Ulnar nerve latency at above-elbow stimulation: a complementary tool for segmental conduction velocity across the elbow.

OBJECTIVE

The purpose of this study was to obtain the latency norms of ulnar nerve stimulating at 10 cm above the elbow as a complementary test of the segmental conduction velocity criteria used in the American Association of Electrodiagnostic Medicine guideline for ulnar neuropathy at the elbow (UNE).

DESIGN

This is a cross-sectional study of 100 subjects recruited to the electrodiagnostic laboratory of an academic medical center and a retrospective analysis of 14 symptomatic patients diagnosed with UNE using the American Association of Electrodiagnostic Medicine practice guideline. Sensory nerve action potential and compound muscle action potential were recorded at the fifth finger and abductor digiti minimi muscle, respectively, with 10-cm above-elbow stimulation and with elbow flexion at 90 degrees. The latency values of ulnar nerve, together with the participant's anthropometric data, were measured and correlated. Afterward, these values were compared with latency values from 14 patients with UNE.

RESULTS

Sensory peak and motor onset latency at 10-cm above-elbow stimulation showed mean ± SD values of 8.3 ± 0.7 and 8.0 ± 0.7 msecs, respectively. Latencies were highly correlated with finger-to-arm length (r = 0.87 for sensory and 0.84 for motor). The cutoff values (twice the standard deviation plus the mean) for "abnormal" are 9.7 and 9.4 msecs for sensory and motor latencies, respectively. Applying the motor cutoff value (9.4 msecs) for abnormal, 12 of 14 symptomatic UNE cases were categorized as abnormal, with a sensitivity of 86%. All 14 patients with UNE showed latencies beyond the upper limit of 95% confidence interval of predicted value based on finger-to-arm length.

CONCLUSIONS

This study provides latency norms of ulnar sensory and motor nerves simulating at 10 cm above the elbow. It can be used as a complementary tool to assess the validity of segmental conduction velocity criteria in the electrodiagnosis of UNE.