Analysis of Compound Muscle Action Potential of Median and Ulnar Nerves: Possible Anatomical Correlation

Objective: To analyze of shape, amplitude of CMAP of both median and ulnar in normal population and to analyze the anatomical innervations of small muscles of hand in normal cadavers. Design: Cross sectional study. Settings: outpatient settings and department of anatomy at Alexandria University. Participants: 300 normal adults (82 males and 218 females) and 30 normal adult cadavers. Main outcome measures: Motor conduction study of both median and ulnar nerves for the included normal subjects. Surface recording of CMAP from thenar and hypothenar muscles respectively with analysis of the shape of the waveform and amplitude. The nerve supply of both thenar and hypothenar muscles were dissected for the included normal cadavers. The main nerve trunk (median or ulnar), the number of branches, their sizes and their sites of entering into the muscles were recorded. Results: The mean age of the studied population was 37.86 ± 8.83 (age range 19-69). The mean amplitude of the median nerve was significantly higher than that of the ulnar nerve (11.785 ± 5.0, 10.45 ± 2.96 respectively, p=0.0001). The mean distal latency of median nerve was significantly higher than that of ulnar nerve (3.38 ± 0.41 and 2.698 ± 0.40 respectively, p=0.0001). The median nerve had mostly dome shaped CMAP rather than double peaked CMAP with significant difference (p=0.0001), while the shape of CMAP of the ulnar nerve was more frequently double peaked rather than dome shaped with significant difference (p=0.0001). The mean distal latency of the median nerve was significantly longer in those with dome shaped CMAP rather than those with double peak CMAP (p=0.002). Similarly, the mean distal latency of the ulnar nerve was significantly longer in those with dome shaped CMAP compared with those with double peak CMAP (p=0.0001). There were no statistical significant differences between the amplitude of dome shaped CMAP and the double peaked CMAP in either the median or the ulnar nerve. The anatomical results showed that abductor pollicis brevis supplied by the median nerve in 90% of specimens, and by both nerves in 10%. The abductor digiti minimi brevis supplied by the deep branch of the ulnar nerve in 90% and by superficial branch of the ulnar nerve in 10%. There was a connection between the median and ulnar nerves in 50% of specimens. Conclusion: The configuration of the CMAP of the median nerve is mostly dome, whereas that of the ulnar is mostly double peaked. Variability in the pattern of innervations of the small muscle of the hand could be a possible etiological factor.