Objective: The purpose of this study was to evaluate the outcomes of long-term botulinum toxin type A (BoNTA) treatment for adductor spasmodic dysphonia (AdSD) and to determine the factors predictive of treatment response by investigating dose stability and average intervals. Design: Retrospective cohort study. Setting: Academic tertiary medical centre. Exposures: A total of 470 patients with adductor spasmodic dysphonia, who received electromyography-guided BoNTA injections over 12 years, were retrospectively enrolled in this study. Main Outcomes and Measures: The patients’ demographic data, baseline voice dynamics and treatment profiles (dose, frequency and intervals) were evaluated. Factors correlating with the dose adjustment ratio (number of increasing dosing/total number of BoNTA toxin injections) and changes in intervals between injections were statistically analysed. Results: A total of 122 patients, who received ≥ 4 injections and whose average treatment interval was < 240 days, were finally evaluated. Of them, 115 (94.3%) were female and seven (5.7%) were male, and the mean age at initial treatment was 34.89 ± 13.07 and 41.14 ± 12.71 years, respectively. On average, patients received 18.00 ± 13.33 injections (1.67 ± 0.60 U/injection) to alternating unilateral vocal folds. The treatment period was 65.07 ± 43.28 months and the mean interval between injections was 4.16 ± 1.28 months. The mean dose adjustment ratio among patients who received ≥ 4 injections was 0.15 ± 0.13, and dose changes occurred 4.36 times/patient. The patients’ age and gender significantly affected the treatment response, where younger or female patients showed greater dosing variability and shorter intervals between injections. However, the baseline voice dynamics (voice handicap index, fundamental frequency, jitter, shimmer, noise-to-harmonic ratio, maximum phonation time and degree of voice breaks) did not predict the dose adjustment ratio or interval changes. In addition, patients with fluctuating doses showed lower age and higher VHI subscale scores, and patients with short-treatment interval (< 100 days) showed higher SDF0. Conclusions: Almost all patients received stable low doses of BoNTA over time, irrespective of the baseline results. Patients’ age, gender and VHI scores were correlated with poor treatment responses, such as frequent dose changes and shorter intervals between injections.
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