Sources of error in estimation of laryngeal airway resistance in persons with spasmodic dysphonia.

Estimation of laryngeal airway resistance is a noninvasive method that has proven useful in the study of people with normal and some types of disordered voices. We were interested in examining more closely the application of this method to persons with spasmodic dysphonia (SD), a voice disorder sometimes associated with fluctuating airflow. We speculated unstable airflow could affect the estimation of subglottal pressure and laryngeal airway resistance. Oral pressure and airflow were collected from 10 subjects with SD and 10 control subjects during repetition of /pi/. The coefficient of variation (COV) of airflow during vowel production was calculated to quantify stability of airflow. The results indicated that although some SD subjects produced steady flows during the syllable repetition task, others exhibited substantially varying flows. Inability on the part of the subject to attain steady flows could compromise the usefulness of a midpoint measure of airflow and/or estimation of subglottal pressure, resulting in sources of error in estimation of laryngeal airway resistance. As a result, of the 10 subjects with SD in this study, laryngeal airway resistance could not be estimated in 6 subjects with unsteady flows. Laryngeal airway resistance was estimated in 4 SD subjects who produced steady airflow. Two of these subjects exhibited high laryngeal airway resistance; the others exhibited normal laryngeal airway resistance.