Impaired Rectal Function
The rectum needs to be of a sufficient volume to store stool until defecation. The rectal walls need to be "compliant" i.e. able to distend to an extent to accommodate stool. Rectal sensation is also required to detect the presence, nature and amount of rectal contents. The rectum must also be able to evacuate its contents fully.
Reduced storage capacity. Surgery involving the rectum (e.g. lower anterior resection, often performed in surgery for colorectal cancer) radiation treatment directed at the rectum, and inflammatory bowel disease can cause scarring, which may result in the walls of the rectum becoming stiff and inelastic. Loss of rectal storage capacity (volume + compliance) may lead to "urge incontinence", where there is an urgent need to defecate as soon as stool enters the rectum, where normally stool would be stored until there was enough to distend the rectal walls and initiate the defecation cycle. Tumors and strictures also may impair reservoir function. Radiation induced incontinence
Reduced rectal sensation. If the sensory nerves are damaged, detection of stool in the rectum is disabled, and one will not feel the need to defecate until too late.
Incomplete evacuation. Normal evacuation of rectal contents is 90-100%. If there is incomplete evacuation during defecation, or residual stool will be left in the rectum and threaten continence once defecation is finished. This is particularly a feature of patients with obstructed defecation and FI. They have difficulty evacuating, often due to anismus (paradoxical contraction or relaxation failure of puborectalis). Once the voluntary attempt to defecate, albeit dysfunctional is finished, the voluntary muscles relax, and residual rectal contents is then able to descend into the anal canal and cause leaking. Other causes of incomplete evacuation include non-emptying defects like a rectocele. Encopresis (FI, especially incontinence of liquids, in children) is frequently associated with anismus.
Read more about this topic: Fecal Incontinence, Etiology
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