What is a sphincter injury in a patient with persistent abnormal rectal sensation three years after excisional hemorrhoidectomy?

What is a Sphincter Injury?

A sphincter injury is structural damage to the internal anal sphincter (IAS), external anal sphincter (EAS), or both, resulting from trauma—most commonly obstetric injury, surgical procedures (including hemorrhoidectomy), or primary muscle degeneration—that disrupts the normal anatomy responsible for maintaining anal continence and sensation. 1

Anatomic Components of Sphincter Injury

The anal sphincter complex consists of two distinct muscular structures that work together to maintain continence:

• Internal anal sphincter (IAS): A smooth muscle structure that functions autonomically and provides approximately 15–20% of resting anal pressure without voluntary control 1, 2
• External anal sphincter (EAS): A striated muscle under voluntary control that reinforces the IAS during squeeze maneuvers 3

Mechanisms of Sphincter Injury After Hemorrhoidectomy

In the context of excisional hemorrhoidectomy, sphincter injury occurs through several distinct pathways:

• Direct mechanical trauma: Sphincter defects have been documented by ultrasonography and anal manometry in up to 12% of patients after hemorrhoidectomy 1
• Excessive retraction injury: The use of excessive retraction with extensive dilation of the anal canal is the primary mechanism responsible for sphincter injury and subsequent incontinence 1
• Pudendal nerve damage: The inferior rectal branches of the pudendal nerve (S2–S4) traverse the intersphincteric space and are at high risk during intersphincteric dissection, producing neuropathic injury that manifests as altered or absent rectal-pelvic sensory perception 4

Clinical Manifestations of Sphincter Injury

The presentation depends on which component is damaged:

Mechanical Sphincter Failure (Typical)

• Fecal incontinence is the most common complication after sphincter injury, with incontinence rates of 2–12% reported after hemorrhoidectomy 1
• Urge-related or diarrhea-associated fecal incontinence results from disruption or weakness of the EAS 3
• Passive fecal incontinence occurs when IAS damage reduces resting anal pressure below the threshold needed to maintain continence 3

Neuropathic Dysfunction (Less Common)

• Altered rectal-pelvic sensation with intact continence represents neuropathic dysfunction rather than mechanical failure 4
• This pattern reflects damage to pudendal nerve branches during intersphincteric dissection 4
• Patients may develop compensatory hypertonicity of the puborectalis and EAS, creating persistent pelvic floor tension that interferes with normal relaxation 4

Diagnostic Evaluation of Sphincter Injury

When sphincter injury is suspected three years after hemorrhoidectomy with persistent abnormal sensation:

• Anorectal manometry quantifies resting pressure (IAS function) and squeeze pressure (EAS function), and can detect paradoxical contraction patterns 4
• Endoanal ultrasound is the gold standard for visualizing structural sphincter defects, classifying injuries as scars (≤30°) or defects (>30°) 5
• High-resolution pelvic MRI can identify unrecognized structural complications and assess for a patulous anal canal, which is associated with more severe injury 6
• Digital rectal examination may reveal localized tenderness over the puborectalis if chronic hypertonicity has developed 4

Important Clinical Pitfalls

Several critical distinctions must be recognized:

• Sensory loss without incontinence indicates neuropathic injury rather than mechanical sphincter failure, requiring physical therapy rather than surgical revision 4
• Reduced sphincter tone on examination should raise high suspicion for occult IAS injury even when the initial surgical diagnosis did not document sphincter involvement 5
• Patients cannot consciously perceive IAS pressure changes because the sphincter functions autonomically without sensory feedback to conscious perception 2
• Additional surgery is contraindicated for neuropathic sensory dysfunction, as the underlying problem is nerve injury and myofascial tension, not mechanical failure 4

Relationship to Anal Cushion Damage

Beyond direct sphincter injury, hemorrhoidectomy can damage the anal endovascular cushions:

• The anal cushions serve as a conformable plug to ensure complete closure of the anal canal and contribute approximately 15–20% of resting anal pressure 1
• A patulous anal canal—identified on torso MRI—is associated with more severe anal injury and independently predicts both anal resting pressure and squeeze pressure increment 6
• Damage to the endovascular cushions may produce a poor anal "seal" and impair the anorectal sampling reflex 3