The syndrome has recently been attributed to neurologic or muscular hypersensitivity of the colon. IBS is part of a broader clinical complex known as functional bowel syndrome, which includes nonulcer dyspepsia, an entity with a constellation of upper gastrointestinal tract symptoms such as upper abdominal pain, nausea, and postprandial bloating. People in whom IBS is diagnosed often have symptoms related to nonulcer dyspepsia at various times in their life, and the reverse is true of those in whom nonulcer dyspepsia is diagnosed. Thus, IBS is not a disease reflecting intrinsic colonic dysfunction. Instead, the disorder appears to be caused by a central neurologic mechanism that manifests itself as symptoms originating in different organs at various times.
Patients often have predominant manifestations of either constipation or diarrhea.
Patients often note loose or frequent bowel movements at the onset of abdominal pain, with relief of pain after defecation. However, with clustering of several symptoms, the likelihood of IBS increases. A noteworthy finding of Manning's study is that patients with predominant constipation usually do not complain of abdominal pain. Additionally, patients with IBS often are awakened by symptoms. This feature is important because nocturnal symptoms are often considered a warning sign of organic disease, yet the study reveals that they often occur with functional disorders as well.