The transient relaxation of the internal anal sphincter in response to rectal distention is believed to play an important role in the continence mechanism. Most anorectal physiology laboratories merely report the rectoanal inhibitory reflex as being either present or absent. This study aimed to assess the parameters of the rectoanal inhibitory reflex in incontinent and constipated patients and healthy control subjects, in an attempt to analyze differences in internal anal sphincter function in these groups. We analyzed each response of the internal anal sphincter to rectal distention with progressively increasing volumes of air at a single site (proximal anal canal).
Fifty-five constipated and 99 incontinent patients and healthy control subjects underwent manometry. Various parameters of the rectoanal inhibitory reflex were analyzed, and percentage sphincter relaxation was calculated at each volume at which rectoanal inhibitory reflex occurred.
There was no difference in the volume of rectal distention required to elicit sensation (P = 0.626) or the rectoanal inhibitory reflex (P = 0.371) in the three groups. There was a significant correlation between the volume required to elicit the rectoanal inhibitory reflex and that at which sensation was first felt only in the incontinent (P = 0.0001) group. Significantly greater sphincter relaxation was seen at each volume (P = 0.001) in the incontinent as compared with the constipated patients. With progressive rectoanal inhibitory reflex, consistently progressive increases in internal anal sphincter relaxation were found only in the incontinent group. This consistent relationship was not seen in the constipated patients or in healthy control subjects.
Assessment of various parameters of the rectoanal inhibitory reflex yielded important information regarding the continence mechanism. Altered responses of the internal anal sphincter in anorectal disorders plays a role in the associated physiologic impairment. This may have significant clinical implications with regard to sphincter-saving resections.