The cornea is the transparent, domed front part of the eye that acts to protect and maintain ocular integrity while allowing light to enter the eye for vision. The normal cornea is composed of three layers. An outer layer of epithelial cells lines the surface and protects the rest of the cornea. The thicker middle layer is called the stroma and contains multiple supportive cells, collagen fibers, and nerve fibers. The inner lining is called the endothelial layer and it functions to help maintain corneal clarity. A corneal ulcer is a break in the outer epithelial layer of the cornea. Although initially painful, uncomplicated ulcers should heal with appropriate therapy in three to four days. Ulcers that persist beyond this period of time or worsen or progress are considered be complicated ulcers.
Primary tissue healing defects are conditions in which the epithelium fails to remain adherent to the underlying stroma after attempts to heal the superficial ulcer. This condition is common in middle-aged and older dogs as well as certain breeds, such as the Boxer. It is also found in dogs that have underlying hormonal or metabolic conditions that may delay healing such as hypothyroidism, diabetes mellitus or Cushing’s syndrome. This type of ulcer is sometimes called a “Boxer” ulcer or an “indolent” ulcer. The basic defect affects the ability of the outer epithelial cells to anchor themselves and remain adherent to the underlying corneal stroma. Although they continually attempt to grow over and cover the ulcer, they are periodically stripped away either by the eyelids with normal blinking or due to active rubbing of the eye by the animal.
Among the most important instruments required to evaluate a corneal ulcer is the slit lamp, a biomicroscope. The slit lamp permits our veterinary ophthalmologists to carefully evaluate the patient’s cornea with a high degree of magnification and resolution equivalent to that of a microscope. Sometimes, a cause for the ulcer, such as an aberrant eyelash or foreign material stuck in the eye can be identified. The ulcer will generally heal uneventfully after removal of the inciting cause. Often the characteristic loose tissue edges of these ulcers will provide the ophthalmologist with enough information to diagnose the disease.
To further evaluate a complicated corneal ulcer, we might obtain samples for bacterial culture, virus isolation, and cytological evaluation.
Arguably the most frustrating ulcer is the indolent ulcer. Although these ulcers do not typically threaten a patient’s sight, they run a protracted course. Medical therapy for indolent ulcers consists of preventative antibiotics and often hyperosmotic agents and offers only a 15% success rate.
The recommended treatment for indolent ulcers is a surgical procedure called a keratotomy, either a linear grid or punctuate pattern can be used. The purpose of this procedure is to first remove all of the loose epithelial cells that are not able to remain attached to the stroma. A very fine needle is then used to make a series of microgrooves or grids into the superficial corneal stroma. This will help to disrupt the abnormal portion of the cornea and allow new epithelial cells to anchor down to healthy cornea below this unhealthy zone. This can be performed either awake under topical anesthesia or under general anesthesia, depending on the patient and the size of the ulcer. Most indolent ulcers will heal after this procedure; however, refractory ulcers might require a repeat of the procedure or a more invasive surgical procedure called a superficial keratectomy. Here the outer layers of the cornea are surgically removed with a specialized corneal blade that peels the unhealthy layer of the corneal stroma away.
Not all indolent ulcer patients require surgical intervention, but it is important to be aware of potential complications if surgery does occur. Including potential anesthetic risks, procedures that involve the cornea seldom give rise to complications, which occur in less than 5% of these cases. Potential complications include, but are not limited to:
Some of these complications can lead to blindness.