Pilonidal sinus disease: MR imaging distinction from fistula in ano

PURPOSE: To describe magnetic resonance (MR) imaging features in patients with proved pilonidal sinus disease and to compare these features with those in a matched group of patients with proved fistula in ano to determine the accuracy with which MR imaging can be used to distinguish between the two diseases. MATERIALS AND METHODS: Seven patients with pilonidal sinus disease underwent MR imaging with a body coil. The site and morphology of sepsis were noted, with particular reference to natal cleft sepsis and deep-seated sepsis, including intersphincteric anal canal sepsis and any enteric communication. Comparison was made with 14 age- and sex-matched patients with fistula in ano. Categoric frequencies were compared to calculate differences between the groups and sensitivities, specificities, and predictive values. RESULTS: All patients with pilonidal sinus had natal cleft sepsis, but five (71%) had sepsis at deep-seated sites more characteristic of fistula in ano. Eight patients with fistula in ano (57%) had natal cleft sepsis that was thought characteristic of pilonidal sinus. No patient with pilonidal sinus had intersphincteric sepsis or an enteric communication, in contrast to all patients with fistula in ano having both (P < .001). Natal cleft sepsis reached the subcutaneous tissues overlying the coccyx and sacrum in only one patient with fistula (7%), in contrast to six (86%) with pilonidal sinus (P < .001). MR imaging had a sensitivity of 86% (six of seven), specificity of 100% (14 of 14), positive predictive value of 100% (six of six), and negative predictive value of 93% (14 of 15) for diagnosis of pilonidal sinus disease. CONCLUSION: MR imaging features of perianal and deep-seated sepsis, characteristic of fistula in ano, are also found in patients with pilonidal sinus, but the absence of intersphincteric sepsis or enteric opening allows reliable MR imaging distinction between the two. ((C)) RSNA, 2003.