A 43-year-old man presented with 5 months of left hand pain, extensor weakness, and dorsal sensory loss. There was no history of trauma, infection, or exercise-related symptoms. Neurologic examination showed atrophy and weakness (4/5 on the Medical Research Council scale) of the left brachioradialis, wrist, and finger extensor muscles. Sensation to pinprick and touch was reduced over the dorsum of the left hand. Nerve conduction velocity showed a reduction in compound muscle action potential (1.0 mV) and sensory nerve action potential (14 μV) amplitudes in the left radial nerve. No conduction blocks or focal slowing were recorded. EMG also showed active denervation of the brachioradialis and forearm extensor muscles, sparing the triceps. An axonal radial sensorimotor neuropathy proximal to the brachioradialis muscle was diagnosed. Left arm MRI, ultrasound, and surgical exploration showed an entrapment of the radial nerve in the upper arm due to a triceps accessory muscle belly (Figures 1 and 2). Muscle anatomical variants are an infrequent cause of radial nerve entrapment, and images are crucial to identify this etiology.1,2
(A) Sagittal STIR, (B) axial STIR, and (C) axial T2 images. The radial nerve (white arrows) has focal thickening and increased signal at the level of the middle third of the humerus. An accessory muscle belly of the triceps (red lines; B, C) is visualized compressing the radial nerve. STIR = short tau inversion recovery.
Left radial nerve (R.n.) ultrasound (A). The R.n. (asterisk) has increased echogenicity and cross-sectional area at the middle third of the humerus; the accessory belly of the triceps muscle (A.m.) is shown. Surgical decompression pre-R.n. release (B) and post-R.n. release (C) confirmed this entrapment etiology.
Author Contributions
P. Bastias: drafting/revision of the manuscript for content, including medical writing for content; major role in the acquisition of data; study concept or design; analysis or interpretation of data. R. Melo: drafting/revision of the manuscript for content, including medical writing for content; analysis or interpretation of data. J.M. Matamala: drafting/revision of the manuscript for content, including medical writing for content; analysis or interpretation of data. N. Earle: drafting/revision of the manuscript for content, including medical writing for content; analysis or interpretation of data. I. Acosta: drafting/revision of the manuscript for content, including medical writing for content; major role in the acquisition of data; study concept or design; analysis or interpretation of data.
Study Funding
No targeted funding reported.
Disclosure
The authors report no relevant disclosures. Go to Neurology.org/N for full disclosures.
Footnotes
Go to Neurology.org/N for full disclosures. Funding information and disclosures deemed relevant by the authors, if any, are provided at the end of the article.
Submitted and externally peer reviewed. The handling editor was Resident & Fellow Section Editor Whitley Aamodt, MD, MPH.
Teaching slides links.lww.com/WNL/C800
- Received October 4, 2022.
- Accepted in final form March 23, 2023.
- © 2023 American Academy of Neurology
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