Five comparative studies met the inclusion criteria (n=347, range 18 to 237): four studies were prospective and one was retrospective. Three studies compared both ultrasound and MRI to surgery and each other; two studies compared ultrasound with MRI only.
Surgery was better than ultrasound (RR 0.88, 95% CI 0.84 to 0.92; three studies, 61 patients) and MRI (RR 0.75, 95% CI 0.66 to 0.84; three studies, 61 patients) for detecting external anal defects. When ultrasound was compared directly with MRI, there was no overall difference between the technologies (RR 1.01, 95% CI 0.93 to 1.09; five studies, 347 patients). MRI was more accurate than ultrasound in patients where defects were validated by surgery (RR 0.83, 95% CI 0.77 to 0.89; three studies, 61 patients). Highly statistically significant heterogeneity was observed for all analyses. Using surgery as the reference standard, sensitivity was approximately 88% for ultrasound and 75% for MRI.