Objective: Recurrent episodes of bloating and visible abdominal distension are common and distressing in irritable bowel syndrome, but the mechanisms are unknown. Patients often note that the distension is most pronounced in the upright posture, suggesting that the bloating may be the result of a decrease or absence of the normal rise in electromyograph activity in the abdominal wall muscles when standing. There are no reports of noninvasive electromyograph recordings of abdominal wall muscles in irritable bowel syndrome. We examined the hypothesis that abdominal distension is the result of relaxation of anterior abdominal wall musculature.
Methods: Studies were performed on patients with irritable bowel syndrome and a history of visible distension (n = 11, mean age 48.6 yr, body mass index 24.8) and normal volunteers (n = 13, mean age 39.9 yr, body mass index 24.6). Surface recordings of muscle activity were made while subjects were lying, performing voluntary contraction of the abdominal wall, and standing. The examiners were blind as to the clinical status of the subjects.
Results: There were no differences in abdominal wall muscle activity (by electromyograph voltage) when comparing patients with irritable bowel syndrome to normal volunteers (e.g., relaxed lower abdomen supine mean electromyograph voltage in irritable bowel syndrome was 14.0 vs 14.6 in controls, p = 0.7, and relaxed lower abdomen standing in irritable bowel syndrome was 29.6 vs 25.2 in controls, p = 0.4). There was increased activity in both groups when contracting the muscles and when standing.
Conclusions: Patterns of abdominal wall muscle activity do not differ between normal subjects and patients with irritable bowel syndrome. However, there is a clear increase in muscle activity in the standing position. Episodic distension is unlikely to be due to permanent anterior abdominal muscle weakness or a persistent inability of the muscles to activate with standing in irritable bowel syndrome.