Elsevier

Psychiatry Research

Volume 250, April 2017, Pages 99-105
Psychiatry Research

What is the prevalence of autism spectrum disorder and ASD traits in psychosis? A systematic review

https://doi.org/10.1016/j.psychres.2017.01.017Get rights and content

Highlights

  • Rates of ASD and ASD traits are elevated in a psychosis population.

  • Despite overlaps, there remains a diagnostic distinction between the two disorders.

  • Large-scale epidemiological studies are required to ascertain rates of co-occurrence.

Abstract

There is increasing evidence to suggest both a symptomatic overlap and a clinically significant degree of co-occurrence between Autism Spectrum Disorders (ASD) and psychotic disorders such as schizophrenia but the nature of such relationships remain unclear. We reviewed the literature reporting prevalence rates of Autistic-like Traits (ALTs) and ASD in populations with a diagnosis of schizophrenia or other psychotic disorder. A search of three large databases was conducted and from this seven studies met the criteria for inclusion. The point prevalence rates for ALTs ranged from 9.6% to 61%, whilst the prevalence rates for diagnosed ASD ranged from <1% to 52% across outpatient and inpatient populations. This suggests that prevalence rates of ALTs and ASD in psychosis populations are much higher than in the general population. This has important implications regarding future research, and clinical implications in terms of ensuring that patients receive the most appropriate diagnosis and treatment.

Introduction

In recent years there has been increasing interest in the relationship between psychotic symptoms, psychotic disorders (such as schizophrenia), and symptoms consistent with Autism Spectrum Disorders (ASD). Historically there was much confusion in this field. A century ago, Bleuler considered “autism” to be one of the four core symptoms of schizophrenia (Askok et al., 2012). In the middle part of the twentieth century, the terms schizophrenia and autism were often used synonymously in childhood. It was not until the 1970's that Kolvin (1971) and Rutter (1972) clearly defined ASD and childhood schizophrenia as separate disorders (Chisholm et al., 2015). They remain classified as separate constructs within current diagnostic classification systems.

ASD is a childhood-onset developmental disorder and its diagnosis is dependent upon evidence of impairments in social communication and social interactions, in addition to persistent restricted, repetitive patterns of behaviour or interests. Publication of the fifth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5) has seen previously distinct subtypes of ASD such as Asperger's Syndrome (AS) subsumed within the category of ASD, under neuro-developmental disorders. The prevalence of ASD is estimated to be approximately one percent of the population (Brugha et al., 2011) and it is well documented that individuals with a diagnosis of ASD are predisposed to a variety of other mental health difficulties and co-morbidities.

The population prevalence of schizophrenia is estimated to be one percent (Bradley et al., 2011) and ASD may present a risk factor for the development of schizophrenia (Nylander et al., 2008). DSM-5 and International Classification of Diseases (ICD-10) criteria for schizophrenia relate to the presence of two of more of the following symptoms: hallucinations, delusions, disorganised speech, disorganised (or catatonic) behaviour, and negative symptoms, present for a significant duration throughout a one month period. Where there is childhood onset of ASD or another communication disorder, a DSM-5 diagnosis of schizophrenia will only be made if there is evidence of prominent hallucinations or delusions for at least one month.

ASD and schizophrenia are both characterised by atypical neurodevelopment of language and difficulties with social interaction and communication; however, individuals diagnosed with Asperger's Syndrome (AS) as per the DSM-IV classification system by definition do not have significant delay in language or cognitive development (Davidson et al., 2014: Raja and Azonni, 2001; Solomon et al., 2008). The impairments in social functioning common to both presentations may be underpinned by overlapping psychological mechanisms such as Theory of Mind (ToM) impairments, which though a feature of both disorders, are not identified in current diagnostic criteria (Chisholm et al., 2015).

ASD symptoms appear to overlap most significantly with negative symptoms of schizophrenia. For example, difficulties with emotional reciprocity, or speech delay or absence observed in ASD may be understood in terms of blunting of affect or alogia (poverty of speech) in schizophrenia, respectively. Catatonic features may also present in both disorders. Phenotypic similarities can therefore result in difficulties with differential diagnosis according to classification systems such as the DSM which are based upon observation of specified clinical symptoms. Conversely, positive symptoms such as hallucinations and delusions are defining features of schizophrenia but are not typically symptomatic of ASD. Age of onset constitutes another significant distinction between the two constructs (Konstantareas and Hewitt, 2001).

Whilst there is some evidence to suggest that ASD and schizophrenia co-occur at elevated rates, there is a lack of systemedatic research on the co-occurrence of these two disorders (Chisholm et al., 2015, Davidson et al., 2014, Del Real et al., 2010). A systematic review attempting to ascertain the prevalence rates of psychosis, anxiety and affective disorders in ASD populations reported prevalence rates of schizophrenia between 0% and 6%. Comparison across studies was complicated by substantial methodological heterogeneity and some of the included studies reported on prevalence rates in child and adolescent populations and therefore may have underestimated the prevalence of symptoms yet to emerge (Skokauskas and Gallagher, 2010). A similar systematic review reported difficulties in drawing accurate conclusions about the co-occurrence of psychosis and Pervasive Developmental Disorders (PDD) due to methodological variation, heterogeneity and issues of selection bias. The authors highlighted the importance of future research in order to understand the potential mechanisms underpinning their co-morbidity (Padgett et al., 2010). There is no published research known to the authors which systematically reviews the prevalence of ASD in adult psychosis populations. It should be acknowledged however, that there have been a number of studies which have explored the relationship between childhood-onset schizophrenia and ASD. A longitudinal study carried out by the National Institute of Mental Health (NIMH) in 2004 found that 25% of 75 children with a diagnosis of childhood-onset schizophrenia had a lifetime diagnosis of ASD: one met criteria for autism, two for AS, and 16 for Pervasive Developmental Disorder Not Otherwise Specified (PDD-NOS) (Sporn et al., 2004). This result was replicated with a larger sample size of 101, showing a similar proportion (28%) with a lifetime diagnosis of ASD (Rapoport et al., 2009). A study carried out by Hallerback et al. (2012) in a Swedish population of patients with a Schizophreniform illness found that 41% met diagnostic criteria for ASD based upon interviews carried out with their parents, with particular focus on the presence of ASD symptomatology in childhood and adolescence.

The present review therefore aims to assimilate all published studies reporting such prevalence rates. The literature pertaining to both psychosis and ASD is primarily diagnostically based at present therefore this review will explore prevalence rates in accordance with the relevant diagnostic categories. Furthermore, as both disorders are proposed to exist on a continuum of severity, it is important to consider not only the prevalence of diagnosed ASD, but also “sub-threshold” ASD symptoms or traits referred to hereafter as Autistic-like Traits (ALTs). ALTs include symptoms consistent with the diagnostic symptom profile for ASD as measured by quantitative ASD symptom scales or concurrence with diagnostic criteria. Importantly, ALTs refer to the presence of ASD symptoms at the time of assessment but may lack evidence or assessment of their presence in childhood, which is essential for a diagnosis of ASD. It is these (often overlapping) traits which form the essential criteria for diagnosis therefore this review may represent a first step in untangling a complex relationship between two overlapping constructs. Understanding the prevalence rate of ALTs and ASD in psychosis populations is an important step towards further understanding the overlaps and distinctions between the constructs, which has significant implications for both psychological and pharmacological interventions, in addition to diagnosis and policy (Chisholm et al., 2015, Mandell et al., 2012).

Section snippets

Methods

Studies were identified through searches of three large electronic databases: PsychINFO, Web of Science and PubMed. No date restrictions were imposed and the search strategy included the following groups of keywords: (a) schizo*, psychos*, psychot* OR delusion*; (b) ASD, asperger* OR autis*; (c) prevalence OR co-morbid*. These groups were then combined using the Boolean operator AND. The systematic literature search was conducted independently by two reviewers on the 4th December 2015.

Studies

Results

The database search identified 1388 records and of these, 324 were removed due to duplication. The remaining 1064 records were screened by title and abstract. Sixteen potential studies for inclusion were subsequently identified therefore full text articles were sought and eligibility was assessed. Of these, seven studies met the criteria and were included in the review. Review of the reference lists did not yield any additional relevant studies. Fig. 1 provides an overview of the review process.

Discussion

Overall, the findings of this review demonstrate elevated prevalence rates of ASD at the diagnostic level and at the trait level in psychotic populations compared to the general population. The co-occurrence of psychosis and ALTs is evidently more frequent than ASD at the diagnostic level. The wide-ranging prevalence rates highlighted by this review are problematic in terms of estimating the actual prevalence rates, and may be in part attributable to the variability in measurement and

Funding information

Funding was provided by The Queen's University of Belfast, School of Psychology, Belfast, Northern Ireland.

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