Ascending complex anal fistula secondary to lower extremity soft tissue infection: a case report

Background Complex anal fistulas remain a surgical challenge due to their intricate anatomy, high recurrence rates, and potential for sphincter injury. We report a rare case of complex anal fistula with multiple external openings extending from the gluteal region to the thigh root, successfully managed by combined incision with drainage tube placement. Case presentation A 57-year-old man presented with recurrent purulent discharge from the perianal region and left lower limb for three months following debridement of a lower extremity soft tissue infection. Physical examination revealed multiple unhealed wounds on the left buttock, thigh root, and lower limb with purulent exudation. A cord-like sinus tract was palpable in the left perianal region extending into the anal canal and communicating with the buttock wound. Pelvic contrast-enhanced MRI demonstrated multiple enhancing sinus tracts within the left gluteal subcutaneous tissue and gluteus maximus muscle, extending superomedially to the posterior anal aspect and inferolaterally along the muscle surface to the dorsal thigh root. Fistulography confirmed communication between the left buttock fistula and the rectum, with contrast entering the rectal lumen approximately 2.0 cm from the anal verge. Colonoscopy revealed two NICE type 2 polyps in the descending and sigmoid colon (3 mm × 3 mm and 6 mm × 7 mm), which were removed by endoscopic mucosal resection; pathology confirmed tubular adenomas. The patient underwent fistulotomy with drainage tube placement for the high trans-sphincteric tract extending to the supralevator space, along with partial sinus tract excision and drainage tube insertion. Intraoperative blood loss was approximately 10 mL. The postoperative course was uneventful, and at six-month follow-up, the wounds had completely healed with no signs of recurrence and normal anal sphincter function. Conclusion This rare case of complex anal fistula with multiple external openings and deep extension to the thigh root highlights the critical role of multimodal imaging (MRI and fistulography) in preoperative planning. Combined incision, drainage, and drainage tube placement achieved successful outcomes with preserved sphincter function.