The role of magnetic resonance imaging in the preoperative assessment of anorectal anomalies.

The role of magnetic resonance imaging (MRI) compared to other investigations in providing information relevant to and affecting surgical management in patients with anorectal anomalies (ARA) was studied in two groups of patients. In group 1 a pilot study was done in 9 patients with colostomy for high ARA awaiting posterior sagittal anorectoplasty. These patients had MRI, plain radiographs, a distal loopogram through the mucous fistula, ultrasound scans of the spinal cord and urinary tract, and a micturating cystourethrogram. The level of bowel and spinal-cord termination was correctly determined in all patients by MRI and conventional investigations. The presence or absence of a fistula, urological abnormalities, and bony abnormalities of the pelvis and spine were better visualized by conventional investigations than with MRI. The striated-muscle complex (SMC) was well-visualized by MRI, the thickness accurately graded, and the findings confirmed at operation. The information on muscle thickness and the presence or absence of a fistula was interesting, but played no part in decisions made about surgical management, and the operative technique was not altered. In group 2 the patients were much older and had MRI for persistent problems, mainly soiling; 6 had low lesions and 6 had high lesions. Three patients with low lesions were found to have intraspinal abnormalities on MRI; 4 were found to have malpositioned bowel in the SMC, which was again accurately visualized and graded. Therefore, in 7/12 patients of this group a surgically correctable abnormality was found. We conclude that MRI has no role as a primary investigation in patients with high ARA, but it is useful if there are suspected spinal-cord problems or persistent problems after definitive surgery.