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.