Background & Aims: Although multiple clinical trials support the efficacy of psychological treatments for reducing irritable bowel syndrome (IBS) symptoms, the mechanisms responsible for symptomatic improvement are unknown. One hypothesis is that psychological treatments work by alleviating comorbid psychological distress implicated in the worsening of bowel symptoms and quality of life. An alternative hypothesis assumes that changes in distress are not strictly a cause but a consequence of IBS that will decrease with symptomatic improvement. Methods: We evaluated these 2 hypotheses by applying structural equation modeling (SEM) to the data set of a large number (n = 147) of Rome II diagnosed participants randomized to CBT, psychoeducation, or wait list. Per Rome guidelines, the primary end point was global improvement of gastrointestinal (GI) symptoms measured 2 weeks after a 10-week regimen. Secondary end points were distress and quality of life (QOL). Results: SEM analyses lend support to a model in which CBT is associated with improvements in IBS symptoms, but that therapeutic gains do not depend on changes in patients' overall level of psychological distress. Symptom severity, but not clinical status (pain catastrophizing, predominant bowel habits, symptom duration, abuse, diagnosable psychiatric disorder) or relevant sociodemographic variables (eg, gender, age), moderated treatment outcome. Conclusion: CBT has a direct effect on global IBS symptom improvement independent of its effects on distress. Improvement in IBS symptoms is associated with improvements in the QOL, which may lower distress. Symptom improvements are not moderated by variables reflecting the mental well-being of IBS patients.
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