Function of the striated anal sphincter during straining in control subjects and constipated patients with a radiologically normal rectum or idiopathic megacolon.

The function of the striated anal sphincter during defaecation straining was recorded by manometry and electromyography (E.M.G.) in 31 constipated patients who were unable to expel a water-filled rectal balloon with effort. This group was divided on the basis of measurement of colonic diameter into those with megacolon and those with a normal sized colon. The latter group was further divided into those with normal transit and those with slow transit. The results were compared with those recorded from 15 control subjects with normal bowel function. With straining, anal pressure fell in 12 of 15 controls while in 3 it increased. In 30 of 31 constipated patients, anal pressure rose paradoxically with straining. Electromyographic recording in controls during straining demonstrated decreased activity in 5, in 4 no change and in 5 an increase in activity. In 28 of 31 constipated patients E.M.G. activity increased with straining. These results suggest that external sphincter contraction during straining occurs in some normal subjects but more frequently among patients with constipation of different types.