122 CHAPTER 5 are a major strength of this study. There are however some potential limitations that need to be addressed. First, there were missing questionnaires and values which potentially might lead to biased results. Multiple imputation was used to adjust for any influencing factors. Second, the currently used questionnaire was not validated, unlike the DIS that was introduced in 2010.25 Previous research has however proven the questionnaire to be sensitive to change over time which was the main interest in this study.2,16-18 Besides, unlike the DIS, the currently used questionnaire also evaluates social and emotional consequences in more detail and quantifies the severity of drooling in multiple positions and daily activities. Despite that, the study would have improved with at least a baseline DIS to compare results to other studies. Future research should validate the questionnaire used in the current study and assess inter- and intra-responder variability. Third, despite randomization, the baseline degree of drooling in several positions and daily activities was not equal between the BoNT-A and 2-DL group. During supported sit, unsupported sit, concentrated activity and strenuous activity the mean percentage of excessive drooling was significantly higher in children treated with BoNT-A. The mean decrease in drooling severity from baseline to 32 weeks after treatment was however significantly higher in children treatedwith 2-DL compared to those treated with BoNT-A in all positions and daily activities suggesting no major influence of the unequal baseline values. Another limitation in this study is the fact that questionnaires were filled out prior to inclusion, possibly leading to an exaggeration of complaints in order to receive treatment. Although wiping and encouragement to swallow might be non-specific for the degree of drooling, both outcomes were included because we feel that it gives insight in perceived daily care. Moreover, there was no blinding for treatment allocation for patients and caregivers, as sham surgery is considered unethical, which may have led to an assessment bias in caregivers favoring one of the two treatments. Future research should consider to use a cross-over design to compare treatment within participants where patients would receive surgery after BoNT-A. Additionally, the questionnaires were not always filled out by the same caregiver. The limited follow-up time is another limitation since the reduction of drooling and impact on daily life after 2-DL seemed to fade slightly from 8 to 32 weeks follow-up.
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