Definition

The ulnar nerve is derived predominantly from the nerve roots of C8 and T1 with a small contribution from C7. The C8 and T1 fibers form the lower trunk of the brachial plexus. The ulnar nerve is the continuation of the medial cord of the brachial plexus at the level of the axilla.

Ulnar neuropathy at the elbow is the second most common entrapment neuropathy. Only carpal tunnel syndrome (median neuropathy at the wrist) is more frequent. The ulnar nerve is susceptible to compression at the elbow for several reasons. First, the nerve has a superficial anatomic location at the elbow. Hitting the “funny bone” (ulnar nerve at the elbow) creates an unpleasant sensation that most people have experienced. If the ulnar nerve is susceptible to subluxation, further injury may result. Second, the nerve is prone to repeated trauma from leaning on the elbow or repetitively flexing and extending the elbow. Poorly healing fractures at the elbow may damage this nerve. Finally, and perhaps most important, the ulnar nerve can become entrapped at the arcade of Struthers, in the cubital tunnel (ulnar collateral ligament and aponeurosis between the two heads of the flexor carpi ulnaris; Fig. 27.1 ), or within the flexor carpi ulnaris muscle. The nerve lengthens and becomes taut with elbow flexion. In addition, there is decreased space in the cubital tunnel in this position. The volume of the cubital tunnel is maximal in extension and can decrease by 50% with elbow flexion. The nerve may also become compromised after a distal humerus fracture, either as a direct result of the fracture or because of an altered carrying angle of the elbow and decreased elbow extension (tardy ulnar palsy). Cyclists are prone to traction and compression of the ulnar nerve at both the elbow and the wrist. Repetitive or incorrect throwing can lead to damage of the ulnar nerve at the elbow. Biomechanical risk factors (repetitive holding of a tool in one position), obesity, and other associated upper extremity work-related musculoskeletal disorders (especially medial epicondylitis and other nerve entrapment disorders) have also been associated with the development of ulnar neuropathy at the elbow.

The cubital tunnel.

From Bernstein J, ed. Musculoskeletal Medicine. Rosemont, IL: American Academy of Orthopaedic Surgeons; 2003.

Symptoms

If the ulnar nerve is entrapped at the elbow, both the dorsal ulnar cutaneous nerve (which arises just proximal to the wrist) and the palmar cutaneous branch of the ulnar nerve will be affected. Patients will therefore complain of numbness or paresthesias in the dorsal and volar aspects of the fifth and ulnar side of the fourth digits. Hand intrinsic muscle weakness may be apparent. In cases of severe ulnar neuropathy, clawing of the fourth and fifth digits (with attempted hand opening) and atrophy of the intrinsic muscles may be noted by the patient ( Fig. 27.2 ). Symptoms may be exacerbated by elbow flexion. Pain may be noted and may radiate proximally or distally.

Froment sign. Note prominent atrophy of the intrinsic muscles.

From Weiss L, Silver J, Weiss J, eds. Easy EMG. New York: Butterworth-Heinemann; 2004.

Physical Examination

The ulnar nerve may be palpable in the posterior condylar groove (posterior to the medial epicondyle) with elbow flexion and extension. A Tinel sign may be present at the elbow; however, it should be considered significant only if the Tinel sign is absent on the nonaffected side. Direct pressure over the ulnar nerve posterior to the medial epicondyle with the elbow in flexion is a sensitive provocative test. The ulnar nerve may be felt subluxing with flexion and extension of the elbow. Sensory deficits may be noted in the fifth and ulnar half of the fourth digits. Atrophy of the intrinsic hand muscles and hand weakness may be noted as well (although this is generally seen in more advanced cases). Wartenberg sign (abduction of the fourth and fifth digits) may occur. The patient should be tested for Froment sign. Here, a patient is asked to grasp a piece of paper between the thumb and radial side of the second digit. The examiner tries to pull the paper out of the patient’s hand. If the patient has injury to the adductor pollicis muscle (ulnar innervated), the patient will try to compensate by using the median-innervated flexor pollicis longus muscle (see Fig. 27.2 ).