Motor unit sound in needle electromyography: Assessing normal and neuropathic units

Motor unit action potentials (MUPs) recorded by a monopolar needle electrod e in normal and neuropathic muscles were computer-simulated. Five experienc ed electromyographers acted as examiners and assessed the firing sounds of these MUPs without seeing them on a display monitor. They judged whether th e sounds were crisp or close enough to accept for the evaluation of MUP par ameters and whether, when judged acceptable, they were neuropathic-polyphas ic. The examiners recognized motor unit (MU) sound as crisp or polyphasic w hen the MUP obtained was 0.15-0.2 mm from the edge of the MU territory. Whe n the intensity of the sound decreased, they were unable to perceive it as crisp. When the intensity exceeded the saturation level of loudspeaker outp ut, the sound was perceived as polyphasic, but the wave form of the MUP was not. When the frequency of the neuropathic MUP was lowered, the examiners were unable to determine whether the MUP was polyphasic. MUPs recognized as acceptable for evaluation can be distinguished by listening to MU sounds. The audio amplifier gain must be appropriately adjusted for each MUP amplit ude in order to assess whether an individual MU sound is crisp or polyphasi c before MUP parameters are measured on a display monitor. (C) 2000 John Wi ley & Sons, Inc.