The Prevalence of Autism Spectrum Disorders in Adult Psychiatric Inpatients: A Systematic Review

1. INTRODUCTION

Autism Spectrum Disorders (ASD) describe a range of neurodevelopmental disorders characterised by difficulties in social interaction and communication, as well as restricted, stereotyped and repetitive behaviours [1]. ASD subtypes include autism, Asperger’s Syndrome (AS) and Pervasive Developmental Disorder Not Otherwise Specified (PDD-NOS); a further two PDD’s exist (Rett’s syndrome and childhood disintegrative disorder), though these are qualitatively distinct from the aforementioned subtypes, and are thus not considered further in this review [2]. Though originating in childhood, the debilitating effects of ASD persist into later life, leading to many individuals requiring high levels of support throughout their adult life [3]. Indeed, whilst most research has historically focussed on ASD in children, in recent years there has been an increasing focus on the clinical needs of adult patients [4]. Despite this, service provision for adults with ASD is still in its relative infancy [5]. An important component of this is ready access to diagnostic services for adults with suspected ASD, as diagnosis is essential in both understanding their needs and planning their subsequent care [6].

Community-based prevalence rates for autism in the general population show considerable variation across studies, though recent systematic reviews and large-scale epidemiological research estimate rates of between 0.7-1.1% [7, 8]. Additionally, the prevalence of autism is significantly higher in people with moderate to profound intellectual disability (ID); Brugha et al. [8] found a prevalence of 39.3% (95% confidence interval 31.0-48.4) for this patient group, compared to 1.0% (95% confidence interval 0.4-2.2) for those with mild ID. For these reasons, ASD represents a major global public health issue, responsible for over 111 Disability Adjusted Life Years (DALY’s) per 100,000 persons [7].

However, while there has been a focus on the prevalence of ASD in the community, its prevalence within an inpatient psychiatric setting is less well established. Mandell et al. [9] (2012) cited several reasons to support the suggestion that ASD may be overrepresented and underdiagnosed among adults within such a setting, including a lack of training among adult psychiatrists in conditions originating in childhood. Additionally, presence of comorbid psychiatric disorders that are more common in people with ASD could potentially complicate the diagnostic picture [9, 10], including depression [11, 12], bipolar disorder [13], anxiety disorders [14], schizophrenia [13, 15], attention deficit hyperactivity disorder [11, 14], alcohol and substance abuse [13], as well as ID [16, 17]. Such conditions could lead to instances of failure to identify ASD where present, as well as misattribution of ASD symptoms to other forms of mental disorder, such as schizophrenia [9, 15]. Finally, the diagnostic criteria for ASD have broadened in recent decades, in contrast to the narrowing of those for schizophrenia [9, 18-20].

Understanding the prevalence of ASD within an inpatient psychiatric setting has significant implications for resource allocation by healthcare providers, as this patient group could potentially represent a target for case identification. Identifying individuals with ASD will ensure that the clinical needs of such individuals are better understood, and taken into account in any subsequent treatment approach [15]. This could potentially lead to improved therapeutic outcomes in both the shorter (e.g reduced length of hospital stay) and longer terms (e.g reduced likelihood of readmission).

The purpose of this systematic review was to evaluate the current evidence pertaining to the prevalence of ASD in adults within an inpatient psychiatric setting. This is the first systematic review on this topic.

2. MATERIALS AND METHODS

3. RESULTS

3.2. Findings of Included Studies

Table 1 summarises the details of all four included studies. Publication dates for such studies ranged from 1982 to 2012, and all were conducted in England or North America. The characteristics of the study samples varied markedly, including both state [9] and secure psychiatric hospitals [22, 23], as well as long-stay treatment facilities for individuals with ID [24]. Limited data were available on the mean age for many included studies (though all were conducted on adult populations), with mean age ranging from 42 to 52 years. For all included studies, the majority of the study population comprised of males, though this varied considerably from 61-100%. All included studies were found via the database searching process, with the exception of Hare et al. [23], identified through the ancestry method. No additional eligible articles were identified through either grey literature searches or consultation with experts in the field.

Table 1.

Author, Year, Location, Reference	Total N Enrolled in Study	N Screened	N Indicated for Diagnostic Assessment	N Subjected to Diagnostic Assessment	Details of Excluded Individuals	Male %	Mean Age in Years	Description of Inpatient Setting	Description of Study Population	Method of Screening	Method of Autism Diagnosis	Diagnostic Criteria Used	Prevalence Estimate	STROBE Score
Hare et al. (1999), England, [23]	1305	1305	240	215	Patients on trial leave to other placements were excluded. Also, some were excluded from initial screening for administrative reasons of no clinical significance. 96% of the special hospital population was screened.	86% (185/215) of those subjected to diagnostic assessment. Data pertaining to the male % of the 1305 individuals originally screened is not available.	41.8 (Age range 20-77 years)	Three secure psychiatric hospitals	All adults. Mixture of non-ID and ID patient, though details regarding the relative proportions of each are not provided within the article.	Autism Spectrum Disorder in Adults Screening Questionnaire (ASDASQ)1 [26].	Handicaps, Behaviours and Skills (HBS) schedule [27] and case note analysis.	ICD-10 [1]	2.4% (31/1305), increased to 4.8% (62/1305) when equivocal cases are taken into account.	15/22
Mandell et al. (2012), USA, [9]	141	141 (though all patients were subjected to diagnostic assessment irrespective of screening score)	141	141	There were 348 residents within the hospital, so only 41% were screened (141/308). Researchers attempted to obtain consent from all residents; 89% of refusal were passive (i.e. patient unable to give consent). Thus, non-capacitous individuals were excluded.	75% (106/141)	52	State psychiatric hospital	Civilly committed patient. 32.6% (46/141) patients had ID. Age range not given.	Social Responsiveness Scale (SRS) [28].	Four step process: 1- Historical charts and electronic records reviewed; 2 – Diagnostic Interview for Genetics Studies (DIGS) [29] conducted for each patient; 3 – Autism Diagnostic Interview – Revised (ADI-R) [30] completed by research reliable clinicians; 4 – case conference review by two independent teams	DSM-IV2 [20]	Overall: 9.9% (14/141). ID subgroup: 19.6% (9/46). Non-ID subgroup: 5.3% (5/95).	16/22
Scragg and Shah (1994), England, [22]	392	392	17	17, though 6 refused to be meet the investigator	Female patient excluded, on basis that Asperger’s Syndrome is reporter to be much more common in males.	100% (392/392)	No data on age of study population, though the study population were adults.	Secure psychiatric hospital (Broadmoor, England)	Adult males. Unclear whether any patients in the study population had ID (details on IQ are only given for 9 patients whom met criteria for AS, none of whom had an IQ consistent with ID).	Examination of case notes	Two stages (after initial screening stage): 1 – Screening Schedule for Autistic Behaviour (Part of the HBS interview schedule) [27]; 2 – Interview by the investigator.	Gillberg and Gillberg (1989) [31] criteria	1.5% (6/392) (95% CI – 0.6 to 3.3%), increased to 2.3% with the addition of equivocal cases (However, this estimate was for Asperger’s syndrome only, rather than all forms of ASD).	12/22
Shah et al. (1982), England, [24]	761	n/a	761	761	Exclusion of 129 patients, due to being non-mobile – ‘their inability to walk unaided limited the possibility of their showing the behaviour pattern characteristic of classic Kanner’s syndrome.’	61% (468/761)	No data for mean age available. Youngest patients were 16 years of age.	Long stay ID hospital3	The entire study population had ID.	n/a	Disability Assessment Schedule [32]	Details not provided within the article.	4% (27 to 34/761). The precise number of participants being diagnosed with ASD is not provided within the article, though they report a prevalence of 4% from a study population of 761.	12/22

¹ Not referred to as the ASDASQ within the article itself, as the tool was (at the time of the Hare et al article being published) an unnamed, unpublished screening tool (which was later named the ASDASQ).
² Confirmed via correspondence with Dr David Mandell.
³ Referred to as a ‘mental handicap hospital’ within the source article.

All but one of the included studies estimated the prevalence of ASD in general, though the methods of diagnosis varied considerably, from review of existing case notes, to using more standardised tools, such as the Disability Assessment Schedule (DAS) [32], Handicaps, Behaviours and Skills Schedule (HBS) [27], and the Diagnostic Interview for Genetics Studies (DIGS) [29] and Autism Diagnostic Inventory-Revised (ADI-R) [30]. In contrast, Scragg and Shah [22] estimated the prevalence of AS only, rather than all subtypes of ASD.

Prevalence estimates for ASD ranged from 4 to 9.9%, with Scragg and Shah [22] estimating a prevalence of 1.5% (or 2.3% when equivocal cases are included) for AS only.

4. DISCUSSION

In this systematic review we aimed to assess the current evidence relating to the prevalence of ASD among adults within inpatient psychiatric settings. There was a relative dearth of eligible studies, supporting the assertion that there is a need for further high quality research in order to provide more definitive evidence regarding the prevalence of ASD in inpatient psychiatric settings.

From this pool of eligible studies, we found wide ranging prevalence estimates, though there appears to be a general trend suggesting ASD is more prevalent in inpatient psychiatric settings relative to community populations. The reasons for such variance in estimates are likely manifold, as there is significant heterogeneity among the eligible studies, in terms of the characteristics of the patient populations studied, the study design, as well as the assessment tools and diagnostic criteria used.

With respect to study populations, it is essential that in studies involving a mixture of ID and non-ID patients, separate prevalence estimates are made, given the sizeable disparity in ASD prevalence estimates between this groups in community-based prevalence studies [7, 8, 4, 3]. As a result, the meaningfulness of prevalence estimate for a mixed ID/non-ID study populations is limited, unless separate data for both groups is also reported. Ideally, patients with ID should be further sub grouped according to severity of ID, as risk of ASD increases with increasing severity of ID [8].

There was limited evidence among eligible studies pertaining to inpatient prevalence among non-ID adult patients; however, the only eligible study by Mandell et al. [9] yielded a prevalence of 5.6%, much higher than widely accepted community prevalence estimates for this group [7]. Clearly more research in non-ID inpatient populations is required to discern whether this result is valid and generalizable.

Additionally, there were no eligible studies focussed on ASD prevalence among groups of patients with discrete mental disorders, such as schizophrenia. This is an important area for future research, as it would shed light on both the extent of ASD comorbidity with specific conditions, as well as further explore diagnostic overshadowing in this context.

The data on inpatients with ID showed significant variability in terms of prevalence estimates, ranging from 4-19.6%. Given the wide-ranging estimates from studies conducted over a considerable time period, it is impossible to assert with confidence whether this represents a significantly different prevalence than in patients with ID in the community setting [43].

All eligible studies were focussed on predominantly male study populations. Of the two studies reporting data on gender differences, one described non-statistically significant differences (p> 0.05) in ASD prevalence between males and females [9] and the other [23] reported a higher prevalence rate among males of those whom were subjected to diagnostic assessment, though did not comment on whether this was statistically significant. Further work needs to be done to determine whether there is a difference in ASD prevalence in the psychiatric inpatient setting, as well as more generally. There has been a long-held belief that ASD is more prevalent in males than females, particularly in those without ID [44], though it is being increasingly questioned whether this represents a ‘true difference’, or is attributable to other factors, such as ASD screening and diagnostic tests being insufficiently sensitive for females [45], and/or females possibly having a greater ability to mask their symptoms (so-called ‘camouflaging’) [46].

Based on the findings from the eligible studies, including those conducted in a forensic setting [22, 23], there appears to be a general trend suggestive of an increased prevalence of ASD within inpatient psychiatric settings. This data from forensic units could however be confounded by other factors, such as a lack of data on the number of individuals with ID in both studies. While it is conceivable that the clinical features present in ASD, such as abnormal social interaction and communication, could predispose someone to offending, there is limited evidence to support the notion that people with ASD are at any greater risk of committing crimes than the non-ASD population, with the possible exceptions of arson and sexual abuse [47]. Furthermore, even if there were an increased risk of offending behaviours in those with ASD, this would not necessarily manifest itself as an increased prevalence within a forensic psychiatric inpatient setting, as many individuals with ASD may remain undiagnosed, avoid conviction for their offences or be dealt with via the criminal justice system route.

Several studies employed a multiple-stage study design, involving use of a screening tool to determine those whom progress to more comprehensive diagnostic assessment. Such an approach has similarly been used in community-based ASD prevalence studies in both child [48, 49] and adult populations [50]. This is logistically sensible, as many of the ASD diagnostic assessment tools are informant dependent and resource intensive, requiring a considerable amount of time and specialist input [50]. For these reasons, a multiple-stage approach enables coverage of a larger patient population per unit of resources [51]. Of course, the value of such an approach is contingent on the validity and reliability of both the constituent screening and diagnostic tests used.

Diagnostic criteria can have a profound effect on the resultant prevalence estimate; as a community population-based study by Cooper et al. [43] demonstrated, where ASD prevalence among the same group of 1,023 individuals with ID varied from 2.0-4.4% depending on the diagnostic criteria used, and yielded a value of 7.5% for clinical diagnosis by a specialist, considered the gold standard. Also, the eligible studies were published across a period of 30 years and over that time the diagnostic criteria and overall concept of ASD as a clinical entity have broadened considerably [9, 20-22]

In conclusion, there is a clear need for robustly conducted epidemiological research pertaining to the prevalence of ASD among adults in psychiatric inpatient settings, as the current evidence base is lacking. Such research needs to carefully consider the assessment tools and diagnostic criteria used, as well as a study design supporting coverage of a large patient group. Additionally, the characteristics of the study population need to be appropriately detailed, with prevalence estimates for ID and non-ID groups and both genders as a bare minimum requirement. Adult psychiatric inpatient populations could indeed potentially provide a target for identifying those with ASD, though it is first essential to first establish whether ASD is truly significantly more prevalent in people within this group relative to the community setting.

REFERENCES