To the Editor At least 3 different issues related to study design, internal validity, and external validity of the trial of onabotulinumtoxinA vs sacral neuromodulation limit the generalizability of the findings.1
First, the authors stated that a recent review of onabotulinumtoxinA and sacral neuromodulation found insufficient evidence that either treatment was superior.2 From a trial design standpoint, when there is uncertainty about relative effectiveness between 2 treatments and when no placebo group is included, the logical next step is to design a noninferiority rather than a superiority trial. Such a trial would ensure that the effectiveness of one treatment is not significantly worse than the other. Although the authors discussed a sample size calculation based on a noninferiority trial design, they performed a superiority trial. Even if one treatment is assumed to be superior to the other, it is unclear which one. In the protocol the authors stated their original hypothesis as “InterStim therapy will result in a greater reduction in daily urge urinary incontinence episodes over the 6-month follow-up period as compared to Botox A injection,” whereas in the article they stated “Is onabotulinumtoxinA superior to sacral neuromodulation in controlling symptoms of refractory urinary incontinence?” This approach is an example of wrong design bias and undermines the validity of the findings.3
McCauley JF, Gonzalez HM, Osborn DJ. OnabotulinumtoxinA vs Sacral Neuromodulation for Urgency Incontinence. JAMA. 2017;317(5):535. doi:10.1001/jama.2016.19563
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