Ulnar Mononeuropathy

Because the ulnar nerve is a mixed nerve, supplying muscles in the forearm and hand and providing sensation over the fourth and fifth digits of the hand, palm, and posterior aspect of the forearm, very specific symptoms are associated with its pathology. Physicians are reminded that the most common site of entrapment is in the wrist (carpal tunnel syndrome) with the elbow being the second most common. Both the axons and the myelin sheaths may be affected, often in a selective manner, which in axonal pathology may involve fascicles to individual muscles, leading to motor unit loss and amplitude/area reduction. Involvement of myelin sheaths (usually as isolated demyelination) presents as slowing of conduction (abnormal temporal dispersion) (39).

Interestingly, men are more susceptible to wrist entrapment than women, a finding that may be of value in the rural setting where trauma of the elbow is a common occurrence, while carpal tunnel-associated trauma (i.e., typing) is less common. Patients commonly present with changes in sensation and individual muscle strength; some present with a clawed posture of the hand(s) (38).

Two signs need to be ascertained: the Froment sign (indicates weakness of the adductor pollicis muscle) and Tinel-2 sign (useful in assessment of carpal tunnel syndrome-associated neuropathic changes). The Froment sign is manifested by activation of the flexor pollicis longus while the patient attempts to pinch the thumb and forefinger or grasping a sheet of paper (the patient may notice this and describe a failure of the thumb to move "on its own" to reach the forefinger). The Tinel-2 sign is elicited by tapping over the carpal tunnel; in a positive sign, this results in a tingling sensation in the distribution of the median nerve (38).

In all patients NCSs with or without EMG are viewed as the ultimate diagnostic and monitoring studies. The NCS measures basic sensory and motor nerve parameters such as latency, amplitude, and conduction velocity. With stimulation above and below the elbow and recording from the main belly of an involved muscle (commonly, abductor digitorum quinti [ADQ] or first dorsal interosseous [FDI]), the neurologist will both localize the site of involvement and decide on its severity. We recommend the use of the "inching" technique (more formally known as the short segment stimulation technique) for increased resolution and differential diagnosis between infracondylar (commonly, in the cubital tunnel) or supracondylar (commonly as the ulnar palsy tarda) conduction blocks (38,39).

Physicians are also reminded of the common (about 25% of the population) anatomical variation, known as the Martin-Gruber anastomosis in which fibers from the median nerve, typically the motor branches, cross over and join with the ulnar nerve in the forearm. This abnormal pattern of innervation may lead to confusing findings (e.g., the larger median CMAP amplitude at the elbow has an initial downward deflection, which is not seen at the wrist). Electrophysiological findings may also ascertain the ongoing loss of muscle fibers via detection of abnormal spontaneous activity (such as fibrillation potentials and fasciculations) (43).

Management

Patients with ulnar nerve damage should be treated aggressively and with a certain degree of creativity and personalization of care. Depending on the general medical health status, medications that address vascular and metabolic components of the neuropathic process are warranted. Pain may respond to nonsteroidal antiinflammatory drugs, opioids, tricyclic antide-pressants, stimulants (e.g., methylphenidate), and many anticonvulsants (e.g., gabapentin). Many patients may need surgical care; thus all patients should be referred for an appropriate consultation. Electrodiagnostic studies should be repeated as needed, especially in cases of severe pathology (with motor amplitude of 10% of normal or a greatly reduced recruitment of motor units, which, in our opinion, is a sign of poor prognosis for recovery) (43).

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