Diagnostic accuracy of MRI for preoperative mapping of anal fistula: a prospective study of 204 patients with surgical correlation.

Abstract

Background: Anal fistula is a chronic abnormality of communication between the anal canal or rectum and the perianal skin. Accurate preoperative assessment is essential for effective operative planning, reducing fistula recurrence and preservation of anal sphincter function. Magnetic resonance imaging (MRI) is now a reliable tool for assessing fistula anatomy and complexity. This study aimed to classify anal fistulas using the Parks, St. James's University Hospital (SJUH), and Garg classification systems, and to evaluate the diagnostic accuracy of MRI compared with surgical findings.

Methods: A prospective study was conducted on 204 patients with a clinical diagnosis of anal fistulas who underwent preoperative MRI and surgery at Hue Central Hospital and Hue University of Medicine and Pharmacy Hospital between October 2022 and December 2024. MRI findings of primary tracts and extensions were recorded. Diagnostic accuracy parameters were calculated using surgical findings as the gold standard.

Results: Among 204 patients (185 men and 19 women), 219 primary fistulous tracts were identified, of which Parks type II (70.3%), SJUH grades IV (40.6%) and Garg grades IV (37.4%) were most frequent. Diagnostic accuracy was excellent for primary tract (97.5%), secondary tract (95.1%), abscess (92.6%), and horseshoe extension (98.0%). The agreement with surgical results was almost perfect for secondary tracts (κ = 0.828) and abscesses (κ = 0.825). MRI also maintained high diagnostic performance in recurrent fistulas, with overall accuracies ranging from 91.4% to 98.6%.

Conclusion: MRI is a highly accurate and valuable imaging modality for anal fistula. It allows comprehensive classification and detailed mapping of primary tracts and extensions. MRI provides comprehensive preoperative mapping and may facilitate individualized surgical planning, particularly for complex or recurrent anal fistulas.