Intramuscular recording in neurodiagnostic studies

Intramuscular (IM) recordings are often used in nerve conduction studies involving deep or proximal muscles and in other electrodiagnostic tests. Little is known of how various needle electrode configurations and placements influence the characteristics of the evoked responses and their clinical interpretation. The present set of investigations sought to characterize the amplitude, waveform, consistency, latency and muscle discrimination capabilities of IM recordings from a variety of muscles using monopolar, concentric and bipolar electrodes. Results indicate that 1) all forms of IM electrodes studied show marked changes in response waveform with changes in position of the electrode tip, but the monopolar electrode configuration shows the least amount of change; 2) the concentric electrode does not differentiate well between deep and superficial muscle responses; 3) the latency of the evoked response does not change with the depth of penetration within a muscle; 4) the latency of response recorded from IM electrodes may be longer, the same or shorter than that recorded from surface electrodes; and 5) monopolar electrodes should be used when onset latency or waveform is the critical parameter to be recorded from deep or atrophied muscles.