Orbital plasmacytoma in a cat.

An Abyssinian cat presented with epistaxis, melaena and retinal vascular changes. Plasma protein levels were markedly elevated and a monoclonal gammopathy was present on serum protein electrophoresis. Based on these laboratory findings, hyperviscosity syndrome was assumed to be responsible for the clinical signs. The cat responded initially to fluid therapy, but relapsed and then developed unilateral exophthalmia. The animal was euthanased and an extramedullary plasmacytoma of the right orbit was diagnosed at postmortem examination.