ULCERATIVE KERATITIS

Inflammation of the cornea (keratitis) is characterised by corneal oedema, cellular infiltration and ciliary congestion.

ULCERATIVE KERATITIS
Corneal ulcer may be defined as discontinuation in normal epithelial surface of cornea associated with necrosis of the surrounding corneal tissue.
Pathologically it is characterised by oedema and cellular infiltration. Common types of corneal ulcers are described below.INFECTIVE KERATITIS
BACTERIAL CORNEAL ULCER
Being the most anterior part of eyeball, the cornea is exposed to atmosphere and hence prone to get infected easily. At the same time cornea is protected
from the day-to-day minor infections by the
  normal defence mechanisms present in tears in the form of lysozyme, betalysin, and other protective proteins. Therefore, infective corneal ulcer may develop when:

• either the local ocular defence mechanism is jeopardised, or
• there is some local ocular predisposing disease, or host's immunity is compromised, or
• the causative organism is very virulent.

Etiology
There are two main factors in the production of purulent corneal ulcer:

• Damage to corneal epithelium; and
• Infection of the eroded area.

However, following three pathogens can invade the intact corneal epithelium and produce ulceration:
Neisseria gonorrhoeae, Corynebacterium diphtheriae and Neisseria meningitidis.
1. Corneal epithelial damage. It is a prerequisite for most of the infecting organisms to produce corneal ulceration. It may occur in following conditions:
i. Corneal abrasion due to small foreign body, misdirected cilia, concretions and trivial trauma in contact lens wearers or otherwise.
ii. Epithelial drying as in xerosis and exposure keratitis.
iii. Necrosis of epithelium as in keratomalacia.
iv. Desquamation of epithelial cells as a result of corneal oedema as in bullous keratopathy.
v. Epithelial damage due to trophic changes as in neuroparalytic keratitis.

2. Source of infection include:
i. Exogenous infection. Most of the times corneal infection arises from exogenous source like conjunctival sac, lacrimal sac (dacryocystitis),
infected foreign bodies, infected vegetative material and water-borne or air-borne infections.

ii. From the ocular tissue. Owing to direct anatomical continuity, diseases of the conjunctiva readily spread to corneal epithelium, those of sclera to stroma, and of the uveal tract to the endothelium of cornea.

iii. Endogenous infection. Owing to avascular nature of the cornea, endogenous infections are of rare occurrence.

3. Causative organisms. Common bacteria associated with corneal ulceration are: Staphylococcus aureus, Pseudomonas pyocyanea, Streptococcus pneumonia, E. coli, Proteus, Klebsiella, N. gonorrhoea, N. meningitis and C. diphtheria.