Assessment of rectal mechanoreceptors in healthy humans by a reflexologic technique

Abstract

Rectal distension studies have suggested the existence of 2 types of mechanoreceptors. Our aim was to confirm this functional duality by using a reflexologic technique, based on the recording of a nociceptive somatic reflex (the Rill reflex) which allows an objective evaluation of visceral sensitivity, during slow ramp or rapid phasic rectal distensions together with application of a local anesthetic. Methods: during a randomized double blind study, the effects of rectal distension on the amplitude of the Rill reflex recorded on the lower limb, were measured in 6 healthy volunteers after intrarectal application of 20 mL of local anesthetic, 2% lidocaine (L), or saline (S). Rectal distensions were performed with an electronic barostat on 4 days separated by a week. Each experimental day, one type of distension was performed with either L or S: rapid (900 mL/min) phasic distension (3 min duration) in a random order at 10, 20 and 30 mmHg, or slow (40 mL/min) ramp distension. The RIII reflex was continuously stimulated and the amplitude of the reflex response recorded before (control, 3 rain), during and after (3 min) each distension, and expressed as percentage of control values. Volumes, pressure and sensations were also measured. Results: (mean-+ SEM) : the RIII reflex threshold of detection was not modified by intrarectal application of L or S. During rapid distension, at 10 mmHg, the Rill response was not modified. At 20 mmHg, a facilitation of the reflex followed by a partial inhibition was obtained with S, and L abolished this inhibition (maximal inhibition : 64 _+_ 9% vs. 109 + 13%; P < 0.05). At 30 mmHg, an inhibition of the response was obtained with S that was partially abolished by L (maximal inhibition : 60 -+ 8% vs. 80 -+ 7%; P = NS). During S, slow ramp distension induced an inhibition of the RIII response that was completely abolished by L (maximal inhibition : 47-+ 8% vs. 100-+ 8%; P<0.05) and sensations during the distensions were significantly lower with L than with S (P < 0.05). Compliance curves were not different between L and S for any type of distensions. Conclusion: The inhibition of the RIll reflex corresponds to the classical phenomenon of counter-irritation triggered by heterotopic stimulation. Our results suggest that slow ramp distensions stimulate exclusively superficial (probably mucosai) mechanoreceptors whereas the effects of rapid distension are more complex as they probably stimulate both superficial and deeper (probably serosal) mechanoreceptors. This emphasizes the interest of the • refiexologic technique for neurophysiological study of gut sensations.