Anorectal Manometry Does Not Test Bladder Sensory Thresholds
Anorectal manometry (ARM) exclusively assesses anorectal sensorimotor function—including rectal sensory thresholds for distension—but does not measure bladder threshold sensations. 1
What Anorectal Manometry Actually Measures
ARM is a diagnostic procedure that uses pressure sensors to evaluate the following anorectal parameters:
Motor Function
- Resting anal canal pressure (internal anal sphincter tone) 2, 3
- Squeeze pressure (external anal sphincter and puborectalis contraction during voluntary effort) 2, 3
- Anal sphincter relaxation during simulated defecation (to detect dyssynergic patterns) 1
- Rectoanal inhibitory reflex (RAIR) in response to rectal balloon distension 4, 2
Sensory Function (Rectal Only)
- First sensation threshold (initial awareness of rectal distension) 1, 5
- Urge to defecate threshold (desire to evacuate) 1, 5
- Maximum tolerable volume (discomfort threshold) 1, 5
- Rectal compliance (distensibility of the rectal wall) 1, 3
Coordination Assessment
- Rectoanal coordination during push maneuvers (to diagnose dyssynergic defecation) 1, 5
- Balloon expulsion test (ability to expel a 50 mL water-filled balloon within 1–3 minutes) 1, 5
Why Bladder Sensation Is Not Assessed
ARM is anatomically and functionally specific to the anorectum. The probe is positioned in the anal canal and rectum, and the balloon distension protocol targets rectal mechanoreceptors only. 6, 2, 5 Bladder sensory thresholds require separate urodynamic testing (cystometry), which measures bladder filling sensations, first desire to void, and maximum cystometric capacity. 7
Clinical Context: Overlapping Pelvic Floor Symptoms
Although ARM does not test bladder function, concurrent incomplete bowel and bladder emptying suggests shared pelvic-floor dyssynergia affecting both systems through common neuromuscular pathways. 7 In such cases:
- ARM identifies defecatory disorders (dyssynergic defecation, rectal sensory impairment) 1, 5
- Urodynamic studies are required separately to evaluate bladder dysfunction 7
- Pelvic-floor biofeedback therapy can address both anorectal and urinary symptoms when pelvic-floor hypertonicity is the underlying mechanism 7, 8
Common Pitfall to Avoid
Do not assume that ARM provides information about bladder function simply because pelvic-floor disorders often affect both bowel and bladder. 7 Patients with overlapping symptoms require coordinated evaluation by gastroenterology (for ARM) and urology/urogynecology (for urodynamic testing). 1, 7