Ulnar neuropathy

Ulnar neuropathy is a disorder involving the ulnar nerve. Ulnar neuropathy may be caused by entrapment of the ulnar nerve at the elbow or wrist with resultant numbness and tingling into the fourth and fifth fingers.

Motor function can be assessed by testing for a positive Froment's sign, or making an OK sign (which the patient will be unable to do). Little finger abduction can be tested as well.

Pathophysiology

The nerve, axon, and myelin can be affected. Within the axon, fascicles to individual muscles may be involved selectively. Axonal involvement leads to motor unit loss and amplitude/area reduction. Conduction block implies impaired transmission through a segment of nerve. In the absence of changes indicating axonal damage, conduction block implies myelin damage to the involved segment. Significant slowing of conduction and/or significant spreading out of the temporal profile of the recorded response (i.e., abnormal temporal dispersion) with preserved axonal integrity suggests demyelination.

Signs and symptoms

Both the onset and progress of the symptoms can be variable. Although the answer is frequently negative, one should ask specifically about trauma and pressure to the arm and wrist, especially the elbow, the medial side of the wrist, and other sites close to the course of the ulnar nerve.

Many patients complain of sensory changes in the fourth and fifth digits. Rarely, a patient actually notices that the unusual sensations are mainly in the medial side of the ring finger (fourth digit) rather than the lateral side, corresponding to the textbook sensory distribution. Sometimes the third digit is also involved, especially on the ulnar (i.e., medial) side. The sensory changes can be a feeling of numbness or a tingling or burning. Pain rarely occurs in the hand. Complaints of pain tend to be more common in the arm, up to and including the elbow area. Indeed, the elbow is probably the most common site of pain in an ulnar neuropathy. Occasionally, patients specifically say “I have pain in my elbow,” “I have pain in my funny bone,” or even “I have pain in this little groove in my elbow,” but usually they are not quite so explicit unless prompted. Patients rarely notice specific muscle atrophy.

Weakness may also be a presenting complaint, but the complaint may be expressed in subtle ways. One traditional sign of ulnar neuropathy is a complaint of weakness. The patient complains that the little finger gets caught on the edge of the pants pocket when he or she puts the hands into the pocket. Because of the ulnar dysfunction the patient cannot adduct the fifth digit tightly against the fourth because of weakness of the interosseous muscles. The muscle that extends the fifth digit at the metacarpal phalangeal joint (extensor digiti quinti) is radially innervated and it inserts on the ulnar side of the joint. Normally this muscle is opposed by ulnar innervated muscles that flex the joints, but with an ulnar neuropathy the muscle is relatively unopposed so it pulls the finger up and to the ulnar side. This is why the finger tends to stick out and get caught on objects.

The patient may also express the complaint of weakness by saying “my grip is weak.” Many of the grip muscles are ulnar. Also, when someone tries to grip powerfully, the hand usually deviates in the ulnar direction under the influence of the flexor carpi ulnaris. If this ulnar deviation is impaired, the grip mechanism does not work optimally even for the muscles that are unimpaired.

Sometimes a patient notices that his pincer grip (pinching with the thumb and index finger) is weak. Two of the key muscles involved in this movement are the adductor pollicis (which adducts the thumb) and first dorsal interosseous, which adducts the index finger. Not only may the pincer grip be weak in an ulnar neuropathy, the median innervated flexor pollicis longus partially compensates for the weakened adductor pollicis and the thumb flexes at the distal joint. Usually a patient does not notice the thumb flexion, but when demonstrated by the examiner, this flexion is considered to be Froment sign.

Causes

Ulnar nerve at or near the elbow:

1. Compression during the perioperative period
2. Elbow fractures with resultant cubitus varus (aka tardy ulnar nerve palsy)
3. Blunt trauma
4. Deformities (e.g., rheumatoid arthritis)
5. Metabolic derangements (e.g., diabetes)
6. Transient occlusion of brachial artery during surgery
7. Subdermal contraceptive implant
8. Venipuncture
9. Hemophilia leading to hematomas
10. Malnutrition leading to muscle atrophy and loss of fatty protection across the elbow and other joints
11. Cigarette smoking

Ulnar neuropathy at Guyon's canal

1. Ganglionic cysts
2. Tumors
3. Blunt injuries with or without fracture
4. Aberrant artery
5. Idiopathic

Epidemiology

Ulnar neuropathy occurs more frequently in men than in women, despite the fact that there are no significant gender differences in the anatomical course of the ulnar nerve. However, the tubercle of the coronoid process is larger and the nerve and blood vessels passing by it are less protected by subcutaneous fat in men than in women. These two anatomical differences between men and women may contribute to the increased frequency of ulnar neuropathy induced by external compression-induced restriction in blood supply at the medial aspect of the elbow in men.^[1]

Footnotes