This syndrome initially gives constipation, and later FI. The pelvic floor is innervated by the pudendal nerve and the S3 and S4 branches of the pelvic plexus. With recurrent straining, e.g. during difficult labour or long term constipation, then stretch injury can damage the nerves supplying levator ani. The pudendal nerve is especially vulnerable to irreversible damage, (stretch induced pudendal neuropathy) which can occur with a 12% stretch. If the pelvic floor muscles lose their innervation, they cease to contract and their muscle fibres are in time replaced by fibrous tissue, which is associated with pelvic floor weakness and incontinence. Increased pudendal nerve terminal motor latency may indicate pelvic floor weakness. The various types of pelvic organ prolapse (e.g. external rectal prolapse, mucosal prolapse and internal rectal intussusception & solitary rectal ulcer syndrome) may also cause coexisting obstructed defecation. Rectum
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 required to detect the presence, nature and amount of rectal contents. The rectum must also be able to evacuate its contents fully. There must also be efficient co-ordination of rectal sensation and relaxation of the anal canal. Rectal storage capacity (i.e. rectal volume + rectal compliance) may be affected in the following ways. Surgery involving the rectum (e.g.
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.