Page 1244 - Problem-Based Feline Medicine

1236 PART 15 CAT WITH EYE PROBLEMS

INTRODUCTION epithelium is missing, or deep if the epithelium and
stroma are eroded. When the epithelium and entire
stroma are eroded as far as Descemet’s membrane, the
MECHANISM?
condition is called a Descemetocele. Descemet’s mem-
The main supportive tissue of the cornea is the stroma, brane is very elastic in the cat. When there is a large
which consists of cells that stretch from one side to stromal deficit, the Descemetocele will be seen as
the other, known as keratocytes, and an intercellular a large bulge of transparent tissue.
matrix that contains collagen fibers and glycosamino-
The integrity of the epithelium is tested with
glycans. The outer surface is covered by an epithelial
a water-soluble dye called fluoresein. The epithelium
layer, about six cells deep, that has a very thin base-
prevents water-soluble substances from entering the
ment membrane. The inner surface has a cell mono-
stroma. If the epithelium is disrupted, the dye will be
layer called the endothelium, which has a very thick
absorbed by the stroma and remain temporarily as
basement membrane called Descemet’s membrane.
a yellow green stain. The dye coloration can be enhanced
The cornea is transparent because it does not have by using ultraviolet light (the blue filter on direct and
a blood supply, contains no pigmented cells, the colla- indirect ophthalmoscopes). Rose bengal is another
gen fibers are laid down in very specific planes and the stain that is used to show minor changes in the epithe-
hydration of the intercellular matrix is controlled. lial layer. It may be useful to show the dendritic lesions
seen in feline herpesvirus-1 keratitis.
The epithelium and endothelium have properties that
prevent fluid from entering the stroma. If the epithe- Corneal ulcers may be caused by trauma, infectious
lium is eroded, the stromal matrix will absorb fluid, diseases, immune processes that affect the epithelium
swell and become cloudy. This is known as corneal and stroma, and by any mechanism that prevents the
edema. Similarly, if the endothelium is compromised, normal tear film from covering the eye.
aqueous humor will enter the stroma and cause corneal
Endothelial disruption may be caused by trauma
edema.
or inflammation within the eye, usually secondary to
Very simply, the cornea is kept alive by the tear film, uveitis (an inflammation of the vascular tissue in the
which provides nutrients for the cells and immune eye). An inherited form of endothelial dystrophy has
mechanisms to protect the eye from disease. Oxygen been described in the Manx cat.
enters the cornea by diffusion through the tear film.
The cornea has a very rich nerve supply emanating
The cornea is the main refractive tissue for the optical from the trigeminal nerve. Nerve endings are found
mechanism of the eye. throughout the cornea up to the level of the basal epithe-
lial cells. Any disruption of the epithelium causes severe
Diseases of the cornea in cats are generally confined to
ocular pain, which is manifested by blepharospasm
the outer epithelium and the supportive stromal tissue.
(eyelid spasm), photophobia (pain from light) and ocu-
The cat cornea heals very well with limited scarring lar guarding. The pain will stimulate excessive tear pro-
and melanin pigment deposition compared to other duction causing a serous ocular discharge.
species.
The cat cornea has some unique diseases caused by
Inflammation of the cornea is known as “keratitis”. degeneration of the stroma, and proliferative lesions,
Keratitis may be acute or chronic, and ulcerative or which develop from immune mechanisms associated
infectious. Inflammation is associated with erosions with systemic reactions to allergic disease.
and edema.
Chronic superficial corneal disease will stimulate
When the epithelium is damaged, a corneal ulcer vascular in-growth from the limbus (the area where
develops. The ulcer may be superficial if only the the cornea merges with the sclera) known as corneal