Local thermal stimulation relaxes hypertonic anal sphincter: evidence of somatoanal reflex

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 sphincter, hence reducing pain. It is our purpose to demonstrate response of the anal sphincter to local thermal stimulation via a somatoanal reflex.

Continuous anorectal manometry tracings were obtained from 15 healthy volunteers, 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 interphalangeal region (control area).

Obvious response to local thermal stimulation was shown by 13.3 percent of volunteers, 36.4 percent of patients with hemorrhoid, and 60 percent of patients with fissure. Heat-sensitive patients who responded to local thermal stimulation were divided to two groups, those with ultraslow waves and those without ultraslow waves. In patients with ultraslow waves, the amplitude of ultraslow waves decreased significantly after local thermal stimulation, with amplitude before local thermal stimulation, (mean +/- standard deviation) 66.2 +/- 30.6 mmHg, and during local thermal stimulation, 43.2 +/- 22.3 mmHg, respectively, P = 0.003. By contrast, in patients without ultraslow waves, the tonic pressure measured before local thermal stimulation and during local thermal stimulation was 74.2 +/- 23.5 and 60.5 +/- 18.5 mmHg, respectively, P = 0.001. The response began at approximately three minutes after local thermal stimulation when the skin temperature was 42.1 +/- 0.3 degrees C. No anal response was observed when the heat pad was applied to the control area. The maximum resting pressure of the heat-sensitive patients was significantly higher than that of the nonresponding patients (97.3 +/- 0.1 vs. 76.9 +/- 23.3 mmHg; P = 0.012).

Local thermal stimulation evokes relaxation of the hypertonic internal anal sphincter through a somatoanal reflex, thus providing an easy and feasible method of clinical application.