INTRODUCTION
The occurrence of anomalous muscles in the region of the wrist is not uncommon but they are seldom associated with compression neuropathy. Anomalous proximal muscle origins or hypertrophy of the lumbrical muscles may limit space available for the median nerve within the carpal tunnel causing compressive neuropathy (Ragoonwansi et al. 2002).
Aberrant muscles in relation to the carpal tunnel have been previously described, some of which were directly implicated in median nerve compression resulting in carpal tunnel syndrome (Cartwright et al. 2014). The majority of the muscle aberrations are from the first or second lumbricals, palmaris longus, flexor digitorum superficialis of the index finger and atypically, palmaris profundus muscles (Redondo et al. 2011).
The lumbrical muscles typically originate from the flexor tendons in the palm and insert into the extensor expansions. Variations are not uncommon and the present study depicts a case where the lumbricals originate much more proximally in the forearm. We report a patient with carpal tunnel syndrome (CTS) of both hands resulting from an abnormal origin of the lumbrical muscles proximal to the carpal tunnel in the forearm arising from both flexor digitorum profundus (FDP) and superficialis (FDS) tendons. Additionally on the left side, there was hypertrophy of the lumbricals to the left ring finger.
CASE REPORT
A 73-year-old retired army veteran presented with numbness, pain, night pain and clumsiness on his right hand for past one year. There was thenar muscle atrophy and numbness over the median nerve area, together with soft tissue fullness proximal to the wrist crease. Numbness was aggravated by active flexion of his fingers. Tinel sign over the carpal tunnel area and Phalen test were positive.
Open carpal tunnel release was performed under general anesthesia in anticipation of exploratory surgery. Intraoperatively, the transverse carpal ligament had no overlying mass or fibrotic thickening. After incising the transverse carpal ligament, a hypertrophic muscle was found compressing on the median nerve. A gentle pull of this muscle confirmed its origin from the first and second lumbricals extending into the carpal tunnel attaching to the transverse carpal ligament as well as the FDP. Additionally, the second lumbrical was hypertrophic causing overcrowding in the carpal tunnel (Figure 1). Other structures in the carpal tunnel were normal. This anomalous muscle was later excised to reduce the compression to the median nerve. Symptoms were relived post-operatively.
One year later, the patient presented with similar complaints and physical findings over his left hand. Magnetic Resonance Imaging (MRI) revealed abnormal muscle tissue within the carpal tunnel. The median nerve was compressed between the transverse carpal ligament and the anomalous muscle. An open carpal tunnel release was performed under general anesthesia. Hypertrophied lumbrical muscles were noted inside the carpal tunnel overcrowding the carpal tunnel (Figure 2), with an abnormal origin from the tendons of both FDP and FDS of ring finger, proximal to the wrist (Figure 3). No median nerve abnormality was noted. The abnormal lumbrical muscle belly was excised (Figure 3).
Post-operatively the patient was satisfied as his pain was relieved. On follow-up, he underwent physiotherapy and his symptoms of numbness gradually improved.
Discussion
The median nerve course and its surrounding anatomical structures are vital while discussing on median nerve compression. The median nerve arises from the C5-T1 roots and enters the arm lateral to the brachial artery between biceps brachii and brachialis. It then crosses the artery descending medial to it before entering the cubital fossa. Once in the cubital fossa, it is medial to the brachial artery and enters between the two heads of pronator teres, deep to the bicipital aponeurosis but superficial to the brachialis. Proximal median nerve compression can be due to pronator teres, lacertus fibrosus and tendinous origin of flexor digitorum superficialis. An anomalous third head of biceps brachii has been reported in a cadaveric dissection tightly encasing the median nerve and brachial artery restricting motion of the two structures (Zar Chi et al. 2017).
The median nerve then continues in the forearm passing between the flexor digitorum superficialis and and flexor digitorum profundus muscles before emerging between the flexor digitorum superficialis and flexor carpi radialis muscle just about 5cm proximal to the carpal tunnel. In the carpal tunnel, the median nerve shares an enclosed space with nine other structures namely the four FDS, four FDP and one flexor pollicis longus tendon. Hence, it is commonly compressed resulting in CTS.
There are four groups of median nerve variations i.e. I-the course of the thenar branches, II-accessory branches at the distal carpal tunnel, III-high division of the nerve including bifid median nerve and IV-accessory branches proximal to the carpal tunnel (Lanz 1977). In a bifid median nerve, an accessory muscle or a median artery may run in-between the division. The accessory muscles reported include the first lumbrical, palmaris longus, FDS muscle (Park et al. 2017; Atoni et al. 2017) and palmaris profundus. Presence of these anatomical variations are frequently associated with median nerve compression (Al-Qattan et al. 2009).
Lumbrical muscles are important for finger movements and is part of the intrinsic muscles of the hand. They are different from other muscles as they connect muscle to muscle forming a connection from the digit flexors to the extensors. Usually, they originate from the FDP tendon, distal to the carpal tunnel, and the muscle belly moves proximally on finger flexion. It is common to encounter variations in lumbrical attachment. Few variations include originating from a metacarpal, from the forearm or from a superficial flexor tendon (instead of FDP). Additionally, the third and fourth lumbricals normally originate from two tendons but may originate from a single tendon (Mehta et al. 1961).
An additional muscle belly for the lumbrical muscle of the left ring finger was seen in our case where the hypertrophic muscle belly compressed the median nerve within the carpal tunnel. CTS can be caused by median nerve compression by anomalous and additional lumbrical muscles (Chaudruc et al. 2000). A similar observation was seen in a study showing a bipennate origin of the first lumbrical, extending from the distal part of forearm with a split insertion (Singh et al. 2001). Several other authors also reported that lumbrical muscle intrusion is a potential cause of CTS (Jabaley 1978; Joshi et al. 2005).
In our case, the lumbrical muscle of the left ring finger had a proximal origin in the carpal tunnel as well as in the forearm. Siegel et al. reported that those patients with idiopathic CTS had lumbrical muscle origins located significantly more proximally in the carpal tunnel. Additionally, the lumbrical muscles were larger and had more proximal origins in those with jobs requiring repetitive hand motions (Siegel et al. 1995).
Other than the abnormal attachments, those with repetitive hand movements and those with CTS were reported to have significantly larger and hypertrophied lumbricals (Joshi et al. 2005). Shimizu et al. reported a case of CTS caused by a hypertrophied lumbrical muscle with flexor synovitis (Shimizu et al. 2015). Such hypertrophied muscle belly can cause overcrowding of the carpal tunnel as they glide proximally along with the flexor tendon during flexion of the fingers or when making a fist. Some reports suggest that it is not the existence of an anomalous muscle per se producing symptoms but hypertrophy due to manual work (Redondo et al. 2011; Ragoonwansi et al. 2002).
Operative treatment is preferred for CTS associated with lumbrical muscle anomaly rather than conservative management. Carpal tunnel release is usually performed, as well as resection of the abnormal muscle belly. It is preferably done under general anesthesia due to the larger surgical exposure. The prognosis is good after operative treatment once the abnormal muscle is resected as this prevents overcrowding of the carpal tunnel. It is best performed at an early stage before irreversible pathological changes occur on the median nerve.
Conclusion
Variations in the anatomy of the lumbrical muscles may produce CTS. Surgeons performing carpal tunnel surgeries should have a sound knowledge of carpal tunnel anatomy and not consider them as simple straight forward operations.