The British Journal of Psychiatry (2008) 193: 364-372. doi: 10.1192/bjp.bp.107.044461
© 2008 The Royal College of Psychiatrists
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Intellectual disability co-occurring with schizophrenia and other psychiatric illness: population-based study

Vera A. Morgan, MSocSc

Neuropsychiatric Epidemiology Research Unit, School of Psychiatry and Clinical Neurosciences, The University of Western Australia

Helen Leonard, MBChB and Jenny Bourke, MPH

Telethon Institute for Child Health Research, Centre for Child Health Research, The University of Western Australia

Assen Jablensky, DMSc

Neuropsychiatric Epidemiology Research Unit and Centre for Clinical Research in Neuropsychiatry, School of Psychiatry and Clinical Neurosciences, The University of Western Australia, Perth, Australia

Correspondence: Vera A. Morgan, The University of Western Australia School of Psychiatry and Clinical Neurosciences, Level 3 Medical Research Foundation Building, Rear 50 Murray Street, Perth, Western Australia, Australia 6000. Email: vmorgan{at}cyllene.uwa.edu.au

Declaration of interest

None. Funding detailed in Acknowledgements.

Background

The epidemiology of intellectual disability co-occurring with schizophrenia and other psychiatric illness is poorly understood. The separation of mental health from intellectual disability services has led to a serious underestimation of the prevalence of dual diagnosis, with clinicians ill-equipped to treat affected individuals.

Aims

To estimate the prevalence of dual diagnosis and describe its clinical profile.

Method

The Western Australian population-based psychiatric and intellectual disability registers were cross-linked (total n=245 749).

Results

Overall, 31.7% of people with an intellectual disability had a psychiatric disorder; 1.8% of people with a psychiatric illness had an intellectual disability. Schizophrenia, but not bipolar disorder and unipolar depression, was greatly over-represented among individuals with a dual diagnosis: depending on birth cohort, 3.7–5.2% of those with intellectual disability had co-occurring schizophrenia. Pervasive developmental disorder was identified through the Intellectual Disability Register and is therefore limited to individuals with intellectual impairment. None the less, pervasive developmental disorder was more common among people with a dual diagnosis than among individuals with intellectual disability alone. Down syndrome was much less prevalent among individuals with a dual diagnosis despite being the most predominant cause of intellectual disability. Individuals with a dual diagnosis had higher mortality rates and were more disabled than those with psychiatric illness alone.

Conclusions

The facility to combine records across administrative jurisdictions has enhanced our understanding of the epidemiology of dual diagnosis, its clinical manifestations and aetiological implications. In particular, our results are suggestive of a common pathogenesis in intellectual disability co-occurring with schizophrenia.


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