Causes of Decreased Anal Tone
Decreased anal tone results from multiple etiologies including aging, childbirth trauma with sphincter injury, neurological lesions (particularly lower motor neuron disease), and specific metabolic conditions, with age and vaginal delivery being the most common contributors in clinical practice.
Primary Causes
Age-Related Decline
Anal resting tone decreases by approximately 0.66 cm H₂O per year of age, while squeeze pressures decline by 0.3 cm H₂O annually 1. This age-related decline occurs regardless of other factors and affects both internal and external sphincter function [@3,@8@].
Obstetric Trauma
Childbirth represents a major cause of decreased anal tone, particularly affecting the external anal sphincter:
- Resting tone decreases by 4.3 cm H₂O per vaginal birth [@6@]
- Squeeze pressures decrease by 3.8 cm H₂O per birth 1
- Instrumental deliveries cause greater damage than spontaneous vaginal delivery
- Both internal and external sphincter defects are associated with reduced pressures
- Cesarean delivery is protective against sphincter damage and tone reduction [@6@]
Neurological Causes
Lower Motor Neuron Lesions (most commonly associated with hypotonia):
- Cauda equina syndrome
- Spinal cord injury at sacral levels
- Results in decreased basal resting sphincter pressures and reduced rectal compliance [@7@]
- Associated with fecal incontinence risk [@4@]
Upper Motor Neuron Lesions (paradoxically may cause hypertonia):
- Spinal or encephalic lesions above the sacral cord
- Can lead to recto-anal dyssynergia and constipation rather than incontinence [@4@]
Metabolic/Genetic Conditions
Infantile-onset Pompe Disease patients on enzyme replacement therapy develop poor anal tone as an emerging complication requiring monitoring for rectal prolapse 2.
Clinical Context and Pitfalls
Important Caveats
- Decreased anal tone is relatively uncommon even in populations you might expect it: only 11.3% had decreased resting tone and 6.3% had decreased squeeze tone in a large study [@3]
- In suspected cauda equina syndrome, 35% of MRI-positive patients and 31% of MRI-negative patients had reduced anal tone, meaning this finding has no diagnostic value for CES 4
- Sexual practices (receptive anal intercourse) do not reliably cause decreased anal tone and digital examination cannot detect such practices [@3]
Associated Conditions Requiring Assessment
When decreased anal tone is identified on anorectal manometry, evaluate for [@1,5]:
- Anal sphincter defects (internal or external) - assess with endoanal ultrasound or MRI
- Rectal sensation abnormalities (hyposensitivity or hypersensitivity)
- Evacuation disorders - particularly in patients with fecal seepage from overflow
- Pudendal neuropathy - bilateral pudendal nerve terminal motor latencies increase with age and vaginal delivery 1
Diagnostic Approach
When decreased anal tone is suspected:
- Anorectal manometry is the primary diagnostic tool to quantify resting and squeeze pressures [@1,5]
- Imaging (endoanal ultrasound or MRI) to identify structural sphincter defects, atrophy, or patulous anal canal [@2@]
- Balloon expulsion test to assess for dyssynergic defecation [@1@]
- Consider electrophysiological testing though validation is still needed [@4@]
Treatment Implications
Biofeedback therapy can increase anal sphincter tone in individuals with fecal incontinence and is effective in 70-80% of appropriately selected patients [@1,5]. This should be considered before surgical interventions or device-aided therapy in most cases.