Introduction

The success of fistula surgery depends on two critical goals:

• Eradication of the fistula tract
• Preservation of continence

Both rely on accurate preoperative anal sphincter assessment, particularly in complex or recurrent cases. For anesthesiologists, knowledge of how anesthetic agents alter sphincter tone, reflex arcs, and the depth of anesthesia is essential. This understanding ensures optimal timing for sphincter evaluation and prevents avoidable postoperative continence issues.

Basic Science Review of the Anal Sphincter

Internal Anal Sphincter (IAS)

• Muscle type: Smooth muscle
• Innervation: Autonomic (sympathetic via hypogastric nerves)
• Control: Involuntary
• Contribution: ~70% of resting tone

External Anal Sphincter (EAS)

• Muscle type: Striated muscle
• Innervation: Somatic (pudendal nerve, S2–S4)
• Control: Voluntary
• Contribution: Maintains tone during stress, coughing, or voluntary squeeze

Puborectalis

• Muscle type: Striated muscle
• Innervation: Somatic (S2–S4)
• Control: Both voluntary and reflexive
• Contribution: Maintains anorectal angle (~80°), essential for continence

Pharmacology of Anesthetic Agents and Sphincter Function

General Anesthesia

Intravenous Agents

• Propofol: GABA-A agonist; decreases voluntary EAS tone, partially preserves IAS tone.
• Thiopentone: GABA-A agonist; rapid loss of sphincter tone and reflexes.
• Ketamine: NMDA antagonist; preserves some reflex tone, may still impair voluntary contraction.
• Etomidate: GABA-A modulator; reduces EAS contraction, minimal cardiovascular depression.

Volatile Agents

• Sevoflurane (MAC ~2%): Reduces EAS tone profoundly at ≥1 MAC, abolishes reflexes.
• Isoflurane (MAC ~1.2%): Dose-dependent loss of tone and reflexes, delayed emergence.
• Desflurane (MAC ~6%): Rapid onset, suppresses skeletal muscle reflexes strongly.

Key Point: At 1.0–1.2 MAC, volatile anesthetics abolish pudendal and pelvic reflexes, making sphincter tone assessment unreliable. IAS tone may persist partially due to autonomic input, but EAS tone is eliminated.

Neuromuscular Blocking Agents

• Rocuronium: Non-depolarizing; abolishes all striated muscle contraction including EAS and puborectalis.
• Succinylcholine: Depolarizing; transient fasciculations followed by flaccid paralysis.

Key Point: As EAS and puborectalis are striated muscles, relaxants abolish their tone. Delay administration until after tone assessment.

Regional Anesthesia

Spinal Anesthesia

• Drugs: Bupivacaine (0.5% hyperbaric), with or without fentanyl.
• Mechanism: Blocks S2–S4, abolishes pudendal (somatic) and pelvic (parasympathetic) fibers.
• Effect: Loss of EAS and puborectalis tone, loss of voluntary and reflexive control.

Caudal or Epidural Anesthesia

• Spread-dependent: If block reaches S2–S4, effects are similar to spinal.
• Duration: Shorter than spinal but still prevents tone testing intraoperatively.

Note: Regional anesthesia is useful for postoperative analgesia but should only be given after sphincter tone assessment.

Sedation and Local Anesthesia

• Midazolam: May reduce voluntary squeeze; light use preserves sphincter tone.
• Dexmedetomidine: Minimal respiratory depression; mild reduction in voluntary contraction; useful for awake assessment.
• Fentanyl: High doses impair cortical input; light sedation preserves sphincter evaluation.
• Local infiltration or pudendal block: Preserves sphincter tone and allows awake assessment.

Depth of Anesthesia and Sphincter Assessment

• Minimal Sedation (Anxiolysis): Patient responds normally; sphincter assessment feasible.
• Moderate Sedation: Purposeful responses to verbal/tactile stimuli; tone assessment partially feasible.
• Deep Sedation: Responses only to repeated stimuli; reflexes diminished; assessment unreliable.
• General Anesthesia: No response to painful stimuli; tone and reflexes abolished.
• Surgical Anesthesia (Plane 3): Complete motor areflexia; sphincter testing impossible.

Clinical Workflow for Anesthesiologists

Preoperative Discussion

• Confirm with surgeon whether sphincter tone assessment is required.
• Review fistula complexity and patient’s continence status.

Before Sedation

• Permit surgeon to perform digital rectal examination (DRE) while the patient is awake.
• Document resting tone and voluntary squeeze.

Anesthetic Plan

• Low/simple fistulas: Spinal or local anesthesia after tone check.
• Complex/high fistulas: General anesthesia after awake DRE.
• Cases with pre-existing incontinence: Local block with sedation; intraoperative assessment possible.
• Day-care surgery: Caudal or pudendal block, tone assessed before sedation.

Documentation

• Record sphincter tone findings and specify whether tone was assessed before anesthesia induction.

Summary

• Anal continence depends on a complex neuromuscular system influenced by anesthetic drugs.
• Both general and regional anesthesia impair sphincter tone and reflexes, preventing accurate assessment after induction.
• Understanding anesthetic pharmacology—especially GABAergic mechanisms, MAC thresholds, and somatic vs autonomic effects—is crucial.
• Collaboration with surgeons and careful anesthesia planning safeguard continence and meet medico-legal standards.

References

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2. Barash PG, Cullen BF, Stoelting RK. Clinical Anesthesia. 9th ed. Wolters Kluwer; 2021.
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4. Duthie GS, Bennett RC. The use of anal manometry and endoanal ultrasound in the assessment of sphincter function. Br J Surg. 1992;79(4):304–307.
5. Corman ML. Colon and Rectal Surgery. 6th ed. Lippincott Williams & Wilkins; 2013.
6. Sanders RD, et al. The neuroscientific foundations of anesthesia: From neuronal circuits to consciousness. Anesth Analg. 2012;114(1):139–153.