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Intellectual disability co-occurring with schizophrenia and other psychiatric illness: population-based study

  • Vera A. Morgan (a1), Helen Leonard (a2), Jenny Bourke (a2) and Assen Jablensky (a3)



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.


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


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


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 overrepresented 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.


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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Corresponding author

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:


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Declaration of interest

None. Funding detailed in Acknowledgements.



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Intellectual disability co-occurring with schizophrenia and other psychiatric illness: population-based study

  • Vera A. Morgan (a1), Helen Leonard (a2), Jenny Bourke (a2) and Assen Jablensky (a3)
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Dual Diagnosis Quandries

Patricia Hogan, Psychiatrist
26 May 2009

To Vera Morgan, Et al, authors of ?Intellectual Disability co- occurring with Schizophrenia and other Psychiatric Illness: Population- based study? [(2008) 193, 364-372], You have made a useful contribution inthe area of intellectual disability/mental illness (ID/MI) dual diagnosis.However, this study, like most in this area, is flawed by inadequate definition of terms.?Intellectual Disability?, the current phrase of fashion for this population, is unsatisfactory, because many individuals in the higher IQ ranges are not disabled. The American Association on Mental Retardation (now AAIDD) definition, probably the most widely used definition, is cited. It gets around the disability issue by requiring that ID individuals must also have ?limitations in adaptive behaviors and skills?.This confounds the ID and MI categories, as such limitations may well be mental illness. Perhaps a better term for studies to use would be ?intellectual impairment?, which, like visual impairment, does not necessarily imply disability. Then all individuals in certain IQ ranges could be included. As it is, a certain portion of the individuals without MI are excluded by the definition. This may inflate the prevalence rates.

Additionally, there is a problem in lumping together all ranges of ID. As you note, MI, particularly schizophrenia is more likely to be diagnosed in the Borderline group and Pervasive Developmental Disorder(PDD) is more likely to be diagnosed in the severe/profound group. Rather than a true reflection of incidence, this may reflect a nosological bias.A strict definition of Schizophrenia is difficult to apply to a non-verbalperson. Historically, PDD and schizophrenia have sometimes been used interchangeably in apparently disturbed and non-verbal individuals, but since the 1990s, at least in the US, there has been a massive shift towards the diagnosis of PDD subcategories, such as autism and Asperger?s.The diagnosis of schizophrenia has an additional stigma which some families find unacceptable. The authors found some trends distinguishing dual diagnosis individuals from those with ID alone. Some of these trends also distinguished borderline from other levels of ID (fewer genetic causes, less Down syndrome, less epilepsy, etc). To distinguish Dual diagnosis from ID alone, probably results should be controlled for IQ level.

You have considered dual diagnosis patients to have more severe MI than other MI patients as indicated by number of hospitalizations, length of hospitalizations, etc. Perhaps this just indicates that treatment and placement options for these patients are poorer. Future studies need to bedone to clarify the unique aspects of this population.

Patricia E. Hogan, D.O., Psychiatrist
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Conflict of interest: None Declared

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Re: Dual Diagnosis Quandries

Vera A. Morgan, Epidemiologist
02 April 2009

We thank Patricia Hogan for her comments challenging current definitions of intellectual disability (ID) and highlighting the difficulty of accurate assessment of psychotic illness in individuals withintellectual disability. With respect to the former, we note the importance of applying standard definitions and nomenclature in the study of the epidemiology of dual diagnosis. The criteria used to define ID affect prevalence rates and the use of IQ criteria alone rather than the dual criteria of IQ and adaptive behaviours will have a marked impact on rates (Whitaker, 2004). We employed the American Association on Mental Retardation dual criteria in our study. The use of dual criteria is the most common approach across services and in research, and is consistent with DSM-IV and ICD-10 definitions. As the American Association on Mental Retardation criteria are the basis of service eligibility in Western Australia, their use ensures a thorough assessment of individuals on the intellectual disability register and greater confidence that cases have been correctly classified in this study. While the difficulty of diagnosing psychosis accurately in individuals with intellectual disability is well-documented (Deb & Weston, 2000; Friedlander & Donnelly, 2004), our paper highlights another pressing issue. The poor recognition of dual diagnosis in affected individuals as a result of the administrative separation between ID and mental health services has led toa serious underestimate of the prevalence of dual diagnosis and has created structural impediments to inter-agency approaches to integrated, person-oriented clinical practice. Critical improvements are needed both in the structure of service provision and in clinical education programs to ensure dual diagnosis is correctly identified and appropriately treated(Bouras & Holt, 2004; Catinari, Vass, Ermilov, & Heresco-Levy, 2005). Otherwise dual diagnosis will continue to be recognised and treatedineffectively or, at worst, missed altogether, with important implicationsfor best practice.

Bouras N, Holt G: Mental health services for adults with learning disabilities. British Journal of Psychiatry 184:291-292, 2004.

Catinari S, Vass A, Ermilov M, et al.: Pfropfschizophrenia in the ageof deinstitutionalization: whose problem? Comprehensive Psychiatry 46:200-205, 2005.

Deb S, Weston S: Psychiatric illness and mental retardation. Current Opinion in Psychiatry 13:497-505, 2000.

Friedlander R, Donnelly T: Early-onset psychosis in youth with intellectual disability. Journal of Intellectual Disability Research 48:540-547, 2004.

Whitaker S: Hidden learning disability. British Journal of Learning Disabilities 32:139-143, 2004.
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Conflict of interest: None Declared

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