Adrenal Medullary Hyperplasia

Diffuse or nodular adrenal medullary hyperplasia appears to precede the development of pheochromocytomas in bulls with C-cell tumors of the thyroid gland. This diffuse proliferation of chromaffin cells is nonencapsulated but compresses the surrounding adrenal cortex. In bulls with prominent diffuse medullary hyperplasia, there are often a few small foci of intense nodular proliferation of medullary cells.

Pheochromocytomas

These tumors of chromaffin cells are almost always located in the adrenal glands. They are the most common tumors in the adrenal medulla of animals; they develop most often in cattle, laboratory rats, and dogs, and are infrequent in other domestic animals. In bulls and rats, pheochromocytomas develop concurrently with calcitonin-secreting C-cell tumors of the thyroid gland, possibly as a neoplastic transformation of multiple types of endocrine cells of neuroectodermal origin in the same individual. Malignant pheochromocytoma designates a medullary tumor that invades through the adrenal capsule into adjacent structures (eg, posterior vena cava) or metastasizes to distant sites (eg, liver, regional lymph nodes, or lungs), or both. Functional pheochromocytomas are reported infrequently in animals; however, several dogs and horses with pheochromocytomas have had tachycardia, edema, and cardiac hypertrophy attributed to excess catecholamine secretion. Clinical signs in dogs may include polyurea and polydipsia. It appears that horses may have a syndrome similar to the multiple endocrine neoplasia noted in humans with concurrent adrenal and thyroid disease.

Although size varies considerably, pheochromocytomas may be large (≥10 cm in diameter) and incorporate most of the affected adrenal. A small remnant of the adrenal gland often can be found at one pole. Smaller tumors are well encapsulated by a thin, compressed rim of adrenal cortex. Large pheochromocytomas are multilobular and variegated, and they may exert pressure on and invade adjacent tissues, particularly the vena cava and aorta. In dogs, ~50% of pheochromocytomas metastasize to the liver, regional lymph nodes, spleen, and lungs.

Due to the lack of routine availability of validated assays for catecholamines in dogs and cats, the diagnosis is often made on the basis of clinical signs and ultrasound. Treatment involves surgery (if feasible) and management of hypertension.

Last full review/revision July 2011 by Robert C. Rosenthal, DVM, PhD, DACVIM (Small Animal, Oncology), DACVR (Radiation Oncology)