The cornea is the clear dome-shaped structure that forms the front surface of the eye. It allows light to enter the eye and helps to focus it on the retina for functional vision. It also protects the structures inside the eye and maintains ocular shape and integrity. A corneal ulcer is simply a break on the surface of the cornea and leads to loss of the outer layer or epithelial surface of the eye. Ulcers have many possible causes: trauma, lacerations, abrasions, drying due to abnormal tear production, eyelid abnormalities, foreign material, parasites, viral infections, insect stings, chemical sprays or inﬂammation, intraocular disease, etc. Often the initial cause cannot be determined but treatment is similar for most corneal ulcers, as they are essentially open wounds on the eye. Signs of ulceration include squinting, tearing, discomfort, watery or mucoid discharge, redness and sensitivity to light. Cloudiness of the cornea or discoloration may also be noted.
A superficial, uncomplicated ulcer usually heals uneventfully in about 1 week with minimal scarring after appropriate medical treatment. A corneal ulcer can become secondarily infected with bacteria, fungi, or a mixture of the two. Infected, deep, or complicated ulcers require more intensive medical treatment and sometimes surgery. Corneal ulcers should be evaluated by a veterinarian. In addition to the exam, the veterinarian may take samples of the ulcer for cytology, bacterial and/or fungal culture. These tests will help determine the most appropriate therapy.
In complicated cases, the ulcer may rapidly deepen or the cornea may start a breakdown process called “melting”. A “melting” cornea can even progress to rupture of the eye. Melting and deep ulcers are considered eye emergencies. Secondary inﬂammation, or uveitis inside the eye, is painful and can contribute to vision-threatening complications. The ulcer may require intensive, frequent treatments for several days to weeks to control infection, reduce melting and control inﬂammation in order to achieve healing and save the globe. Deep and complicated corneal ulcers generally result in more noticeable scarring after healing. Large or dense white scars can cause visual impairment and the horse’s athletic performance or intended use may be affected.
Deep or melting ulcers that worsen despite therapy may require surgery in addition to medical therapy to save the eye. These surgeries usually involve grafting of additional supportive tissue or biosynthetic material, such as a ﬂap of conjunctiva (tissue from the “pink” part of the eye), into the ulcer. This helps to fill in the tissue defect, bring in a blood supply, and hasten healing. These grafts usually incorporate into the corneal tissue in a few weeks to months but also may result in a permanent scar.