EXPRESS: Functional extra-adrenal paraganglioma in a cat

Case summary: An 11-year-old neutered male domestic shorthair cat was referred for diabetes mellitus and recurrent episodes of respiratory distress. Physical examination identified an abdominal mass, while routine laboratory testing was unremarkable. Cardiac evaluation revealed cardiomyopathy with a hypertrophic phenotype. Thoracic and abdominal computed tomography identified a heterogeneous mass (5.6 cm) in the right caudal abdominal region, with normal-appearing adrenal glands. Urinary normetanephrine-to-creatinine and metanephrine-to-creatinine ratios were markedly increased (3164 µg/g and 271 µg/g, respectively), representing 51-fold and 15-fold increases compared with 13 clinically healthy control cats, strongly supporting the suspicion of a functional extra-adrenal paraganglioma. Following phenoxybenzamine treatment, exploratory laparotomy was performed. Due to cardiovascular instability, characterized by tachycardia and hypertension, tumor resection was not feasible; however, incisional biopsy samples were successfully obtained. Immunohistochemical findings confirmed the diagnosis of a neuroendocrine tumor, with immunoreactivity for chromogranin A and neuron-specific enolase. The absence of immunoreactivity for pan-cytokeratin (AE1/AE3) and the presence of S100 protein staining in sustentacular cells were consistent with an extra-adrenal paraganglioma. This is the first reported functional extra-adrenal paraganglioma in a cat with biochemical confirmation of catecholamine excess based on urinary metanephrine- and normetanephrine-to-creatinine ratios. Relevance and novel information: This case highlights the diagnostic utility of urinary metanephrine-to-creatinine ratios as a non-invasive tool for detecting catecholamine excess in cats. It also underscores the importance of considering paraganglioma in cats with episodic respiratory distress, hypertrophic phenotype cardiomyopathy, and the presence of an extra-adrenal mass.