Laryngeal electromyography in the diagnosis of laryngeal nerve injuries.

A practical technique of electromyographic (EMG) sampling of the cricothyroid and thyroarytenoid (vocalis) muscles is described. Quantitative motor unit action potential (MUAP) analysis on a group of seven healthy subjects revealed a mean amplitude of 426 microV for the vocalis and 500 microV for the cricothyroid muscle. Mean MUAP duration for the vocalis muscle was 3.5 msec and 4.4 msec for the cricothyroid. Normal activation patterns are reviewed for each muscle. The results of EMG studies in 18 consecutive patients referred to the EMG laboratory with chronic hoarseness was reviewed. There was good agreement between findings at laryngoscopy and EMG, although more widespread involvement was seen on EMG than was suspected on the basis of laryngoscopy. Vocal cord immobility (apparent paralysis) was not usually accompanied by the total absence of MUAPs. The procedure was well tolerated without local anesthesia and provides information about peripheral nerves which may be of prognostic value.