Tumors of the Adrenal Medulla
Pheochromocytomas occur in domestic species, including dogs and cats. Incidental masses in the area of the adrenal glands are being discovered with greater frequency because of the increased use of abdominal ultrasound and other imaging techniques. Pheochromocytoma, although rare, should be a differential diagnosis whenever such a mass is identified. Pheochromocytomas arise from the adrenal medullary chromaffin cells that normally synthesize and secrete the catecholamines epinephrine and norepinephrine. These tumors have been identified more often in dogs than in cats, usually affect only one gland, and tend to occur in older animals. They are often locally invasive (may result in thrombus formation in the adjacent vena cava) and metastasize in ~25% of cases. Diagnosis of pheochromocytoma is challenging. Clinical signs are nonspecific and may appear sporadically, possibly related to periodic or intermittent release of catecholamines. In addition, signs may vary depending on which catecholamine predominates. Signs reported in most dogs include weight loss, anorexia, depression, weakness, and occasional collapse. Dyspnea and tachycardia may be seen, and hypertension is common; indirect blood pressure measurement can help establish the diagnosis. Imaging, such as ultrasonography or CT scans, is very useful in confirmation of a suspected adrenal mass. Although surgical removal is the treatment of choice, animals with pheochromocytoma present an anesthetic risk secondary to the cardiovascular effects of the catecholamines. Surgery is additionally complicated by the tendency toward local invasion of these tumors and their proximity to large vessels.
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.