What pressure threshold can the adult patient perceive in the internal anal sphincter after pudendal‑nerve‑sparing sphincteroplasty?

Conscious Perception of Internal Anal Sphincter Pressure

Patients cannot consciously perceive any pressure changes generated by the internal anal sphincter (IAS) because it functions autonomically without providing sensory feedback to conscious awareness. 1

Physiologic Basis for Lack of Perception

• The IAS is a smooth-muscle structure that operates under autonomic control, maintaining resting anal tone without voluntary input 1
• The IAS contributes approximately 15–20% of total resting anal pressure, but this contribution occurs entirely outside conscious perception 1
• Continence is preserved when the combined pressure from IAS resting tone and external anal sphincter (EAS) voluntary contraction exceeds intrarectal pressure, regardless of whether the patient consciously perceives this pressure gradient 1

Sensory Innervation Anatomy

• The inferior rectal branches of the pudendal nerve (S2–S4) provide sensory innervation to the anal canal and perianal region, but these sensory fibers detect touch, pain, and temperature—not the smooth muscle tone of the IAS 2
• Pudendal nerve blockade studies demonstrate that blocking these sensory branches reduces vaginal resting pressures from 19 ± 10 mmHg to 15 ± 10 mmHg and squeeze pressures from 61 ± 29 mmHg to 37 ± 24 mmHg, confirming the nerve's role in voluntary EAS function rather than IAS perception 3

Clinical Implications After Sphincteroplasty

• Following pudendal-nerve-sparing sphincteroplasty, patients may experience altered sensations due to neuropathic injury to the inferior rectal branches, but these changes reflect damage to sensory fibers innervating the anal canal surface—not perception of IAS pressure itself 2
• Manometric studies show that IAS disruption correlates with significantly decreased mean maximum resting pressures (P = 0.023), but patients do not consciously detect this pressure reduction 4
• After anatomical sphincteroplasty, mean maximal resting pressure increases from 27.6 mmHg to 41.7 mmHg (p < 0.001), yet this improvement in IAS function occurs without patients consciously perceiving the pressure change 5

Distinguishing IAS Function from Conscious Sensation

• A patulous anal canal—which reflects severe IAS injury—is associated with decreased anal resting pressure (P < 0.01) and independently predicts anal squeeze pressure increment (P < 0.01), but patients do not perceive the IAS contribution to these measurements 6
• Internal sphincter injury predicts anal resting pressure (P < 0.01), while external sphincter injury predicts squeeze pressure (P = 0.02), but only the voluntary EAS contraction is consciously perceived 6

Common Pitfall to Avoid

• Do not confuse altered perianal sensations (dysesthesia, numbness, or neuropathic pain) after sphincteroplasty with an inability to perceive IAS pressure—the former reflects pudendal nerve injury affecting surface sensation, while the latter is a normal physiologic state because IAS pressure was never consciously perceptible in the first place 2, 1