Internal anal sphincter: An anatomic study

Control of Motility in the Internal Anal Sphincter

The internal anal sphincter (IAS) plays an important role in the maintenance of fecal continence since it generates tone and is responsible for > 70% of resting anal pressure. During normal defecation the IAS relaxes. Historically, tone generation in gastrointestinal muscles was attributed to mechanisms arising directly from smooth muscle cells, ie, myogenic activity. However, slow waves are now known to play a fundamental role in regulating gastrointestinal motility and these electrical events are generated by the interstitial cells of Cajal. Recently, interstitial cells of Cajal, as well as slow waves, have also been identified in the IAS making them viable candidates for tone generation. In this review we discuss four different mechanisms that likely contribute to tone generation in the IAS. Three of these involve membrane potential, L-type Ca²⁺ channels and electromechanical coupling (ie, summation of asynchronous phasic activity, partial tetanus, and window current), whereas the fourth involves the regulation of myofilament Ca²⁺ sensitivity. Contractile activity in the IAS is also modulated by sympathetic motor neurons that significantly increase tone and anal pressure, as well as inhibitory motor neurons (particularly nitrergic and vasoactive intestinal peptidergic) that abolish contraction and assist with normal defecation. Alterations in IAS motility are associated with disorders such as fecal incontinence and anal fissures that significantly decrease the quality of life. Understanding in greater detail how tone is regulated in the IAS is important for developing more effective treatment strategies for these debilitating defecation disorders.

Preoperative MRI sphincter morphology and anal manometry: can they be markers of functional outcome following anterior resection for rectal cancer?

AIM

Good functional outcome following anterior resection (AR) for rectal cancer is an important clinical goal, but its prediction has proven difficult. Assessments such as anal manometry have been advocated as a potential tool but functional anatomy as depicted on MRI has not been investigated. This study looked at whether sphincter complex measurements recorded from preoperative staging MRIs and preoperative anal manometry have any correlation with functional outcome.

METHOD

Consecutive patients with rectal adenocarcinoma underwent preoperative manometric assessment and MRI staging. MRIs were assessed with regard to anorectal angle, puborectalis thickness, canal length and external and internal anal sphincter thickness. Functional outcome was categorized into three groups according to the number of adverse postoperative symptoms (frequency, urgency, leakage, diarrhoea, use of pads, use of antidiarrhoeal medication): 0, 1 and ≥ 2. This was evaluated 1 year following surgery and 6 months following stoma reversal where applicable. Univariate analysis of an ordinal regression model was performed with significance at the 5% level.

RESULTS

Thirty patients were assessed. No single preoperative manometric parameter proved significant (P > 0.05). Only puborectalis thickness showed a significant (P = 0.01) relationship with the number of adverse symptoms suffered postoperatively. On receiver operating characteristics analysis, a cut-off value of 3.5 mm gave an optimal sensitivity of 0.5 (95% CI, 0.17-0.83) and specificity of 0.86 (95% CI, 0.64-0.96).

CONCLUSIONS

Measurements of the puborectalis thickness on preoperative staging MRIs for rectal cancer may help predict functional outcome following AR. Prospective assessment of larger numbers with a fully validated continence score are required to evaluate these findings further.

Should we care about the internal anal sphincter?

The internal anal sphincter is currently regarded as a significant contributor to continence function. Four physiological and morphological aspects of the internal anal sphincter are presented as part of the current evidence base for its preservation in anal surgery. 1) The incidence of continence disturbance following deliberate internal anal sphincterotomy is underestimated, although there is presently no prospective imaging or physiologic data supporting the selective use of sphincter-sparing surgical alternatives. 2) Given that the resting pressure is a measure of internal anal sphincter function, its physiologic representation (the rectoanal inhibitory reflex) shows inherent differences between incontinent and normal cohorts which suggest that internal anal sphincter properties act as a continence defense mechanism. 3) Anatomical differences in distal external anal sphincter overlap at the point of internal anal sphincter termination may preclude internal anal sphincter division in some patients where the distal anal canal will be unsupported following deliberate internal anal sphincterotomy. 4) internal anal sphincter-preservation techniques in fistula surgery may potentially safeguard postoperative function. Prospective, randomized trials using preoperative sphincter imaging and physiologic parameters of the rectoanal inhibitory reflex are required to shape surgical decision making in minor anorectal surgery in an effort to define whether alternatives to internal anal sphincter division lead to better functional outcomes.

Pharmacology of the internal anal sphincter and its relevance to faecal incontinence

1 The internal anal sphincter (IAS) has a spontaneous tone and is the main contributor to the maintenance of faecal continence. The spontaneous resting tone exhibited by the sphincter can be modified by neurotransmitters from the autonomic and enteric nervous systems. 2 In this review, the influence of the sympathetic and parasympathetic nervous systems on IAS tone are discussed and the putative roles of nitric oxide, carbon monoxide, vasoactive intestinal peptide and adenosine triphosphate in non-adrenergic non-cholinergic transmission are considered. 3 Faecal incontinence is a common condition that places a heavy financial burden on the health service and severely affects patients' quality of life. Resting anal pressure is reduced in patients with faecal incontinence and agents that increase sphincter tone tend to relieve symptoms. The results of clinical studies of the use of phenylephrine to treat faecal incontinence are reviewed. 4 It is concluded that the IAS is a potential target for drug development for the treatment of faecal incontinence.