Autism/autism spectrum disorders (DSM–IV)

Autism shares the same DSM criteria for abnormal social interaction and behaviour as Asperger disorder, but requires additional impairments in communication (Box 3). Delay or impairment in social interaction, communication or behaviour must arise before age 3 years. In DSM–IV, if both autism and Asperger disorder diagnoses can be made, the autism diagnosis takes precedence. Unlike Asperger disorder (which excludes individuals with delays in cognitive abilities or other skills), autism can occur at all levels of ability: the majority (70%) of cases have associated learning disabilities and almost half have an IQ below 50.

In a prevalence study of autism, Wing & Gould (1979) identified a large number of children who failed to meet the diagnostic criteria for classic autism, but had a triad of impairments involving social interaction, communication and imagination, with additional repetitive stereotyped activities. This triad of symptoms, termed the ‘autistic spectrum’, was recognised at all levels of intelligence and is included in DSM–IV as ‘pervasive developmental disorder not otherwise specified’, and in ICD–10 as ‘atypical autism’, ‘other pervasive developmental disorders' or ‘pervasive developmental disorders, unspecified’. The estimated prevalence of autism spectrum disorders (autistic spectrum disorders) may be as high as 91 per 10 000.

At present it is not clear whether Asperger syndrome is distinct from the autistic spectrum disorders. Diagnostic uncertainty arises in patients of near normal cognitive ability (total IQ >70) with autistic spectrum disorders, who are described as having high-functioning autism (HFA). Differences are reported between Asperger syndrome and HFA, for example, verbal skills being significantly greater than non-verbal ones in Asperger syndrome – the opposite of the pattern reported in HFA. This differentiation may be simplistic, as it depends on the diagnostic system used. A sample defined using Wing or Gillberg criteria for Asperger syndrome would include children with abnormal or delayed language and, by definition, worse verbal skills than a DSM or ICD sample, which would exclude these individuals. Attempts to separate Asperger syndrome or HFA categorically based on presence or absence of language delay are artificial. Comparison studies have also failed to control adequately for IQ differences between samples in many cases. Current research data do not convincingly support the separation of Asperger syndrome and the autistic spectrum disorders as distinct disorders. Both Asperger syndrome and autistic spectrum disorders are about five times more common in boys than in girls; segregate within the same families; appear strongly genetic (American Psychiatric Association, 2000); and share similar comorbidity (Reference Gillberg and BillstedtGillberg & Billstedt, 2000). If autism and Asperger syndrome differ at all, it is in the degree of impairment rather than in having discrete, specific and independent features. This is supported by outcome studies; both are associated with social difficulties persisting into adulthood, but these are less severe in Asperger syndrome. Combining Asperger syndrome and autistic spectrum disorders into an autistic spectrum is a better way forward.

In addition to the diagnostic criteria mentioned, a number of assessment instruments are available or in development for use with people with Asperger syndrome. These include the Asperger Syndrome Screening Questionnaire (ASSQ; Ehlers et al, 1999) and the Pervasive Developmental Disorder Questionnaire (PDD–Q; Baron-Cohen et al, 1996). The ASSQ is designed for completion by parents or teachers to screen for Asperger syndrome and other high-functioning autism spectrum disorders in school-age children. The PDD–Q is an 18-item questionnaire for parents, which includes questions specifically designed to identify clinical characteristics of Asperger syndrome.

These instruments were all designed for screening purposes, not to differentiate Asperger syndrome from HFA, but to identify higher-functioning individuals within the autistic spectrum. Each requires further work to fulfil psychometric requirements for sensitivity, specificity, reliability and validity. Of existing instruments used for the diagnosis of autistic spectrum disorders, the Autism Diagnostic Interview – Revised (ADI–R; Lord et al, 1994) may be useful in assessing individuals for Asperger syndrome. This instrument consists of three scales corresponding to the social, communication and behavioural impairments and is based on parent report. The Autism Diagnostic Observational Schedule – Generic (ADOS–G; Lord et al, 1999), a supplementary standardised interview and observational assessment may also be helpful. Accurate diagnosis in younger children (under 2 years of age) is difficult, despite the neurodevelopmental nature of these disorders. The Checklist for Autism in Toddlers (CHAT; Baird et al, 2000) may prove useful in identifying children at risk. Howlin (2000) provides a more extensive appraisal of available assessment instruments. Full assessment of Asperger syndrome requires a multi-disciplinary approach, as outlined in Box 4.

Pervasive developmental disorder not otherwise specified (DSM–IV)

Another diagnosis of relevance is pervasive developmental disorder not otherwise specified (PDDNOS). This is characterised by “a severe and pervasive impairment in the development of reciprocal social interaction or verbal and non-verbal communication skills or when stereotyped behaviour, interests and activities are present” (American Psychiatric Association, 1994: p. 77). This category in DSM–IV needed revision as the inclusion of the word ‘or’ twice greatly diluted its meaning and grossly widened it as a category. DSM–IV–TR has corrected this error, requiring there to be an impairment in reciprocal social interaction associated with an impairment in communication skills or with stereotyped behaviour, interests or activities. In the past, particularly in the USA, PDDNOS was used as a synonym for Asperger syndrome, although for parents this title is unhelpful and confusing.

Other pervasive developmental disorders (DSM–IV)

Neither Rett disorder nor childhood disintegrative disorder are part of the autistic spectrum, and they are unlikely to represent sources of diagnostic confusion. Rett disorder is most common in girls and is characterised by apparently normal development in the first 5 months of life, with subsequent deceleration of head growth, loss of previously acquired hand skills, loss of social engagement, poorly coordinated gait and language problems (American Psychiatric Association, 2000). Similarly, childhood disintegrative disorder presents with loss of language, motor skills and bowel and bladder control following a 2-year history of normal development (American Psychiatric Association, 2000). In each case the diagnosis should be excluded by a detailed developmental history and physical examination.

Schizophrenia (DSM–IV)

Schizophrenia is a disorder in which psychotic symptoms (delusions or hallucinations), thought disorder and so called ‘negative symptoms' cause social and/or occupational dysfunction over time. Because individuals with Asperger syndrome have normal cognitive ability, restrictive behaviours and impairments in social interaction and communication can be misinterpreted as evidence of schizophrenia. People with Asperger syndrome have difficulty understanding the subtleties of social behaviour, but this should not be confused with evidence of psychotic disorder. In a clinical setting, asking individuals with Asperger syndrome whether they hear voices may induce a positive response, and they might concur that they hear voices “when people aren't there”, but they may be refering to the voices of people in an adjacent room. Deficiencies in concrete thinking and in understanding how other minds think may cause patients with Asperger syndrome to misinterpret what is said to them, and they might as a result be labelled paranoid. Misinterpreting social contacts can also lead to inappropriate emotional responses, contributing to this impression. Persons with Asperger syndrome sometimes speak their thoughts out loud, which again can be misinterpreted by a psychiatrist.

Language abnormalities associated with autistic spectrum disorders include substitutions, literalness, problems with prosody, staccato speech and monotonous speech that is excessively pedantic and focused on details or obsessive questions. A tendency to direct the conversation towards obsessions could easily be mistaken for evidence of associative loosening. A comparison of thought disorder and affective flattening in patients with autism and with schizophrenia found that they did not differ in terms of affective flattening, and that adult patients with autism showed poverty of speech, poverty of content and perseveration (Reference Ramsey, Andresen and RapoportRamsey et al, 1986). The autism group showed significantly less derailment and illogicality, suggesting that they would be unlikely to meet DSM or ICD criteria for thought disorder in schizophrenia.

Social and communication deficits can be interpreted as evidence of negative symptomatology, so it is important when assessing functioning to establish premorbid ability. These conditions obviously differ in age at onset, developmental history and mental state examination. In DSM–IV, pervasive developmental disorder is an exclusion condition for schizophrenia and it should be suspected in atypical or non-responsive cases. Schizophrenia can co-occur in autistic spectrum disorders, but the additional diagnosis is made only if prominent delusions or hallucinations are present for at least 1 month (less with treatment). Despite an absence of epidemiological studies of psychiatric comorbidity in autistic spectrum disorders, it has been suggested that delusions or auditory hallucinations may be more common than in the general population, but the prevalence of schizophrenia (at 0.6) is comparable to general population levels.

Bleuler (1911), founder of the modern concept of schizophrenia, described four primary symptoms necessary for the disorder (the four As): ambivalence, loosening of associations, disturbance of affect and autism, which he defined as dependence on an internal unrealistic world. Both he and Kraepelin (1919) defined subgroups with social withdrawal and affective flattening, ‘oddness' and ‘eccentricity’, being timid with a narrow circle of interests and cold relations to companions, and lacking sympathy or attachment. From these descriptions the concept of simple schizophrenia, considered by some to be a diagnostic waste-basket, entered the lexicon. The symptoms described are equally applicable to autistic spectrum disorders, and the subtype ‘simple schizophrenia’ has been removed from DSM–IV. Its retention in ICD–10 is a likely source of diagnostic confusion.

Schizoid personality in childhood (DSM-IV)

Schizoid personality in childhood is defined by solitariness, lack of empathy, emotional detachment, increased sensitivity, at times paranoid ideation, and single-minded pursuit of special interests. All these features are seen in Asperger syndrome, and comorbid issues (depression or behaviour problems in particular) are likewise similar for both conditions. On the basis of evidence presented in Wolff's (1998) discussion of schizoid personality in childhood, we have concluded that there is significant overlap between schizoid personality in childhood and Asperger syndrome.

Schizotypal personality disorder (DSM–IV)

The DSM–IV diagnosis of schizotypal personality disorder depends on odd beliefs or magical thinking, bizarre fantasies or preoccupations, odd thinking and speech, odd, eccentric or peculiar behaviour and appearance, lack of close friends and social anxiety. All of these criteria can also occur in Asperger syndrome, and Wolff (1998) regards “Asperger syndrome and schizoid/schizotypal disorders as interchangeable terms that identify roughly the same group of children”. The conditions do differ in at least three important respects. First, there appears to be an increased rate of develop-ment of schizophrenia in schizotypal personality disorder. Second, schizotypal personality disorder and schizophrenia co-occur in families and appear genetically related. Third, prospective research of children at high risk of schizophrenia (Reference Erlenmeyer-Kimling, Rock and RobertsErlenmeyer-Kimling et al, 2000) suggests that some individuals later diagnosed with schizotypal personality disorder developed without impairments in reciprocal social interaction and communication.

Semantic pragmatic disorder

Semantic pragmatic disorder (Reference Rapin, Allen and KirkRapin & Allen, 1983) is probably not an uncommon cause of misdiagnosis. Children with autistic spectrum disorders often have some language difficulties and many will attend a speech therapist in the first instance and receive a diagnosis of semantic pragmatic disorder. This is characterised by “near-normal vocabulary, grammar, and phonology, but language use is abnormal in content and function and comprehension is also impaired. There are considerable difficulties in initiating or sustaining a conversation, making cohesive links in conversation from topic to topic, and words are used out of context” (Reference Szatmari, Schopler, Mesibov and KunceSzatmari, 1998: p. 71). Is this an exact description of the language problems of Asperger syndrome (Reference WingWing, 1981)? These describe pragmatic language difficulties. Wing's criterion for a reduction in quality and quantity of babbling refers to expressive language difficulties; a delay in speech acquisition and impoverishment of content are receptive–expressive language problems; and defective imaginative play is a receptive–pragmatic language difficulty. The definition of semantic pragmatic disorder includes no reference to problems of social and emotional interaction or to repetitive and stereotyped patterns of behaviour. The existence of semantic pragmatic disorder as a separate entity with clinical validity is questionable.

Deficits in attention, motor control and perception

The core features of deficits in attention, motor control and perception (DAMP; Gillberg et al, 1982) include a cross-situational disturbance of attention, gross and fine motor dysfunction and perceptual dysfunctions not accounted for by associated mental retardation or cerebral palsy. DAMP and Asperger syndrome are similar and can occur together. Overlapping features include: the condition being more common in boys, perceptual problems, a failure to adjust volume and pitch of voice and motor clumsiness (although the latter is not recognised in the Asperger disorder criteria). Whether they represent the same population is uncertain, as attention difficulties are not part of the definition of either Asperger syndrome or disorder, and neither are associated with delay in cognitive development. DAMP can also have significant speech and language difficulties, e.g. articulation problems, hypotonia of the mouth and certain variants of stuttering. Heredity appears to play a much lesser role in DAMP than in Asperger syndrome. It would appear that criminality is more common at follow-up in persons with DAMP than in those with Asperger syndrome. Gillberg (1995) found that about half of adults with DAMP had significant and persistent problems that included criminal offences.

Multidimensionally impaired disorder

Criteria for multidimensionally impaired disorder (MDI; Kumra et al, 1998) include a poor ability to distinguish fantasy from reality, impairment in interpersonal skills and multiple deficits in processing information. Fitzgerald (1998) has argued that MDI should be categorised with autism or Asperger syndrome because of the overlapping symptomatology. Kumra et al (1998) consider MDI an atypical variant of childhood-onset schizophrenia, as they share a similar pattern of cytogenetic abnormalities, neuropsychological deficits, structural brain abnormalities, smooth-pursuit eye-tracking abnormalities, premorbid history and elevated rates of schizophrenia spectrum disorder in first-degree relatives. The nosological status of MDI is uncertain, but we feel that the clinical diagnosis of Asperger syndrome offers far greater opportunities to engage with appropriate educational, psychological and psychiatric services (Reference FitzgeraldFitzgerald, 1998).

Cerebellar affective syndrome

Cerebellar affective syndrome (Reference Schmahmann and ShermanSchmahmann & Sherman, 1998) presents with impairment of executive functions such as planning, set shifting, abstract reasoning and working memory. It also includes difficulties with spatial cognition, including visuo-spatial organisation with disinhibited or inappropriate behaviour. It differs from Asperger syndrome in that it is a late-onset condition. The persons so far studied with this condition range in age from 23 to 74 years. They presented with post-infectious cerebellitis, cerebellar tumours and strokes. Differential diagnosis is easily made on history-taking. This is a differential diagnosis that should be considered in older patients.

Dyslogia

The syndrome of dyslogia was described by Jordan (1972) as the inability to apply logic and common sense in decision-making. Individuals with this difficulty make decisions based on partial facts and have difficulty in integrating data into a working whole. They have social difficulties similar to those of individuals with Asperger syndrome, and dyslogia may simply describe the same population.

Developmental learning disability of the right hemisphere (social–emotional learning disorder)

This disorder (Denckla et al, 1983) could be seen as the product of a lack of communication between neurologists and psychiatrists since there is such an overlap between this condition and Asperger syndrome. Children with the condition have difficulty understanding social and emotional information.

Non-verbal learning disability

Non-verbal learning disability (Reference Myklebust and MyklebustMyklebust, 1975) is characterised by deficits in perception, coordination, socialisation, non-verbal problem-solving and understanding of humour, but well-developed rote memory. As many people with Asperger syndrome have this disability, a primary diagnosis of Asperger syndrome is often preferred and is certainly the most clinically useful. This is an example of excessive diagnostic splitting, although non-verbal learning disability can occur with other disorders.