The pathogenesis of CSK is thought to be a cell-mediated immune response to corneal altered antigens (or antigens altered
by ultraviolet light), but it is not a typical autoimmune disease in that there are not antibodies to normal self-proteins.1 One study demonstrated that dogs with CSK have an increased number of corneal mast cells and that these mast cells have more
intense degranulation than the mast cells in the corneas of normal dogs or dogs with keratitis caused by other factors.1 In addition, in comparison with clinically normal dogs, dogs with CSK have significantly increased hypersensitivity-like
cellular responses against corneal antigens.6 These findings support the theory that CSK is an immune-mediated disease. The cornea contains tissue-specific antigens that
are modified by ultraviolet light. It may be that these altered antigens induce a cell-mediated immune response against the
modified cornea. Further support of an immune-mediated pathogenesis of CSK comes from a study in which researchers used immunohistochemical
staining to detect immunoglobulin in the corneas of dogs with CSK.1 The pattern of corneal staining that the researchers detected was diffuse in the stroma and uncharacteristic of that of autoimmune
diseases. Perilimbal immunoglobulin deposition was present, suggesting that conjunctival-associated lymphoid tissue and Langerhans'
cells might be involved in the disease process.1 These cells are probably involved with immune processing and the production of the antibodies responsible for CSK.
Treating CSK is directed at controlling the immune response and preserving vision. Because no cure is available, lifelong
therapy is necessary. Initial therapy usually involves topical or subconjunctival corticosteroid administration. One percent
prednisolone acetate or 0.1% dexamethasone ophthalmic preparations, both potent anti-inflammatory and immunosuppressive agents,
should be administered two to four times a day, depending on the initial severity of the lesions. If owner compliance with
the medication schedule is a problem or if the lesions are advanced when therapy is initiated, a subconjunctival injection
of a medium-or long-acting corticosteroid may be given. This type of injection does have some risk because if a corneal ulcer
develops, there is no way to extract or discontinue the medication. Owners should be warned of this risk and taught to recognize
the clinical signs of corneal ulcers (e.g. blepharospasm, epiphora, purulent ocular exudates). Owners should bring dogs in immediately if clinical signs occur. Adding
topical cyclosporine (0.2% ointment [Optimmune] to 2% oil [formulated by a compounding pharmacy]) to the treatment regimen
is widely done because it appears to be beneficial and may allow a reduced topical corticosteroid dosage. 7
The affected dog's response to therapy and prognosis for preserving vision varies with its breed and genetic makeup, the dog's
age at the onset of the disease, the altitude and geographic location of the dog's residence, owner compliance with drug administration,
and the severity of the lesions at the initiation of therapy. If the dog lives at a lower altitude (< 4,000 ft), the prognosis
for preserving vision with medical therapy alone is good.5 The younger the dog is at the onset of disease, the poorer the prognosis because the dog has more time for the disease to
progress. Obviously, an owner's diligence with drug administration plays an important role in controlling the disease.
FIGURE 6. A strontium-90 beta applicator in place on the cornea of a German shepherd that is undergoing radiation therapy
for severe CSK.
If the affected dog lives at a high altitude or if the disease becomes uncontrolled, surgery, radiation therapy, or both may
be indicated. Strontium-90 beta radiation may be used to halt endothelial cell and pigment proliferation (Figure 6).8 This type of radiation only penetrates 1 mm, so the CSK plaque must be relatively thin for the treatment to be effective.7 After radiation therapy, the eye will improve in a week and maximum clearing of the cornea will occur by 30 days.8 Neither extended hospitalization nor isolation is required, as residual radiation is not a factor. If the lesions advance
to the point that the cornea is severely scarred, a superficial keratectomy may be performed to totally remove the lesions.
Beta radiation can then be applied to the limbus to prevent postsurgical neovascularization.8
The prognosis for preserving this dog's vision is guarded, as is the prognosis for all patients at Colorado State University
(because of the altitude) that are diagnosed with CSK, but because treatment was initiated relatively early and because the
owners are conscientious, we think that the dog will be visual for many years.
The photographs and information for this case were provided by Juliet R. Gionfriddo, DVM, MS, DACVO, and Cynthia Powell, DVM,
MS, DACVO, Department of Veterinary Clinical Sciences, College of Veterinary Medicine and Biomedical Sciences, Colorado State
University, Fort Collins, CO 80524.