As colonic contents move into the rectum, the rectum distends. The IAS relaxes, allowing rectal contents to come into contact with specialized sensory epithelium in the anal canal. This is called anorectal sampling. The EAS contracts via a parasympathetically mediated reflex. The EAS exerts a greater percentage of the anal tone in the presence of rectal distension. If evacuation of the rectum is not appropriate, sympathetically mediated inhibition of the smooth muscle of the rectum and voluntary contraction of the EAS and puborectalis musculature occur. The bolus of material is pushed back up the rectum to the rectal reservoir. Contraction of the puborectalis increases the anorectal angle and keeps the bolus above the IAS.

Anal incontinence (AI) is the involuntary loss of flatus, liquid or solid stool; fecal incontinence (FI) is the involuntary loss of liquid or solid stool. Among women aged 18–65, anal incontinence is reported by 28.4% and fecal incontinence by 15.9%. In women over the age of 40, fecal incontinence is reported by 24%. Women are affected 8:1 over males. Less than 20% of patients with fecal incontinence actually report it to their physician. In a study of 200 women seen in our urogynecology clinic, we found that women were more likely to report this symptom in a written questionnaire when compared to direct questioning by the physician.

The most important risk factors for fecal incontinence include increasing age, increasing BMI and irritable bowel syndrome. Other risk factors are diabetes mellitus, history of stroke, spinal cord trauma, degenerative nervous system disorders, history of pelvic and/or anorectal surgery, and inflammatory diseases of the bowel.

Childbirth is a common predisposing factor to fecal incontinence in younger women; however, the effect of childbirth on rates of fecal incontinence disappears with age. Besides disruption of the internal or external sphincter by a third- or fourth-degree perineal laceration, which has been shown to increase the risk of postpartum fecal incontinence, damage to the pudendal nerve through overstretching and/or prolonged compression and ischemia may play a role. Fecal incontinence in the postpartum period is more common after a fourth-degree than a third-degree perineal laceration. As many as 50% of women who sustain a third- or fourth-degree obstetric tear experience some impairment of anorectal function after primary repair. Obstetric risk factors predisposing to anal sphincter lacerations include episiotomy, use of forceps or vacuum assistance, occiput posterior presentation, fetal weight > 4000 g, and prolonged second stage of labor.

It is important to obtain a thorough history, including the onset, frequency, type of stool lost, history of obstetric tears, trauma, surgery, and medical conditions predisposing to incontinence. A careful physical exam including a vaginal exam, evaluation of the perineum, and a neurologic exam should be performed. Resting tone represents the IAS and squeeze pressure represents the EAS. The EAS is palpable as a 1.5–2 cm moderately firm doughnut-shaped mass within the perineal body. Usually one index finger is inserted rectally and the thumb of the same hand or the other index finger is used to identify the anal sphincter and to determine if there are any defects.

Endoanal ultrasound

Sensitivity and specificity of ultrasound for detecting sphincter defects are 98–100% for the EAS and 95.5% for the IAS. Normal thickness of the EAS is 7.5–9 mm and it appears as a hyperechoic circumferential ring. The IAS has a normal thickness of 6–7 mm and appears as a hypoechoic or sonolucent ring that is medial to the EAS. Endoanal ultrasonography is the single most important study for determining treatment options for a patient with fecal incontinence because it identifies the presence of sphincter defects that may be amenable to surgical repair.

Anal manometry