Which method is best for imaging of perianal fistula?

Background: Successful surgery for perianal fistula is contingent upon accurate pre-operative classification of the primary tract and its extensions. We aimed to find, using "evidence based medicine" (EBM) methods, the optimal technique for fistula classification: MRI, anal endosonography (AES) or clinical examination. Methods: A clinical question was derived, "In patients suspected of having perianal fistula, how does MRI compare to AES and clinical assessment for discriminating simple from complex disease". A search of primary literature and secondary evidence resources was performed and expert opinion sought. Inclusion criteria were blinded prospective studies (level 2b +) of patients undergoing preoperative MRI, clinical examination +/- AES using a clinical outcome based reference standard. Retrieved literature was appraised using EBM methods. Results: The highest-ranking evidence found was level 1b. MRI is more sensitive 0.97(CI 0.92-1.01) than clinical examination, 0.75(0.65-0.86) but comparable to AES, 0.92(0.85-0.99) for discriminating complex from simple disease. The positive LR for MRI confirming complex disease is 22.7 compared to 2.1 and 6.2 for clinical examination and AES, respectively. Conclusion: MRI is the optimal technique for discriminating complex from simple perianal fistula, although AES is superior to clinical examination, and may be used if MRI availability is restricted.