Schizophrenia spectrum disorders
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 (Ramsey 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 (Erlenmeyer-Kimling et al, 2000) suggests that some individuals later diagnosed with schizotypal personality disorder developed without impairments in reciprocal social interaction and communication.

Attention-deficit hyperactivity disorder (DSM–IV)
Attention-deficit hyperactivity disorder (ADHD) presents with inattention, distractability, fidgetiness, impulsivity and hyperactivity. Persons with HFA spectrum disorders may be hyperactive, impulsive, have a short attention span and share similar executive function deficits as patients with ADHD. The conditions differ in that ADHD lacks the classic impairment in reciprocal social interaction, narrow interests, repetitive routines and non-verbal problems of Asperger syndrome. In accordance with a hierarchical rule in DSM–IV, a person meeting the criteria for a pervasive developmental disorder cannot be diagnosed as having ADHD. This is not the case in ICD–10, in which a dual diagnosis of Asperger syndrome and ADHD is possible.

Gillberg & Ehlers (1998) point out that children who meet criteria for ADHD may also meet the full criteria for Asperger syndrome. They mention one study, in which 21% of children with severe ADHD met the full criteria for Asperger syndrome and 36% showed autistic traits. A developmental history is usually sufficient to separate ADHD from Asperger syndrome, but ADHD can present as soon as the child can walk, and it is important to consider that impulsivity can interfere with social relationships, making children appear unempathic. Indeed, children with ADHD can be so easily distracted that they appear to be in a world of their own and therefore seem socially disconnected. It is not surprising, therefore, that children with Asperger syndrome are not uncommonly misdiagnosed as having ADHD, since it is often the attention and hyperactive problems that parents first observe.

Obsessive compulsive disorders (DSM–IV)
The core features of obsessive–compulsive disorder (OCD) are recurrent and persistent thoughts, impulses or images that are experienced at some time during the disturbance as intrusive and inappropriate and that cause marked anxiety or distress. The individual recognises that these are a product of his or her own mind. Compulsions involve repetitive behaviours or mental acts that a person feels driven to perform to reduce stress associated with some dreaded event or situation. An adult can recognise that they are excessive or unreasonable, but children cannot (American Psychiatric Association, 1994).

These phenomena, including the urge to count and manipulate numbers, to repeat the same action over and over, are similar to the repetitive routines associated with Asperger syndrome. Individuals with both conditions display ritualistic behaviour and resistance to change. Where they differ is that persons with Asperger syndrome have obsessive interests that are not experienced as ego-dystonic and, indeed, are often enjoyed. Baron-Cohen (1989) was critical of the use of the term obsession in persons with autism because the subjective phenomena of resistance to repetitive activities could not be discerned in autism. He suggested instead the phrase ‘repetitive activities’. OCD generally has a much later onset and lacks the poor social emotional reciprocity, empathy problems and social skills difficulties of people with Asperger syndrome (Szatmari, 1998). Detailed analysis of current symptoms and an early developmental history are the key to making a correct diagnosis.

Affective disorders
Despite some overlap in symptomatology (including social withdrawal, lack of emotional response and loss of interest in relationships), affective disorders differ in representing a distinct change from premorbid functioning, and typically are associated with onset in adulthood. Epidemiological studies of psychiatric comorbidity are lacking in individuals with autistic spectrum disorders, but depression, anxiety disorders and bipolar disorder occur more commonly than in the general population and represent substantial morbidity (Gillberg & Billstedt, 2000).

Other diagnostic categories
Many other overlapping categories are unrepresented in DSM–IV or ICD–10 and may be a source of confusion for families and professionals alike. Terms such as semantic pragmatic disorder, non-verbal learning difficulty and developmental learning disability of the right hemisphere have arisen as different specialities have struggled independently to categorise individuals with social disabilities who do not meet criteria for classic autism.

Semantic pragmatic disorder
Semantic pragmatic disorder (Rapin & 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” (Szatmari, 1998: p. 71). Is this an exact description of the language problems of Asperger syndrome (Wing, 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 (Fitzgerald, 1998).

Multiple complex developmental disorder
The defining characteristics of multiple complex developmental disorder (MCDD; Cohen et al, 1987) are shown in Box 5. Thought disorder and affective dysregulation are more characteristic of MCDD subjects, whereas problems in social interaction, communication and behavioural adjustment are more typical of subjects with autistic disorder. As the core features can also occur in Asperger syndrome its nosological status is uncertain.

Cerebellar affective syndrome
Cerebellar affective syndrome (Schmahmann & 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 (Myklebust, 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.