Hyperviscosity syndrome with IgM monoclonal gammopathy and hepatic plasmacytoid lymphosarcoma in a cat

A 19‐month‐old cat investigated because of depression, anorexia, polyuria, and polydipsia was found to have hepatomegaly, retinal haemorrhages, and distended tortuous retinal veins. Prolonged bleeding occurred after venipuncture, and serum was hyperviscous due to an immunoglobulin M monoclonal gammopathy. Prothrombin and activated partial thromboplastin times were prolonged, thrombin clotting time was only slightly prolonged and the activities of coagulation factors VII, VIII and IX were normal. The cat died suddenly before the origin of the immunoglobulin could be determined. Hepatic plasmacytoid lymphosarcoma was found at post‐mortem examination. The pathophysiology of hyperviscosity syndrome is discussed and the observations in this cat are compared with those reported in the dog.