Corneal Ulcers, Canine and Feline - contd.
The cornea, or front of the eye, has three very important functions. It acts as a supportive barrier to keep the internal structures in place within the eye. The cornea allows light to pass into the eye and thus through the lens to the retina at the back of the eye. It also bends the incoming light rays to aid the lens in focusing the incoming light.
The cornea is composed of five microscopically identifiable layers. From outermost to innermost, the layers are called the epithelium, Bowman's membrane, the stroma, Descemet's membrane and the endothelium. Bowman's membrane and Descemet's membrane are basement membranes that support the epithelium and the endothelium, respectively. When the cornea is injured, the extent to which these layers are involved will determine the severity of the injury, the treatment required and the prognosis for saving the eyesight in the injured eye.
Corneal ulcers may be graded according to the depth of the ulcer and the corneal tissue layers lost. Corneal ulcers are superficial if only the outer, epithelial layer and Bowman's membrane is missing. If the ulcer also has up to one half of the underlying stroma absent as well, the ulcer is considered shallow to moderate in depth. Corneal ulcers with more than one half of the stroma missing are deep ulcers.
The specific cellular events underlying the healing of corneal wounds will depend on which layers of the cornea are denuded. Superficial scratches of the outer layer of the cornea, or epithelium, that do not penetrate deeply into this layer usually heal without veterinary medical treatment. Healing takes two to three days. The denuded epithelium in corneal ulcers heals by epithelization. In this process, intact epithelial cells move to the defect and undergo mitosis, or simple cell division, and the new cells close the crater in the epithelial layer. This normally takes seven to 10 days. If Bowman's membrane is also ulcerated, healing will take longer; this basement membrane is made up of connective tissue that takes longer to heal. A superficial ulcer generally will heal uneventfully in several weeks if it does not become infected. However, all corneal ulcers are susceptible to bacterial infection, which will delay healing. Additionally, delayed healing may be associated with increased scar formation, so veterinary treatment is necessary to facilitate as close to ideal healing as possible.
Deeper ulcers involving the stroma will take a number of weeks to heal. During stromal healing, blood vessels from the outer rim of the cornea may grow on the floor of the ulcer, a process called neovascularization. This process promotes deep healing by providing extra nutrients to the damaged area. During this process scar tissue will form. In some untreated ulcers the new blood vessels will regress and eventually disappear and scar tissue will have remodeled upon completion of healing so that vision is not impaired. However, in many cases of untreated deep corneal ulcers, scar tissue sufficient to severely impair vision remains. For this reason, treatment aimed at minimizing scar formation and idealizing healing is always indicated. Residual new blood vessels and scar tissue potentially can result in vision impairment because these tissues are not transparent and the normal clarity of the cornea can be lost.
An ulcer extending to Descemet's membrane is especially dangerous. Without the stromal layer backing it up, Descemet's membrane, can herniate outward. Herniation of this corneal layer is called a descemetocele. This weakened condition of these corneal layers can result in rupture of Descemet's membrane and structural failure of the endothelium. When this occurs, the aqueous humor -- the fluid in the anterior chamber of the eye immediately behind the cornea -- flows out and the outer part of the eyeball collapses. This is a surgical emergency that requires immediate attention to save the eye.
Some corneal ulcers may be refractory to treatment or may heal, only to re-ulcerate repeatedly. These ulcers have a defect in the epithelial cells of the outer cornea. This defect may have a genetic basis.
