## Anatomical Distribution of Pheochromocytomas **Key Point:** The adrenal medulla is the site of origin in approximately 80–85% of all pheochromocytomas, whether sporadic or hereditary. ### Rule of 10s (Modified for Modern Data) | Feature | Percentage | |---------|------------| | Adrenal medulla | 80–85% | | Extra-adrenal (paragangliomas) | 15–20% | | Bilateral adrenal involvement | 5–10% (mainly in MEN 2A/2B and NF1) | | Malignant pheochromocytoma | 10% | ### Extra-Adrenal Sites (Paragangliomas) When pheochromocytomas arise outside the adrenal medulla, they originate from chromaffin tissue along the **sympathetic chain**: 1. **Organ of Zuckerkandl** (most common extra-adrenal site; ~40% of paragangliomas) - Located at the bifurcation of the abdominal aorta - Usually regresses after age 1 year but can retain chromaffin cells 2. **Carotid body** (~10% of paragangliomas) - Parasympathetic paraganglioma (rarely catecholamine-secreting) 3. **Bladder wall** (~5% of paragangliomas) - Presents with hematuria and hypertensive episodes during micturition 4. **Thorax, neck, pelvis** (remaining sites) **High-Yield:** In sporadic pheochromocytoma, the adrenal medulla accounts for >80% of cases. Extra-adrenal paragangliomas are more common in hereditary syndromes (SDH mutations, NF1) and carry higher malignancy risk (~30–40%). **Clinical Pearl:** Patients with extra-adrenal pheochromocytomas (paragangliomas) should be screened for germline mutations in *SDH* genes (SDHA, SDHB, SDHD, SDHAF2), as up to 70% of extra-adrenal cases are hereditary.
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