by Kristin Bailey, DVM, DCVS-SA
Adrenal masses are generally diagnosed when specific clinical signs prompt investigation and subsequent identification of a tumor, or a mass is found incidentally on imaging performed for another reason. Rarely, adrenal gland masses can rupture resulting in hemorrhage that requires emergent surgery. There are four general types of adrenal tumors in small animals - functional adrenocortical tumors causing hyperadrenocorticism, functional adrenocortical tumors secreting hormones other than cortisol, pheochromocytoma secreting catecholamines, and nonfunctional tumors.
When an adrenal mass is identified, determining whether the tumor is functional is an important step. This can be accomplished somewhat easily if clinical signs of hyperadrenocorticism are present in combination with results of endocrine testing (low dose dexamethasone suppression test, endogenous ACTH, etc) that supports the diagnosis. Clinical signs and exam findings associated with excessive catecholamine release from a pheochromocytoma can include episodic collapse, weakness, tachycardia, systemic hypertension, arrhythmias, seizures, etc. These findings are generally intermittent and unpredictable and therefore may not be identifiable during the veterinary visit. Any patient with an adrenal tumor should have a blood pressure measurement performed, but systemic hypertension is not pathognomonic for pheochromocytoma as patients with Cushing’s can occasionally also have hypertension. Measurement of serum catecholamine levels is generally not reliable due to their sporadic secretion and short half-life. As a result, definitive diagnosis of a pheochromocytoma can be difficult prior to adrenalectomy with histopathology, and presumptive diagnosis is ultimately based on clinical signs and exam findings.
Ultrasound is the primary imaging modality used for the identification of adrenal masses. Dogs with adrenal-dependent hyperadrenocorticism have unilateral adrenomegaly with or without atrophy of the contralateral gland. Unilateral adrenomegaly with a normal contralateral gland is seen with pheochromocytoma. Vascular invasion is about 7 times more likely with pheochromocytoma than adrenocortical tumors and can be seen on ultrasound with reasonable accuracy. Contrast-enhanced CT can be performed in cases where more information is necessary for surgical planning or prognosis.
In general, adrenalectomy is recommended for any patient with a functional tumor or those with characteristics of malignancy. Characteristics of malignancy include any signs of vascular invasion and large tumor size. The presence of metastatic disease in the abdomen or chest is obviously indicative of malignancy, but surgery would not generally be pursued in those patients. The literature suggests that masses greater than 2-3cm in maximal dimension are more likely to be malignant than those smaller than 2cm, but there can be quite a bit of variation in adrenal size in normal dogs. If a small mass is seen or there is only slight enlargement of one pole of the gland, imaging can be repeated over time to monitor for progression. A definitive diagnosis of benign vs malignant cannot be made without histopathology.
If pheochromocytoma is suspected and surgery will be pursued, initiation of phenoxybenzamine therapy is generally recommended to block the effects of chronic exposure to catecholamines. Phenoxybenzamine is an alpha-blocker that acts to reduce vascular resistance and subsequently blood pressure. Therapy is generally continued for 2 weeks before adrenalectomy but varies based on surgeon preference. Medical treatment of hyperadrenocorticism is not generally necessary prior to adrenalectomy but can be done.