Clinical and electrophysiological characteristics of larynx in myasthenia gravis

We investigated the clinical and electrophysiological characteristics of the larynx in patients with myasthenia gravis (MG).

Thirty-two cases of MG were analyzed. The laryngeal behaviors and characteristics of laryngeal electromyography were evaluated, and voice assessment and repetitive nerve stimulation (RNS) tests were conducted.

The initial symptoms of patients were ptosis and/or diplopia in 28, dysphagia and slurred speech in 3, and limb weakness in 1. Only 8 patients had slight hoarseness and vocal fatigue, and 4 patients had positive laryngeal signs. When compared with normal subjects, the patients with MG had worse results on the acoustic analysis of shimmer, their normalized noise energy was greater, and the harmonics-to-noise ratio and maximum phonation time were significantly lower. The RNS findings were positive in 28 patients and negative in 4 patients. The average (+/-SD) amplitude decrement was 31.9% +/- 19.1%. The mean number of involved laryngeal muscles was 2.22 +/- 1.35. Cricothyroid muscles were involved in 26 cases, thyroarytenoid muscles were involved in 14 cases, posterior cricoarytenoid muscles were involved in 6 cases, and lateral cricoarytenoid muscles were involved in 2 cases.

Although few patients presented with laryngeal symptoms as their initial symptoms, most patients with MG exhibited asymmetry and abnormal findings on laryngeal electromyography. The RNS test for laryngeal muscles is a more sensitive indicator for the diagnosis of MG, especially in the cricothyroid muscle.