Abstract

BACKGROUND: The optimal imaging modality of obstetric anal sphincter injuries (OASIs) needs to take into consideration convenience, availability and ability to assess the sphincter morphology. Endoanal ultrasound is currently regarded as the reference standard but is not widely available in obstetric units. Exoanal alternatives exist, such as three-dimensional (3D) introital or transperineal ultrasound, which are already readily available in most obstetrics and gynecology units. OBJECTIVES: The primary objective was to evaluate the diagnostic accuracy of 3D introital and 3D transperineal ultrasound compared to 3D endoanal ultrasound 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 diagnosis of anal sphincter defect on imaging to symptoms of anal incontinence, and to assess patient discomfort experienced for each imaging modality STUDY DESIGN: A cross-sectional study of 250 women who sustained OASIs, all underwent 3D introital, transperineal and endoanal ultrasound. Introital and transperineal ultrasound were assessed using tomographic ultrasound imaging. All 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 as reference standard. Diagnostic test characteristics of introital and transperineal ultrasound using these optimal cut-offs were calculated. RESULTS: Optimal cut-off for a significant external anal sphincter defect was ≥3/7 slices; sensitivity and specificity were 0.65 and 0.75 on introital and 0.70 and 0.69 on transperineal ultrasound respectively. Optimal cut-off for a significant internal anal sphincter defect was ≥2/5 slices; sensitivity and specificity were 0.59 and 0.84 on introital and 0.43 and 0.97 on transperineal ultrasound. The Area Under the Curve for diagnosing external and internal anal sphincter defects ranged from 0.70 - 0.74 (p<0.001) for introital and transperineal. Positive predictive value for external and internal sphincter defects ranged from 0.37-0.63 and negative predictive value ranged from 0.85-0.93 for transperineal and introital ultrasound. Endoanal ultrasound was the only modality for a defect to correlate with symptoms; mean modified St Mark's score 2.4 (SD 4.1) for defect sphincter and 0.9 (SD 2.7) for intact sphincter (p<0.01). Introital and transperineal ultrasound were associated with less discomfort than endoanal ultrasound. CONCLUSION: Endoanal ultrasound remains the most accurate diagnostic imaging modality. With low positive predictive values, introital and transperineal ultrasound are not suitable for identifying sphincter defects; however high negative predictive values show a good ability to detect an intact sphincter. The optimal cut-off number of slices on tomographic ultrasound imaging for external and internal anal sphincters allows for standardisation of a significant defect. In women with a history of OASI, introital and transperineal ultrasound are suitable to screen for an intact sphincter if endoanal ultrasound is not available. Women with defects seen should then have endoanal ultrasound to verify the diagnosis.