What is rectal sensory impairment and what are its causes?

Rectal Sensory Impairment: Definition and Etiology

Definition

Rectal sensory impairment (also termed rectal hyposensitivity) is a diminished perception of rectal distension diagnosed during anorectal physiologic testing, characterized by elevated sensory threshold volumes during balloon distension—specifically, a first-sensation threshold >60 mL or an urge-to-defecate threshold >120 mL. 1, 2, 3

• Rectal hyposensitivity is defined clinically when one or more sensory threshold volumes are elevated beyond the normal range (mean plus two standard deviations) during rectal balloon distension 4, 3
• However, consensus guidelines now recommend that more than one sensory parameter must be outside the normal range before diagnosing a sensory abnormality, given the subjective nature of these assessments 5, 1
• The condition reflects either true impairment of afferent nerve function or altered rectal wall biomechanical properties that affect sensory perception 2, 3

Clinical Presentation and Associations

Rectal hyposensitivity most commonly presents with:

• Constipation (48% of cases), often with reduced awareness of rectal filling and inability to perceive the urge to defecate until the rectum is markedly distended 1, 4
• Combined constipation and fecal incontinence (27% of cases), where overflow incontinence occurs due to lack of sensory feedback 1, 4
• Isolated fecal incontinence (20% of cases) 4
• In 30-40% of patients with defecatory disorders, rectal hyposensitivity coexists with dyssynergic defecation 1

Critically, impaired rectal sensation may be the only identifiable abnormality on physiologic testing in 48% of constipated patients and 31% of incontinent patients, suggesting it plays a primary etiologic role in symptom generation. 4

Pathophysiologic Mechanisms and Causes

Primary Afferent Pathway Disruption

Pelvic nerve injury is a major etiologic factor:

• 38% of patients with rectal hyposensitivity have a history of previous pelvic surgery 4
• 13% have a history of spinal trauma or spinal cord lesions 1, 4
• Neurologic conditions such as Parkinson's disease, autonomic neuropathy, and myopathy can impair colonic motility and rectal sensation 1

Secondary Rectal Wall Dysfunction

Altered rectal biomechanics contribute to sensory impairment:

• Elevated rectal compliance (increased distensibility) is found in patients with absent sensation during sensorimotor response testing, suggesting rectal wall dysfunction 6
• Higher balloon volumes are required to induce rectoanal inhibitory reflex (p=0.008) and contractile reflex (p=0.001) in hyposensitive patients compared to controls 6
• The diagnosis of rectal hyposensitivity may be susceptible to misinterpretation when altered rectal wall properties are present, as increased compliance can elevate sensory thresholds without true nerve dysfunction 2

Behavioral and Functional Factors

• Abnormal toilet behavior and chronic stool withholding may contribute to rectal hyposensitivity, though evidence is limited 2
• 22% of patients have a history of anal surgery, which may alter local sensory pathways 4

Disrupted Gut-Brain Pathways

Patients with rectal hyposensitivity demonstrate abnormal sensorimotor responses:

• In 43% of hyposensitive subjects, the onset of sensorimotor response is associated with completely absent sensation 6
• In 57%, only transient rectal sensation occurs during sensorimotor response 6
• The amplitude, duration, and magnitude of sensorimotor response are paradoxically higher in hyposensitive patients (p<0.05), suggesting compensatory mechanisms or disrupted afferent signaling 6

Diagnostic Approach

Anorectal manometry with rectal sensory testing is the essential first-line diagnostic evaluation:

• The International Anorectal Physiology Working Group (IAPWG) protocol measures both motor function and rectal sensory thresholds during stepwise graded balloon distension 5, 1
• Three sensory thresholds are assessed: first sensation, desire/urge to defecate, and maximum tolerable volume 7
• Altered rectal sensation should be evaluated through rectal sensory testing (balloon distension or mucosal electrosensitivity), not sphincter pressure measurements alone 8

Common Pitfalls

• Do not diagnose rectal hyposensitivity based on a single abnormal sensory threshold; at least two parameters must be outside the normal range to establish a pathologic diagnosis 5, 1
• Elevated sensory thresholds may reflect increased rectal compliance rather than true afferent nerve dysfunction—correlation with clinical symptoms and other physiologic findings is essential 2, 3
• Depression is associated with elevated first rectal sensory threshold volumes and predicts poor response to biofeedback therapy; screen for and address psychological comorbidity 5, 8

Treatment Implications

Biofeedback therapy targeting sensory adaptation exercises is the most effective treatment for rectal hyposensitivity:

• Sensory retraining biofeedback is associated with objective improvement in rectal sensory function and symptomatic relief in both constipation and fecal incontinence 5, 2
• 76% of patients with refractory anorectal symptoms achieve adequate relief with biofeedback therapy 5, 9
• Patients with lower baseline rectal sensory thresholds (better preserved sensation) are more likely to respond to therapeutic interventions, while elevated first rectal sensory threshold volume independently predicts poor biofeedback efficacy 5, 8