Behaviour
Most pheochromocytomas are benign and are located in one adrenal gland, although they can occur in both adrenal glands. These tumours range in size from a few millimeters to 15 centimeters and large pheochromocytomas often invade blood vessels and adjacent tissues. Local invasion into adjacent blood vessels is observed in up to 56% of dogs and this behaviour is often related to malignancy. However, based on the latest definition of the WHO, pheochromocytomas are only considered malignant when distant metastases are present.
In approximately 20% of dogs with pheochromocytoma, metastases are described in regional lymph nodes, liver, spleen, pancreas, lung, heart, central nervous system, kidney, and bone.
Approximately 50% of dogs with pheochromocytoma also have another endocrine tumour. These are tumours that involve endocrine glands, such as adrenal or pituitary tumours causing Cushing’s syndrome (most common), followed by parathyroid tumours, thyroid tumours, and insulinomas.
Prevalence
Pheochromocytoma is usually diagnosed in older dogs and is uncommon. Currently it is estimated that pheochromocytoma accounts for 0.01% to 0.1% of all canine tumours. This prevalence might be an underestimation because the diagnosis of pheochromocytoma via bloodwork has only recently become possible; the diagnosis can now be made faster and easier than before.
Breed predisposition
There is no obvious apparent sex or breed predisposition, although brachycephalic dog breeds (such as Bulldog, Pug) seem to be predisposed to develop a pheochromocytoma.
Symptoms
Pheochromocytomas are characterized by the overproduction of catecholamines (hormones) such as epinephrine and norepinephrine.
Clinical signs are usually related to the direct actions of secreted catecholamines and/or the space‐occupying or invasive nature of the adrenal mass. Hormone secretion from the tumour is unpredictable and episodic and consequently, the clinical picture varies considerably. Episodes of increased catecholamine release may occur several times per day or only at intervals of weeks to months. The triggers for catecholamine secretion in dogs are unknown. Pheochromocytoma should be considered a potentially life-threatening disease that can result in collapse and sudden death due to massive catecholamine release or tumour rupture.
The clinical manifestations of catecholamine excess are often non‐specific and may resemble other disorders that are more common than pheochromocytoma. General symptoms may include weakness, abdominal pain, anorexia, and discomfort. Other symptoms (related to the cardiorespiratory system) can include panting, a very fast heartbeat, an irregular heartbeat, and fainting. Systemic hypertension, one of the hallmarks of pheochromocytoma, is present in approximately 50% of affected dogs at the time of initial examination.
The neuromuscular system may be affected by catecholamine‐induced spasm of blood vessels, haemorrhage within the central nervous system, or brain metastases. As a consequence, dogs may present with pacing, disorientation, anxiety, and seizures. Paresis (partial paralysis), lameness, and pain have been reported as a result of metastases to the vertebral canal or bone. Amongst the miscellaneous clinical signs of pheochromocytoma, drinking and urinating more are common.