Spinal cord injury (SCI) is a life-altering event with various consequences. Bowel dysfunction is highly prevalent and occurs in at least 60% of patients because of an SCI.1 The incidence of SCI ranges from 490 to 526 per million in developed countries.2 Males are more likely to experience an SCI than females in the United States: male: female ratio of 3.95:1.3 Age relative to an SCI follows a bimodal distribution – first peak at 15-29 years and second over 65 years of age.2 Spinal cord lesions affect anal sphincter function which can lead to neurogenic constipation and/or fecal incontinence.4 Sixty-eight percent of SCI patients experience abnormal defecation and 20% require digital stimulation.1 A majority of SCI patients report constipation(56% - 80%) and fecal incontinence (42% - 75%).5
The International Standards for Neurological Classification of Spinal Cord Injury (ISNCSCI) evaluates global injury severity. The scale assesses the SCI level and grade (A-E), corresponding to the spinal lesion level and extent of remaining sensorimotor function. In a meta-analysis from Khorasanizadeh et al., 19,460 SCI subjects were included: Grade A (49.8%), Grade B (15.9%), Grade C (24.6%), and Grade D (9.7%).6 Table 1 describes the differences in ASIA Impairment Scale grades.7 In a study of 142 SCI subjects, there was a more significant association between injury severity and neurogenic bowel dysfunction (NBD). Subjects in the ASIA A group had a 12.8-fold more significant risk of severe NBD than those in the ASIA D group.8
Anorectal manometry (ARM) coupled with the Balloon Expulsion Test (BET) is a test that evaluates rectal, anal, and pelvic floor muscle function. ARM assesses the pressure in the anal canal at rest, during voluntary effort (squeeze), and attempted evacuation (simulated defecation). High-resolution manometry pressures in the anal canal and rectum are presented as color-contour plots. The HR-ARM catheter (Diversatek Healthcare, Milwaukee, WI) is 4 mm in diameter of 8 sets of radially and orthogonally arranged pressure sensors spaced 1 cm apart. The HR-ARM catheter configuration consists of five pressure sensors in the anal canal spaced 10 mm apart, two sensors on the distal end used to measure rectum pressure and balloon inflation, and one external reference sensor outside the body to assess atmospheric pressure.9
The internal anal sphincter is a circular, smooth muscle innervated via the sacral nerves responsible for ensuring the anal canal closure at rest.10 During the first stage of the evacuation of feces, migrating contractions propel the colonic contents into the rectum. The rectum becomes distended and initiates the rectoanal inhibitory reflex (RAIR), which results in rectal contraction and internal anal sphincter relaxation.11 The RAIR is manometrically characterized by the relaxation or loss of anal canal pressure during rectal balloon distension and the reflex's lowest anal canal pressure point (residual pressure). Amplitude reduction is the change in pressure from resting to residual pressure. It has been demonstrated that the amplitude reduction and duration of the RAIR are dependent on the frequency and volume of rectal distension.12 The RAIR response contains various measurement components: the basal internal anal sphincter pressure, excitation latency, amplitude reduction, inhibitory reflex duration, and return toward internal anal sphincter basal pressure.12,13 Digital-rectal stimulation (DRS) facilitates defecation in patients with SCI. Data demonstrated that anal dilation stimulates stretch receptors in the internal anal sphincter, causing an increase in rectal pressure mediated by an anorectal reflex.11 A study of nine SCI subjects illustrated that rectal pressure did not significantly rise with increasing anal distension after a certain threshold, indicating that high pressures may not be necessary to cause bowel evacuation.11 However, digital stimulation may induce transient rectal pressure relaxation providing compliance for evacuation.
There is little data within the SCI literature defining anorectal function utilizing high-resolution ARM. Mazo et al., assessed a cohort of SCI subjects with neurogenic bowel by ARM and demonstrated lower anal squeeze pressures than controls.14 Additionally, SCI subjects had significantly lower maximum squeeze pressure than healthy controls. Elevated sensory thresholds for all distension parameters, first sensation, urgency, maximum tolerated) were reported for SCI subjects compared to healthy volunteers.13
Based on limited studies, SCI subjects tend to have a longer duration of the RAIR reflex and less reduction of the IAS compared to healthy controls.13 However, this data does not compare RAIR parameters in SCI subjects undergoing high-resolution anorectal manometry (HR-ARM) compared to healthy control RAIR metrics. Additionally, further investigation of correlations and associations between RAIR parameters and SCI injury duration, location, and AIS levels is required.
Objectives
To compare RAIR parameters in SCI subjects vs. published healthy control values. Explore correlations and associations between RAIR parameters and SCI injury duration, SCI location, and AIS levels to provide physiological evidence for using DRS as a tool for bowel evacuation.