Defective E2 electrode lead gives low-amplitude compound muscle action potential.

INTRODUCTION/AIMS: Low-amplitude compound muscle action potential (CMAP) suggests a neuromuscular pathology. Low amplitude will also result from a defective E1 electrode or its lead, that is, a technical artifact. The aim of this study was to investigate the effect of a defective E2 electrode lead on the CMAP.

METHODS

The CMAP was recorded using standard nerve conduction methodology and all electrode leads connected properly. Signals were then recorded when either the E1 or the E2 electrode lead was disconnected from the amplifier. This simulated a defective electrode lead. Studies were performed in four nerves of a healthy subject.

RESULTS

CMAP amplitude was reduced as expected when E1 was disconnected. Surprisingly, the amplitude fell by more than 65% when the E2 lead was disconnected, although E1 was properly connected.

DISCUSSION

E1 and E2 electrodes contribute to the CMAP. A defective recording electrode lead to E1 or E2 results in a low-amplitude CMAP. The amplitude drop observed with a disconnected E2 lead was far greater than the signal recorded by the E2 electrode. This occurs due to the amplifier's inherent property to reduce the voltage difference between the E1 and E2 inputs. When E2 lead is defective, the CMAP will be an attenuated version of the signal recorded by the E1 electrode, and vice versa. When low-amplitude CMAP amplitude is observed in all conduction studies, technical artifact should be considered before exploring the pathological basis for the abnormal results.