Definition
The cornea is the transparent, shiny membrane which makes up the front of the
eyeball. Think of it as a clear window. To understand a corneal ulcer,
you must first understand how the cornea is constructed.
The cornea is comprised of three layers. The most superficial layer is the
epithelium. This layer is comprised of many very thin layers of
cells. Below the epithelium is the stroma, and the deepest layer is
DescemetÃs membrane. Because all of these layers of the cornea are clear,
it is not possible to see them without special stains which colour particular
cells and highlight them when looked at with a microscope.
An erosion of few layers of the epithelium is called a corneal erosion or a
corneal abrasion. A corneal ulcer is an erosion through the entire epithelium
and into the stroma. If the erosion goes through the epithelium
and stroma to the level of Descemet's membrane, a descemetocele exists.
If Descemet's membrane ruptures, the liquid inside the eyeball leaks out, the
eye collapses and irreparable damage can occur.
Mechanism of occurance
There are several causes for corneal ulcers in dogs. The most common is
trauma. An ulcer may result from blunt trauma, such as a dog rubbing its
eye on the carpet, or due to a laceration, such as a cat scratch. The
second most common cause is chemical burn of the cornea. This may happen
when irritating shampoo gets in the eye.
Less common causes of corneal ulcers include bacterial infections, viral infections,
and other diseases. These may originate in the eye or develop secondary to disease
elsewhere in the body. Examples of other diseases include Epithelial Dystrophy (a
weakening of the cornea which can be inherited in breeds such as the Boxer),
Keratoconjunctivitis Sicca (drying of the cornea due to abnormal tear
formation), which occurs in breeds such as the West Highland White Terrier and
diseases of the endocrine system (diabetes mellitus, hyperadrenocorticism, and
hypothyroidism).