A 52-year-old man with hypertension presents with episodic severe headaches, profuse sweating, and palpitations lasting 15–20 minutes. Clinical examination reveals BP 180/110 mmHg during an episode and 145/90 mmHg between episodes. Plasma metanephrines are elevated. Which investigation is most appropriate to localize the source of catecholamine excess?

Explanation

## Investigation of Choice for Pheochromocytoma Localization ### Clinical Context This patient presents with classic **paroxysmal hypertension, diaphoresis, and palpitations** with biochemically confirmed catecholamine excess (elevated plasma metanephrines). The next step is **anatomical imaging** to localize the tumor. ### Why CT Abdomen and Pelvis is Correct **Key Point:** Once biochemical confirmation of pheochromocytoma is established (elevated plasma metanephrines), the **first-line imaging modality for localization is CT abdomen and pelvis with contrast**. This is recommended by the Endocrine Society Clinical Practice Guidelines and endorsed in Harrison's Principles of Internal Medicine (21e, Ch. 405). **High-Yield:** CT has **~90–100% sensitivity** for adrenal pheochromocytomas (which account for ~85–90% of cases) and provides excellent anatomical detail regarding tumor size, extent, and relationship to adjacent structures — all critical for surgical planning. ### Investigation Hierarchy for Pheochromocytoma | Step | Investigation | Purpose | |------|---|---| | 1 | Plasma metanephrines / 24h urine metanephrines | **Biochemical confirmation** (already done — positive) | | 2 | **CT abdomen and pelvis with contrast** | **First-line anatomical localization** (NEXT STEP) | | 3 | MRI abdomen | Alternative if CT contraindicated (pregnancy, contrast allergy) or for extra-adrenal/metastatic disease | | 4 | 123I-MIBG scintigraphy or 68Ga-DOTATATE PET | Functional imaging for extra-adrenal, multifocal, or metastatic disease; or when CT/MRI inconclusive | | 5 | Genetic testing | If familial syndrome suspected (MEN2, SDH mutations, NF1) | **Clinical Pearl:** Per Endocrine Society guidelines, CT (or MRI) is the **initial imaging study of choice** after biochemical confirmation. Functional imaging (MIBG, 68Ga-DOTATATE PET) is reserved for cases where anatomical imaging is negative, inconclusive, or when metastatic/multifocal disease is suspected. MIBG is NOT the first-line localization tool. ### Why Other Options Are Incorrect **Plasma free catecholamines (supine, 30 min rest):** This is a **biochemical test**, not a **localization study**. The diagnosis is already biochemically confirmed by elevated plasma metanephrines; further biochemical testing does not localize the tumor. **Clonidine suppression test:** Used to **exclude false-positive plasma metanephrines** in borderline cases (e.g., anxiety, medications). Not indicated when metanephrines are clearly elevated and clinical suspicion is high. It is a **confirmatory biochemical test**, not a localization tool. **123I-MIBG scintigraphy:** A valuable **functional imaging** modality with ~90% sensitivity and ~99% specificity, but it is **not the first-line localization study**. It is reserved for: (a) negative or inconclusive CT/MRI, (b) suspected extra-adrenal or metastatic disease, (c) pre-therapeutic assessment before 131I-MIBG therapy. Starting with functional imaging before anatomical imaging is not standard practice per current guidelines. [cite:Harrison 21e Ch 405; Endocrine Society Clinical Practice Guideline on Pheochromocytoma/Paraganglioma 2014]