Canine corneal diseases: secrets for transparency greater than the federal stimulus (Proceedings)
Corneal pathologic responses
The normal cornea is clear, and any alteration in clarity signifies pathology. Possible pathologic changes include corneal vessels, edema, pigment, lipid or calcium deposits, inflammatory cell infiltrates, destruction from degradative enzymes, and scarring. Such changes are usually non-specific and incited by numerous causes. Pathologic responses can occur singularly, but more often several occur simultaneously. Because corneal pathologic responses are usually secondary rather than primary, resolution is best achieved by identification and treatment of the primary cause.
Chronic superficial keratitisChronic superficial keratitis (or pannus) is an immune-mediated keratitis with genetic and environmental influencing factors. High altitude and ultraviolet radiation are the greatest environmental risk factors, and the most severe cases in this country occur in states such as Utah and Colorado. There is an obvious breed predisposition for German Shepherds and Shepherd crosses, but it can occur in any breed. It is also common in the Greyhound. It usually begins as a bilateral pink or red inflammatory lesion of the inferotemporal cornea and is often symmetrical. However, it can begin in other corneal quadrants and be asymmetrical. Histologically, the corneal infiltrate is characterized by plasma cells, lymphocytes, and blood vessels. With progression, the entire cornea can be affected and result in markedly diminished vision or blindness. Corneal pigment and fibrosis are prominent features in chronic cases. The nictitans can be involved simultaneously or exclusively (e.g., atypical pannus or plasmoma). Age of onset is usually 3-5 years but may occur at anytime. Pannus can be more difficult to control in dogs affected at a young age. Diagnosis is based on clinical appearance, breed predilection, and corneal or conjunctival cytology. Cytology usually reveals a preponderance of lymphocytes and plasma cells.
Like most immune-mediated diseases, pannus is a disease to be controlled rather than cured. Topical corticosteroid, cyclosporine, or tacrolimus are the primary treatments. The preferred steroid preparations are those with 1% prednisolone acetate or 0.1% dexamethasone. The required frequency of administration varies with the time of year and severity of pannus but is usually 2-4 times daily. Subconjunctival steroids can be administered as an adjunct to topical treatment, for refractory cases, or for dogs difficult to treat. Triamcinalone, methylprednisolone, and betamethasone are similarly effective, but conjunctival granuloma formation may be less likely to occur after betamethasone injection. Topically applied cyclosporine in concentrations of 0.2, 1 or 2%, or tacrolimus in concentrations of 0.02 or 0.03%, are also effective treatments. Some pannus cases are effectively controlled with cyclosporine or tacrolimus alone. In other cases, their use allows the steroid treatment to be reduced, thereby minimizing undesirable side effects. Treatments can often be reduced during the winter months but must be increased again during the spring and summer months. Beta-irradiation and lamellar keratectomy are additional treatment options, but they are no longer commonly employed. Plasma cells and lymphocytes are particularly sensitive to beta-irradiation, and radiation is an effective treatment for difficult cases. However, stringent licensing requirements for the Strontium-90 probe have made irradiation an impractical treatment.
Scleritis/Episcleritis (aka Nodular Granulomatous Episclerokeratitis, or NGE)
This is an immune-mediated condition that occurs as a nodular to diffuse inflammation of the sclera or episclera. It can be unilateral or bilateral. Often only one quadrant is affected, and because of the nodular appearance, scleritis can be mistaken for a neoplasm. However, scleral neoplasms other than limbal melanoma are rare. There is a breed predilection for Cocker Spaniels, but any breed can be affected. Lymphocytes, plasma cells, and histiocytes are typical histologic features. The adjacent cornea is usually affected with a variable degree of vessels, inflammatory cell infiltrates, and secondary lipid degeneration. Deep necrotizing scleritis is rare but can cause serious intraocular disease (e.g., uveitis and retinal detachment). Diagnosis is usually by clinical appearance alone, but scleral biopsy can be performed to confirm the diagnosis. Immune function tests (e.g., ANA, Coomb's, etc.) are often negative and of little benefit. Treatment usually involves a combination of topical and subconjunctival steroids or systemic treatment. Effective systemic treatments include prednisone, azathioprine, and combination treatment with tetracycline and niacinamide. Oral or topical cyclosporine may be effective for some cases. Long-term treatment is likely to be required.