There are three types of muscle. Skeletal muscle is attached to bones and used for movement. Cardiac muscle is present in the heart. Smooth muscle forms the tissue of hollow internal organs such as blood vessels, the intestines and bladder. Smooth muscle is involuntary, it is not usually under conscious control. The gut has smooth muscle (see this image from The Open University).
IBS patients experience excess contractions or ‘spasms’, leading to pain and the sensation of abdominal bloating or ‘trapped wind’. These are not true spasms as the onset is gradual. The production of ribbon stools is a consequence of a gut that is contracted and unable to relax, producing long, narrow stools. This would also lead to the need to strain to defecate, since a partially contracted gut has little scope for further contractions to propel its contents. This is similar to the school bully who offers to shake hands and crushes their victim’s fingers. If offered one or two fingers they are unable to exert much pressure.
As well as a propensity of the gut to contract there is abnormal motility in the colon, the small bowel and the oesophagus of IBS patients. IBS patients have a gut that contracts more readily and a gut that has abnormal motility. This abnormal motility will be addressed in a later chapter on the Enteric Nervous System.
Patient experience and research confirms there is abnormal smooth muscle contraction throughout the gut of IBS patients.
Is there evidence to suggest IBS patients have a more generalized smooth muscle disorder?
IBS patients frequently suffer from ‘irritable bladder‘.
Abnormal contraction of bronchial smooth muscle leads to reduced lung capacity and symptoms of asthma. Compared to the general population, IBS patients have a higher prevalence of respiratory symptoms (33.8% v 5.8%) and asthma (15.8% v 1.45%). Note that a propensity to abnormal smooth muscle contraction is not the underlying cause of asthma, rather a contributory factor.
Dysmenorrhoea – painful menstruation typically involving abdominal cramps and dyspareunia (painful sexual intercourse) are both more common in IBS patients.
Thus, there is evidence that:
IBS patients have a generalized smooth muscle disorder.
Next we look at Abnormal Pain Response.