The neurogenic hypothesis of stress incontinence.

There are a number of causes of incontinence. The common forms of urinary incontinence, faecal incontinence or double incontinence, are stress related, in that voiding of urine or faeces occurs in response to a sudden increase in pressure in the bladder or anorectum that is not opposed by an adequate pressure increase in the sphincteric region. This weakness of the sphincter mechanism is due to chronic partial denervation of the striated sphincter muscles of the pelvic floor, comprising the external and sphincter muscle and puborectalis (puboanalis) components of the voluntary anal sphincter musculature, and the periurethral and intramural components of the urinary striated sphincter musculature. Denervation of these muscles occurs progressively following injury initiated during childbirth and then sustained by repeated stretch-induced injury during straining behaviour at stool. Age-related changes to this innervation may also be important. Weakness of the pelvic floor, and perineal descent during straining, lead to secondary changes in the anatomy of the bladder neck, of the anorectal angle, and of the smooth muscle of the internal urinary and anal sphincters. The cystometric and anal manometric changes found in patients with stress incontinence are secondary to this neurogenic weakness of the pelvic floor.