Rectal heat thresholds: a novel test of the sensory afferent pathway

Currently, rectal sensation may be measured by balloon distention or mucosal electrostimulation. This study investigated the application of a graded heat stimulus to the rectum using a novel thermal probe as a further method of evaluating rectal sensory afferent mechanisms.

A thermal probe specially designed in our institution was used to test rectal heat sensitivity in 31 healthy subjects. This was compared with all other standard anorectal physiologic measurements. Repeatability studies were also performed.

Heat stimulation of the mid rectum elicited sensory responses in all subjects. The most common reported response was not heat but a sharp or prickling sensation. The median rectal heat threshold was similar in males (median, 47 degrees C; range, 44-50 degrees C) compared with females (median, 45 degrees C; range, 43-50 degrees C; P > 0.05). There was a high degree of repeatability with rectal heat and balloon distention thresholds, but not electrostimulation thresholds. A strong correlation was found between rectal heat thresholds and defecatory desire (r = 0.71; P < 0.001) and maximum tolerable volumes (r = 0.8; P < 0.001) measured with balloon distention.

This is the first demonstration of a repeatable sensory response to heat stimulation in the rectum of normal subjects. Strong correlation between heat thresholds and balloon distention to maximum tolerable volumes and defecatory desire suggest common sensory afferent pathway excitation. Heat stimulation is a simple technique that has a high degree of repeatability and may be an objective assessment of polymodal nociceptor function in the rectum.