Anomalous Radial Nerve Innervation to the Biceps Brachii: A Case Report With Electrophysiological Confirmation

The radial nerve typically innervates the posterior compartment muscles of the arm and forearm, while the biceps brachii is innervated by the musculocutaneous nerve. We report a novel case of radial nerve innervation to the biceps brachii in a 27-year-old woman, identified during surgical fixation of a left humeral shaft fracture. Intraoperative exploration revealed an anomalous branch from the radial nerve to the biceps brachii. Postoperative nerve conduction studies confirmed its functionality, with compound muscle action potentials (CMAP) recorded from the biceps brachii following radial nerve stimulation (latency: 2.3 ms, amplitude: 0.26 mV). Comparatively, musculocutaneous nerve stimulation produced a stronger response (latency: 3.45 ms, amplitude: 2.66 mV), indicating its primary role in biceps innervation. Reduced CMAP amplitudes in the triceps and extensor digitorum communis suggested possible mild radial nerve injury related to the fracture. This anatomical variation may provide compensatory innervation in musculocutaneous nerve injuries, highlighting its clinical relevance in upper limb surgery and electrophysiological assessments.