Site-dependent thenar compound muscle action potential: Comparison between surface and needle recordings.

Background: Within the thenar eminence, the median nerve innervates three muscles: abductor pollicis brevis (APB), flexor pollicis brevis (FPB), and opponens pollicis (OP). Of these muscles, APB was often considered as the sole contributor to the thenar compound muscle action potential (CMAP).

Objective: To evaluate subcomponents of the thenar CMAP from the median nerve innervated muscles.

Methods: Surface and needle CMAPs were recorded in normal human subjects from three recording sites: proximal (site-I), middle (site-II), and distal (site-III) aspects of the thenar eminence when the median nerve was activated at the wrist.

Results: In the site-I and -II, both the surface and needle CMAPs shared many similar characteristics although the needle CMAPs were larger (∼ 5 folds) and briefer (∼ 60%, needle/surface duration). In addition, on the surface recording, the CMAP was larger (by ∼ 1.9 mV) when recorded from the site-I comparing to that of the site-II. In the site-III, the surface recordings registered a delayed (by ∼ 3.8 ms) CMAP. The muscle fiber action potential (MFAP) study suggested a predominant FPB contribution in the site-III.

Conclusions: The optimal recording site for APB derived thenar CMAP is the site-I and for FPB is the site-III. The CMAPs registered by the needle recordings are more robust than the surface ones.