a1 Department of Psychological Medicine, Otago University, Wellington, New Zealand
a2 Center for Health Studies, Group Health Cooperative of Puget Sound, Seattle, WA, USA
a3 Health Services Research Unit, Institute of Municipal d'Investigacio Medica (IMIM), Barcelona, Spain
a4 Department of Psychiatry, University of Leipzig, Germany
a5 SUNY Stony Brook, Stony Brook, New York, USA
a6 Institute for Development, Research, Advocacy and Applied Care (IDRAAC), Beirut, Lebanon
a7 Department of Mental Health, AUSL di Bologna, Italy
a8 Department of Neurosciences and Psychiatry, University Hospital, Gasthuisberg, Leuven, Belgium
a9 Hôpitaux de Paris, Paris, France
a10 Department of Psychiatry, University College Hospital, Ibadan, Nigeria
a11 Sant Joan de Deu-SSM, Barcelona, Spain
a12 Shanghai Mental Health Centre, Shanghai, People's Republic of China
a13 Department of Health Care Policy, Harvard Medical School, Boston, MA, USA
a14 Mental Health Services, Ministry of Health, Jerusalem, Israel
a15 National Institute of Psychiatry, Mexico City, Mexico
a16 Department of Rural and Indigenous Health, School of Rural Health, Monash University, Australia
a17 Department of Psychiatry, University Medical Centre, Groningen, The Netherlands
a18 Development Rehabilitation System FSC, Saldarriaga Concha Foundation, Bogota, Colombia
a19 Department of Public Health, Jichi Medical University, Tochigi, Japan
a20 Harvard School of Public Health, Boston, MA, USA
Background The relationship between mental and physical disorders is well established, but there is less consensus as to the nature of their joint association with disability, in part because additive and interactive models of co-morbidity have not always been clearly differentiated in prior research.
Method Eighteen general population surveys were carried out among adults as part of the World Mental Health (WMH) Survey Initiative (n=42 697). DSM-IV disorders were assessed using face-to-face interviews with the Composite International Diagnostic Interview (CIDI 3.0). Chronic physical conditions (arthritis, heart disease, respiratory disease, chronic back/neck pain, chronic headache, and diabetes) were ascertained using a standard checklist. Severe disability was defined as on or above the 90th percentile of the WMH version of the World Health Organization Disability Assessment Schedule (WHODAS-II).
Results The odds of severe disability among those with both mental disorder and each of the physical conditions (with the exception of heart disease) were significantly greater than the sum of the odds of the single conditions. The evidence for synergy was model dependent: it was observed in the additive interaction models but not in models assessing multiplicative interactions. Mental disorders were more likely to be associated with severe disability than were the chronic physical conditions.
Conclusions This first cross-national study of the joint effect of mental and physical conditions on the probability of severe disability finds that co-morbidity exerts modest synergistic effects. Clinicians need to accord both mental and physical conditions equal priority, in order for co-morbidity to be adequately managed and disability reduced.
(Received May 15 2007)
(Revised January 21 2008)
(Accepted February 13 2008)
(Online publication March 26 2008)