Factors Influencing Botulinum Toxin Dose Instability in Spasmodic Dysphonia Patients

Objective. Many patients with spasmodic dysphonia (SD) see consistent effects from botulinum toxin (BTX) injections of the same dose, whereas others require dosage changes over time. We sought to determine whether demographics (age and gender) or environmental factors (smoking) affect the long-term stability of BTX dosing in these patients. Study Design. Retrospective review. Methods. Charts of all patients undergoing BTX injection for adductor SD were reviewed. Dosage change, defined as whether there was any difference in total dosage used between two beneficial injections, was used as a measure of dosing stability. Beneficial injections were indicated by a voice rating score of at least three of four and any non-zero duration of improved voice. Logistic regression analysis was performed to determine whether age, gender, smoking status, or duration of treatment correlated with odds of having a dosage change. Results. A total of 211 patients were ultimately included. Age, gender, and smoking status were all found to have no correlative effect on dosing stability. The only factor that was predictive of dose stability was the number of previous beneficial injections, as every additional injection led to decreased odds of a change in dosage for the next injection (odds ratio = 0.964; 95% confidence interval = 0.947-0.981). Conclusions. Dosage of BTX injections for long-term treatment of SD has a significant propensity to remain stable over time. Factors such as age, gender, and smoking status do not appear to influence the dosage stability. These findings should allow for better patient counseling regarding expectations for their long-term treatment.