Comparing the diagnostic accuracy of 3 ultrasound modalities for diagnosing obstetric anal sphincter injuries

The optimal imaging modality of obstetric anal sphincter injuries needs to take into consideration convenience, availability, and ability to assess the sphincter morphologic condition. Endoanal ultrasound imaging currently is regarded as the reference standard, but it is not widely available in obstetric units. Exoanal alternatives exist, such as 3-dimensional introital or transperineal ultrasound imaging, which are already readily available in most obstetrics and gynecology units. The primary objective was to evaluate the diagnostic accuracy of 3-dimensional introital and 3-dimensional transperineal ultrasound imaging compared with 3-dimensional endoanal ultrasound imaging as the reference standard for the detection of anal sphincter defects in women who sustained obstetric anal sphincter injuries. The secondary objective was to correlate a diagnosis of anal sphincter defect on imaging to symptoms of anal incontinence, and to assess patient discomfort that is experienced for each imaging modality. A cross-sectional study was conducted of 250 women who sustained obstetric anal sphincter injuries, all of whom underwent 3-dimensional introital, transperineal, and endoanal ultrasound imaging. Introital and transperineal ultrasound imaging were assessed with tomographic ultrasound imaging. All of the women completed a validated modified St Mark’s Score and Visual Analogue Score for discomfort. Optimal cut-off values for a significant defect on tomographic ultrasound imaging were defined as those with the greatest sensitivity and specificity based on receiver operating characteristic curves with endoanal ultrasound imaging as the reference standard. Diagnostic test characteristics of introital and transperineal ultrasound imaging were calculated with the use of these optimal cut-offs. Optimal cut-off for a significant external anal sphincter defect was ≥3 of 7 slices; sensitivity and specificity were 0.65 and 0.75 on introital imaging and 0.70 and 0.69 on transperineal ultrasound imaging. Optimal cut-off for a significant internal anal sphincter defect was ≥2 of 5 slices; sensitivity and specificity were 0.59 and 0.84 on introital imaging and 0.43 and 0.97 on transperineal ultrasound imaging. The area under the curve for the diagnosis of external and internal anal sphincter defects ranged from 0.70–0.74 (P