Fecal Incontinence - Physiologic Continence Mechanism

Physiologic Continence Mechanism

To understand FI, it is helpful to study the normal continence mechanism. The rectum is believed to act as a reservoir to store stool until it fills past a certain volume, at which time the defecation reflexes are stimulated. In continent individuals, defecation can be temporarily delayed until it is socially acceptable. The rectum is able to expand to a degree to accommodate this function (rectal compliance). Distension of the rectal walls creates the awareness of needing to defecate.

The point at which the rectum joins the anal canal is known as the anorectal ring, which is at the level that the puborectalis muscle (part of the pelvic floor muscle, levator ani) loops around the bowel from in front, where the puborectalis attaches to the pubic bone. This arrangement means that when puborectalis is contracted, it pulls the junction of the rectum and the anal canal forwards, creating an angle in the bowel called the anorectal angle. This angle prevents the movement of stool stored in the rectum moving into the anal canal. It is thought to be responsible for gross continence of solid stool. Some believe the anorectal angle is one of the most important contributors to continence.

Conversely, relaxation of the puborectalis reduces the pull on the junction of the rectum and the anal canal, causing the anorectal angle to straighten out (increase). A squatting defecation posture is also known to increase the anorectal angle, meaning that less effort is required to defecate when in this position. This has led to the recommendation that a squatting position be used by specific patient groups, for example those with constipation or rectal outlet obstruction (obstructed defecation, e.g. anismus).

The anal canal can be defined functionally as the distance between the anorectal ring and the end of the internal anal sphincter (IAS). The IAS forms the walls of the anal canal. It is an involuntary, smooth muscle. In normal persons it is contracted at all times except when there is a need to defecate. This means that the IAS contributes more to the resting (i.e. when not consciously contracting) tone of the anal canal than the external anal sphincter (EAS). The IAS is responsible for creating a watertight seal, and therefore provides continence of liquid stool elements. Hemorrhoids are vascular cushions within the anal canal. These structures are present during health, but when they become inflamed, they are termed piles. They are also thought to contribute to continence, but to a lesser degree than the IAS (15 and 55% of the resting anal tone respectively). During valsalva maneuvers (sneezing, coughing, etc.), the hemorrhoidal vascular cushions fill with blood to resist the increased pressure from rectal contents.

Distension of the rectum causes the IAS to relax (rectoanal inhibitory response, RAIR) and the EAS initially to contract (rectoanal excitatory reflex, RAER). The relaxation of the IAS is an involuntary response.

The EAS, by contrast is made up of skeletal (or striated muscle) and is therefore under voluntary control. It is able to contract vigorously for a short time. Contraction of the external sphincter can defer defecation for a time by pushing stool from the anal canal back into the rectum (essentially a type of reverse peristalsis). Voluntary contraction of the EAS produces a doubled pressure in the anal canal (maximal squeeze pressure). Contraction of EAS and puborectalis together also acts to lengthen the anal canal.

The mechanisms and factors contributing to normal continence are therefore many and inter-related:

• Consistency of stool- stool is easier for the muscles to control if it is solid. Liquid stool is more likely to leak out than solid stool.
• Size of formed stools- small, hard stool requires more muscular contraction to expel than bulky, softer stools
• Specialized anti-peristaltic function of the last part of the sigmoid colon, which keeps the rectum empty most of the time
• Rectal capacity - the rectum must be able to accommodate and stretch to a degree (rectal compliance)
• Pelvic floor muscles - to maintain the anorectal angle when contracted
• The IAS and the hemorrhoidal vascular cushions together give a watertight seal to the anal canal
• Sensation in the lining of the rectum and the anal canal to detect when there is stool present, its consistency and quantity
• Normal rectoanal reflexes and defecation cycle which completely evacuates stool from the rectum and anal canal
• Normal internal anatomy of the anal canal and rectum, with no non-emptying reservoirs which may cause incomplete evacuation of stool, e.g. a rectocele

Problems affecting any of these mechanisms and factors may be involved in the etiology of FI.