Corneal ulceration

The cornea is the clear surface of the eye. It consists of a thin outer layer the epithelium, the thicker middle layer the stroma, and an inside lining made up of the basement membrane (Decemet's membrane) and the one cell thick endothelium. Any damage to the cornea that causes a break in the outer surface is termed a corneal ulcer. The size and depth of the ulcer will determine both the treatment and likely outcome.

Superficial ulcers are where the epithelium has only shallow damage. The animal may present showing signs of a painful eye, with weeping and blepharospasm (partially closed eyelids). As shallow ulcers may be difficult to detect a dye called fluorescein, which will highlight the ulcer as a green patch on the cornea is placed in the eye. The epithelium is a very effect barrier against infection so even shallow ulcers can leave the cornea open to infection with bacteria. Some bacteria such as Pseudomonas can produce enzymes (chemicals) called proteases which are capable of dissolving the stroma, creating a much deeper more serious ulcer. Topical antibiotic drops are used in the eye for treatment of superficial ulcers to prevent infection. Healing in uncomplicated cases should be rapid taking 3-4 days.
Deep corneal ulcers represent a more serious risk of permanent damage to the eye, with a higher chance of infection and inflammation extending to the inside of the eye. As the ulcer becomes deeper it can reach the point where Decement's membrane may bulge into the defect or the cornea may actually perforate, leading to some degree of collapse of the eye and damage to the internal structures.

Deep ulcers require topical and systemic (tablets etc) antibiotics. Atropine drops are used to dilate the pupil which helps to reduce pain. The third eyelid is usually pulled across the effected eye and stitched under the top eyelid to protect the damaged cornea.
More severe cases may have a flap of the conjunctiva (lining of eyelid) stitched across the ulcer to help promote healing. Initial healing to fill the defect in the cornea may take up to 4-6weeks. Once this has been achieved and the ulcer no longer takes fluorescein stain corticosteroids may be used to reduce the scarring.

Some ulcers can be very slow to heal despite being relatively shallow. These are called
indolent ulcers and are characterised by having a build up of dead cells in the ulcer which need to be removed in order to promote healing. Two surgical techniques are available. The first technique is a grid keratotomy, where fine incisions are made in a grid pattern across the cornea to stimulate healing of the ulcer. The second technique requires an operating microscope to cut off the top layers of the cornea and completely remove the ulcer. The two techniques have success rates of 80% and 95% respectively. The third eyelid is temporarily sutured closed in both cases to protect the eye whilst healing.

In all cases of corneal ulceration identifying and correcting the cause is a critical aspect of treatment. The range of causes is extensive and includes both extrinsic factors such as trauma and foreign object; and intrinsic factors such as eyelashes rubbing on the eye.