How do you diagnose a neurogenic cause of constipation?

Diagnostic Approach for Neurogenic Causes of Constipation

Digital rectal examination and assessment of anal sphincter tone and perineal sensation are the most important first-line tests to diagnose neurogenic constipation. 1

Clinical Assessment

History

• Duration and pattern of constipation
• Presence of straining during defecation
• Sensation of incomplete evacuation
• Need for digital manipulation to assist evacuation
• Recent changes in bowel patterns
• Presence or absence of urge to defecate
• Fecal incontinence or rectal leakage
• Current and previous laxative use 1

Physical Examination

• Digital rectal examination (DRE) - crucial for identifying:
  • Sphincter tone (lax with colonic hypotonia suggests neurogenic cause)
  • Rectal sensation
  • Ability to contract and relax pelvic floor muscles
  • Presence of impacted feces 1
• During DRE, assess:
  • Resting tone of sphincter
  • Augmentation during squeezing effort
  • Puborectalis muscle contraction
  • Patient's ability to "expel examiner's finger" 1
• Observe perineum during simulated evacuation:
  • Patulous opening of anal verge suggests neurogenic constipation
  • Assess for normal descent during simulated evacuation 1
• Test anal reflex with light pinprick or scratch 1
• Complete neurological examination if spinal cord compression is suspected 1

Specialized Testing

When initial assessment suggests neurogenic cause or when standard management fails after 4-6 weeks:

1. Anorectal manometry - evaluates:

   • Anal sphincter weakness
   • Rectal sensation issues
   • Rectoanal inhibitory reflex
   • Paradoxical contraction of pelvic floor muscles 2

2. Colonic transit studies - diagnose slow-transit constipation 1, 2

   • Useful to differentiate between slow transit and evacuation disorders

3. Anal imaging (ultrasound or MRI) - identifies:

   • Sphincter defects
   • Muscle atrophy 2

4. Defecography (conventional or MRI) - evaluates:

   • Pelvic floor descent
   • Rectocele
   • Rectal intussusception 1

Specific Tests for Neurogenic Causes

• Neurological examination - assess for:

  • Anal sphincter tone (lax with colonic hypotonia)
  • Rectal sensation
  • Lower extremity reflexes
  • Sensory deficits 1

• Small-fiber function tests:

  • Pinprick sensation
  • Temperature sensation 1

• Large-fiber function tests:

  • Lower-extremity reflexes
  • Vibration perception
  • 10-g monofilament testing 1

• Autonomic function testing - for suspected autonomic neuropathy:

  • Heart rate variability with deep breathing
  • Orthostatic blood pressure measurements 1

Diagnostic Algorithm

1. Initial evaluation:

   • Complete history focusing on bowel patterns
   • Digital rectal examination with assessment of sphincter tone and sensation
   • Neurological examination

2. If neurogenic cause suspected based on initial evaluation:

   • Proceed to specialized testing (anorectal manometry, colonic transit studies)
   • Consider autonomic function testing if diabetic or other autonomic neuropathy suspected

3. If standard management fails:

   • Referral for defecography and/or anal imaging
   • Consider referral to neurologist for electrophysiological testing

Specific Neurogenic Causes to Consider

• Spinal cord injury/disease: Assess for history of trauma, level of injury, and other neurological symptoms 3, 4

• Multiple sclerosis: Look for other neurological symptoms and disease history 4

• Diabetic autonomic neuropathy: Assess for other autonomic symptoms (erectile dysfunction, orthostatic hypotension) 1

• Parkinson's disease: Assess for characteristic motor symptoms 3

• Congenital defects: Consider in pediatric patients with lifelong symptoms 5

Pitfalls and Caveats

• A normal digital rectal examination does not exclude defecatory disorders 1
• Electrophysiological testing or referral to a neurologist is rarely needed except when clinical features are atypical 1
• Metabolic tests (thyroid-stimulating hormone, serum glucose, creatinine, calcium) have low diagnostic utility but are often performed 1
• Colonoscopy should not be performed unless age-appropriate cancer screening is due or alarm symptoms are present 1
• Neurogenic bowel dysfunction can present with both constipation and fecal incontinence, sometimes simultaneously 3

By following this systematic approach, clinicians can effectively diagnose neurogenic causes of constipation and implement appropriate management strategies to improve patient outcomes.