Transfer of brachialis muscle branch of the musculocutaneous nerve to the median nerve: an electrophysiological study

Objective To trace the origination of brachialis muscle branch at the root level by means of intra-operative electrophysiological study and discuss the rationale of transfer of brachialis muscle branch of the musculocutaneous nerve to the median nerve. Methods EMG was done in 10 cases of contralateral C_7 nerve root transfer. After exploration of the normal brachial plexus, the C_5～T_1 nerve roots were stimulated to record compound muscle action potential (CMAP) of the brachialis muscle. Amplitude and latenct period of CMAP were calculated. Brachialis muscle branch transfer was carried out in one case of C_7 level spinal cord injury. Pre-operative examination showed normal shoulder abduction and elbow flexion and absence of finger flexion. Brachialis muscle branch of the musculocutaneous nerve was transferred to the anterior interosseous nerve of the median nerve. Results CMAP was recorded in the brachialis muscle when C_5～C_7 nerve roots of the normal brachial plexus were stimulated. Average latent period of CMAP were 6.32±1.50ms, 6.51±1.36ms, and 6.99±1.33 ms respectively. Average amplitude of CMAP were 2.81±3.17mV, 3.01±3.20mV, and 1.55±1.99mV respectively. No CMAP was elicited when C_8 or T_1 was stimulated. Among these, the highest CMAP amplitude was recorded when C_6 was stimulated indicating that nerve fibers innervating the brachialis muscle were mainly originated from C_6 nerve root although there was no statistically significant difference among the C_ 5～7 roots. One and half year follow-up of the nerve transfer case revealed recovery of thumb and index finger flexion. Conclusion The nerve fibers innervating the brachialis muscle originate from C_5～C_7 nerve roots. When it is transferred to the median nerve which mainly originates from C_8～T_1, the results should be reliable.