Reflex Bowel (Neurogenic Reflex Bowel) Definition
Reflex bowel, also called upper motor neuron (UMN) bowel, is a type of neurogenic bowel dysfunction that occurs when spinal cord injury is located above the sacral spinal cord segments (above S2-S4), resulting in loss of voluntary control over defecation but preservation of spinal reflex activity in the colon and anal sphincters 1.
Key Pathophysiologic Features
In reflex bowel, the injury disrupts the connection between the brain and the sacral spinal cord, but the sacral reflex arc remains intact. This creates a distinct pattern:
- Preserved spinal reflexes: The rectoanal inhibitory reflex and other local reflexes continue to function 2
- Loss of voluntary control: Patients cannot voluntarily initiate or suppress defecation
- Reflex-mediated peristalsis: The colon can still generate coordinated contractions in response to local stimulation 1
- Increased anal sphincter tone: The external anal sphincter maintains or increases its resting tone due to unopposed spinal reflex activity 2
Clinical Presentation
Patients with reflex bowel typically experience 2:
- Constipation (present in approximately 86% of patients with injuries above T7)
- Significant difficulty with defecation due to inability to coordinate abdominal pushing with anal relaxation
- Moderate fecal incontinence (mean Wexner score 4.5) - less severe than in lower motor neuron bowel
- Delayed colonic transit time, particularly in the left colon and rectosigmoid region
- Inability to increase intra-abdominal pressure effectively for evacuation
Distinction from Lower Motor Neuron (Areflexive) Bowel
This contrasts sharply with lower motor neuron bowel, which occurs when injury affects the sacral spinal cord, cauda equina, or pelvic nerves 1. In areflexive bowel:
- Spinal reflexes are absent
- The anal sphincter is flaccid with low resting tone
- Fecal incontinence is typically more severe (mean Wexner score 7.2) 2
- Colonic transit is more severely delayed
- Manual removal of stool is often necessary 3
Management Implications
The presence of intact reflexes in reflex bowel allows for digital-rectal stimulation to trigger reflex peristalsis and anal sphincter relaxation, which is the cornerstone of bowel care in these patients 1. This technique involves gentle circular motion with a gloved, lubricated finger in the rectum to stimulate the reflex arc and facilitate evacuation. This approach is ineffective in areflexive bowel, where manual removal is typically required 3.
The level of injury matters: patients with injuries above T7 have additional challenges due to impaired ability to generate intra-abdominal pressure for defecation 2.