TARTARIC ACID-INDUCED COUGH AND THE SUPERIOR LARYNGEAL NERVE EVOKED POTENTIAL1

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Abstract
The purpose of this study was to stimulate the laryngeal cough reflex using a nebulized, mild chemical irritant and to record an associated laryngeal evoked potential from the internal branch of the superior laryngeal nerve. The laryngeal evoked potential was obtained on ten normal subjects from the right internal branch of the superior laryngeal nerve. The electrodiagnostic setup included an active electrode placed just below the hyoid bone with a 4-cm separation and distal reference. A ground electrode was placed between the active and reference electrodes. The receptors and internal branch of the superior laryngeal nerve were stimulated by inhalation of a nebulized 20% solution of tartaric acid and normal saline. The time line was triggered by a pneumatic switch on initial inspiration of the nebulized tartaric acid. The electrodiagnostic settings were set at a sweep speed of 1 ms/division, a gain of 10 to 20 μV/division, and 20 to 2,000 filters. There were 132 variables recorded from the internal branch of the superior laryngeal nerve of the ten subjects. The mean peak distal latency was 1.66 ± 0.42 ms with a 1.6 median, 1.6 mode, and 0.17 variance. The duration was 0.41 ms, and amplitude was 5.19 ± 2.91 μV. In conclusion, the laryngeal evoked potential, the afferent component of the involuntary cough reflex, can be recorded from the internal branch of the superior laryngeal nerve after inhalation of tartaric acid-induced cough.