Pheochromocytoma: A High-Yield USMLE Review

Pheochromocytoma is a catecholamine-secreting tumor arising from chromaffin cells of the adrenal medulla. Episodic release of norepinephrine (and variable epinephrine) produces the hallmark paroxysmal symptoms that make this a favorite USMLE topic. Recognizing it matters because both diagnostic biopsy and improper drug sequencing can trigger a fatal catecholamine crisis.

Presentation

• Paroxysmal (episodic) hypertension — the classic pattern, though sustained hypertension occurs in about 50% of patients
• Pounding headache during episodes
• Perspiration (diaphoresis)
• Palpitations
• Pallor rather than flushing during episodes, driven by norepinephrine-mediated α1 peripheral vasoconstriction

Diagnosis

• Plasma free metanephrines or 24-hour urine fractionated metanephrines — the first-line biochemical confirmation of catecholamine excess (all adrenal incidentalomas should be screened for pheochromocytoma)
• CT/MRI to localize the tumor once biochemical excess is confirmed — pheochromocytomas characteristically show high unenhanced attenuation (>10 HU) and delayed contrast washout, features that distinguish them from benign lipid-rich adrenal adenomas rather than predicting malignancy
• Malignancy in pheochromocytoma is defined by the presence of metastases (in sites where chromaffin tissue is normally absent, e.g., bone, liver, lung, lymph nodes), NOT by imaging attenuation, size, or washout — functional imaging such as I-123 MIBG or FDG-PET helps detect metastatic/extra-adrenal disease
• Genetic testing is recommended for all patients, since up to 40% of pheochromocytomas harbor germline mutations
• Screen for medullary thyroid cancer / MEN 2 when a thyroid nodule is present in a pheochromocytoma patient

Management

• Preoperative alpha-blockade for 10–14 days with phenoxybenzamine (irreversible) or selective α1-blockers such as doxazosin or prazosin to normalize BP and prevent intraoperative hypertensive crisis
• Add a beta-blocker only after adequate alpha-blockade if tachycardia is present, plus volume expansion with salt and fluids to correct chronic volume depletion
• Laparoscopic adrenalectomy for most unilateral tumors, with postoperative monitoring for hypotension and hypoglycemia
• For malignant/metastatic disease: I-131 MIBG therapy, chemotherapy, or tyrosine kinase inhibitors

High-yield

• The 4 P's: Paroxysmal hypertension, Pounding headache, Perspiration, and Palpitations
• Rule of 10s: 10% bilateral, 10% malignant, 10% extra-adrenal (paragangliomas), 10% familial, 10% in children — though up to 40% are now known to have germline mutations
• Never give a beta-blocker before alpha-blockade — unopposed alpha stimulation causes severe hypertension
• Chromaffin cells are neural crest–derived; the medulla is a modified sympathetic ganglion secreting predominantly norepinephrine, with variable epinephrine depending on PNMT (extra-adrenal paragangliomas lack PNMT and make only norepinephrine)
• Pheochromocytoma is associated with MEN 2 — always check for medullary thyroid cancer, and preoperative preparation drops perioperative mortality from historically >40% to <3%

Pitfalls

• Never biopsy an adrenal mass before ruling out pheochromocytoma — a biopsy can provoke a fatal catecholamine surge
• Do not start a beta-blocker before achieving alpha-blockade, as unopposed alpha stimulation triggers a hypertensive crisis
• Do not assume normal catecholamines exclude the diagnosis — between paroxysms levels may be normal
• Don't try to diagnose malignancy from imaging — high unenhanced attenuation and delayed washout are typical of benign pheochromocytomas; malignancy requires demonstrated metastases
• Don't reverse the secretory pattern: pheochromocytomas predominantly release norepinephrine, not epinephrine — a classic exam trap
• Don't overlook the thyroid: a nodule in a pheochromocytoma patient raises concern for MEN 2 and mandates screening for medullary thyroid cancer before any thyroid surgery