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

Abstract

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.

Introduction

The radial nerve, arising from the posterior cord of the brachial plexus (C5-T1 nerve roots), innervates the posterior compartment muscles of the arm and forearm. It courses through the triangular interval, along the medial humerus, and into the radial groove, before piercing the lateral intermuscular septum to enter the anterior compartment. In the elbow region, it provides motor innervation to the anconeus, extensor carpi radialis longus, and brevis muscles, and subsequently bifurcates into deep and superficial branches in the forearm.^1,2 Proximal variations of the radial nerve in the arm are rare, with most documented anomalies occurring distally.^2,3 For instance, in the forearm and hand, variations such as the Froment-Rauber anomaly, where the radial nerve may give rise to an anomalous branch innervating intrinsic hand muscles, are well documented. ⁴ Typically, the biceps brachii is innervated by the musculocutaneous nerve, and radial nerve innervation to this muscle has not been reported. ² Knowledge of such variations is critical for upper limb surgeries, electrodiagnostic evaluations, and nerve transfer procedure.^1,5 This case report describes a previously undocumented instance of radial nerve innervation to the biceps brachii, discovered during surgical fixation of a humeral shaft fracture, with electrophysiological confirmation of its functional significance.

Case Report

A 27-year-old woman presented with a left humeral shaft fracture following a traumatic incident, requiring surgical intervention. An anterolateral approach was used, with a skin incision extending from the deltopectoral groove to the interval between the biceps brachii and the mobile wad (extensor carpi radialis muscles). The fascia was incised and extended proximally; the biceps was retracted medially, and the mobile wad laterally. During exploration, the radial nerve was identified and traced distally. Proximal exploration revealed an anomalous branch from the radial nerve innervating the biceps brachii muscle (Figures 1 and 2). All radial nerve branches were preserved during the procedure. The fracture was reduced and stabilized with plates and screws, and the surgical site was closed conventionally.

Figure 1.

Intraoperative anterolateral view of the left arm during humeral shaft fracture fixation, showing the radial nerve with an anomalous branch innervating the biceps brachii muscle.

Figure 2.

Intraoperative anterolateral view of the left arm during humeral shaft fracture fixation. The fracture is stabilized with a plate and screws.

Postoperative nerve conduction studies were performed to assess the functional significance of this anomalous branch. The radial nerve was stimulated in the axillary fossa, in the depression formed by the coracobrachialis and the medial border of the triceps muscles. Compound muscle action potentials (CMAP) were recorded from the biceps brachii using both surface and needle electrode techniques. For surface recording, the active electrode (E1) was placed at the midpoint of the biceps brachii muscle, and the reference electrode (E2) was positioned over the biceps brachii tendon in the antecubital fossa. For needle recording, a standard concentric needle electrode was inserted into the muscle 24 cm distal to Erb’s point. The musculocutaneous nerve was also stimulated to evaluate its contribution to biceps innervation, and the radial nerve was tested at other sites (recording from the triceps and extensor digitorum communis [EDC]) to assess its overall function. The electrophysiological findings are summarized in Table 1. Stimulation of the radial nerve elicited a CMAP response in the biceps brachii, with a latency of 2.3 ms (surface) and 2.21 ms (needle), and amplitudes of 0.26 mV and 0.25 mV, respectively, confirming functional radial nerve innervation to the muscle. In contrast, musculocutaneous nerve stimulation produced a CMAP with a latency of 3.45 ms and an amplitude of 2.66 mV, indicating its primary role in biceps innervation. The CMAP waveforms are presented in Figures 3a to 3e. Notably, radial nerve recordings from the triceps and EDC showed reduced amplitudes (1.65 mV and 1.48 mV, respectively), suggesting possible mild nerve injury, potentially related to the humeral fracture.

Table 1.

Electrophysiological Findings of Motor Nerve Conduction Studies, Showing Latency, Amplitude, and Area of CMAP Responses Recorded From the Biceps Brachii, Triceps, and EDC Following Stimulation of the Radial and Musculocutaneous Nerves.

#	Stim.Site	Rec.Site	Latency (ms)	Amp. (mV)	Area (mV ms)
Left	Radial	EDC	4.98	1.48	2.73
Left	Radial	Triceps	1.7	1.65	9.11
Left	MC	Biceps brachii	3.45	2.66	12.26
Left	Radial (axillary)	Biceps brachii	2.3	0.26	1.48
Left	Radial (axillary) recording by concentric needle electrode	Biceps brachii	2.21	0.25	1.35

Note. CMAP = compound muscle action potentials; EDC = extensor digitorum communis; MC = musculocutaneous.

Figure 3.

Compound muscle action potential waveforms recorded during nerve conduction studies.

Note. (a) Radial nerve stimulation (axillary fossa) with recording from the extensor digitorum communis; (b) radial nerve stimulation (axillary fossa) with recording from the triceps; (c) musculocutaneous nerve stimulation (Erb’s point) with recording from the biceps brachii; (d) radial nerve stimulation (axillary fossa) with surface electrode recording from the biceps brachii; and (e) radial nerve stimulation (axillary fossa) with needle electrode recording from the biceps brachii. These waveforms confirm supplementary radial nerve innervation to the biceps brachii.

Discussion

The radial nerve’s anatomical course makes it susceptible to injury, particularly in the context of humeral shaft fractures and nerve compression. ² Understanding its anatomy and potential variations is essential for upper limb surgeries, electrodiagnostic assessments, and nerve transfer procedures following peripheral nerve injuries. ⁶ While variations in the radial nerve’s communications with other nerves are well documented, branching anomalies in the arm are rare. ⁷ For instance, Claassen et al, ³ in a study of 167 arms, identified only one case of radial nerve variation, characterized by nerve thinning. Maślanka et al ² reported radial nerve innervation to the coracobrachialis during a cadaveric dissection, potentially due to aberrant embryonic development. ² Variations in the sensory branches of the radial nerve have also been described. ⁸

This case report documents a previously unreported variation of the radial nerve innervating the biceps brachii, identified during surgical fixation of a humeral shaft fracture. The functional significance of this branch was confirmed through electrophysiological studies. Stimulation of the radial nerve in the axillary fossa elicited a CMAP response in the biceps brachii, with a latency of 2.3 ms (surface electrode) and 2.21 ms (needle electrode), and amplitudes of 0.26 mV and 0.25 mV, respectively (Table 1, Figures 3d and 3e). In contrast, musculocutaneous nerve stimulation produced a stronger CMAP response (amplitude 2.66 mV, latency 3.45 ms; Figure 3c), indicating its primary role in biceps innervation. The low amplitude of the radial nerve–mediated response suggests that this anomalous branch plays a supplementary role in biceps innervation. This variation may have significant clinical implications, such as providing compensatory innervation in cases of musculocutaneous nerve injury, which could preserve biceps function and influence surgical or rehabilitative strategies.

In addition, electrophysiological evaluation revealed reduced CMAP amplitudes in the triceps (1.65 mV; Figure 3b) and EDC (1.48 mV; Figure 3a) following radial nerve stimulation, suggesting possible mild nerve injury, potentially related to the humeral shaft fracture or intraoperative manipulation. This finding underscores the importance of comprehensive nerve assessments in patients with upper limb fractures, as subclinical nerve injuries may affect functional outcomes.

Due to intraoperative constraints, precise measurements of the nerve branch’s diameter and length were not obtained. In addition, to preserve anatomical structures, nerve stimulation was not performed during surgery. However, the electrophysiological confirmation, supported by detailed CMAP waveforms, strengthens the significance of this finding. This case highlights the need for awareness of rare anatomical variants during clinical evaluations, diagnostic imaging, and surgical interventions, particularly in the context of upper limb procedures.

Conclusion

This case report describes a novel anatomical variation of the radial nerve innervating the biceps brachii, with electrophysiological confirmation of its functionality. The supplementary innervation may offer clinical benefits in cases of musculocutaneous nerve injury, emphasizing the importance of recognizing such variants in upper limb surgery and diagnostic evaluations.