Syndrome of inappropriate antidiuretic hormone secretion in a cat with a putative Rathke's cleft cyst.

An 11-year-old spayed female domestic shorthair cat was evaluated for anorexia, lethargy and weight loss of 6 days' duration. Bilateral mydriasis, absent menace response, slow-to-absent pupillary light reflexes, bilateral retinal detachment, intermittent horizontal nystagmus, intermittent ventral strabismus and systemic hypertension were present. Biochemical analysis revealed severe hyponatremia, severe hypochloremia and mild hypokalemia. Multifocal central nervous system disease was suspected based on optic, trigeminal sensory (ophthalmic branch), vestibulocochlear and possible oculomotor nerve dysfunction. Thoracic radiographs showed mild cardiomegaly without evidence of congestive heart failure. Ultrasound revealed mild pleural and peritoneal effusion. A cause of the severe hyponatremia was not identified, and it persisted despite fluid therapy. Syndrome of inappropriate antidiuretic hormone secretion (SIADH) was suspected as the cause of hyponatremia. Humane euthanasia was elected owing to continued clinical decline. Serum hyposmolality, urine hyperosmolality, natriuresis and lack of confirmed renal, thyroid and pulmonary disease aided in the presumed diagnosis of SIADH. Post-mortem histopathology of the brain revealed degeneration of the hypothalamus and optic tracts, along with a prominent fluid-filled craniopharyngeal duct (putative Rathke's cleft cyst) separating the pars distalis and the pars intermedius. The hypothalamic degeneration, possibly secondary to a Rathke's cleft cyst, was hypothesized to be the cause of presumptive SIADH in the patient. Although rare in occurrence, Rathke's cleft cyst should be included as a differential diagnosis in dogs and cats with signs of pituitary dysfunction.