Infracoccygeal/transperineal window: new method to prenatally diagnose and classify level of anal atresia.

Objectives: To introduce a two-dimensional sonographic method to assess the fetal anus, and to evaluate the feasibility of this method to diagnose anal atresia prenatally and identify the presence or absence of anoperineal fistula (in males) and anovestibular fistula (in females).

Methods: This was an observational study of suspected cases of anal atresia referred to a single center in Israel between August 2018 and October 2023. In addition to conventional evaluation of the perineum in the axial plane, fetuses referred to our center for suspected malformation were scanned with a new method termed the 'infracoccygeal/transperineal window'. This window consisted of a midsagittal view of the fetal pelvis, including the distal rectum and the anal canal. Normal anatomy was confirmed when the anal canal was continuous with the rectum and terminated at the expected location on the perineum. In female fetuses, the normal anal canal runs parallel to the vaginal canal and diverges posteriorly, terminating at the perineal skin, distant from the vestibule. In male fetuses, the normal anal canal diverges posteriorly in relation to the corpora cavernosa, terminating at the perineal skin, distant from the scrotum. High anal atresia was identified when a blind-ending rectal pouch was demonstrated in the pelvis without a fistula to the perineum or vestibule. Low anal atresia was determined when a rectal pouch was continuous with an anteriorly deflected fistula. In females, the fistula converges with the vaginal canal, terminating at the vestibule; in males, the fistula deflects anteriorly, terminating at the base of the scrotum. Postnatally, the diagnosis and type of anal atresia were confirmed through physical examination with direct visualization of the fistula, radiographic studies, surgical examination and/or postmortem autopsy.

Results: Of the 16 fetuses diagnosed prenatally with anal atresia, eight were suspected to have low anal atresia and eight were suspected to have high anal atresia. The median gestational age at diagnosis was 23 (range, 14-37) weeks. All cases showed additional structural malformation. Eleven patients opted for termination of pregnancy, of which four had low anal atresia and seven had high anal atresia. Postnatal confirmation was not available in four cases due to curettage-induced mutilation or in-utero degradation following selective termination of the affected twin, leaving 12 cases for analysis, of which seven were diagnosed with low anal atresia and five with high anal atresia. In these 12 cases, all prenatal diagnoses were confirmed as correct, rendering 100% sensitivity and 100% specificity in this high-risk fetal population.

Conclusions: The infracoccygeal/transperineal window is an effective method to detect and classify the level of anal atresia prenatally. This may improve prediction of postnatal fetal continence and optimize prenatal counseling. © 2024 International Society of Ultrasound in Obstetrics and Gynecology.