This may give side effects of FI, diarrhea and steatorrhea. Overflow incontinence
This may occur when there is a large mass of feces in the rectum (fecal loading), which may become hardened (fecal impaction). Liquid stool elements are able to pass around the obstruction, leading to incontinence. Megarectum (enlarged rectal volume) and rectal hyposensitivity are associated with overflow incontinence. Hospitalized patients and care home residents may develop FI via this mechanism, possibly a result of lack of mobility, reduced alertness, constipating effect of medication and/or dehydration. Pathophysiology
The mechanisms and factors contributing to normal continence are multiple and inter-related. The puborectalis sling, forming the anorectal angle (see diagram), is responsible for gross continence of solid stool. The IAS is an involuntary muscle, contributing about 55% of the resting anal pressure. Together with the hemorrhoidal vascular cushions, the IAS maintains continence of flatus and liquid during rest. The EAS is a voluntary muscle, doubling the pressure in the anal canal during contraction, which is possible for a short time. The rectoanal inhibitory reflex (RAIR) is an involuntary IAS relaxation in response to rectal distension, allowing some rectal contents to descend into the anal canal where it is brought into contact with specialized sensory mucosa to detect consistency. The rectoanal excitatory reflex (RAER) is an initial, semi-voluntary contraction of the EAS and puborectalis which in turn prevents incontinence following the RAIR.
Congenital
Anorectal anomalies and spinal cord defects may be a cause in children. These are usually picked up and operated upon during early life, but continence is often imperfect thereafter. Anal canal
The functioning of the anal canal can be damaged, traumatically or atraumatically. The resting tone of the anal canal is not the only factor which is important; both the length of the high pressure zone and its radial translation of force are required for continence. This means that even with normal anal canal pressure, focal defects such as the keyhole deformity can be the cause of substantial symptoms. External anal sphincter (EAS) dysfunction is associated with impaired voluntary control, whereas internal anal sphincter (IAS) dysfunction is associated with impaired fine tuning of fecal control. Lesions which mechanically interfere with, or prevent the complete closure of the anal canal can cause a liquid stool or mucous rectal discharge. Such lesions include piles (inflamed hemorrhoids), anal fissures, anal cancer or fistulae. Obstetric injury may tear the anal sphincters, and some of these injuries may be occult (undetected). The risk of injury is greatest when labor has been especially difficult or prolonged, when forceps are used, with higher birth weights or when an midline episiotomy is performed. Only when there is post operative investigation of FI such as endoanal ultrasound is the injury discovered. FI is a much under-reported complication of surgery. The IAS is easily damaged with an anal retractor (especially the Parks anal retractor), leading to reduced resting pressure postoperatively.