Local thermal stimulation relaxes hypertonic anal sphincter - Evidence of somatoanal reflex

PURPOSE: Although it is generally believed that warm perineal baths reduce pain resulting from anal fissure, complicated hemorrhoids, or anal surgery, the exact mechanisms remain unclear. Because hypertonicity of the internal anal sphincter contributes to increasing pain in these conditions, it has been postulated that warm perineal baths could help to relax the anal sphin cter, hence reducing pain. It is our purpose to demonstrate response of the anal sphincter to local thermal stimulation via a somatoanal reflex. METHO DS: Continuous anorectal manometry tracings were obtained from 15 healthy v olunteers, 22 patients with hemorrhoid, and 20 patients with anal fissure. Local thermal stimulation was achieved by applying a heat pad on the right infragluteal region (local area), and subsequently on the right first inter phalangeal region (control area). RESULTS: Obvious response to local therma l stimulation was shown by 13.3 percent of volunteers, 36.4 percent of pati ents with hemorrhoid, and 60 percent of patients with fissure. Heat-sensiti ve patients who responded to local thermal stimulation were divided to two groups, those with ultraslow waves and those without ultraslow waves. In pa tients with ultraslow waves, the amplitude of ultraslow waves decreased sig nificantly after local thermal stimulation, with amplitude before local the rmal stimulation, (mean +/- standard deviation) 66.2 +/- 30.6 mmHg, and dur ing local thermal stimulation, 43.2 +/- 22.3 mmHg, respectively, P = 0.003. By contrast, in patients without ultraslow waves, the tonic pressure measu red before local thermal stimulation and during local thermal stimulation w as 74.2 +/- 23.5 and 60.5 +/- 18.5 mmHg, respectively, P = 0.001. The respo nse began at approximately three minutes after local thermal stimulation wh en the skin temperature was 42.1 +/- 0.3 degrees C. No anal response was ob served when the heat pad was applied to the control area. The maximum resti ng pressure of the heat-sensitive patients was significantly higher than th at of the nonresponding patients (97.3 +/- 0.1 vs. 76.9 +/- 23.3 mmHg; P = 0.012). CONCLUSIONS: Local thermal stimulation evokes relaxation of the hyp ertonic internal anal sphincter through a somatoanal reflex, thus providing an easy and feasible method of clinical application.