Intraabdominal abscess caused by actinomycosis in a patient with mesenteric fibromatosis of the small intestine: report of a case.

A 38-year-old man presented to our Emergency Department with acute severe abdominal pain; 3 days after, a mesenteric mass had been detected by abdomino-pelvic computed tomography. Emergency laparotomy revealed a mesenteric mass with focal surface rupture. Microscopically, the mesenteric mass was composed of fibroblast-like spindle cells with intervening marked collagen deposits. These spindle cells were positive for nuclear β-catenin and negative for CD34, c-kit, smooth muscle actin, and S-100 protein. We diagnosed the mesenteric lesion as deep fibromatosis, consistent with mesenteric fibromatosis (MF). Serial sections of the ruptured portion showed abscess formation with a peripheral radiating pattern of microcolonies, proven to be actinomycosis by Grocott's methenamine silver stain and gram staining. One glandular fragment was found in the adhered muscle layers and a foreign body reaction was seen within the abscess cavity. We speculate that MF involved the serosal adhesions between the adjacent bowel walls and the subsequent fibrous pulling might have created the connection between the bowel lumen and the bowel wall. This disrupted barrier led to the Actinomyces infection. To our knowledge, this is the first report of sporadic MF leading to the formation of an abscess cavity with rupture and actinomycosis.