Mri in evaluating atrophy of the external anal sphincter in patients with fecal incontinence maaike p. the smooth muscle internal anal sphincter (ias) is primarily responsible for the anal resting tone, contributing 85 of the anal resting pressure. The striated external anal sphincter (eas) makes a small contribution towards the resting anal pressure, but is primarily responsible for the voluntary contraction of the anal sphincter 11. The value of external anal sphincter electromyography for the diagnosis of multiple system atrophy. Author information (1)service de neurologie, inserm u-330, centre hospitalier universitaire, bordeaux, france. The term sphincter atrophy refers mostly to external anal sphincter (eas) atrophy, as the eas is the most important factor for maintaining fecal continence. Eas atrophy, often due to pudendal neuropathy caused by stretch injury during childbirth or chronic constipation 2, 3, is an important cause of fecal incontinence. Mri in evaluating atrophy of the external anal sphincter in patients with fecal incontinence. Terra mp(1), beets-tan rg, van der hulst vp, deutekom m, dijkgraaf mg, bossuyt pm, dobben ac, baeten cg, stoker j. Atrophy of the external sphincter is easily detected on coronal mr images by comparing the thicknesses of all anal muscles. Endoanal mr imaging is superior to endoanal ultrasonography because of the multiplanar capability and higher inherent contrast resolution of the former. , the ani externus, sphincter ani internus, and externalinternal anal sphincters also benefit in relaxing the sphincter muscles you have to tighten and squeeze the anus at the time of lying down, sitting or standing. Comparative study on diagnostic significance of urethral sphincter versus external anal sphincter electromyography in patients with multiple system atrophy. The thickness of the anal sphincter muscles and the length of the anal canal were measured, and age- and sex-related correlations were studied. Results sex-related differences included a significantly shorter external sphincter in women than in men both laterally (mean, 27.).