Analysis of stapedial reflex in neuromuscular diseases.

An impedance audiometric analysis of the stapedial reflex was carried out in 17 patients with motor neuron disease (MND), 11 with myasthenia gravis (MG), and 3 myotonic dystrophy (MD). The features examined were the threshold and latency parameters and the response pattern of the reflex. All groups had mean reflex thresholds within normal limits (80-90 dB hearing level). The MND group showed a lengthened mean reflex latency (L1). The MG group showed an unstable response pattern to 10-second tone stimuli and a lengthened L1. Half the group demonstrated abnormal decay of the reflex. The MD group showed a reduction in the growth of averaged reflex in response to stimuli with short interstimulus intervals (2.6 s) and the L1 and C50 were lengthened.