Anatomy and Function of the Internal Anal Sphincter in Continence
Primary Anatomical Structure
The internal anal sphincter (IAS) is an involuntary ring of smooth muscle that generates the majority of resting anal tone, contributing over 70% of resting anal pressure and serving as the primary barrier preventing passive fecal and flatal incontinence. 1, 2
The IAS consists of flat rings of smooth muscle bundles stacked sequentially, resembling slats of a Venetian blind, with an average of 26 ring-like slats per anal canal, each covered by its own fascia. 3 These smooth muscle fibers coalesce at three equidistant points around the anal canal (at 1,5, and 9 o'clock positions when viewed anteriorly), forming three columns that extend into the lumen and play a crucial role in closing off the anal canal. 3
Functional Role in Continence Maintenance
Tone Generation Mechanisms
The IAS maintains continuous basal tone through four distinct mechanisms:
- Electromechanical coupling via L-type calcium channels, involving summation of asynchronous phasic activity, partial tetanus, and window current 1
- Myofilament calcium sensitivity regulation independent of membrane potential changes 1
- Slow wave activity generated by interstitial cells of Cajal, which are now recognized as fundamental regulators of IAS motility 1
- Protein kinase C (PKC) pathway signaling, which is the primary mediator of both basal tone and acetylcholine-induced force generation in human IAS 4
Neurological Modulation
- Sympathetic motor neurons significantly increase IAS tone and anal pressure 1
- Inhibitory motor neurons (particularly nitrogenic and vasoactive intestinal peptidergic) abolish contraction during normal defecation 1
- The IAS is anatomically distinct from deep pelvic autonomic nerves, being a superficial structure confined to the anal canal level 5
Clinical Implications in Patients with Constipation and Straining
Pathophysiology of IAS Dysfunction
In patients with chronic constipation and straining, the IAS often develops hypertonicity, which creates a vicious cycle:
- Increased IAS tone reduces anodermal vascular blood flow, potentially leading to ischemic complications like anal fissures 6
- Protective guarding patterns develop during painful defecation and persist even after the acute problem resolves 7, 8
- The IAS extends approximately 1.2 cm cephalad from the proximal margin of the external anal sphincter, with 1.7 cm of overlap between the two sphincters 6
Consequences of Structural or Functional IAS Damage
Structural injury or functional weakness of the IAS results in passive incontinence of feces and flatus, as the muscle is responsible for maintaining the closed anal canal at rest. 2
- Sphincter defects have been documented in up to 12% of patients after hemorrhoidectomy, primarily due to excessive retraction and extensive dilation of the anal canal 6
- Incontinence rates after hemorrhoidectomy with sphincter manipulation range from 2-12% 5
- The IAS is thin and pale pink, lying in close proximity to the anorectal mucosa, making it vulnerable to iatrogenic injury during anorectal procedures 6
Management Considerations for Hypertonicity
When IAS hypertonicity contributes to constipation or anal fissures:
- Calcium channel blockers (topical diltiazem or nifedipine) are more effective than glyceryl trinitrate for reducing IAS tone, with healing rates of 65-95% and fewer systemic side effects 6
- Treatment should continue for at least 6 weeks, with pain relief typically occurring after 14 days 6
- Manual dilatation is absolutely contraindicated due to unacceptably high incontinence rates (30% temporary, 10% permanent) 6, 8
Critical Pitfall
Lateral internal sphincterotomy, while effective for chronic anal fissures, carries wound-related complications in up to 3% of patients and can result in permanent alterations in sphincter function. 6, 7 Botulinum toxin injection represents a safer alternative, achieving 75-95% cure rates with no risk of permanent incontinence 5