Pheochromocytoma and Paraganglioma

Pheochromocytoma and paraganglioma are rare neuroendocrine tumors arising from chromaffin cells that produce catecholamines (epinephrine and norepinephrine). Pheochromocytomas originate from the adrenal medulla, while paragangliomas arise from extra-adrenal sympathetic or parasympathetic ganglia along the paravertebral sympathetic chain. These tumors affect approximately 1-4 per million people annually with a prevalence of 1-5 per 10,000 in hypertensive populations; peak incidence occurs in the third to fifth decades, though pediatric cases and hereditary syndromes can present earlier. Clinical significance is paramount because undiagnosed pheochromocytoma carries substantial morbidity and mortality (hypertensive crises, myocardial infarction, stroke, sudden death), yet diagnosis dramatically improves outcomes with appropriate medical and surgical management. Approximately 30-40% of cases are associated with hereditary syndromes (MEN2A/2B, NF1, SDH mutations), making genetic screening essential. Understanding the diagnostic algorithm, biochemical pathophysiology, and perioperative management is critical for board examinations and clinical practice.

Chromaffin cells possess the enzymatic machinery to synthesize and store catecholamines in secretory granules. Pheochromocytomas arise from malignant transformation of these cells, acquiring the ability to produce excessive catecholamines. The biosynthetic pathway proceeds: tyrosine → L-DOPA (via tyrosine hydroxylase) → dopamine (via DOPA decarboxylase) → norepinephrine (via dopamine β-hydroxylase) → epinephrine (via phenylethanolamine N-methyltransferase in adrenal medulla). Catecholamines are stored in chromaffin granules via vesicular monoamine transporter 2 (VMAT2) and released in response to acetylcholine stimulation of nicotinic receptors or spontaneous granule exocytosis. Tumoral catecholamine production far exceeds physiologic levels, overwhelming normal feedback mechanisms and causing systemic α- and β-adrenergic effects.

Approximately 40% of pheochromocytomas and paragangliomas harbor germline or somatic mutations in susceptibility genes, fundamentally altering the pathophysiology. Key mutated genes include: (1) SDHA, SDHB, SDHC, SDHD, SDHAF2 encoding succinate dehydrogenase complex subunits—mutations impair mitochondrial oxidative metabolism, causing pseudohypoxia and activation of HIF1α pathway; (2) RET proto-oncogene associated with MEN2A/2B, activating tyrosine kinase signaling; (3) NF1 causing neurofibromatosis type 1, with loss of neurofibromin (a RAS GTPase-activating protein) leading to constitutive RAS signaling; (4) VHL (von Hippel-Lindau) causing hypoxia inducibility through impaired HIF1α degradation; (5) MAX and PHD1/2 (prolyl hydroxylases). The pseudohypoxia model is particularly important: SDH mutations prevent conversion of succinate to fumarate, causing accumulation of succinate which stabilizes HIF1α even under normoxic conditions. This HIF1α-driven signaling promotes angiogenesis, cell proliferation, and altered metabolism. SDH-mutant tumors (especially SDHB) carry higher malignancy risk (20-40% develop metastases) and younger presentation age.

Excessive catecholamines activate α1-adrenergic receptors on vascular smooth muscle (causing peripheral vasoconstriction, increased peripheral vascular resistance, and hypertension), α2-adrenergic receptors (inhibiting norepinephrine release, potentially paradoxically), and β1-adrenergic receptors (increasing heart rate and contractility). The sustained elevation of catecholamines causes: (1) smooth muscle vasoconstriction leading to hypertension (often paroxysmal); (2) increased myocardial oxygen demand and coronary vasoconstriction predisposing to angina or MI; (3) ventricular arrhythmias from enhanced automaticity; (4) catecholamine cardiomyopathy from chronic toxicity (Takotsubo cardiomyopathy during acute crisis); (5) insulin suppression and hyperglycemia through β3-adrenergic inhibition of insulin secretion; (6) increased lipolysis and metabolic rate through β3-adrenergic effects on adipose tissue. Importantly, unopposed α-adrenergic stimulation from initial norepinephrine release (which doesn't cross the blood-brain barrier as effectively as epinephrine) before β-effects become apparent explains hypertensive crises and reflex bradycardia. During crises, the ratio of epinephrine to norepinephrine shifts, causing severe vasoconstriction followed by potential vasodilation.

Catecholamines are metabolized via monoamine oxidase (MAO) to aldehyde intermediates, then by aldehyde dehydrogenase to acid metabolites: norepinephrine → 3-methoxy-4-hydroxyphenylglycol (MHPG) and normetanephrine, and epinephrine → metanephrine. These plasma free metanephrines and 24-hour urine metanephrines accumulate in tumor patients and serve as diagnostic biomarkers. The metabolism is critical to understand because certain medications (tricyclic antidepressants, decongestants) and foods (tyramine-rich foods) increase catecholamine levels or interfere with metabolism, potentially triggering hypertensive crises or confounding diagnostic testing.

Some pheochromocytomas and paragangliomas produce additional peptide hormones including ACTH (causing ectopic ACTH syndrome with profound hypokalemia), calcitonin, VIP (vasoactive intestinal peptide causing diarrhea), somatostatin, and PTHrP. These contribute to complex presentations beyond catecholamine excess.

Most pheochromocytomas and paragangliomas occur sporadically without identifiable hereditary predisposition. Somatic mutations accumulate in chromaffin cells during adulthood. Risk increases with age and environmental exposures remain incompletely understood. The adrenal medulla is the most common site for sporadic pheochromocytomas (90% of cases), while extra-adrenal paragangliomas typically occur at specific anatomic sites.

MEN2A and MEN2B are autosomal-dominant syndromes caused by germline RET proto-oncogene mutations. Pheochromocytoma develops in 50% of MEN2 patients, often multiple and bilateral in adrenal glands. MEN2A features medullary thyroid carcinoma (>95% penetrance, usually earliest manifestation), pheochromocytoma (50%), and primary hyperparathyroidism. MEN2B includes medullary thyroid carcinoma, pheochromocytoma, mucosal neuromas, and marfanoid habitus. Pheochromocytomas in MEN2 present earlier (mean age 40 years) and rarely become malignant (<5%). RET mutations are oncogenic through constitutive tyrosine kinase activation, promoting cell proliferation and survival.

NF1 is an autosomal-dominant syndrome affecting approximately 1 per 3000 people; pheochromocytoma develops in 1-5% of NF1 patients. The NF1 gene encodes neurofibromin, a RAS GTPase-activating protein (GAP). Loss of neurofibromin function causes constitutive RAS/MAPK signaling, promoting cell growth. NF1-associated pheochromocytomas are typically benign, though bilateral disease occurs in ~30% of cases. Presentation occurs across wide age range (mean ~40 years), and malignancy risk is low.

Mutations in genes encoding succinate dehydrogenase (SDH) complex subunits cause hereditary paraganglioma-pheochromocytoma (HPGL) syndromes. SDHB mutations (most common, ~30% of familial cases) confer highest malignancy risk (20-40% metastasize), typically present as extra-adrenal paragangliomas in para-aortic region, bladder, or mediastinum, and occur at younger ages (mean 35-40 years). SDHD mutations (maternal transmission pattern—disease occurs only in paternal allele carriers) cause primarily extra-adrenal paragangliomas with lower malignancy risk (~5-15%). SDHA, SDHC, SDHAF2 mutations are less common but similarly predispose to paragangliomas. The pseudohypoxia mechanism (impaired SDH function causing HIF1α stabilization) drives aggressive phenotype.

Germline VHL mutations cause autosomal-dominant VHL disease characterized by hemangioblastomas, renal clear cell carcinoma, pancreatic neuroendocrine tumors, and pheochromocytoma (10-20% of VHL patients). VHL protein normally ubiquitinates HIF1α for proteasomal degradation. VHL mutations cause HIF1α accumulation, increasing expression of VEGF, PDGF, and erythropoietin, promoting angiogenesis and cell proliferation. VHL-associated pheochromocytomas are typically bilateral, benign, and present around age 40 years.

Rare autosomal-dominant mutations in MAX impair its ability to form complexes with MYC, disrupting cell cycle regulation. MAX-mutant pheochromocytomas present as bilateral adrenal tumors with lower malignancy risk, predominantly in males (paternal inheritance pattern).

Mutations in prolyl hydroxylase domain proteins 1 and 2 (PHD1, PHD2) impair HIF1α hydroxylation, causing HIF stabilization similar to SDH mutations. These are rare causes of familial paraganglioma.

Rare associations include Carney complex (PRKAR1A mutations causing cardiac myxomas and pigmentation abnormalities), Familial Hyperparathyroidism with Jaw Tumor Syndrome (CDC73 mutations), and concurrent malignancies in patients with germline cancer susceptibility genes.

Hypertension occurs in 90% of pheochromocytoma patients, making it the most common presenting manifestation. Sustained hypertension results from chronic norepinephrine-mediated vasoconstriction and activation of the renin-angiotensin system. Approximately 50% of patients experience paroxysmal (episodic) hypertensive crises characterized by sudden onset severe hypertension (often >200 mmHg systolic) lasting minutes to hours. Crisis episodes are triggered by: (1) physical activity or exercise; (2) abdominal palpation during examination (hence "rule of palpation"); (3) urination (particularly with bladder paragangliomas); (4) foods rich in tyramine (aged cheeses, cured meats, soy sauce); (5) medications (sympathomimetics, tricyclic antidepressants, metoclopramide, SSRIs); (6) spontaneous tumor catecholamine release; (7) micturition in patients with bladder paragangliomas. During crises, patients develop severe headache (often described as thunderclap-like, secondary to sudden blood pressure elevation), profuse diaphoresis (often drenching), palpitations, chest or abdominal pain, anxiety or sense of impending doom, tremor, and pallor. Between episodes, some patients have normal blood pressure, though office readings often remain elevated.

Beyond hypertension, catecholamine excess causes multiple cardiovascular complications. Tachycardia and palpitations result from β1-adrenergic stimulation and increased automaticity. Chest pain or angina reflects increased myocardial oxygen demand combined with catecholamine-induced coronary vasoconstriction. Acute coronary syndrome and myocardial infarction can occur even in young patients without underlying coronary disease, representing a critical diagnostic pearl—unexplained MI in young normotensive individuals should prompt pheochromocytoma screening. Atrial or ventricular arrhythmias including atrial fibrillation and ventricular tachycardia occur secondary to enhanced automaticity. Catecholamine cardiomyopathy (Takotsubo or stress cardiomyopathy) causes acute left ventricular dysfunction with chest pain, dyspnea, and ECG changes mimicking acute MI but with apical ballooning on echocardiography. Acute fulminant cardiogenic shock can develop during hypertensive crisis, particularly when sudden vasodilation occurs after initial vasoconstriction. Hypertensive nephropathy with acute renal dysfunction or chronic renal failure develops from sustained hypertension. Aortic dissection represents a catastrophic complication from sudden severe hypertension.

Patients frequently report severe headache (occurring in 80% of hypertensive patients), often described as pounding, occipital, or thunderclap in character, worsened during crises. Anxiety and panic attacks are extremely common, sometimes so prominent that patients receive psychiatric diagnoses before correct medical diagnosis; the combination of palpitations, tremor, sweating, and sense of impending doom during crises mimics panic disorder. Tremor (fine, rapid, essential tremor-like) results from β2-adrenergic effects on skeletal muscle. Insomnia and sleep disturbance occur from sympathetic activation. Rarely, psychosis or acute behavioral changes accompany severe hypertensive crises. Patients often undergo extensive psychiatric evaluation before pheochromocytoma diagnosis, representing a common clinical trap.

Weight loss despite normal or increased appetite occurs in 20-40% of patients secondary to hypermetabolism driven by β3-adrenergic effects on adipose tissue lipolysis and uncoupled mitochondrial oxidative phosphorylation. Hyperglycemia and diabetes mellitus develop in 30-40% of patients from catecholamine-mediated suppression of insulin secretion (via β3-adrenergic receptors on pancreatic beta cells) and increased hepatic glucose production. Heat intolerance and excessive sweating result from increased metabolic rate and sympathetic activation; diaphoresis during crises is often profuse and drenching. Fatigue and weakness are common, though not always proportional to symptom severity.

Abdominal pain and discomfort occur in 40-60% of patients; when paragangliomas arise in the bladder or retroperitoneal space, pain may be localized. Nausea, vomiting, and abdominal distension reflect sympathetic inhibition of gastrointestinal motility. Constipation is common from reduced bowel motility. Conversely, some patients experience diarrhea, particularly with VIP-secreting tumors.

Dyspnea and shortness of breath occur during hypertensive crises from increased myocardial oxygen demand, catecholamine-induced pulmonary vasoconstriction, or secondary to acute cardiomyopathy and pulmonary edema. Acute pulmonary edema with flash pulmonary edema can develop during severe crises, particularly when sudden vasodilation occurs after initial intense vasoconstriction, causing acute left ventricular failure.

When tumors secrete ACTH, patients manifest severe hypokalemia (often <2.5 mEq/L), severe hy

Metastatic disease (NCCN): ¹³¹I-MIBG (iobenguane I-131) in MIBG-avid tumors, ¹⁷⁷Lu-DOTATATE peptide receptor radionuclide therapy, cytotoxic chemotherapy with cyclophosphamide/vincristine/dacarbazine, or tyrosine kinase inhibitors.

Contraindicated/avoid: unopposed beta blockers, glucagon, metoclopramide, tricyclic antidepressants, sympathomimetics, and percutaneous biopsy. Lifelong biochemical surveillance follows resection.