Acoustic Reflex Response during Curare-Induced Weakness

The acoustic stapedial reflex (AR) was measured in five healthy subjects during intravenous administration of d-tubocurarine in four equal doses totaling 0.2 mg/kg. The behavior of the reflex in the face of progressive muscle weakness was assessed. Amplitude changes in the AR elicited by pulsed tones were compared to measurements of maximum respiratory pressures reflecting respiratory muscle strength. Total curare dose averaged 16.1 mg and produced symptoms progressing from mild diplopia with the initial dose to absence of hand-grip and head-lifting ability with the final dose. Peak-to-peak amplitude of the AR decreased progressively with each dose of curare. Effects were not strictly additive but showed the largest change with the last dose. Decreases in expiratory muscle strength closely paralleled changes in the AR, whereas changes in inspiratory muscle strength were less marked than the AR, especially with mild weakness. The dose-dependent reduction of the AR corresponded to changes in respiratory muscle as well as general muscle strength. Therefore we concluded that the AR is a valid and sensitive index of neuromuscular transmission which has an advantage over most tests of muscle strength since subject effort and cooperation do not influence test results.