Relationship between sphincter morphology on endoanal MRI and histopathological aspects of the external anal sphincter

Atrophy of the external anal sphincter can be shown only on endoanal magnet ic resonance imaging (MRI). Until now no study has compared the morphologic al endoanal MRI findings with histopathological aspects of the external ana l sphincter. The aim of this study was to validate the MRI interpretation o f the external anal sphincter using histology as a "gold standard." In this prospective study 25 consecutive unselected women (median age 48 years, ra nge 27-72) with fecal incontinence due to obstetric trauma were assessed pr eoperatively with endoanal MRI. All patients underwent anterior sphincterop lasty within 6 months of the preoperative assessment. During sphincter repa ir, a biopsy specimen was taken both from the left and right lateral parts of the external anal sphincter. Interpretation of MRI was performed by one of the radiologists (J.S.), and biopsy specimens were evaluated by the path ologist (W.J.M.). Both were blinded to the interpretation of the other. MRI revealed external anal sphincter atrophy in 9 of the 25 patients (36%). Hi stopathological investigation confirmed these findings in all but one. In o ne additional patient atrophy was detected on histological investigation wh ile the morphology of the external anal sphincter was classified as normal on MRI. In detecting sphincter atrophy endoanal MRI showed 89% sensitivity, 94% specificity, 89% positive predictive value, and 94% negative predictiv e value. MRI correctly identified sphincter morphology in 23 of 25 cases (9 2%). This study demonstrates that endoanal MRI accurately identifies normal and abnormal external anal sphincter morphology. Endoanal MRI is therefore a valuable preoperative diagnostic tool.