192 GENERAL DISCUSSION Strengths and limitations of the overall study which limit the extent to which the results are generalizable One of the strengths of this thesis is that the effect, impact of drooling on daily life and care, social interaction and self-esteem, and cost-effectiveness of 2-DL were thoroughly evaluated in a prospective, controlled, partly-single blinded setting. Measurements for chapters 2, 5, and 6 were regularly obtained at baseline, 8, and 32 weeks in a prospective, standardized manner with both objective as subjective outcome measurements. This thesis also has several limitations. First, long-term follow-up was evaluated in a cross-sectional setting wherein only subjective measures were assessed. Although there was no correlation between time to follow-up and effect, there was a wide variation in time to follow-up (1 – 10 years) limiting the extent to which the results are generalizable. Secondly, although one of the strengths of this thesis is that we included an objective outcome, DQ could be subject to the observer’s paradox. Third, neither patients, caregivers, nor study investigators were blinded for treatment allocation. This could have introduced bias. Nevertheless, masked objective DQ was closely related to unmasked DQ, and there was consistency between the subjective VAS and objective DQ, which both suggest limited bias. Fourth, although this is the largest cohort of patients that underwent 2-DL, the cohort size is still relatively small. The small cohort size limited the number of variables to be included in the prediction model. Also, it was impossible to analyze all aspects of caregivers’ impression of self-esteem due to the limited patients. Fifth, a nonvalidated questionnaire was used to study the impact of drooling on daily life and care, social interaction, and self-esteem instead of the validated Drooling Impact Scale (DIS). The current questionnaire nevertheless evaluates social and emotional aspects in more detail. Above all, previous research has proven the sensitivity of the current questionnaire to change over time which was the core interest of chapter 7. Sixth, all studies were conducted in a single center limiting the extent to which the conclusions are to be generalized. The patient characteristics were, however, extensively reported, and patients are generally comparable.66 Moreover, 2-DL is a relatively simple procedure thoroughly explained in this thesis and previous
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