Background: During our study of the defecation mechanism, we found that the external anal sphincter contracted not only upon rectal inflation but also upon deflation. As this reflex relationship was reproducible, it was studied to elucidate its clinical significance in the light of its function.
Methods: A catheter with a condom at its end was introduced into the rectum of 16 healthy volunteers (mean age 45.2 years). The EMG response of the external anal sphincter to rapid rectal inflation and deflation was studied by means of a concentric needle electrode inserted into the muscle. The procedure was repeated in eight subjects after sphincter infiltration with xylocaine or saline.
Results: The external sphincter contracted twice on rectal distension: once on inflation and another time on deflation. The amplitude increased and latency decreased with increasing rectal inflation, while neither was affected on deflation. The anesthetized sphincter did not respond, while the saline-infiltrated sphincter responded to rectal distension.
Conclusions: The deflation reflex functions to interrupt or terminate the act of defecation. It may prove of diagnostic significance in defecation disorders.