Phlegmon, Not Abscess: Multimodal Transperineal Ultrasound Altering Surgical Approach in a Case of Complex Perianal Fistulizing Crohn's Disease

Complex perianal Crohn's disease often requires functional assessment beyond anatomy. We report a 30-year-old man with transsphincteric fistulae and supralevator extension. Point-of-care transperineal ultrasound with contrast and shear-wave elastography showed avid rim enhancement and mixed stiffness in a left ischioanal lesion, favoring phlegmon over liquefied abscess. Magnetic resonance imaging confirmed complex tracts; endoscopic ultrasound deemed internal drainage unsafe. On examination under anesthesia, a seton was placed, and only a limited incision was performed, yielding minimal pus. Multimodal transperineal ultrasound provided real-time vascular and tissue characterization, altered surgical planning, and avoided unnecessary wide drainage while complementing MRI.