Case Report: Feline adrenal pheochromocytoma with a synaptophysin-positive, chromogranin A-negative immunophenotype.

Pheochromocytoma is a rare adrenal medullary neoplasm in cats, with limited published cases and diagnostic challenges due to variable clinical presentation. An 8-year-old neutered male domestic shorthair cat presented with severe watery diarrhea, lethargy, and anorexia. Diagnostic evaluation revealed chronic kidney disease, pyelonephritis, and a large cystic right adrenal mass identified on ultrasonography and computed tomography. Endocrine testing did not support hyperadrenocorticism or hyperaldosteronism, and functional assessment of catecholamine excess was not performed. Surgical adrenalectomy was elected due to mass size and rupture risk. Histopathological examination demonstrated a medullary adrenal neoplasm composed of polygonal cells arranged in characteristic packets. Immunohistochemistry revealed synaptophysin positivity with negative chromogranin A staining, supporting a diagnosis of pheochromocytoma. Postoperative recovery was uneventful, and the cat remained normotensive and clinically stable at one year, with chronic kidney disease managed medically. This case underscores the diagnostic complexity of feline pheochromocytoma and highlights the importance of integrating imaging, histopathology, and immunohistochemistry, as chromogranin A negativity does not exclude this diagnosis.