Clinical examination, endosonography, and MR imaging in preoperative assessment of fistula in ano: Comparison with outcome-based reference standard

PURPOSE: To prospectively evaluate the relative accuracy of digital examination, anal endosonography, and magnetic resonance (MR) imaging for preoperative assessment of fistula in ano by comparison to an outcome-derived reference standard. MATERIALS AND METHODS: Ethical committee approval and informed consent were obtained. A total of 104 patients who were suspected of having fistula in ano underwent preoperative digital examination, 10-MHz anal endosonography, and body-coil MR imaging. Fistula classification was determined with each modality, with reviewers blinded to findings of other assessments. For fistula classification, an outcome-derived reference standard was based on a combination of subsequent surgical and MR imaging findings and clinical outcome after surgery. The proportion of patients correctly classified and agreement between the preoperative assessment and reference standard were determined with trend tests and kappa statistics, respectively. RESULTS: There was a significant linear trend ( P <.001) in the proportion of fistula tracks ( n = 108) correctly classified with each modality, as follows: clinical examination, 66 (61%) patients; endosonography, 87 (81%) patients; MR imaging, 97 (90%) patients. Similar trends were found for the correct anatomic classification of abscesses ( P <.001), horseshoe extensions ( P <.003), and internal openings ( n = 99, P <.001); endosonography was used to correctly identify the internal opening in 90 (91%) patients versus 96 (97%) patients with MR imaging. Agreement between the outcome-derived reference standard and digital examination, endosonography, and MR imaging for classification of the primary track was fair ( kappa = 0.38), good (kappa = 0.68), and very good (kappa = 0.84), respectively, and fair (kappa = 0.29), good (kappa = 0.64), and very good (kappa = 0.88), respectively, for classification of abscesses and horseshoe extensions combined. CONCLUSION: Endosonography with a high-frequency transducer is superior to digital examination for the preoperative classification of fistula in ano. While MR imaging remains superior in all respects, endosonography is a viable alternative for identification of the internal opening.