Which method is best for imaging of perianal fistula?

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.

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.

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.

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.