Rectoanal reflexes and sensorimotor response in rectal hyposensitivity

Abstract

Purpose: Rectal hyposensitivity commonly causes anorectal disorders, but its underlying mechanism is unknown. We hypothesized that subjects with rectal hyposensitivity have altered rectoanal reflexes and/or sensorimotor response.

Methods: We performed stepwise graded balloon distensions of the rectum in 30 subjects with constipation and rectal hyposensitivity and in 23 healthy controls. Thresholds for first sensation, desire, and urgency to defecate were assessed. The lowest balloon volume that evoked rectoanal inhibitory reflex, rectoanal contractile reflex, and sensorimotor response and manometric characteristics and rectal compliance were examined.

Results: Reflex responses were present in all subjects. The balloon volumes were higher in subjects with rectal hyposensitivity for inducing rectoanal inhibitory reflex (P = .008) and contractile reflex (P = .001) compared with controls. All controls showed a sensorimotor response, but in 13 hyposensitive subjects (43%) the onset of sensorimotor response was associated with absent sensation and in 17 (57%), with a transient rectal sensation. Thresholds for eliciting sensorimotor response were similar between patients and controls, but the amplitude, duration, and magnitude of response were higher (P < .05) in patients. Rectal compliance was similar between controls and hyposensitive subjects with transient sensation but higher (P = .001) in subjects with absent sensation.

Conclusions: Constipated subjects with rectal hyposensitivity demonstrate higher thresholds for inducing rectoanal reflexes and abnormal characteristics of sensorimotor response. These findings suggest either disruption of afferent gut-brain pathways or rectal wall dysfunction. These altered features may play a role in the pathogenesis of bowel dysfunction in rectal hyposensitivity.

Figure 1

An example of the sensorimotor reflex (SMR). Three consecutive sets of phasic rectal balloon distensions in one subject are shown. In the first distension, there is an increase in rectal pressure (p1) with transient reflex relaxation of anal sphincter pressure (p3), demonstrating the RAIR. During the second distension a transient reflex contraction of the external anal sphincter – the recto-anal contractile reflex (RACR) is seen. In the third sequence, in addition to the RAIR and RACR, the subject reported a desire to defecate and simultaneously an anal contractile response is seen overlying the anal relaxation. Reproduced with permission from Dis Colon Rectum 2007; 50: 1639-1646.

Figure 2

Examples of the SMR. Figure A shows a SMR (black dotted arrow) evoked with 90 cc of rectal balloon distension and associated with a desire to defecate (DD +) in a healthy control. Figure B shows a SMR evoked with 90 cc and associated with a transient (blunted) rectal sensation in a patient with rectal hyposensitivity. Figure C shows a SMR evoked with 90 cc, but in this case the response is present but the subjects reported no rectal sensation. Note that the SMR is more prominent in subjects B and C when compared to healthy controls.

Figure 3

Figure showing the relationship between the changes in rectal balloon volume and the intrarectal pressure (compliance) in healthy volunteers (HV), patients with RH and normal rectal compliance (NC) and patients with RH and abnormal increased compliance (IC ) or excessive laxity of the rectal wall.

Figure 4

Relationships between rectal hyposensitivity, sensorimotor response and abnormal increase rectal wall compliance. Most patients (80%) with a normal rectal wall compliance had transient sensation with SMR. In contrast, 66% of subjects with an increased compliance of the rectal wall, had absent rectal sensation during the onset of an SMR.

Figure 5

Relationship between Sensorimotor Response and Rectal Wall Compliance