Internal Anal Sphincter Contribution to Resting Pressure and Sexual Function
The internal anal sphincter (IAS) provides approximately 70–80% of resting anal canal pressure but has no role whatsoever in sexual arousal or the perception of rectal stimulation during sexual activity. 1
Resting Anal Canal Pressure
The IAS is the dominant contributor to baseline anal tone:
- Normal resting anal pressure averages 73 ± 27 cm H₂O, with the IAS generating the majority of this pressure through sustained smooth muscle contraction. 1, 2
- The IAS is responsible for more than 70% of resting anal pressure, maintaining continence at rest through involuntary tonic contraction. 3, 4
- In patients with anal fissures, IAS hypertonia elevates resting pressure to approximately 114 ± 17 cm H₂O, demonstrating the sphincter's capacity to modulate baseline tone. 2
Complete Absence of IAS Involvement in Sexual Function
The IAS operates under autonomic control and remains in its baseline involuntary state throughout sexual activity—it does not participate in arousal, sensation, or voluntary pelvic floor contraction. 1
Autonomic vs. Somatic Pathways
- Sympathetic fibers increase IAS tone to maintain continence, while parasympathetic fibers mediate the rectoanal inhibitory reflex during defecation; neither autonomic pathway participates in sexual arousal or ejaculatory pressure generation. 1
- Sexual arousal and ejaculation are driven exclusively by voluntary contraction of pelvic floor muscles (external anal sphincter, puborectalis, bulbospongiosus) via somatic pudendal nerve pathways (S2–S4), not by autonomic control of the IAS. 1
Clinical Evidence of Functional Separation
- Reduced IAS tone—whether from structural injury, neurogenic dysfunction, or surgical sphincterotomy—results in passive fecal incontinence (leakage of stool and flatus at rest) without affecting sexual function. 1
- After lateral internal sphincterotomy for anal fissure, minor flatus incontinence occurs in approximately 1–10% of patients; any postoperative sexual dysfunction is attributable to external sphincter or pelvic-floor involvement, not to changes in internal sphincter pressure. 1, 2
Clinical Implications After Low-Grade Fistulotomy
Expected Continence Impact
- Passive incontinence (involuntary leakage of flatus or liquid stool at rest) may occur if the fistulotomy divides a portion of the IAS, because the sphincter's contribution to resting tone is reduced. 1, 4
- Structural injury or functional weakness of the IAS results in passive incontinence of feces and flatus, not in alterations of voluntary squeeze pressure or sexual sensation. 4
No Impact on Sexual Arousal or Sensation
- Pelvic floor muscle tension during sexual activity should not be conflated with IAS function; the IAS remains in its baseline involuntary state throughout sexual activity. 1
- Autonomic neuropathy associated with diabetes mellitus or Parkinson's disease can diminish resting IAS tone, leading to incontinence but not to sexual dysfunction. 1
Common Pitfall to Avoid
Do not attribute postoperative sexual dysfunction to IAS injury. If a patient reports altered sexual sensation or difficulty with arousal after fistulotomy, evaluate for:
- External anal sphincter or puborectalis injury, which are under voluntary somatic control and do contribute to pelvic floor contraction during sexual activity. 1, 5
- Pudendal nerve injury, which would impair voluntary squeeze and sensation in the anogenital region. 1
- Psychological factors related to perineal surgery, pain, or fear of incontinence during intimacy.
The IAS itself has no sensory or motor role in sexual function, so its division or dysfunction will not directly impair arousal or the ability to sense rectal stimulation during sexual activity. 1