Feeling bad in more ways than one: comorbidity patterns of medically unexplained and psychiatric conditions.

<bold>Background: </bold>Considerable overlap in symptoms and disease comorbidity has been noted among medically unexplained and psychiatric conditions seen in the primary care setting, such as chronic fatigue syndrome, low back pain, irritable bowel syndrome, chronic tension headache, fibromyalgia, temporomandibular joint disorder, major depression, panic attacks, and posttraumatic stress disorder.<bold>Objective: </bold>To examine interrelationships among these 9 conditions.<bold>Design: </bold>Using data from a cross-sectional survey, we described associations and used latent class analysis to investigate complex interrelationships.<bold>Participants: </bold>3,982 twins from the University of Washington Twin Registry.<bold>Measurements: </bold>Twins self-reported a doctor's diagnosis of the conditions.<bold>Results: </bold>Comorbidity among these 9 conditions far exceeded chance expectations; 31 of 36 associations were significant. Latent class analysis yielded a 4-class solution. Class I (2% prevalence) had high frequencies of each of the 9 conditions. Class II (8% prevalence) had high proportions of multiple psychiatric diagnoses. Class III (17% prevalence) participants reported high proportions of depression, low back pain, and headache. Participants in class IV (73% prevalence) were generally healthy. Class I participants had the poorest markers of health status.<bold>Conclusions: </bold>These results support theories suggesting that medically unexplained conditions share a common etiology. Understanding patterns of comorbidity can help clinicians care for challenging patients. [ABSTRACT FROM AUTHOR]