Irritable bowel syndrome (IBS) is a functional condition of the gastrointestinal tract, which presents with diarrhea/constipation, abdominal pain, and bloating symptoms. It is most commonly observed in the age group between 20 and 40 years, and is twice more common in women.

It can cause significant psychiatric symptoms because of the unpredictability and severity with which symptoms come on after a period of remission, which may last anywhere from days to months.

However, this disorder has many other manifestations, and co-existing conditions are not uncommon. These may be responsible for 75% of symptoms serious enough to warrant a doctor’s visit. These other illnesses may be gastrointestinal or psychiatric, or may represent somatization disorders (where psychological stress is reflected in somatic symptoms across several organs and systems of the body).

Interplay between Physical and Psychological Factors

The common occurrence of these disorders in the same subset of patients suggests that they are linked by one feature which is essential to their manifestation. This is thought to be psychological in nature. That is, even though they are not different expressions of the same condition, they are all signs of an underlying psychological morbidity. Both body and mind work together under the influence of psychological stress to produce the constellation of disorders to which IBS belongs.

Another explanation is that this condition is not caused by a single entity but as part of a closely related group of etiological factors. Among those with IBS, some patients have a chiefly physical basis for their symptoms, while for others, the mental disturbance is primary. When there are several comorbid conditions with IBS, it suggests that the patient belongs to the latter group, where psychological etiology is more important.

Many of these patients have at least one additional bodily disorder already diagnosed, and may even have other functional complaints. It is interesting to note that those with multiple functional disorders are at the upper end of the spectrum for IBS severity. They tend to have more mental distress, a lower quality of life, and take more time off work related to their illness.

Common Comorbid Conditions

Common comorbid conditions include:

• Fibromyalgia: between 35 and 70 percent of patients with fibromyalgia also have IBS
• Chronic fatigue syndrome: 14% of these patients have IBS
• Psychological disorders such as generalized anxiety disorder, panic disorder, major depression, bipolar disorder, and schizophrenia: these are also found to have a higher incidence in patients with IBS, with the risk of having IBS being multiplied from 5- to 6-fold
• Functional dyspepsia: 15-40% have IBS
• Gastro-esophageal reflux disorder (GERD): 36% of these patients have IBS
• Chronic pelvic pain: 30-80% of women with chronic pelvic pain have IBS
• Sexual dysfunction: 41% of women with IBS report loss of libido, dyspareunia, and aggravation of the IBS symptoms after a sexual experience

Mechanism of Comorbidity

IBS and other functional disorders may in some cases, if not all, be different manifestations of the same underlying disease process. This means that one mechanism produces various clinical syndromes, based upon the interaction with other factors. The importance of the association with psychological illness has led to a focus on treatment which includes this vital area as well. Screening tests for this area also need to be developed.

In short, social, genetic, and psychological factors interact in producing the symptoms of IBS, through their effects on the primary endocrine and regulatory axis of the body - the hypothalamus-pituitary-adrenal axis. Other mechanisms include the development of increased sensitivity to visceral sensations, or an increased mental vulnerability which lays the patient open to experiencing pain and distress disproportionately.

Therefore IBS is commonly associated with one or more co-morbid conditions, often in the digestive tract, but often outside it. Such patients experience more severe symptoms, require more health care, and have a lower quality of life. In addition, they have many times higher rates of psychological symptoms such as depression or somatization disorder.