Anismus

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Diagnosis and reasoning

This middle aged lady has presented with persistent constipation which has remained unresponsive to most standard measures, including a high-fibre diet, increased fluid intake, and over-the-counter laxatives. A careful scrutiny of her history reveals no red flags (such as hematochezia or marked weight loss) which might suggest malignancy or other serious conditions. In fact, her symptoms are more suggestive of a primary origin, and meet the Rome III diagnostic criteria for functional constipation. Note also the presence of a seemingly high anal sphincter tone at rest and hard impacted stools; these findings are more in favor of outlet obstruction rather than inertia of the entire colon. In particular, the high sphincter tone might potentially indicate the presence of anismus, a well known type of functional constipation which is characterized by paradoxical sphincter contraction. Unfortunately, a significant number of healthy individuals also manifest increased sphincter tone during a rectal examination; therefore this sign is not pathognomonic of anismus. However, given the clinical suspicion, it does make sense to investigate further in this direction. Before resorting to the specialized tests necessary to do so, it makes sense to rule out certain common secondary causes of constipation. Thyroid disease is common in females of this age, and constipation is a classic sign of hypothyroidism; thus, thyroid function tests are much in order. However, these turn out to be normal. Metabolic derangements (such as hypercalcemia) can also precipitate constipation; conversely, longstanding constipation can give rise to the same; however, all tests in this regard are normal as well. While constipation can also occur secondary to neurological, rheumatologic and psychiatric conditions, her history and examination are not suggestive of such an etiology; nor is she on any drugs which might cause this. Anorectal manometry combined with a balloon expulsion test is probably a good first-line gastroenterological study in this patient. This reveals a paradoxical increase in anal sphincter pressure (as opposed to relaxation, which is the normal response), and an inability to generate propulsive forces sufficient to expel the balloon out of rectum; these are in concordance with the clinical suspicion of anismus. Note also that the preservation of the rectoanal inhibitory reflex excludes adult onset, short segment Hirschsprung's disease (which is another potential cause of outlet obstruction). Colonoscopy is not indicated in this patient, given the absence of red flags for malignancy; note in particular that colonoscopy usually yields very little information in patients who are suspected to have functional constipation. Biofeedback therapy is the treatment of choice in patients with anismus. Anorectal myomectomy, in which the puborectalis muscle is weakened by partial surgical division, was popular before introduction of biofeedback therapy, but is no longer used except as a last resort. Both colectomy and fecal diversion do not resolve the underlying problem of inappropriate contraction of the pelvic muscles; even worse, unnecessary surgical handling can aggravate the condition.


Discussion

Constipation is an extremely common complaint, affecting up to 30% of the adult population; many of these cases are due to functional constipation, where no underlying structural or metabolic cause can be identified. Anismus (which is also known as pelvic floor dyssynergia) is a leading cause of functional constipation; these patients experience inappropriate contraction (rather than relaxation) of the pelvic floor muscles during attempted defecation, with the puborectalis muscle often particularly affected; this is termed "paradoxical sphincter contraction" (PSC). While the exact etiology underlying PSC is still unclear, a recent postulation is that frequent and severe straining leads to loss of voluntary control of the muscles involved in defecation, and an adaptive increase in the muscle tone of the pelvic floor. If the above is true, PSC is then best considered a consequence rather than a cause of constipation. Another school of thought is that functional defecation disorders including anismus may develop due to psychological distress; this is supported by a study revealing a strong association between anismus and a history of physical or sexual abuse. Note also that anismus is more likely to develop after stressful conditions such as pregnancy and childbirth; while there is no denying the physical strain and the subsequent weakening of muscles brought forth by these events, the psychological aspect is also not negligible. Key symptoms of anismus include an inability to initiate defection, prolonged straining, passage of lumpy or hard stools, a sensation of incomplete evacuation, a feeling of anorectal obstruction or blockade, and the use of maneuvers to facilitate defecation including application of pressure over the perineum during defecation, and manual disimpaction. Unfortunately, the above symptoms are general to functional constipation; no single finding or grouping of findings is specific to anismus. Examination of these patients may demonstrate an increased sphincter tone; however, this too is not specific to the condition. Physiological tests are key to establishing the diagnosis of anismus; anorectal manometry, the balloon expulsion test, defecography, electromyography (EMG) of the puborectalis muscle, and colonic transit studies are investigations used in this regard. Anal manometry typically demonstrates an increase in anal sphincter pressure during attempted defecation, while balloon expulsion test shows failure to expel the balloon; unfortunately, the use of these tests is limited by the fact that they themselves can trigger PSC in some normal individuals. Similarly, retention of markers in the rectosigmoid segment in colonic transit studies or paradoxical recruitment of puborectalis muscle in EMG does not necessarily correlate with a diagnosis of anismus. Presumably, a positive result obtained from multiple investigations (as opposed to a single test) is diagnostically more accurate since a finding is unlikely to be an artefact if verified by several different techniques. However, this should be tempered by the fact that most of the above investigations are only available in specialized settings, and are often expensive. Thus, when anismus is suspected, it perhaps makes more sense to start treatment with the standard recommendations for all constipated patients (i.e. a high fiber diet, adequate fluid intake, regular exercise, and cautious use of laxatives); detailed investigations are probably indicated only in patients who fail to respond to the above measures. Biofeedback therapy is considered the initial treatment of choice for anismus. The primary target of biofeedback therapy is to restore the normal pattern of defecation. This involves teaching patients to increase their intra-abdominal pressure by contracting their abdominal muscles during defecation, while at the same time relaxing the pelvic muscles. This is usually achieved by an instrument based learning process (expelling an air filled rectal balloon); occasionally, sensory retraining (in which patients are taught to sense the low-threshold rectal filling sensation) is also used. Reportedly the majority of patients who complete biofeedback therapy experience significant improvement of their symptoms and quality of life. In some cases, psychotherapy may need to be added to address any psychological distress present. If these primary measures fail, more invasive techniques such as intramuscular injection of botulinum toxin to the puborectalis muscle, chronic low-amplitude sacral nerve stimulation to modify the electrical activity of pelvic floor or partial division of puborectalis muscle can be considered.


Take home messages

  1. Constipation affects up to 30% of the adult population, with functional (primary) constipation comprising most of these cases.
  2. Anismus, a subdivision of functional defecation disorder is thought to be due to paradoxical sphincter contraction (PSC) and is a strikingly over-diagnosed condition.
  3. Colonic transit studies, anorectal manometry, the balloon expulsion test, defecography, and electromyography (EMG) of the puborectalis muscle are the standard investigations for diagnosis of anismus; unfortunately, no single test is diagnostic.
  4. Biofeedback therapy is the preferred initial treatment for these patients.

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