EP692 - Usefulness of endoanal ultrasound in the characterization of anal fistulas.

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Purpose/Background: Endoanal ultrasound is a fundamental imaging study in the evaluation of anal fistulas, providing a detailed and accurate visualization of the anal canal anatomy and sphincter complex. Currently, it is considered one of the methods of choice for classifying and evaluating anal fistulas.
The accuracy of endoanal ultrasound (EAU) in identifying complex fistulas has been demonstrated in multiple studies, reporting sensitivity rates of up to 90% and specificity rates above 85%. EAU allows for the evaluation and assessment of the sphincter complex muscles, essential for minimizing postoperative fecal incontinence, by providing precise information on their integrity and the involvement of the external and internal sphincters. Additionally, it facilitates the detection of secondary tracts that may not be evident on clinical examination or conventional imaging studies. Its ability to differentiate between simple and complex fistulas is crucial for guiding therapeutic decisions and selecting the relevant surgical treatment, aiming to improve clinical outcomes and minimize recurrence rates.

Methods/Interventions: Descriptive, analytical cross-sectional, observational and retrospective study.

Results/Outcomes: A total of 35 patients were evaluated, 24 (68.6%) males and 11 (31.4%) females, with a mean age of 43.74 years (range 28-73 years, SD 10.5 years). The type of fistula was classified according to different aspects. Clinically, the most frequent types were 14 (40%) low transsphincteric fistulas, 13 (37.1%) high transsphincteric fistulas, and 5 (14.3%) intersphincteric fistulas. Using endoanal ultrasound (EAU), the classification was 19 (54.3%) low transsphincteric fistulas, 11 (31.4%) intersphincteric fistulas, and 5 (14.3%) high transsphincteric fistulas.
The internal fistula opening location was determined using three views: 360-degree, biplanar, and surgical findings. In the 360-degree view, the most frequent locations were posterior in 18 (51.4%) cases, anterior in 7 (20%) cases, and left anterior in 4 (11.4%) cases. Using the biplanar view, the locations were mid-high third in 20 (57.1%) cases, mid-low third in 13 (37.1%) cases, and upper third in 2 (5.7%) cases. Surgically, the most frequent locations were anterior to 1 cm, posterior to 1 cm and to 2 cm in 5 (14.3% each) cases.
EAU revealed cavities in 10 (28.6%) patients and blind tracts in 14 (40%) patients. Additionally, 15 (42.9%) patients had a history of previous fistula surgery, and 20 (57.1%) patients had seton placement. During surgery, 16 (45.7%) patients had high transsphincteric fistulas, 11 (31.4%) patients had low transsphincteric fistulas, and 8 (22.9%) patients had intersphincteric fistulas. Furthermore, 9 (25.7%) patients had fistulous cavities, and 12 (34.3%) patients had blind tracts.

Conclusion/Discussion: Endoanal ultrasound in our patient series showed a concordance rate above 70%. However, the sample size is insufficient to make decisions regarding intersphincteric fistulas. For transsphincteric fistulas, it is essential to report the distance to the internal fistula opening. Endoanal ultrasound is a highly sensitive and specific, low-cost technique for decision-making in anal fistulas. We consider that biplanar ultrasound should be used to better characterize the distance of the internal fistula opening for transsphincteric fistulas.