Canine Pheochromocytoma Study Guide

Overview and Clinical Importance

Pheochromocytomas are functional neuroendocrine tumors arising from the chromaffin cells of the adrenal medulla. Although uncommon in dogs (occurring in less than 0.01% of the canine population), awareness of this tumor type is critical due to its life-threatening complications resulting from excessive catecholamine secretion. The inconsistent and paroxysmal nature of clinical signs, combined with the potential for catastrophic cardiovascular events during anesthetic induction or surgical manipulation, makes pheochromocytoma one of the most challenging endocrine tumors to diagnose and manage in veterinary medicine.

Anatomy and Pathophysiology

Adrenal Gland Structure

The adrenal gland consists of two functionally and embryologically distinct regions. The outer adrenal cortex, derived from mesoderm, produces three classes of steroid hormones from its three zones: mineralocorticoids (zona glomerulosa), glucocorticoids (zona fasciculata), and androgens (zona reticularis). The inner adrenal medulla, derived from neural crest ectoderm, produces catecholamines (epinephrine, norepinephrine, and dopamine) from chromaffin cells. Pheochromocytomas arise specifically from these medullary chromaffin cells.

Catecholamine Synthesis and Regulation

Normal catecholamine synthesis follows a four-step enzymatic pathway: tyrosine hydroxylase (rate-limiting step) converts tyrosine to L-DOPA, aromatic L-amino acid decarboxylase produces dopamine, dopamine beta-hydroxylase creates norepinephrine, and phenylethanolamine N-methyltransferase converts norepinephrine to epinephrine. In healthy animals, sympathetic nervous system stimulation via acetylcholine triggers catecholamine release through a tightly regulated negative feedback mechanism.

In pheochromocytomas, this regulatory system fails. The tumor lacks normal innervation and negative feedback control, resulting in autonomous, episodic, or continuous catecholamine hypersecretion. Most pheochromocytomas predominantly secrete norepinephrine (unlike normal adrenal medulla which secretes approximately 85% epinephrine), though secretory patterns vary by individual tumor.

Adrenergic Receptor Effects

Catecholamine excess produces clinical signs through overstimulation of alpha and beta adrenergic receptors throughout the body:

Epidemiology and Signalment

Pheochromocytomas are diagnosed predominantly in older dogs with a mean age of approximately 10-11 years at diagnosis. There is no consistent sex predilection, though some studies suggest a slight male predominance. Any breed can be affected, but there is no strong breed predisposition documented in the literature, unlike some other endocrine tumors.

These tumors are often incidental findings. More than 50% of canine pheochromocytomas are identified during investigation of concurrent disease, routine abdominal ultrasound for unrelated complaints, or at necropsy examination. This high incidental detection rate reflects the nonspecific and intermittent nature of clinical signs, which can be easily attributed to other more common conditions.

Exam Focus: Remember the '10% tumor' rule from human medicine, which partially applies to dogs: approximately 10% are bilateral, approximately 10% are extra-adrenal (paragangliomas), approximately 10-25% are malignant (metastatic), and clinical signs may be absent or intermittent in a significant percentage of cases.

Tumor Behavior and Metastatic Potential

Local invasion is a hallmark feature of pheochromocytomas. These tumors frequently invade into the caudal vena cava, creating tumor thrombi that can extend cranially toward the right atrium. Vascular invasion is observed in the majority of cases and does not necessarily indicate malignancy, though it significantly complicates surgical excision. The tumor may also directly invade surrounding structures including kidneys, liver, and retroperitoneal tissues.

The metastatic rate for pheochromocytomas is reported at less than 25% of cases, though only the presence of metastases definitively confirms malignancy. Common metastatic sites include regional lymph nodes, liver, lungs, spleen, kidneys, bone, and the central nervous system. Histologic features cannot reliably predict metastatic behavior, though larger tumor size, vascular invasion, and certain histologic patterns (high mitotic rate, necrosis, capsular invasion) may suggest more aggressive biological behavior.

Clinical Signs and Presentation

Clinical manifestations of pheochromocytoma result from two primary mechanisms: excessive catecholamine secretion causing systemic adrenergic effects, and local effects from the space-occupying mass with potential vascular invasion or compression of adjacent structures. Signs may be constant, episodic, or completely absent.

Cardiovascular Signs

Systemic hypertension is present in approximately 50% of dogs at the time of diagnosis. However, the paroxysmal nature of catecholamine release means blood pressure may be normal during examination. Hypertension greater than 160 mmHg systolic or greater than 95 mmHg diastolic should raise suspicion. Other cardiovascular manifestations include tachyarrhythmias (supraventricular or ventricular), palpitations, bounding pulses, and in severe cases, hypertensive crisis with potential for myocardial infarction, stroke, or cardiac arrest.

Neuromuscular and Neurologic Signs

Neurologic signs may result from catecholamine-induced vascular spasm, intracerebral hemorrhage, or brain metastases. Clinical manifestations include weakness, lethargy, collapse, syncope, seizures, pacing, disorientation, anxiety, and ataxia. Posterior paresis or lameness may occur secondary to vertebral or bone metastases.

Respiratory Signs

Panting, tachypnea, and dyspnea are common respiratory manifestations resulting from increased metabolic rate, anxiety, or pulmonary edema secondary to hypertensive crisis. Respiratory distress may also indicate pulmonary metastases.

Gastrointestinal Signs

Vomiting, abdominal pain, diarrhea, and anorexia may occur. Abdominal pain can result from the tumor mass itself, tumor hemorrhage, or gastrointestinal effects of catecholamine excess.

Other Clinical Signs

Polyuria and polydipsia are frequently reported. Additional signs include fever (from hypermetabolism), exercise intolerance, weight loss, and muscle tremors.

Diagnostic Approach

Diagnosis of pheochromocytoma requires a high index of suspicion combined with demonstration of both excessive catecholamine production and an adrenal mass on diagnostic imaging. The diagnosis is often challenging due to nonspecific clinical signs and the need for specialized biochemical testing.

Routine Laboratory Testing

Complete blood count, serum biochemistry panel, and urinalysis typically show no consistent abnormalities specific for pheochromocytoma. Mild elevations in liver enzymes, hyperglycemia (from catecholamine-induced glycogenolysis and gluconeogenesis), or stress leukogram may be observed but are nonspecific.

Blood Pressure Measurement

Indirect blood pressure measurement is essential. Systolic blood pressure greater than 160 mmHg or diastolic greater than 95 mmHg supports the diagnosis, though normal blood pressure does not exclude pheochromocytoma due to episodic secretion. Serial measurements may capture paroxysmal hypertension.

Biochemical Testing for Catecholamine Excess

The preferred diagnostic approach involves measurement of plasma or urinary metanephrines (metanephrine and normetanephrine), which are the stable metabolites of epinephrine and norepinephrine. Metanephrines are continuously produced within the tumor regardless of catecholamine release, providing more consistent diagnostic sensitivity than measuring catecholamines themselves.

Critical consideration: Biochemical testing should be performed before initiating phenoxybenzamine therapy, as alpha-blockers cause false-positive elevation of plasma normetanephrine. Other drugs that may interfere include metoclopramide, beta-blockers, calcium channel blockers, and sympathomimetics.

Diagnostic Imaging

Abdominal Ultrasonography

Abdominal ultrasound is typically the first imaging modality used to identify an adrenal mass. A diagnosis of adrenal mass is made when the gland exceeds 1.5 cm in maximum width, loses its normal kidney-bean shape, and demonstrates asymmetry compared to the contralateral adrenal. Pheochromocytomas are often large (frequently greater than 3 cm), heterogeneous in echotexture due to hemorrhage or necrosis, and may show invasion into the caudal vena cava with tumor thrombus extending cranially. Normal ultrasound appearance does not exclude pheochromocytoma.

Computed Tomography (CT)

Contrast-enhanced CT is the gold standard for surgical planning and metastasis screening. CT provides superior anatomic detail of vascular invasion, tumor extent, and local invasion. Characteristic CT features include: tumor size typically greater than 3 cm, heterogeneous appearance with areas of hemorrhage and necrosis, intense heterogeneous enhancement in arterial phase (often greater than 110 HU), moderate washout in delayed phases, and attenuation value on non-contrast CT typically greater than 10 HU (distinguishing from lipid-rich adenomas).

Thoracic Radiography

Three-view thoracic radiographs are essential to screen for pulmonary metastases before pursuing surgical therapy.

Nuclear Scintigraphy

123I-MIBG (metaiodobenzylguanidine) scintigraphy is a functional imaging technique that exploits the structural similarity between MIBG and norepinephrine, resulting in specific uptake by neuroendocrine tissues. While highly specific for pheochromocytoma, MIBG scintigraphy has limited availability, requires 24-72 hours for image acquisition, lacks anatomic detail, and may have false-negative results in some tumors. It is most useful for detecting extra-adrenal paragangliomas and metastatic disease when anatomic imaging is equivocal.

Fine Needle Aspiration

Ultrasound-guided fine needle aspiration of adrenal masses can differentiate cortical from medullary origin but carries significant risk in pheochromocytoma cases. Potential life-threatening complications include pain, hemorrhage, severe hypertensive crisis from sudden massive catecholamine release, and cardiac arrhythmias. Cytology cannot reliably distinguish benign from malignant neuroendocrine tumors. Many clinicians avoid FNA when pheochromocytoma is suspected based on imaging and biochemical findings.

Differential Diagnoses

When an adrenal mass is identified, the primary differential diagnoses include:

Treatment and Management

Surgical Adrenalectomy - Treatment of Choice

Surgical excision is the only definitive treatment for pheochromocytoma and is indicated for dogs with clinical signs attributable to catecholamine excess, evidence of local invasion with potential for complications, or malignant characteristics on imaging. Complete tumor removal can reverse clinical signs related to catecholamine hypersecretion and prevents complications from progressive tumor growth. Dogs surviving the perioperative period can achieve prolonged survival (years), with good long-term prognosis in the absence of metastatic disease.

Adrenalectomy for pheochromocytoma is technically challenging. The tumor is often large, friable, highly vascular, and frequently invades the caudal vena cava requiring venotomy and thrombectomy. The perioperative mortality rate ranges from 15-37% due to catastrophic hemorrhage, severe hypertensive crises, life-threatening arrhythmias, and hypotension following tumor removal.

Preoperative Medical Management with Alpha-Blockade

The cornerstone of preoperative medical management is phenoxybenzamine, a noncompetitive, irreversible, nonselective alpha-adrenergic receptor antagonist. Phenoxybenzamine blocks both alpha-1 and alpha-2 receptors, preventing the cardiovascular effects of circulating catecholamines.

Beta-blocker therapy: If tachyarrhythmias persist despite adequate alpha-blockade, a beta-blocker (atenolol 0.25-1 mg/kg PO q12-24h or propranolol 0.15-0.5 mg/kg PO q8h) may be added. Critical: Beta-blockers must NEVER be initiated before alpha-blockade, as this would result in unopposed alpha-adrenergic stimulation causing severe hypertensive crisis.

Anesthetic Considerations

Anesthetic management is critical for successful outcomes:

Drugs to avoid: Morphine (histamine release may trigger hypotension), ketamine (sympathomimetic effects), halothane (sensitizes myocardium to arrhythmias)

Recommended agents: Fentanyl or hydromorphone for analgesia, propofol or alfaxalone for induction, isoflurane or sevoflurane for maintenance

Intraoperative monitoring: Continuous ECG, direct arterial blood pressure monitoring, capnography, pulse oximetry, temperature

Medications on hand: Phentolamine (IV alpha-blocker for hypertensive crisis), esmolol or lidocaine (for ventricular arrhythmias), vasopressors (for post-removal hypotension), IV fluids

Perioperative Complications

Medical Management for Nonsurgical Candidates

When surgery is not feasible due to extensive local invasion, distant metastases, concurrent disease, or owner preference, medical management options include:

Phenoxybenzamine: Long-term administration (1 mg/kg PO q12h adjusted to effect) to control clinical signs of catecholamine excess, manage hypertension, and improve quality of life. Survival times with medical therapy alone are not well documented.

Toceranib phosphate (Palladia): A tyrosine kinase inhibitor with antiangiogenic and antiproliferative effects. Dosage: 2.4-2.9 mg/kg PO every other day (Monday-Wednesday-Friday schedule). May provide tumor control in some cases with inoperable or metastatic disease. Adverse effects include diarrhea, decreased appetite, neutropenia, and lameness.

Supportive care: Antihypertensive medications as needed, beta-blockers for arrhythmias, pain management, monitoring for complications

Prognosis and Survival

Prognosis for pheochromocytoma depends on multiple factors including tumor size, presence of metastases, extent of vascular invasion, successful surgical excision, and perioperative complications.

Prognostic Factors

Causes of Death

Most dogs with pheochromocytoma die from complications related to:

• Catecholamine excess (hypertensive crisis, stroke, cardiac arrhythmias, heart failure)
• Tumor thrombus with vascular obstruction or thromboembolism
• Local tumor invasion into kidneys, liver, or major vessels
• Tumor rupture with acute hemorrhage and hypovolemic shock
• Metastatic disease
• Perioperative complications during adrenalectomy

Exam Focus: The perioperative mortality rate for pheochromocytoma adrenalectomy ranges from 15-37%, which is significantly higher than for other adrenal tumors. This high mortality reflects the cardiovascular instability from catecholamine surges during surgical manipulation. Dogs that survive the perioperative period and have no metastatic disease often achieve survival times of several years with good quality of life.