A 52-year-old man with a 10-year history of hypertension presents with episodic severe headaches, profuse sweating, and palpitations lasting 20–30 minutes. Which single finding best distinguishes pheochromocytoma from primary aldosteronism as the cause of his secondary hypertension?

## Distinguishing Pheochromocytoma from Primary Aldosteronism ### Clinical Presentation Overlap Both pheochromocytoma and primary aldosteronism cause secondary hypertension and can present with resistant hypertension. However, their pathophysiology and biochemical signatures are distinct. ### Comparison Table | Feature | Pheochromocytoma | Primary Aldosteronism | |---------|------------------|----------------------| | **Catecholamine excess** | Yes (↑↑ metanephrines) | No (normal) | | **Aldosterone excess** | No (normal) | Yes (↑↑ aldosterone) | | **Hypokalemia** | Rare | Common (↓ K^+^) | | **Metabolic alkalosis** | Absent | Present | | **Episodic symptoms** | Yes (headache, sweating, palpitations) | No (asymptomatic or mild) | | **Plasma renin** | Normal or ↑ | Suppressed | | **24-h urine metanephrines** | ↑↑ (diagnostic) | Normal | | **Plasma free metanephrines** | ↑↑ (diagnostic) | Normal | ### Key Discriminating Feature **Key Point:** The **elevated 24-hour urinary metanephrines or plasma free metanephrines** is the single best discriminator. This directly reflects catecholamine excess, which is the defining pathophysiology of pheochromocytoma and is absent in primary aldosteronism. ### Why This Is the Best Answer **High-Yield:** Metanephrines are the O-methylated metabolites of catecholamines (epinephrine and norepinephrine). They are: 1. **Highly sensitive and specific** for pheochromocytoma (>95% sensitivity) 2. **Stable** in plasma and urine (unlike catecholamines, which are unstable) 3. **Diagnostic gold standard** for ruling in pheochromocytoma 4. **Completely normal** in primary aldosteronism **Mnemonic:** **"PHEOCHROMOCYTOMA = Plasma/urine Metanephrines Elevated"** — remember that the biochemical hallmark is catecholamine excess, not aldosterone excess. ### Clinical Pearl **Clinical Pearl:** The episodic symptoms (headache, sweating, palpitations) in this patient are classic for pheochromocytoma and reflect acute catecholamine surges. Primary aldosteronism typically presents insidiously with asymptomatic hypertension and hypokalemia. ### Why Other Options Are Less Discriminating - **Hypokalemia with metabolic alkalosis:** This is the hallmark of primary aldosteronism, not pheochromocytoma. It would point AWAY from the correct diagnosis. - **Suppressed plasma renin activity:** Seen in primary aldosteronism due to volume expansion and aldosterone-mediated sodium retention. In pheochromocytoma, renin is typically normal or elevated. - **Resistant hypertension:** Both conditions can present with resistant hypertension. This is a feature of severity, not a discriminator between the two. ```mermaid flowchart TD A[Secondary Hypertension with Episodic Symptoms]:::outcome --> B{Biochemical Pattern?}:::decision B -->|↑ Metanephrines, Normal Aldosterone| C[Pheochromocytoma]:::outcome B -->|Normal Metanephrines, ↑ Aldosterone| D[Primary Aldosteronism]:::outcome C --> E[Measure 24-h urine metanephrines<br/>or plasma free metanephrines]:::action D --> F[Measure aldosterone/renin ratio<br/>Confirm with saline suppression]:::action E --> G[If elevated: CT/MRI abdomen<br/>for tumor localization]:::action F --> H[If elevated: CT/MRI adrenal<br/>for adenoma vs hyperplasia]:::action ``` [cite:Harrison 21e Ch 297]