A new electrode placement for recording the compound motor action potential of the first dorsal interosseous muscle.

Objective: This paper describes a new electrode placement for recording compound muscle action potentials (CMAPs) of the first dorsal interosseous muscle (FDI) to determine the distal motor latency (DML) and study nerve conduction of the ulnar nerve across the wrist.
Methods: The DML to the FDI was evaluated bilaterally in 90 subjects after stimulation 1cm proximal to the distal wrist crease and at the palm. The CMAP was recorded with a pair of disposable surface electrodes fixed over the FDI and wrist.
Results: The CMAP never exhibited a positive initial deflection, with a gain of 0.5 mV per division. DML to the FDI was 2.65±0.26 ms (mean±SD), and CMAP amplitude was 14.7±3.3 mV. A prolonged DML was taken as 3.4 ms (mean+3 standard deviation [SD]).
Conclusions: This new electrode placement offers more reproducible results for determining the DML to the FDI as it provides the shortest DML, and the tightest SD values.
Significance: This result is obtained through the respect of fundamental rules for CMAP recording, as it shows no positive wave at the onset of the CMAP of the FDI. Its use should improve the diagnosis of ulnar nerve lesions at the wrist and more especially of the deep motor branch.
(Copyright © 2011 Elsevier Masson SAS. All rights reserved.)