Assessment of external anal sphincter morphology in idiopathic fecal incontinence with endocoil magnetic resonance imaging.

The failure of external anal sphincter repair may relate to sphincter atrophy where muscle fibers are replaced by fat, seen on MRI due to the differing signals returned by fat and muscle tissue. Manometry, electrophysiology, and MRI with an endocoil were performed on 34 fecally incontinent patients with intact sphincters on endosonography. The area of the external sphincter was measured in the midcoronal plane, and the percentage fat content calculated. Sphincter muscle area correlated strongly with squeeze pressure (P < 0.001) but not with percentage fat content. There was no relationship between percentage fat and age, weight, anal sensation, squeeze pressure, sphincter length or width, or pudendal nerve terminal motor latency. There was a trend for smaller sphincters to contain a higher percentage fat content (P = 0.059). MRI has established a relationship between function and external sphincter bulk, but not fat content, although smaller muscles may contain more fat.