Multiparametric Transperineal Ultrasound Features of Perianal Fistulas in Pediatric Crohn's Disease

OBJECTIVES: To evaluate and compare the multiparametric transperineal ultrasound (TPUS) features of perianal fistulas at baseline and upon achieving magnetic resonance imaging (MRI)-confirmed radiologic healing in pediatric Crohn disease. METHODS: This retrospective study included 24 children (22 boys and 2 girls; mean age, 13.0 ± 2.3 years [range, 9-19]) with 45 perianal fistulas evaluated between January 2019 and July 2025. All patients with fistulas who underwent at least 2 TPUS and pelvic MRI assessments and achieved complete healing on MRI were enrolled. Baseline and follow-up TPUS images were evaluated in consensus by 2 radiologists for multiple parameters, including diameter, echogenicity relative to the internal anal sphincter (IAS), vascularity, internal pattern (layered versus homogeneous), anisotropy, presence of pus, air bubbles, abscesses, and adjacent inflammation. Changes in these findings were compared using paired t-tests and McNemar tests. RESULTS: At baseline (n = 45), most fistulas demonstrated higher echogenicity than that of the IAS (73%), exhibited a layered pattern (80%) and internal vascularity (94%) and frequently showed pus (40%) and adjacent inflammation (78%). Despite achieving complete MRI-defined radiologic healing, perianal fistulas remained detectable by TPUS in the majority of cases (60%, 27/45), while the other 18 fistulas (40%) were no longer detectable. However, the 27 persisting fistulas demonstrated significant multiparametric evolution, including reductions in diameter (p < .001), loss of the layered pattern (p < .001), increased anisotropy (p < .001), decreased vascularity (p = .002), and resolution of pus (p = .001) and adjacent inflammation (p < .001). CONCLUSION: Radiologic healing in pediatric perianal Crohn disease is characterized by distinct multiparametric TPUS changes, including diameter reduction, loss of layered pattern, emergence of anisotropy, and resolution of inflammatory markers. While TPUS serves as a valuable adjunct to MRI for assessing treatment response, the persistent sonographic detectability of fistulas despite MRI-defined healing warrants further validation in larger cohorts and long-term follow-up studies.