The adrenal gland consists of two distinct regions, the outer cortex and inner medulla. The cortex consists of three distinct layers that produce mineralocorticoids (aldosterone), glucocorticoids and androgens. Collectively these hormones are sometimes referred to as corticosteroids. The medulla produces catecholamines including epinephrine, norepinephrine and dopamine. The synthesis and release of catecholamines is stimulated by stressful stimuli such as exercise, hemorrhage, hypotension, hypoxemia, and hypoglycemia amongst others.^1,2 The normal adrenal medulla converts most norepinephrine to epinephrine which is the primary product excreted.¹ Once released into circulation, catecholamines are bound to proteins such as albumin and quickly metabolized by various tissues into inactive metabolites such as metanephrine, normetanephrine and vanillymandelic acid. ^1,2 The intact catecholamines and their metabolites are excreted in the urine.² Excess intracellular catecholamines are also metabolized within the adrenal medulla. This explains why pheochromocytomas excrete a large number of metabolites directly into circulation.^1,2 Catecholamines stimulate adrenergic receptors on various tissues. Some effects include vasoconstriction, vasodilation, increased heart rate and force of contraction, increased renin release, decreased intestinal motility, increased glycogenolysis in the liver, bronchiolar dilatation and lipolysis.²

Signalment

Pheochromocytomas are uncommon but found in dogs much more often than in cats. In fact there have only been 4 reports of pheochromocytomas causing clinical signs to the author's knowledge.^3,4,5,6 Animals are usually older at the time of diagnosis.^1,7,8

Neuroendocrine tumors are tumors of organ systems that utilize both hormones and chemical messenger systems. Neuroendocrine tumors of the adrenal medulla that produce excess catecholamines are called pheochromocytomas. Pheochromocytomas arise from chromaffin cells in the adrenal medulla. Unilateral involvement occurs in most all animals but bilateral disease has been reported.^7,8 Rarely, concurrent adrenocortical disease is present.^9,10,11 Approximately 50% of pheochromocytomas are considered malignant based on their behavior.^7,8,12 These tumors may invade adjacent vessels (caudal vena cava, phrenicoabdominal artery/vein, adrenal artery/vein, renal artery/vein and hepatic vein) or metastasize to the regional lymph nodes, liver, spleen, kidneys, pancreas, peritoneum, lungs, heart, spinal cord, bone and brain.^7,8,12,13,14

Clinical Signs

These tumors may be an incidental finding and in a previous report was found in up to 60% of dogs at necropsy so most animals with pheochormocytomas exhibit no clinical signs.^7,8 In animals with signs, they may be non-specific and intermittent. Signs include lethargy, weight loss, anorexia, panting, polydispia, polyuria, abdominal distention, and collapse.¹ Additional clinical findings may include pale mucous membranes, tachycardia and hypertension.¹

General Laboratory Findings

Routine laboratory abnormalities are uncommon but may include a non-regenerative anemia of chronic disease, regenerative anemia (if hemorrhage), hyperglycemia, and elevated liver enzymes.¹ Proteinuria and variable concentration might be found on urinalysis due to the effects of catecholamines on hypertension and vasopressin secretion, respectively.¹