Conscious Perception of External Anal Sphincter Pressure on the Internal Anal Sphincter
Patients cannot consciously perceive pressure changes in the internal anal sphincter (IAS), regardless of external anal sphincter (EAS) contraction, because the IAS 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 autonomically and contributes only 15-20% of total resting anal pressure without voluntary control. 1
Conscious perception of IAS pressure changes is physiologically impossible because the sphincter does not transmit sensory information to conscious awareness pathways. 1
Even when the EAS contracts voluntarily and increases total anal canal pressure, the patient perceives only the voluntary contraction of the EAS itself, not any mechanical effect on the underlying IAS. 1
What Happens When IAS Pressure Drops
A drop in IAS resting pressure would not produce a perceivable change in "fullness" sensation because the IAS itself is not consciously perceived. 1
However, reduced IAS tone triggers compensatory hypertonicity of the EAS and puborectalis muscles, which patients may perceive as increased pelvic floor tension or discomfort rather than reduced fullness. 2
Manometric studies demonstrate that when IAS pressure is abnormally low, patients show paradoxically increased squeeze pressure from compensatory EAS contraction, confirming that the body attempts to maintain continence through the voluntary sphincter. 2
Clinical Context: What Patients Actually Perceive
Rectal distension activates conscious perception through stretch receptors in the rectal wall, triggering a transient EAS contraction that patients can feel, followed by IAS relaxation that occurs without conscious awareness. 3
The sensation of "fullness" originates from rectal distension and rectal wall stretch receptors, not from sphincter pressure changes. 3
Studies in populations with reduced IAS tone (resting pressures of 70.7 mm Hg vs. 91.4 mm Hg in controls) show no complaints of altered sensation or perceived fullness, despite objectively lower sphincter pressures. 4
Critical Pitfall to Avoid
Do not conflate continence mechanism with conscious perception—continence is preserved when combined IAS and EAS pressure exceeds intrarectal pressure, but this pressure gradient operates entirely below the threshold of conscious awareness. 1
Patients reporting altered pelvic sensations after procedures affecting the IAS (such as lateral sphincterotomy) are experiencing pelvic floor muscle tension and protective guarding patterns, not direct perception of IAS pressure changes. 5