Internal Anal Sphincter Pressure Reduction in Conservative Lateral Internal Sphincterotomy
In a conservative (minimal-depth) lateral internal sphincterotomy, internal anal sphincter resting pressure is reduced by approximately 25–45 mmHg, bringing elevated pre-operative pressures (typically 85–140 mmHg) down to post-operative levels of 75–110 mmHg. 1, 2, 3
Quantitative Pressure Changes by Study
The magnitude of pressure reduction depends on the pre-operative sphincter tone and the extent of sphincterotomy performed:
Standard LIS technique: Pre-operative mean resting pressure of 138 ± 28 mmHg drops to 86 ± 15 mmHg at 1 month post-operatively (reduction of ~52 mmHg), then gradually recovers to 110 ± 18 mmHg at 12 months (net reduction of ~28 mmHg from baseline). 1
Conservative approach: Pre-operative pressure of 87.6 ± 12.3 mmHg decreases to 78.1 ± 11.3 mmHg at 2 months (reduction of ~10 mmHg), which is comparable to controlled anal dilatation outcomes. 3
Minimal sphincterotomy: Pre-operative pressure of 59.99 mmHg reduces to 32.43 mmHg post-operatively (reduction of ~27 mmHg), though this study population had lower baseline pressures than typical chronic fissure patients. 2
Calibrated Sphincterotomy Approach
A more conservative, tailored technique divides the internal sphincter based on pre-operative manometry findings to minimize over-treatment: 4
- Mild hypertonia (50–60 mmHg): Divide 20% of sphincter length
- Moderate hypertonia (60–80 mmHg): Divide 40% of sphincter length
- Severe hypertonia (>80 mmHg): Divide 60% of sphincter length
This calibrated approach achieved 97.6% cure rates with only 0.4% gas incontinence, and post-operative pressures remained above 30 mmHg in 96.2% of patients, preserving continence function. 4
Physiologic Context
The pressure reduction interrupts the pain-spasm-ischemia cycle that perpetuates chronic anal fissures: 5
- Normal resting pressure: 73 ± 27 mmHg in healthy controls 1
- Chronic fissure pressure: Elevated to 114 ± 17 cmH₂O (approximately 85–140 mmHg range across studies) 5, 1
- Target post-operative pressure: 75–110 mmHg, which remains above normal but eliminates pathologic hypertonia 1
Clinical Implications
The goal of conservative LIS is not to normalize sphincter pressure to healthy control levels, but rather to reduce pathologic hypertonia sufficiently to restore anodermal blood flow while preserving continence. 1 Post-operative pressures that remain 10–40 mmHg above normal controls are associated with excellent healing rates (>95%) and minimal incontinence risk (1–3%). 5, 1
Recovery Pattern
Sphincter tone demonstrates partial recovery over the first post-operative year: 1
- 1 month: Maximal pressure reduction (nadir ~86 mmHg)
- 3–6 months: Gradual tone recovery begins
- 12 months: Plateau at ~110 mmHg, still significantly below pre-operative baseline but above normal controls
This recovery pattern indicates that the internal sphincter undergoes adaptive remodeling after division, though it never fully returns to pre-operative hypertonic levels—which is therapeutically desirable. 1