Ulnar Neuropathy

Excerpt

The ulnar nerve has multiple potential compression sites along its course. Although the elbow is the most common site of compression, clinically significant injury may also occur at the wrist, forearm, and upper arm. Prevention of compression, along with early diagnosis and treatment, is essential for optimizing prognosis, as treatment outcomes are often disappointing once axonal damage has developed.

The C8 and T1 nerve roots merge to form the lower trunk of the brachial plexus. This trunk continues as the medial cord and gives rise to the ulnar nerve. The ulnar nerve then courses along the upper arm medial to the brachial artery and in proximity to the median nerve. At the middle and lower 3rd of the arm, the ulnar nerve passes through the Arcade of Struthers, a fibrous canal composed of the medial head of the triceps brachii muscle and its aponeurotic expansion, which extends into the intermuscular septum and the internal brachial ligament.

At the elbow, the ulnar nerve courses posteriorly and enters the retroepicondylar groove between the medial epicondyle and the olecranon process. The nerve then enters the cubital tunnel through the cubital tunnel retinaculum (CTR), a band approximately 4 mm wide attached from the medial epicondyle to the olecranon at the proximal edge of the tunnel roof. Distally, the nerve passes beneath the humeroulnar aponeurotic arcade, also known as the Osborne band, a dense aponeurosis between the tendon attachments of the flexor carpi ulnaris (FCU). The area beneath the humeroulnar aponeurotic arcade constitutes the cubital tunnel (see Image. Osborne Ligament in the Cubital Tunnel). The nerve then traverses the belly of the FCU muscle and exits through the deep flexor-pronator aponeurosis.

At the forearm, the ulnar nerve innervates the FCU and the flexor digitorum profundus. At the midforearm to distal forearm, the palmar ulnar cutaneous (PUC) branch separates and enters the hand ventral to the Guyon canal, providing sensory innervation to the hypothenar skin. Distal to the PUC bifurcation, the dorsal ulnar cutaneous (DUC) branch emerges from the main trunk, curves around the ulna, and supplies sensory innervation to the dorsal medial hand, the medial one-half of the fourth digit, and the fifth digit.

The main trunk of the ulnar nerve enters the Guyon canal at the level of the distal wrist crease. The proximal wall of the canal is formed by the pisiform bone, and the distal wall by the hook of the hamate. The roof consists of the palmaris brevis muscle, while the floor is formed by the transverse carpal ligament, the hamate, and the triquetrum bone. At the canal outlet, the pisohamate hiatus forms a thick band connecting the hook of the hamate to the pisiform bone. Within the Guyon canal, the ulnar nerve separates into a superficial sensory branch and a deep palmar motor branch. The superficial sensory branch provides sensory innervation to the palmar aspects of the medial one-half of the fourth digit and the fifth digit.

Before exiting through the pisohamate hiatus, motor fibers branch from the deep palmar motor branch to innervate the hypothenar muscles, including the abductor digiti minimi, flexor digiti minimi, opponens digiti minimi, and palmaris brevis. The deep palmar branch further provides motor innervation to the adductor pollicis, deep head of the flexor pollicis brevis, third and fourth lumbricals, and the 3 palmar and 4 dorsal interossei.