Autism is a neurodevelopmental disorder characterized by impairment in social interaction, in communication skills, and in behaviour, which is restricted and repetitive. In this article, we review the history of autism’s classification and how this classification has affected its increased prevalence. We examine current diagnostic categories, methods of assessment, clinical correlates, and comorbid conditions. Finally, we discuss epidemiology and prognosis in autism spectrum disorders. Two companion papers discuss neuroimaging, genetic, and therapeutic aspects (1,2).
In 1943, Leo Kanner published his seminal paper on autism. He clearly described 11 children who were socially isolated, with “autistic disturbances of affective contact,” impaired communication, and behavioural inflexibility (3). He coined the term “infantile autism” and discussed etiology in terms of biological processes when much attention was focused on analytic theories (4). His paper did not receive much attention, and children with these problems were diagnosed with childhood schizophrenia (5). The choice of the word “autism” may have created some confusion, because Bleuler first used the word to describe a mental state of fantastical, self-centred thought processes as symptomatic of schizophrenia. This implies a previously developed cognitive state and subsequent regression; it therefore belies the arrested social development, leading to isolation, described by Kanner (6).
The DSM Classification System
Autism did not become a diagnostic entity in its own right until the DSM-III was published, in 1980 (7). The DSM-III criteria used “objective, unambiguous, and precisely defined operational criteria” (8) and did not focus on causality. The term used to describe the disorder was “infantile autism”; criteria included onset before age 30 months, lack of responsiveness to other human beings, gross impairments in communication and language, and bizarre responses to the environment (7). The condition was one of a group of disorders brought together under a new term, “pervasive developmental disorders (PDD),” with “pervasive” indicating that the developmental disability affected or pervaded all spheres of the child’s life. This term has been widely accepted and is currently used, although it is neither transparent to parents nor satisfactory to the clinical and research community (9). Other PDD subtypes were described but were discarded in subsequent DSM revisions.
The DSM-III-Revised changed “infantile autism” to “autistic disorder” and created 16 criteria in the areas of social development, communication, activities, and interests (10). Age criteria were qualified to before or after age 3 years, and “atypical” was changed to “not otherwise specified” (NOS) to avoid confusion with other concepts of atypical personality development put forth by Rank, in 1949 (see 11 for a discussion). A national field trial attempted to determine scoring rules for autistic disorder (12). With these changes in criteria, the DSM-III-R became overly inclusive, created false-positive diagnoses, and differed widely from the criteria set down in the ICD-10 (13).
DSM and ICD
Because it was critical that future research should compare different populations, it was necessary to align the DSM and ICD classifications. To achieve this, a literature review, a reanalysis of existing data, and a large multicentre international field trial were undertaken. The ICD-10 included Rett’s disorder and childhood disintegrative disorder (CDD) under the umbrella of PDDs. It also separated research and clinical criteria (Table 1). Studies using ICD-10 criteria showed that they correlated well with expert clinical diagnoses (14). The DSM-IV classification system kept the research and clinical criteria together but brought criteria into line with the ICD-10 and included the other PDDs (15). Hence, the 2 systems’ descriptions of the different disorders are now close (Table 1).
Childhood Disintegrative Disorder (CDD)
Asperger’s Disorder (AD)
A neuropsychological diagnosis of nonverbal learning disabilities (NLD) has many symptoms in common with a diagnosis of AD but is not well recognized in psychiatric literature. Differences in right and left hemispheric functioning differentiate NLD from high-functioning autism, and neuropsychologic testing may help to distinguish these entities (21).
PDD Not Otherwise Specified (NOS)
Critique of the DSM-IV
Problems still exist with the DSM-IV system. The criteria are less stringent than the ICD-10 research criteria and are therefore more inclusive. Among the disorders, autistic disorder is the most clearly defined, while AD and PDD NOS are less so. Diagnosing AD is extremely difficult if the DSM criteria are strictly followed, because many affected children also meet the criteria for autism (22,23). The criteria for PDD NOS are even less well delineated. Parents and clinicians tend to drop the NOS and refer to children in this group as suffering from PDD. This diagnosis is not specific and creates many false positives, with ramifications not only for research but also for service providers and schools. However, there is a high degree of international consensus regarding concepts about autism, compared with other psychiatric conditions.
Assessing Autism Spectrum Disorders
Autism Diagnostic Assessment
The clinical evaluation should also include a physical exam with growth measurements (especially head circumference), a neurological examination, and an examination of the skin using a Wood’s lamp to look for depigmented markings, such as those seen in tuberous sclerosis. For children with abnormal findings on a neurological examination, an EEG and magnetic resonance imaging (MRI) are necessary. Children with definite loss of language skills need a sleep-deprived EEG to evaluate the possibility of Landau–Kleffner syndrome. Finally, an audiogram is needed to rule out any hearing loss that may account for the language delay.
Laboratory investigations may include testing for fragile X syndrome and chromosome analysis, although a positive yield is low and may not warrant routine testing. More recently, molecular genetics testing using fluorescence in situ hybridization (FISH) techniques has demonstrated abnormalities on chromosomes 7 and 15 (25). Other laboratory investigations for immune deficiency or metabolic problems have very low yield (less than 5%) and are not warranted unless clinical symptoms or signs are present (26). The same is true of tests for antigliadin antibodies that look for gluten enteropathy. Unless the child has a history of diarrhea, anorexia, or failure to thrive, the test yield will be extremely low.
Diagnostic Assessment Tools
The Autism Diagnostic Interview-Revised (ADI-R) (29) is a detailed parent interview divided into 4 main subject areas: communication, reciprocal social interaction, play, and developmental history. Items are scored on an algorithm to give cut points for autism. This instrument has shown good sensitivity and specificity, and it is now considered a necessary component of a diagnostic evaluation, particularly for research.
As a companion to the ADI-R, the Autism Diagnostic Observation Schedule-Generic (ADOS-G) (30) was devised to provide evaluators with a standard direct-observation measure. The ADOS-G has 4 modules designed for children, adolescents, and adults with varying levels of language ability. The procedure takes about an hour and uses specific play interactions called “presses.” Each of these presses is coded and scored on an algorithm. Scores in communication and reciprocal social interaction, as well as the total score, fall above or below a cut point for autism or the autism spectrum. The ADOS-G has good reliability for determining children in the autism spectrum.
Both of these tests require training and reliability checks, especially if they are to be used for research. However, teaching for the ADOS-G is available to clinicians who wish to become familiar with the test and its administration.
The most frequently used scale for adaptive behaviour is the Vineland Adaptive Behavior Scale (VABS) (36). This scale assesses 4 areas of functioning—communication, daily living skills, socialization, and motor skills—and can be helpful in diagnosis.
Speech and Language Assessment. Assessing the child’s communication skills is important, particularly in children with a diagnosis of either high-functioning autism or semantic- pragmatic language disorder. Listing the numerous available language tests is beyond the scope of this paper, but commonly used ones are the Peabody Picture Vocabulary Test-Revised (PPVT-R) (37) and the Reynell Developmental Language Scales (RDLS) (38).
Occupational Therapy Assessment. Although motor skills are often less affected in autism than are other developmental skills, many children have problems in fine or gross motor functioning and motor coordination. The Beery–Buktenica Developmental Test of Visual-Motor Integration (VMI) (39) scores graphic and motor skills, perceptual accuracy, and hand eye coordination. Other useful tests are the WeeFIM instrument (functional independence measure) (40), which can be used with young children, and the Peabody Developmental Motor Scales (PDMS) (41). Finally, it is important to evaluate sensory problems, such as acoustic or tactile problems, to understand how children with autism respond to environmental stimuli. The Sensory Integration and Praxis Test (SIPT) (42) is one tool to assess this; however, it may not be needed for every child.
Family Assessment. There is no doubt that a family caring for a child or adult with autism is under much stress, owing to the demands of an individual with special needs and the lack of support in the health and education system. To obtain knowledge about the autistic patient’s environment, clinicians should evaluate the parents’ awareness of their child’s needs, their knowledge of autism, their ability to cope with the extra parenting demands, and the extent of family and community support. As in any family, parents with other stressors, such as financial difficulties or relationship problems, will have more difficulties meeting the needs of their child. A social worker can provide the family with support and community resources particular to their needs.
The prevalence rate for autism and autism spectrum disorders has increased worldwide over the past decade (45). In epidemiologic studies done in the 1960s, autism rates tended to be determined based on severe impairment of language, social interaction, and behaviour. Such cases, known as “classical autism,” had prevalence rates of 4/10 000. These rates have changed with changing diagnostic criteria as well as parents’ and professionals’ increased awareness of developmental problems.
Prevalence Rates for Autistic Disorder
Prevalence Rates for Autism Spectrum Disorders
From Fombonne’s review, the most recent estimate for all autism spectrum disorders, taken together, is 27.5/10 000. Owing to the methodological vagaries, this estimate is thought to be conservative and somewhat imprecise. Fombonne notes improved case finding methods and diagnostic precision in the 3 most recent studies reviewed (done in 2000 and 2001): for all PDDs, including autistic disorder, these studies show prevalence rates between 57.9 and 67.5/10 000 and a convergence in the range of 60/10 000 (Table 2). However, these studies varied in the reported rates of autistic disorder, PDD NOS, and AD, despite the use of the ADI-R and the ADOS. Therefore, it is still difficult to get a sense of prevalence rates for PDD subtypes.
Whether there is an increased incidence of autism has yet to be determined. Surveys attempting to find incidence rates used referral statistics that were confounded by changes in referral patterns or did not account for the changes in diagnostic classification during the study period. Therefore, it is not possible at this time to say that the incidence of autism has increased. Further, because the condition is somewhat rare, future large-scale population studies will be needed to detect a small change in incidence rates (46,47).
Clinical Correlates of Autism
Cognitive deficits observed in the studies reviewed by Fombonne indicated that 30% of subjects had mild-to-moderate impairment, while 40% had severe levels of mental retardation, leaving 30% with normal IQ. Thus, most children with autism have mental retardation. However, the 3 surveys done in 2000 and 2001 reported lower rates of impairment, with 29% to 60% of subjects having normal IQ. This finding raises the possibility that early detection and intervention may make a difference in the overall rate of mental retardation, but it needs more research to be clearly ascertained.
Regression in Autism
It has been known for many years that autism can present as either a lack of language development or as a loss of previously acquired language, usually single words. This differs from the loss of language skills previously defined in CDD. The ADI defines early regression of language: it indicates that the child should have 5 meaningful words used over a period of 3 months, which he or she then ceases to use, causing the parents to become concerned about language development. This pattern occurred in 25% of children studied in a British sample of 473 children suffering from autism (50).
The issue of regression in autism came to the forefront as part of the measles, mumps, rubella (MMR) vaccine controversy. A 1998 article appearing in Lancet described a small group of children with autism who had diarrhea (referred to as autistic enterocolitis) and who lost previously acquired developmental skills after receiving an MMR vaccine at age 15 months (51). However, another study found that 17% of the children had bowel symptoms and that this rate did not change over the 20-year period for which data were examined and which included the introduction of the MMR vaccine in 1988 (50). Additional epidemiologic studies examining this question have not confirmed a link between the onset of autism and the administration of the MMR vaccination (49,52–55). A recent population study undertaken in Denmark, where almost all children are accounted for through a government registry, found no difference in the relative risk of autism between the vaccinated (n = 440 655) and unvaccinated (n = 96 648) groups (56). To date, this is one of the best studies demonstrating the lack of association; however, there has been some criticism of the methodology, which the authors are in the process of correcting (57,58).
There has also been recent controversy about the relation between high mercury levels in children with autism and the use of thimerosal in vaccines. The hypothesis is that vulnerable children will develop neurodevelopmental problems secondary to the neurotoxic effects of mercury. However, a recent review demonstrates that the clinical presentation and neuropathology of autism and mercury toxicity differ (59). In addition, there is no evidence that children exposed to mercury from any source have a higher rate of autism. Thimerosal has not been present in vaccines in Canada since 1992, except for one preparation of the Hepatitis B vaccine that children receive at birth, but this preparation contains mercury levels well below safety estimates (that is, 12.5 micrograms Hg) (60). Thimerosal was removed from vaccines in the US in 1999.
Medical conditions were reported in 15 of the surveys reviewed by Fombonne (46). The rates of medical conditions associated with autism ranged from 0% to 16.7%, with a mean of 6%. This included disorders such as tuberous sclerosis, cerebral palsy, and Down syndrome. According to a study by Dykens and Volkmar, 4% of people with autism have fragile X syndrome (26).
Rates of epilepsy in autism range from 5% to 38.3%. The highest rates occur in adolescents and adults, where about one-third can have seizures (62,63). However, in one study, 39% of a sample of 41 children with autism and seizure disorder were under age 3 years (64).
Mental retardation is an important predictive factor for the development of seizures in children with autism. In one study (64), over 80% of the children with an IQ of less than 50 had seizures, which is consistent with other studies (65,66). In a study of children with autism and normal or mild cognitive deficits, the cumulative probability was 0.06, which is similar to that for children with language disorder (62). The risk of epilepsy is higher in Rett’s disorder and CDD, being in the order of 75% to 90% (62).
Individuals with autism of varying degrees of severity experience problems that may require intervention and could mask the diagnosis in higher-functioning patients. These problems include anxiety, depression, oppositional behaviour, hyperactivity, poor attention, tics, and compulsive behaviours (67). It is unclear whether these symptoms constitute a second diagnosis or whether they are associated problems, because the symptoms may be transient and not as severe as the full-blown disorder. The degree to which the problem interferes with the ability of the child to function will dictate whether treatment should be undertaken (68).
Of these disorders, depression is very common, although prevalence rates have not yet been determined. Depression as a condition comorbid with autism may reflect a population with a family history of depression or an increased frequency of negative life events, such as parental marital discord (69). Children with autism and epilepsy may also have higher rates of depression. Depression in the presence of autism may be expressed as increased behavioural disturbances, or exacerbated compulsive behaviours, or anxiety; symptoms may not be verbalized because of the communication impairment. Whether women are more affected than men, as in the general population, is not known.
Anxiety is manifested at an early age in autism spectrum disorders, but the prevalence is unknown. When symptoms are severe, appropriate behavioural or pharmacologic treatment will alleviate some of the difficulties for the patient and the family.
In children of all ages, attention and hyperactivity problems cooccur in autism spectrum disorders and sometimes confuse the diagnosis (67). When symptoms are present, an approach to treatment may be similar to that for attention-deficit hyperactivity disorder (ADHD).
Although the presence of tics with autism or AD has been reported clinically, the exact prevalence of this disorder is not known. A study by Baron-Cohen and others found a prevalence rate of 8.1% in a population of adolescent children with autism attending a specialized school (70). These authors note that tics are difficult to distinguish in this population but “are usually short-lived, contextually inappropriate, and interrupt the flow of behaviour or speech,” in contrast to stereotypic behaviours (70).
There have been some reports of individuals suffering from autism having schizophrenia, but most studies looking at cooccurrence have shown that numbers are low. One review of 163 individuals with autism found a single individual with schizophrenia, resulting in a rate of 0.6%, which is comparable to the rate for the general population (71). The DSM-IV suggests that schizophrenia can be diagnosed in an individual with autism when hallucination or delusions have been present for at least 1 month. However, the 2 disorders may be difficult to differentiate when a patient has odd obsessions. These can look like delusional thought disorder, except that, usually, ideas of reference or other distortions of thinking are absent.
Outcome research in autism has been marred by methodological difficulties, making statements about prognosis tentative. Just as epidemiologic research has been hampered by the lack of precise diagnostic tools, so has outcome research. This type of research is further challenged by the variation in autism disorders, particularly with respect to language and cognitive abilities. Tsatsanis recently reviewed this topic and concludes that there are few prospective, longitudinal, population-based studies (72). However, from the available data reviewed, it is becoming clear that the diagnosis of autism is stable, but the outcome in school, work, or social functioning is varied. Of individuals suffering from autism, 75% have a poor outcome, and 25% have a better prognosis. Acquisition of language before age 6 years, IQ levels above 50 (9), and having a special skill, such as expertise in computers, predict good outcome. Howlin found that, with respect to independent living and employment, there was not much difference between high- functioning individuals with autism and those with AD (73). For people with severe autism, independent living and social functioning are unlikely; for those with higher-functioning autism, the jobs acquired are often below their education level and are found with the help of families or friends. A few adults with autism marry or have close relationships, but most have few friends.
In a comparison of outcome studies before and after 1980, Howlin found that individuals in the later studies tended to have better overall competence and higher language levels (73). In addition, more had jobs (20% vs 5%) and lived independently (12% vs 0%). Although these outcomes are still a cause for concern, it appears that earlier diagnosis and intervention has improved the outcome for individuals suffering from autism. Of critical importance is the need for services for autism sufferers of all ages. With greater numbers of diagnosed individuals, health, education, and community resources are in great demand and currently overburdened. As adults in the near future, these large numbers of individuals will need support, including help with living arrangements and job opportunities.
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Manuscript received and accepted June 2003.
1. Assistant Professor of Psychiatry, McGill University Hospital Centre, Montreal, Quebec.
2. Harris Professor of Child Psychiatry, Pediatrics, and Psychology, Yale Child Study Center, New Haven, Connecticut.
Address for correspondence: Dr L Tidmarsh, Department of Psychiatry, Montreal Children’s Hospital, 4018 Ste Catherine St W, Montreal, QC H3Z 1P2
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