Anismus, also known as pelvic floor hypertonicity, pelvic floor dyssynergia, dyssynergic defaecation, or paradoxical puborectalis contraction, is a condition in which the external anal sphincter and the puborectalis muscle, one of the core pelvic floor muscles, contract rather than relax during an attempted bowel movement. The puborectalis muscle forms a sling around the back of the lower part of the rectum and is normally tense (contracted). This muscle should be able to relax and allow the angle between the rectum and the anal canal to straighten so that stool can pass successfully. This relaxation does not happen in people with anismus.
Why some people develop anismus is not well understood. People with anismus often get a sensation of blockage or resistance to passing stool. This can be painful and lead to obstructive constipation. Other problems may include faecal impaction (hard dry stools that stay in the rectum) and megarectum (enlargement of the diameter of the rectum). People with anismus typically strain when attempting to have a bowel movement, which further irritates the pelvic floor muscles. Men and women with anismus may need to put a finger inside the anus and apply pressure to allow a bowel movement.
Anismus is diagnosed by clinical examination of the rectum and some additional tests, including anorectal physiology and proctography.
Many patients report improvement in their symptoms with pelvic floor retraining, which helps the sphincter muscles to relax when going to the toilet. Alternatively, some patients benefit from Botulinum toxin injection into the puborectalis muscle. The first injection helps make the diagnosis. If anismus is the cause of the patient’s symptoms, a beneficial effect may be evident within the first week, but may take up to six weeks to appear. If there is no change in symptoms after six weeks, no further injection is given. If the injection helps, a repeat injection is given, and it is this second injection that gives long-term benefit, which may last for years or be permanent. If the initial Botulinum toxin injection is unsuccessful, there may be an underlying internal rectal prolapse, and an examination under anaesthesia in the operating theatre would be performed to confirm this.
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