Perianal Sonographic Approach to Fistula-in-Ano and Perianal Abscesses: A Pictorial Review

Fistula-in-ano and perianal abscesses are common conditions in pediatric patients. Recently, the perianal ultrasound approach has been reported as a useful method for evaluating anal and perianal lesions. However, this technique remains unfamiliar to many sonographers. In this pictorial review, we aimed to describe the basic sonographic approach for assessing fistula-in-ano and perianal abscesses. A sagittal view of the anal canal is obtained by placing the probe on the proctodeum parallel to the intergluteal cleft, providing a longitudinal image of the anus and rectum. The probe is then gently swung from right to left to identify anatomical structures and detect fistula-in-ano or perianal abscesses. When necessary, the probe is rotated perpendicular to the gluteal cleft to obtain additional transverses, offering further diagnostic information. An abscess appears as a hypoechoic (low-echoic) mass surrounded by echogenic subcutaneous tissue and a hypervascular rim due to inflammation. The presence of a fluid-filled, low-echoic mass indicates pus accumulation, prompting surgical incision. The fistula-in-ano typically extends from a rectal crypt to the dermis, and abscess formation may occur along this tract. Perianal ultrasonography is a valuable, noninvasive tool for evaluating perianal lesions, and sonographers should become familiar with this approach. Throughout the procedure, patient comfort and pain management are essential, and further diagnostic imaging or intervention may be warranted when necessary.