Diagnosing Pheochromocytoma Without an Adrenal Mass
Begin with biochemical testing using plasma free metanephrines or 24-hour urinary fractionated metanephrines, followed by functional imaging with 123I-MIBG scintigraphy to localize extra-adrenal paragangliomas, which are the most likely diagnosis when no adrenal mass is present 1.
Biochemical Confirmation First
The absence of an adrenal mass suggests you're dealing with an extra-adrenal paraganglioma rather than a typical adrenal pheochromocytoma. However, the diagnostic approach remains biochemically driven:
- Measure plasma free metanephrines OR 24-hour urinary fractionated metanephrines as your initial test 1
- These tests have the highest sensitivity (urinary normetanephrine reaches 96.9% sensitivity) 2
- For plasma metanephrines, draw blood with the patient supine and use position-matched reference intervals to avoid false positives 1
- Use liquid chromatography with mass spectrometry or electrochemical detection methods for optimal accuracy 1
Critical Preanalytical Considerations
Watch for factors causing false positives: certain medications (tricyclic antidepressants, sympathomimetics), stress, and improper blood collection technique. If results are moderately elevated (not dramatically high), consider the clonidine suppression test to differentiate true catecholamine excess from physiologic elevation 3.
Localization Strategy for Extra-Adrenal Disease
Once biochemical diagnosis is confirmed, the imaging approach differs significantly from typical adrenal pheochromocytoma:
Primary Imaging Sequence
CT or MRI of chest, abdomen, and pelvis - CT is suggested for initial anatomical imaging, but MRI is superior when you suspect extra-adrenal location 1
- Extra-adrenal paragangliomas occur along the sympathetic chain from skull base to pelvis
- Common sites: organ of Zuckerkandl (near aortic bifurcation), bladder wall, thorax, head and neck
123I-MIBG scintigraphy - This functional imaging is particularly valuable for extra-adrenal tumors 1, 4
When Standard Imaging is Negative
If CT/MRI and MIBG are non-localizing but biochemistry is clearly positive:
- Consider 18F-FDG PET or 18F-FDOPA PET for additional functional imaging 4
- These are particularly useful in hereditary syndromes and metastatic disease
- Re-evaluate for head and neck paragangliomas (carotid body, jugular, vagal)
Genetic Testing is Mandatory
Recommend genetic testing in ALL patients with confirmed pheochromocytoma/paraganglioma 1. This is critical because:
- Up to 30-40% of cases are hereditary 3, 5
- Extra-adrenal location increases likelihood of SDHx mutations
- SDHB mutations specifically predict metastatic potential - these patients need more aggressive surveillance 4
- SDHD mutations predict multifocal disease 4
- Genetic results guide family screening and long-term surveillance intensity
Common Pitfalls to Avoid
- Don't biopsy a suspected paraganglioma before ruling out catecholamine excess - this risks hypertensive crisis and arrhythmias 4
- Don't assume negative adrenal imaging rules out pheochromocytoma - 10-15% are extra-adrenal 1
- Don't stop at negative MIBG if biochemistry is strongly positive - consider alternative functional imaging
- In younger patients (≤40 years), extra-adrenal and multifocal disease is more common, requiring whole-body imaging 4
Clinical Context Matters
Patients with extra-adrenal paragangliomas may have:
- Less dramatic hypertensive symptoms than adrenal pheochromocytomas
- Higher rates of dopamine secretion (measure methoxytyramine if available) 4
- Greater metastatic potential, especially with SDHB mutations (requiring lifelong surveillance) 4
The diagnostic sequence is: biochemical confirmation → comprehensive anatomical imaging → functional imaging (MIBG) → genetic testing → multidisciplinary surgical planning 1, 5.