A foregoing thenar muscular branch of the median nerve

Abstract

We present a 62-year-old female patient who had an anatomic variation in the median nerve of the left hand. During surgery for releasing the left carpal tunnel, an abnormally high level of origin of the thenar muscular branch of the median nerve was detected, at 2.5 cm above the proximal border of transverse carpal ligament. It traveled between the medial side of the flexor carpi radialis tendon and median nerve and entered the carpal tunnel. After exiting the carpal tunnel distally, the nerve, was noted to course towards the thenar area. Such variations in the median nerve should be kept in mind while performing carpal tunnel release.

INTRODUCTION

Motor branch or recurrent branch of the median nerve, also called as the thenar branch, innervates the intrinsic muscles of the thumb (abductor pollicis brevis, opponens pollicis, and the superficial head of the flexor pollicis brevis). It has an important role in the movements of the thumb and thus in many activities of the hand. Accordingly, hand surgeons pay attention not to injure the motor branch while performing carpal tunnel surgery.[1,3] Herein, a rare anatomic variation regarding the motor branching of the median nerve is being reported.

CASE REPORT

During surgery for releasing the left carpal tunnel in a 62-year-old lady, the thenar muscular branch of the median nerve was observed to originate 2.5 cm above the proximal border of the transverse carpal ligament [Figure 1]. It traveled along the medial side of the flexor carpi radialis tendon. By entering the carpal tunnel, the motor branch, along with the median nerve, was noted to course towards the thenar area distal to the transverse carpal ligament. It entered the thenar compartment in an intramuscular fashion [Figure 2]. The medial and lateral divisions of the median nerve were normal in appearance under the carpal tunnel and there were no pseudoganglion formation.

Figure 1.

Intraoperative photograph demonstrating motor branching of the median nerve proximal to the transverse carpal ligament

Figure 2.

Illustration showing the course of motor branching of the median nerve

DISCUSSION

The motor branch of the median nerve was, in relation to the transverse carpal ligament, was found to be of extraligamentous, subligamentous, transligamentous types in 46%, 31%, 23%, respectively in 246 carpal tunnel release operations by Lanz.[3] Siverhuset et al.[4] while studying 72 hands of 36 cadavers, reported the classical distal (extraligamantous-subligamantous) branching in 86%, and transligamentous motor branching in the rest. While Linburg and Albright,[5] Rockwell[6] reported multiple motor branching anomalies, Graham[7] referred to a case with motor branching in the ulnar region. Lanz[3] and Gruber[8] found upper level median and lateral division branching (in the distal forearm) in four cases. Eiken et al.[9] reported three similar cases and Kessler reported one case with the same anomaly.[10] However, all those reports were of medial and lateral divisions of the median nerve at the upper level.[3] The upper level branching of only the motor branch is reported in the series of Olave et al.[1] in one case. Likewise in our case, only the motor branch branched out from the median nerve distally in the forearm. The sensory branch of the median nerve (medial and lateral divisions) was inside the carpal tunnel.

While releasing carpal tunnels a proper understanding of the anatomic variations of the motor branch is paramount in order not to cause iatrogenic injuries that may compromise hand functions severely.