Effects of botulinum toxin on vocal tract steadiness in patients with spasmodic dysphonia

Since laryngeal botulinum toxin (BTX) injections have become the treatment of choice for spasmodic dysphonia, the purpose of this study was to examine its effects on the stability of the upper vocal tract as compared to the effects on glottic stability. Two different acoustic methods were used to analyze voice samples from 16 patients with adductor-type spasmodic dysphonias before and after BTX therapy and from a normal control group. Independent acoustic analyses were used to determine laryngeal and upper vocal tract stability. The results showed significantly higher values for the standard deviation of fundamental frequency (SDF0), reflecting laryngeal instability, for the patient group than for the control group and an impressive improvement for the patients after BTX therapy. Further, the equally high values of SDF0 for the initial second and a second from the midsegment of phonation were differentially reduced by BTX therapy, resulting in a normal pattern of laryngeal stability during sustained phonation. The variability of the first and second formants, reflecting upper vocal tract instability, showed higher values for the patients compared with the control group, but this difference was not statistically significant. The present findings showed that BTX injections to the thyroarytenoid muscle had no discernible effect on stability of the upper vocal tract.