Causes of Fecal Incontinence
Fecal incontinence is primarily caused by bowel disturbances (particularly diarrhea), anal sphincter trauma, rectal urgency, and burden of chronic illness, rather than obstetric history alone. 1
Pathophysiological Mechanisms
Fecal incontinence occurs when the normal anatomy or physiology that maintains the structure and function of the anorectal unit is disrupted. It typically results from multiple pathogenic mechanisms rather than a single factor 2. Key mechanisms include:
Sphincter-Related Causes
- Anal sphincter trauma/dysfunction:
- Obstetrical injury (most common cause of sphincter disruption) 2
- Prior anorectal surgery
- External anal sphincter (EAS) weakness causing urge-related or diarrhea-associated incontinence
- Internal anal sphincter (IAS) dysfunction affecting resting anal pressure
Neurological Causes
- Pudendal neuropathy: Diminishes rectal sensation leading to fecal impaction and overflow 2
- Neurological disorders:
- Stroke
- Multiple sclerosis
- Spinal cord injury or disease
- Diabetes (causing peripheral neuropathy)
- Dementia 1
Bowel/Stool Consistency Issues
- Diarrheal states: The strongest independent risk factor (OR=53) 1
- Fecal impaction with overflow 3
- Inflammatory bowel disease causing anorectal inflammation 1
Anatomical Defects
- Major anatomic defects requiring surgical correction:
- Rectovaginal fistula
- Full thickness rectal prolapse
- Fistula in ano
- Cloacalike deformity 1
- Rectocele: Independent risk factor (OR=4.9) 1
Other Medical Conditions
- Cholecystectomy: Independent risk factor (OR=4.2) 1
- Stress urinary incontinence: Independent risk factor (OR=3.1) 1
- Higher BMI: Independent risk factor (OR=1.1 per unit increase) 1
- Current smoking: Independent risk factor (OR=4.7) 1
- Advanced age 1
- Disease burden (co-morbidity count) 1
- Decreased physical activity 1
Post-Surgical Causes
- Upper GI surgery:
- Vagus nerve damage
- Gastric resection leading to rapid emptying
- Bile salt malabsorption
- Bacterial overgrowth
- Reduced absorptive capacity due to gut bypass 1
Clinical Subtypes of Fecal Incontinence
Defecatory Disorders
- Characterized by impaired rectal evacuation from:
- Inadequate rectal propulsive forces
- Increased resistance to evacuation
- High anal resting pressure ("anismus")
- Incomplete relaxation or paradoxical contraction of pelvic floor and external anal sphincters ("dyssynergia") 1
Overflow Incontinence
- Common in patients with cognitive or behavioral issues
- Learning difficulties
- Neurological or spinal disease
- Severe fecal loading causing overflow diarrhea 1
Common Pitfalls in Diagnosis
- Failure to identify underlying causes: Especially diarrhea, which is the strongest risk factor 3
- Misdiagnosis as IBS: Unrecognized cases of fecal incontinence are sometimes misdiagnosed as diarrhea-predominant IBS 1
- Inadequate anorectal testing: Not performing appropriate tests (manometry, sphincter imaging) can lead to misdiagnosis 3
- Overlooking evacuation disorders: Failing to identify evacuation disorders that cause fecal leakage with overflow of retained stool 3
Evaluation Approach
- Detailed characterization of bowel habits and circumstances surrounding incontinence 1
- Careful dietary history to identify poorly absorbed sugars and caffeine 1
- Anorectal examination to identify impaction and rectal prolapse 1
- Specialized testing when conservative measures fail:
- Anorectal manometry to identify anal weakness and sensory issues
- Anal imaging (ultrasound or MRI) to identify sphincter defects 3
Understanding these multifactorial causes is essential for developing an effective treatment plan that addresses the specific mechanisms contributing to an individual's fecal incontinence, ultimately improving quality of life and reducing morbidity.