Magnetic resonance imaging in the workup of patients with perianal fistulas

BackgroundPerianal sepsis and fistulas are common lesions. Fistula-in-ano is a tedious problem because of high recurrence rates and severe unexpected complications postoperatively. Many imaging modalities are used to evaluate this area. Magnetic resonance imaging (MRI) has been considered the gold standard procedure for perianal fistula assessment, i.e., it provides the surgeon with an accurate roadmap to select the best surgical approach, determines the extent of sphincter division, and estimates the risk of postoperative incontinence. We carried out a prospective diagnostic accuracy study involving 50 patients (mean age, 42.44 years) with perianal fistulas who underwent pelvic MRI with a 1.5 Tesla scanner using multiple sequences, including diffusion and post-contrast series that were either local or systemic. This study aimed to evaluate the diagnostic validity of pelvic MRI/MR fistulogram in perianal fistula assessment, identify perianal fistula-associated findings by MRI, and optimize the MRI technique with maximum technical safety.ResultsIntersphincteric fistulas were the most common type of perianal fistula observed based on Parks' classification. Intersphincteric, trans-sphincteric, extrasphincteric, and suprasphincteric fistulas were found in 30 (60%), 12 (24%), 4 (8%), and 4 (8%) patients, respectively. According to St. James' classification, 21 (42%), 9 (18%), 8 (16%), 4 (8%), and 8 (16%) patients had perianal fistula grades I, II, III, IV, and V, respectively. The combination of variable MR sequences and MR fistulogram increased the diagnostic validity of MRI examinations. Our results correlated with surgical results (reference standard) with perfect interobserver reliability.ConclusionsOf all imaging modalities, MRI has become a prerequisite for a successful surgery of a perianal fistula. MRI can identify: (a) fistula morphological details, (b) the relationship between the fistulous tract and the anal sphincter, (c) fistula wound healing, (d) an active versus chronic scarred fistula, (e) postoperative stigmata, and (f) a perianal fistula from its mimics. MRI with variable sequences and MRI fistulogram are successful combinations that increase diagnostic efficiency with technical safety by avoiding both ionizing radiation and systemic gadolinium.