopyon remains sterile so long as the Descemet's membrane is intact.
6. Iris may be slightly muddy in colour.
7. Pupil may be small due to associated toxin– induced iritis.
8. Intraocular pressure may some times be raised (inflammatory glaucoma).