Effect of demyelinating ulnar nerve injury on strength and fatigue.

OBJECTIVES

Conduction block (CB) from focal neuropathy is often associated with weakness and fatigue in affected muscles. Ulnar neuropathy at the elbow (UNE) provides an excellent model to examine the relationships between electrophysiologically defined CB and quantitative measurement of weakness and fatigue.

METHODS

Eight healthy control subjects (47 ± 14 years) and nine patients (53 ± 3 years) with clinical and electrophysiological features of UNE with CB were studied. All underwent bilateral, ulnar motor nerve conduction studies recording from the first dorsal interosseous muscle as well as quantitative measurement of strength and fatigue of the first dorsal interosseous with a custom dynamometer.

RESULTS

Strength and fatigue were similar in the dominant and nondominant hands of control subjects and unaffected limb in patients. Varying degrees of conduction block (14-62%, mean 36%) and conduction slowing (31 m/s ± 7) were observed in those with UNE. CB was associated with significant reductions in strength (42%) and fatigue (23%) on a timed fatigue task. The reductions in strength (r = 0.74) and fatigue (r = 0.60) were strongly correlated with the degree of CB.

CONCLUSIONS

CB in UNE defined by electrophysiological criteria was strongly correlated with weakness and fatigue in the first dorsal interosseous. Fatigue may be simply related to the reduction in strength, but activity or frequency dependent CB may also contribute.