SUN-509 A Pheo False Alarm - Deception with Cyclobenzaprine

Abstract Disclosure: M.A. Stezzi: None. K. Desai: None. G. Umarji: None. N.V. Kagita: None. S.A. Jabbour: None. A 45-year-old male with a history of hypertension and prediabetes presented to endocrinologyfor evaluation of possible pheochromocytoma. A month prior, he was seen by his primary care provider for episodic palpitations accompanied by heat flashes. Given his symptoms, workup up for a pheochromocytoma was obtained. Plasma metanephrines were 69.1 pg/ml (0.0-88.0 pg/ml)and normetanephrines 210.7 pg/ml (0.0-218.9 pg/ml). 24- hour urine metanephrines were 300 ug/24 hr (58-276 ug/24 hr) and normetanephrines were 1105 ug/24 hr (156-729 ug/24 hr).Computed Tomography of the abdomen with unremarkable adrenal glands. A iοbеոgսаոe I-123nuclear scan was ordered to rule out a paraganglioma. When he saw us in the endocrine clinic, we discovered that around the time of his testing, he had injured his back and went to the emergency room. He was given a short course of cyclobenzaprine for pain relief. The blood and urine tests were all performed while taking cyclobenzaprine. He also denied any pertinent family history. He denied any other medications or supplements other than amlodipine. Physical examination was unrevealing. Blood pressure 130/80 mmHg; heart rate 70/min. Repeat 24-hour urine metanephrines off the cyclobenzaprine for 2 weeks were 126 ug/24 hr and normetanephrines were 304 ug/24 hr. Cyclobenzaprine is a centrally acting skeletal relaxant that shares a similar chemical structure to tricyclic antidepressants, differing only by one double bond in the tricyclic ring system. Tricyclic antidepressants are known to interfere with the collection of urine metanephrines by inhibiting reuptake of norepinephrine. Breakdown products of norepinephrine include normetanephrines, leading to a false elevation when measuring urinary levels. Measurement of plasma fractionated metanephrines has a sensitivity of 97% and specificity of 93%. The sensitivity of 24-hour urinary free metanephrines is 100% and the specificity is 94%. Given these high percentages, it is moreimportant to be thorough with a patient’s medication history.This case demonstrates the importance of assessing every part of a patient’s history when evaluating for pheochromocytomas/paragangliomas. This patient happened to have had a course of cyclobenzaprine coincide with his 24-hour urine collection. This drug is not well known to be in the tricyclic antidepressant category as its use is primarily for muscle spasms. He had unnecessary CT scan and he also could have undergone unnecessary additional testing such as magnetic resonance imaging or functional imaging techniques. In conclusion, proper history is a critical part in making or ruling our certain diagnoses and can save a patient from being on the wrong medications or undergoing unnecessary testing. Presentation: Sunday, July 13, 2025