Re: Comparison of diagnostic criteria for significant anal sphincter defects between endoanal and transperineal ultrasound

We read with interest the article of Okeahialam et al. comparing the diagnostic criteria for significant anal sphincter defect between endoanal (EAUS) and transperineal (TPUS) ultrasound1. The authors found that TPUS shows excellent agreement with EAUS in the identification of defects of the external anal sphincter. Given that the former method is non-invasive and that suitable equipment is much more widely available, this suggests that EAUS is obsolete for the diagnosis of maternal birth trauma. However, we have concerns about the conclusion of the paper that a cut-off angle of 30◦ should not be used for TPUS in the diagnosis of a significant residual anal sphincter defect. The issue of how to define a ‘significant’ defect of the external anal sphincter remains unresolved. We previously attempted to validate the 30◦ criterion for translabial ultrasound, but failed2, largely due to the limited association between anal sphincter trauma and anal incontinence. We were successful in defining similar criteria for levator trauma, likely due to a stronger association between defects and morbidity and the lack of postnatal repair which is a major confounder. Anal sphincter defect angle needs to be validated against the symptom of anal incontinence which, to our knowledge, has not been done successfully for any imaging method used for the assessment of the anal sphincter. We consider it inappropriate to ‘validate’ anal sphincter defect angle obtained on TPUS against defect angles obtained on EAUS. Since an endoprobe distorts and distends the anal canal, the measurements will not match. This does not invalidate defect angle measurements on exoanal ultrasound, especially when EAUS criteria have not been properly validated. There are some additional issues related to the study which we would like to raise. The authors seem to compare apples with oranges, in that the largest angle size of any identified anal sphincter defect was used for analysis. This is not the methodology described for exoanal ultrasound of the anal sphincter3 and, to our knowledge, this measure has not been tested against symptoms of anal incontinence. As an aside, the provided TPUS illustrations do not comply with international standardization criteria4. It also seems unusual to us that Dr Ka Woon Wong who analyzed the TPUS images is not included in the list of authors. Authorship is about taking responsibility, therefore, how is it possible that the authors take responsibility for assessments they did not in fact perform? Finally, Okeahialam et al. suggest they can distinguish between a gap in the muscle due to missed obstetric anal sphincter injury and a scar after anal sphincter repair. Large defects, especially if they appear symmetrical and undistorted, suggest a missed tear, but it is expected that there will be a collagenous bridge between the muscle ends regardless of whether repair was highly successful, poor or was not performed. Of course, there will be a non-contractile scar of varying distensibility. In summary, the study of Okeahialam et al. shows that anal sphincter defect angles vary between EAUS and TPUS, which is not unexpected. However, their findings do not support conclusions regarding which method is more predictive or useful in clinical practice.