Ulnar Neuropathy

Excerpt

The ulnar nerve has several potential compression sites along its course. Although the elbow is the most common site of compression, the ulnar nerve is also susceptible to injury at the wrist, forearm, and upper arm. Prevention of compression and early diagnosis/treatment is important for its prognosis because the treatment outcome is usually disappointing once the nerve has axonal damage.

The C8 and T1 nerve roots merge to form the lower trunk of the brachial plexus which continues as the medial cord to give rise to the ulnar nerve. The ulnar nerve then courses along the upper arm medial to the brachial artery, in proximity to the median nerve. Just above the elbow, the ulnar nerve courses posteriorly to pass through the retroepicondylar groove between the medial epicondyle and olecranon process. It then passes underneath the humeroulnar aponeurotic arcade (HUA), which is a dense aponeurosis between the tendon attachments of the flexor carpi ulnaris (FCU). The area beneath the HUA is also called the cubital tunnel. The nerve then passes through the belly of the FCU muscle and out through the deep flexor-pronator aponeurosis. At the forearm, it innervates the FCU and the flexor digitorum profundus (FDP). At the mid to distal forearm, the palmar ulnar cutaneous branch (PUC) splits from the ulnar nerve and enters the hand ventral to the Guyon canal and gives sensory innervation to the skin at the hypothenar area. Distal to the bifurcation of the PUC, the dorsal ulnar cutaneous (DUC) branch separates from the main trunk, curves around the ulna, and provides sensory innervation to the dorsum of the skin of the medial hand, medial half of the fourth digit, and fifth digit.

The main trunk of the ulnar nerve enters through the Guyon’s canal at the level of the distal wrist crease. The proximal wall of Guyon’s canal is formed by the pisiform bone and the distal wall by the hook of the hamate. The roof is formed by the palmaris brevis muscle, and the floor is formed by the combination of the transverse carpal ligament, the hamate, and the triquetrum bone. A thick band is formed at the outlet (pisohamate hiatus) connecting the hook of the hamate to the pisiform bone. In the canal, the nerve separates into the superficial sensory branch and the deep palmar motor branch. The superficial sensory branch provides sensory innervation to the palmar aspects of the medial half of the fourth digit and the fifth digit. Before the nerve exits through the pisohamate hiatus, the motor fibers branch off from the deep palmar motor branch to innervate the hypothenar muscles (abductor digiti minimi, flexor digiti minimi, opponens digiti minimi, and palmaris brevis). The deep palmar branch gives motor innervation to the adductor pollicis, the deep head of the flexor pollicis brevis, the third and fourth lumbricals, and the three palmar and four dorsal interossei muscles.