16 Faecal incontinence (leakage of stool)

Case

Mrs C.B. is a 58-year-old woman who presents complaining of worsening faecal incontinence over 2 years. She had noticed soiling of her underclothes two or three times per week and needed to wear a pad. This occurred particularly if the stool was loose but also occasionally with formed stool. She also had incontinence to flatus. The incontinence took the form of urgency, with inability to defer defecation and soiling if she did not make the toilet in time. At other times there was involuntary seepage without awareness.

In terms of risk factors for incontinence, she had three vaginal deliveries, the first being complicated by a third degree tear. She was not aware of haemorrhoidal or rectal prolapse, and there had been no prior surgery for an anal condition.

On digital anorectal examination the sphincter felt weak and the sphincter ring felt intact. There was no external rectal prolapse, or internal prolapse seen at rigid sigmoidoscopy when asking the patient to strain down. Anorectal manometry showed a low resting and squeeze pressure, and normal rectal compliance. Pudendal nerve conduction studies showed delayed conduction along the nerves, and endoanal ultrasound showed a thin sphincter anteriorly, but the sphincter was otherwise complete. The findings indicated neurogenic weakness of the internal and external sphincter as the cause of incontinence.

Management and progress: Mrs B was prescribed psyllium (Metamucil™) and loperamide (Imodium™) to attempt to bulk up the stools and make them more solid. She was given biofeedback treatment to try to strengthen the sphincter muscles. There was no significant improvement in her incontinence. She was then treated with sacral nerve stimulation and had a good result with the test electrode; she then went on to have a permanent stimulator inserted, with good effect.

Introduction

Faecal incontinence (leakage of gas or stool) is a common condition, occurring in 10% of the adult population. Men and women are affected in equal proportions, although in men the incontinence is generally much less severe. Incontinence can cause restriction of lifestyle or even social isolation. In many cases, it is the factor resulting in an elderly person requiring entry into a nursing home. It has been estimated that only one-fifth of people with incontinence consult their doctor about the problem—the remainder fail to do so, citing as their reason embarrassment or the belief that no treatment is available. A recent analysis has shown that treatment of faecal incontinence in Australia costs several hundred million dollars annually and places a significant financial burden on the community. It is therefore vital to understand the condition, and improve treatment as well as methods of prevention.

Definitions

Continence is defined as the ability to control the onset of rectal evacuation. Minor incontinence refers to mucus discharge or incontinence of liquid or solid stool less than once per week. Major incontinence is loss of liquid or solid stool more than once a week, or sufficiently frequently to be causing social embarrassment for the patient. A more objective numerical continence scoring system, taking into account the exact frequency and nature of incontinence, is now commonly used in specialist centres and a simplified version is helpful for routine clinical assessment (Table 16.1).

Table 16.1 Simplified Wexner scoring system for faecal incontinence (0–20): 0 = normal continence, 20 = severe incontinence

It is important to focus on the exact nature of the incontinence as described above in order to decide whether the condition warrants full investigation and treatment.

History

The causes of faecal incontinence are summarised in Box 16.1. A good history about the nature of the incontinence will often provide clues about the cause. Physical examination and tests of anorectal function will determine whether the sphincter is normal, and will identify the likely pathology in most cases. The three most common causes of faecal incontinence requiring surgical treatment are rectal prolapse, sphincter trauma and neurogenic incontinence. These conditions account for the majority of patients who suffer severe longstanding symptoms.

The features in the history that need to be established in these patients are:

• Is the incontinence for solids, liquids or gas? Determine whether complaints of incontinence reflect only perianal soiling by mucus, or whether there is leakage of stool. Mucus seepage occurs with local anal conditions such as anal mucosal prolapse and haemorrhoids.

• Is it passive incontinence, where there is involuntary seepage of stool without awareness? Passive incontinence occurs with internal sphincter weakness.

• What is the frequency and duration of the incontinence, and has it been progressive?

• Is urgency present? Is the patient able to defer defecation for a reasonable time? Urgency and incontinence may occur when the sphincter is normal, such as in proctitis and rectal cancer, or with diarrhoea. With diarrhoea, incontinence results from rapid transit of stool through the lower colon. It may be preceded by lower abdominal pain prior to the incontinent event. Importantly, urgency and incontinence are also caused by external sphincter weakness.

• Does the incontinence date from a particular medical event, such as perianal surgery or a difficult vaginal delivery? Perianal trauma may not present with incontinence immediately after the event, but rather many years later. Thus, the anal sphincter mechanism can compensate for a badly damaged sphincter due to anal surgery (e.g. for fistula), an unsatisfactorily repaired episiotomy or third-degree tear, or prolonged stretching of the pudendal and sacral nerves during second-stage vaginal delivery.

• Are there any other symptoms of anal or rectal disease?

Examination

The patient is examined in the left lateral position with the hips flexed to allow the examiner good access to the perineum and anal region. The area is inspected for soiling, and for excoriation of the perianal skin as evidence of chronic irritation secondary to incontinence. Any external prolapse at the anus (Box 16.2) and an asymmetrical sphincter caused by trauma and perianal scarring are looked for.

Digital anorectal examination is carried out next and the resting tone (produced by the internal sphincter) is noted. The functional length of the anal canal in centimetres should be estimated. The strength of the pelvic floor muscles is assessed by pressing posteriorly and laterally. External sphincter strength is assessed by asking the patient to contract the sphincter and, thereafter, to cough. A shortened, weak sphincter is found in neurogenic incontinence. A weak sphincter of normal length is caused by a sphincter defect due to trauma. The sphincter is examined circumferentially between the thumb and index finger and a defect may be palpable.

The rectal mucosa is then examined circumferentially for proctitis or a tumour; this should not be done at the beginning of the examination since discomfort from deep pelvic examination may impair the subsequent ability of the patient to cooperate with contraction and relaxation of the sphincter.

The patient is next asked to bear down while the examiner looks for rectal prolapse. The ischial tuberosities are palpated and the plane of the perineum assessed in relation to the position of an imaginary line drawn between the tuberosities. If the perineum descends below this line when the patient is asked to bear down, this is evidence for abnormal descent associated with neurogenic weakness of the pelvic floor muscles. If rectal prolapse is suspected but not demonstrated in this position, the examination should be repeated with the patient squatting over a paper towel or sitting on a commode.

If there is a history suggestive of a neurological condition, a full neurological examination should be carried out.

Proctoscopy and sigmoidoscopy

At proctoscopy, anal mucosal prolapse and haemorrhoids can be diagnosed. Sigmoidoscopy is essential to exclude rectal pathology including proctitis and rectal tumours (Ch 22). The patient is asked to strain down on the sigmoidoscope and prolapse of the rectal wall can often be detected in this way.

Physiological Anorectal Assessment

Before considering physiological investigation of anal incontinence it is important to consider the physiology of continence. Continence is maintained by a complex process that has both sensory and motor components.

Physiology of continence

Normal continence depends on an interaction of the following factors.

The anal canal is surrounded by a muscular tube that produces a high-pressure zone exceeding the pressure in the rectum. The sphincter contains two layers (Fig 16.1): an inner layer of involuntary smooth muscle (internal sphincter) and an outer layer of skeletal muscle (external sphincter). The internal sphincter is a distal continuation of the circular muscle of the rectum and is in a state of constant contraction, maintained by a process of intrinsic muscle stimulation. Relaxation of the muscle occurs during defecation, mediated by a local neural reflex within the wall of the anorectum in response to distension of the rectum by the faecal bolus, as well as by extrinsic autonomic control via the presacral sympathetic nerves. The external sphincter is under voluntary control but also contracts involuntarily in response to an increase in intraabdominal pressure via a spinal reflex through the anterior horns of the S2–S4 spinal segments and the Onuf nucleus in the spinal cord.