Encyclopedia of Autism Spectrum Disorders

Living Edition
| Editors: Fred R. Volkmar

Atypical Autism

  • Kylie M. GrayEmail author
Living reference work entry
DOI: https://doi.org/10.1007/978-1-4614-6435-8_1635-3

Short Description or Definition

Atypical autism is often described as a subthreshold diagnosis, presenting with some symptoms of autism but insufficient to meet criteria for a diagnosis of childhood autism (or autistic disorder). Alternatively, atypical autism can be diagnosed when there is a late onset of symptomatology. Atypical autism (as defined by ICD-10) is seen as being equivalent to the DSM-IV-TR diagnostic category of pervasive developmental disorder not otherwise specified (PDD NOS). DSM-5 does not have a separate diagnostic category for PDD NOS.

Like PDD NOS, atypical autism is poorly defined, resulting in a research literature that can be difficult to interpret and conclusions difficult to reach. Atypical autism, as defined by the ICD, lacks operationalized diagnostic criteria, resulting in inconsistencies and variability in the way in which the diagnosis is applied. Although it now appears to be more common than autistic disorder, in general it remains poorly understood. This is likely due, in no small part, to the lack of a clear definition. Although it is often assumed that findings relating to autism apply to atypical autism, the lack of operationalized diagnostic criteria has undoubtedly hampered specific research into this diagnostic category and contributed to inconsistent findings across studies. Studies often fail to describe how they operationalized or defined their samples of atypical autism or PDD NOS. The ICD-10 provides specifiers to further define the diagnosis of atypical autism (see section “Categorization”); however, studies generally do not use these specifiers. Difficulties therefore remain in interpreting and comparing findings across studies. The broadening of the PDD NOS category in DSM-IV (Volkmar et al. 2000) has also contributed to difficulties in interpretability of results across studies, although with DSM-IV-TR (American Psychiatric Association 2000) this was remedied. Further definition of atypical autism or PDD NOS in research (see, e.g., Mandy et al. (2011)) would assist with furthering knowledge in this area.

This entry will focus on research studies involving individuals with atypical autism. Where necessary, this is supplemented with research findings from samples with PDD NOS.


The category of pervasive developmental disorder (PDD) was introduced in DSM-III (American Psychiatric Association 1980) and included the subthreshold diagnosis of atypical PDD, which subsequently became pervasive developmental not otherwise specified (PDD NOS) in DSM-III-R (American Psychiatric Association 1987). Reflecting thinking at the time, ICD-9 categorized autism (299.0 Infantile Autism) under the category of childhood psychoses and included a code for other specified early childhood psychoses, including atypical childhood psychosis (299.8) (World Health Organisation 1978). With the revision of these classification systems to the DSM-IV (American Psychiatric Association 2000) and ICD-10 (World Health Organisation 1992), the systems shared a common approach to coding and were seen as conceptually the same (Volkmar 1998).

The ICD-10 (World Health Organisation 1992) provides diagnostic criteria for atypical autism (F84.1) under the category of pervasive developmental disorders. The diagnosis is for cases where age of onset is after the age of three (criteria the same for childhood autism except for age of onset), or all three sets of criteria for childhood autism are not met (subthreshold). Criteria in the domains of abnormalities in reciprocal social interaction, or communication, or restricted, repetitive, and stereotyped patterns of behavior, interests, and activities are the same as for childhood autism (F84.0) except that it is not necessary to meet the criteria for number of areas of abnormality. Specifiers can then be used to indicate atypicality in age of onset (F84.10), atypicality in symptomatology (F84.11), or atypicality in both age of onset and symptomatology (F84.12). The DSM-IV (American Psychiatric Association 2000) defines PDD NOS as including atypical autism.

The ICD-10 also has two additional diagnoses, namely, other pervasive developmental disorder (F84.8, with no diagnostic criteria specified) and pervasive developmental disorder, unspecified (F84.9). The latter disorder is defined as a residual category for cases where there is a lack of information or contradictory findings, but where symptomatology fits the general description for a pervasive developmental disorder. The ICD-10 diagnoses of atypical autism, other pervasive developmental disorder, and pervasive developmental disorder, unspecified are considered to be broadly equivalent to the DSM-IV-TR (American Psychiatric Association 2000) diagnosis of PDD NOS.

In the current DSM (DSM-5; American Psychiatric Association 2013), the category of PDD NOS has been subsumed under autistic spectrum disorder, with the instruction to give the DSM-5 diagnosis of autism spectrum disorder to those with a well-established diagnosis of PDD NOS. Concerns have been raised regarding whether children and adolescents with DSM-IV diagnoses of PDD NOS or ICD-10 diagnoses of atypical autism would meet the DSM-5 diagnostic criteria for autism spectrum disorder. Using draft criteria, a number of studies reported concerningly low rates (3–28.3%) of cases of PDD NOS/atypical autism meeting the DSM-5 criteria for autism spectrum disorder (Barton et al. 2013; Mandy et al. 2011; Mayes et al. 2013; McPartland et al. 2012). Kim et al. (2014) reported a higher rate (63%) and Huerta and colleagues found that the DSM-5 diagnostic criteria resulted in improved specificity compared to the DSM-IV criteria for PDD NOS (Huerta et al. 2012). It has been speculated that children without repetitive, restricted, or stereotyped behaviors previously diagnosed with PDD NOS may meet the diagnostic criteria for the new DSM-5 Social Communication Disorder category (Ozonoff 2012; Skuse 2012). Prospective research studies using the DSM-5 diagnostic criteria are needed to explore these issues.

Draft guidelines for ICD-11 (due for release in 2018), mirror the DSM-5, subsuming atypical autism into the single diagnostic category of autism spectrum disorder (WHO, GCP Network 2017).


Atypical autism is rarely the focus of prevalence studies, and differing labels and combining of groups other than autistic disorder can make the extraction and interpretation of prevalence figures difficult. A number of population and birth cohort studies have included figures on the prevalence of atypical autism. The UK-based studies in children have reported differing prevalence figures of 10.5/10,000 (Lingam et al. 2003), 10.9/10,000 (Williams et al. 2008), and 27/10,000 (Baird et al. 2000), while a birth cohort study (6-year-olds) in Stockholm reported a prevalence of 22/10,000 (Fernell and Gillberg 2010). A study in the Faroe Islands (considered a genetic isolate) reported a population prevalence of atypical autism of 0.12%, while acknowledging that this is possibly an underestimate particularly in terms of higher functioning children (Ellefsen et al. 2007). A Danish population study reported separate prevalence rates for atypical autism (3.3/10,000) and PDD NOS (14.6/10,000), which when taken together are similar to those rates reported by Fernell and Gillberg (2010) and Baird et al. (2000). A South Korean study provided a prevalence estimate of 1% for PDD NOS (Kim et al. 2011). Using data from the national Danish register, reported rates of Gender ratios have been reported by a very small number of studies, with a higher proportion of males with autistic disorder compared to atypical autism, 6.5:1 compared to 3.8:1 in Stockholm (Fernell and Gillberg 2010), and no reported gender differences between PDD NOS (85.3% male) and autistic disorder (85.9% male) in a birth cohort of 4–6-year-olds in Stafford in the UK (Chakrabarti and Fombonne 2005).

A series of review studies by Fombonne, most recently in 2009, reviewed 43 prevalence surveys, 17 of which provided separate estimates of the prevalence of atypical autistic syndromes (PDD NOS and atypical autism) (Fombonne 2009). Fourteen of these studies reported a higher prevalence of atypical autism syndromes compared to autistic disorder, 37.1/10,000 and 20.6/10,000 respectively. Like the prevalence of autism, the reported prevalence of atypical autism has increased over time. Similarly, this increase is typically discussed in relation to changes in diagnostic criteria, increased awareness, diagnostic substitution, changes in special education policies, and increases in the availability of services. What is, however, clear from these studies is that there is a significantly large population of children with atypical autism who have treatment needs similar to those of children with autism.

Natural History, Prognostic Factors, Outcomes

A small number of studies have investigated the early signs and symptoms in children later diagnosed with atypical autism, with mixed results. One study looked at first symptoms and diagnosis in children with atypical autism, comparing the parent-reported onset of symptomatology to that of children diagnosed with childhood autism (Oslejskova et al. 2007). Significant group differences were found in age of first symptoms, with parents of children with atypical autism reporting first symptoms at an average of 36.7 months (compared to 23.5 months for children with childhood autism). There were however no significant group differences in age at diagnosis. In contrast, Walker et al. reported no difference between autism and PDD NOS in terms of age at which abnormalities were first identified by parents (2004). Two epidemiological studies found that atypical autism was diagnosed later than childhood autism, with atypical autism generally diagnosed at 5–6 years of age and childhood autism at 3–4 years (Fernell and Gillberg 2010; Lingam et al. 2003).

Research has demonstrated that outcome in autism and other pervasive developmental disorders is associated with the acquisition of expressive language skills by the age of 5–6 years, cognitive ability, and early social-communicative skills (Gillberg and Steffenburg 1987; Kobayashi et al. 1992; Mundy et al. 1990; Nordin and Gillberg 1998; Sigman and Ruskin 1999). Longitudinal studies have reported that initial diagnosis (i.e., atypical autism or PDD NOS compared to autistic disorder) is not related to outcomes (Baghdadli et al. 2007; Turner et al. 2006) and therefore has limited use in predicting developmental outcomes. However, Moulton et al. (2016) reported that a diagnosis of PDD NOS at age 2 was associated with better outcomes at age 4 relative to those children with a diagnosis of autistic disorder, likely due to lower rates of autism symptomatology, particularly restricted and repetitive behaviors.

Clinical Expression and Pathophysiology

The reliability and stability of the diagnoses of atypical autism and PDD NOS has been questioned. In a study of subtypes of pervasive developmental disorders in children, Mahoney et al. (1998) reported interrater agreement for diagnoses of Asperger’s disorder, autism, and atypical autism across three raters. Kappa values revealed good agreement for the diagnosis of autism (0.55), Asperger’s disorder (0.56), and non-PDD (0.67), but poor agreement in the case of atypical autism (0.18). Consistent with the results of studies in children with atypical autism, research in toddlers with autism and PDD NOS has reported good agreement between clinicians on the diagnosis of autism, but low rates of agreement for PDD NOS (Chawarska et al. 2007; Stone et al. 1999).

In relation to diagnostic stability, research has focused on individuals with PDD NOS. While diagnoses of autistic disorder have been shown to be relatively stable in toddlers, the same is not true of PDD NOS (Chawarska et al. 2007; Stone et al. 1999; Turner et al. 2006; van Daalen et al. 2009). A meta-analysis of the diagnostic stability of PDD NOS reviewed eight studies, reporting higher rates of stability for a diagnosis of autistic disorder compared to PDD NOS (Rondeau et al. 2010). It was concluded that a diagnosis of PDD NOS prior to 36 months was unstable (35% stability) over time, highlighting the need for reassessment. It has been suggested that low diagnostic stability may be attributable to the later emergence of stereotyped and repetitive behaviors in young children (Kleinman et al. 2008; Sutera et al. 2007).

The lack of operationalized diagnostic criteria for atypical autism and the variability in which the diagnosis is applied have possibly resulted in a significant amount of heterogeneity in the presentation of individuals; as such, there is as yet no consensus regarding the symptom profile for atypical autism or PDD NOS (Mandy et al. 2011). Two studies have examined symptom profiles in children with atypical autism, focusing on high-functioning children with atypical autism, Asperger’s disorder, and childhood autism (Kanai et al. 2004; Kurita 1997). In a comparison of children with high-functioning atypical autism and childhood autism, symptom patterns were examined using the Childhood Autism Rating Scale (CARS) (Kurita et al. 1989), rated by clinicians blind to the child’s diagnosis. The children with atypical autism scored significantly lower on the CARS total score. There were no significant group differences on 11 of the 15 CARS items. After controlling for IQ and total CARS score, the children with atypical autism were found to be significantly less impaired on two items of the CARS (relationships with people and general impressions) and were more impaired in anxiety reaction compared to the children with childhood autism. In a comparison of high- functioning atypical autism and Asperger’s disorder, the Asperger’s disorder group was significantly less impaired than the atypical autism group on total CARS score, imitation, visual responsiveness, auditory responsiveness, and nonverbal communication (Kurita 1997). Overall, these findings are consistent with the idea of atypical autism being a subthreshold diagnosis for children with a significant degree of impairment, but not to the degree that criteria for childhood autism are met.

Further information on symptom presentation comes from studies with children with a diagnosis of PDD NOS. Consistent with the results of the studies with children with atypical autism, a number have reported generally finding children with PDD NOS to have significantly less impairment in the social, communication, and restricted and repetitive symptom domains compared to children with autistic disorder (Fodstad et al. 2009; Walker et al. 2004). de Bruin et al. (2006) reported that children with PDD NOS have similar cognitive profiles as children with autism, although in contrast Walker et al. (2004) found that children with PDD NOS scored better than children with autism on measures of adaptive behavior and nonverbal reasoning and problem-solving skills. An investigation of communication impairments using the Children’s Communication Checklist (Bishop 1998) with children with high-functioning autism, Asperger’s disorder, and PDD NOS found that while all groups demonstrated significantly more impairment than the typically developing control group, there was little difference across the autism subtypes. In a comprehensive study, Mandy et al. (2011) operationalized the definition of PDD NOS and compared the symptom profiles of children with autistic disorder, Asperger’s disorder, and PDD NOS on independent measures of symptomatology. They found that the overwhelming majority (97%) of children with PDD NOS presented with a symptom profile characterized by significant impairment in social interaction and communication skills without repetitive stereotyped behavior. The remaining children presented with a symptom pattern of significant social impairment and repetitive stereotyped behavior without communication impairment. These results are inconsistent with the view of PDD NOS being a condition with marked heterogeneity. The children with PDD NOS demonstrated significantly less routinized and repetitive behaviors, sensory difficulties, feeding, and visuospatial problems compared to the children with autistic disorder and Asperger’s disorder. With PDD NOS now subsumed under the DSM-5 diagnostic category of autism spectrum disorder (ASD), it may be that individuals presenting with marked impairments in social interaction and communication, without repetitive stereotyped behavior, will not meet the DSM-5 diagnostic criteria for ASD.

High rates of comorbid mental health problems have been reported in atypical autism and PDD NOS. A Danish study compared a sample of 89 individuals diagnosed as children with atypical autism to a matched control sample from the general population (Mouridsen et al. 2008). Using the Danish Psychiatric Register, they demonstrated that over a 36-year follow-up period, elevated rates of co-occurring psychiatric diagnoses were found in those with atypical autism. The most prevalent of these was schizophrenia spectrum disorder. High levels of depression, anxiety, and disruptive behavior disorder have been reported in children with PDD NOS (de Bruin et al. 2007; Pearson et al. 2006), highlighting the importance of considering comorbid mental health problems when conducting diagnostic assessments for atypical autism.

It has been reported that while comorbid medical conditions in autism are associated with degree of intellectual disability, they may be more frequent in individuals with atypical autism, although results are mixed across studies (Gillberg and Coleman 1996; Juul-Dam et al. 2001; Rutter et al. 1994). A study by Hara (2007) found no differences between individuals with autism and atypical autism in terms of epilepsy. Biological research on atypical autism and PDD NOS, including neuroimaging and genetic studies, has overall found no evidence for differences between these conditions and autistic disorder (Towbin 2005).

Evaluation and Differential Diagnosis

The assessment process for atypical autism is the same as that recommended for autism and other pervasive developmental disorders. In making a differential diagnosis, whether the criteria are met for a diagnosis of autism or Asperger’s disorder needs to be considered, and degree of intellectual disability needs to be taken into account. Differentiating atypical autism from language disorder is also important. It has been demonstrated that children with PDD NOS can be differentiated from children with language disorders on the basis of more severe social impairment and a greater need for routines and order (Mayes et al. 1993). Research with children with a significant degree of disruptive behavior has also highlighted the need to consider a diagnosis of atypical autism. In a cohort of primary school-aged children, significant impairments in social and communication domains were identified in children with significant disruptive behavior, with 28% meeting criteria for a diagnosis of atypical autism (Donno et al. 2010).

Differentiating ADHD and atypical autism in young children can be problematic, with children often first diagnosed with ADHD (Jensen et al. 1997). In a retrospective study, parents of children with PDD NOS or ADHD reported on the symptoms of their children in their first 4 years (Roeyers et al. 1998). Early differences were infrequent, although children with ADHD showed more hyperactive behaviors during the 7–12-month period; this difference was not maintained as the PDD NOS children became active with age. As children aged, the difference became more apparent, with children with PDD NOS demonstrating more pronounced social difficulties, withdrawal, anxiety, stereotyped motor behaviors, unusual behaviors, and better scores on cognitive assessments compared to children with ADHD (Jensen et al. 1997; Luteijn et al. 2000; Roeyers et al. 1998; Scheirs and Timmers 2009).


As for autism, treatment for individuals with atypical autism needs to include a range of services and approaches. Behavioral, educational, and developmental approaches to the treatment of communication deficits, social difficulties, and behavior problems have been demonstrated to result in improvements for individuals with autism and are likely to be helpful for individuals with atypical autism. Although there are no drugs that specifically treat autism, medication and medication in combination with parent training approaches have been shown to reduce severe behavior problems such as aggression, self-injurious behavior, severe tantrums, and irritability (King 2000; Research Units on Pediatric Psychopharmacology (RUPP) Autism Network 2005a, b, 2009).

Early intervention has been highlighted as a specific area of importance in the treatment of children with autism. Treatment gains have been demonstrated in adaptive functioning, developmental skills, symptom severity, and behavior problems (Dawson et al. 1998; Howlin et al. 2009; Rogers and Vismara 2008). Training parents to implement early intervention programs has also demonstrated gains in communicative behavior, knowledge of autism, parent communication style, parent-child interaction, child behavior problems, and a reduction in parent stress and mental health problems (McConachie and Diggle 2005; Tonge et al. 2006; Whittingham et al. 2009). Improvements can also be made in teaching joint attention, symbolic play, and imitation skills to very young children (Drew et al. 2002; Kasari et al. 2001, 2006). Although important gains have been made in the development of evidence-based early interventions, less is known about the role played by mediating or moderating variables in treatment outcomes. Importantly, the impact that early childhood intervention may or may not have on adult outcomes remains unknown.

Research on treatment approaches specifically for individuals with atypical autism is lacking; it is assumed that treatment needs and approaches are similar to those for individuals with autism.

Whether existing evidence-based treatments produce greater effects in individuals with atypical autism remains an area for further research. As is the case for autism, it has been concluded that no single treatment approach or method has been shown to be effective for PDD NOS (Towbin 2005), with treatment approaches needing to take into account the specific strengths, impairments, and needs of each individual.


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Authors and Affiliations

  1. 1.Centre for Developmental Psychiatry and Psychology, Department of Psychiatry, School of Clinical Sciences at Monash HealthMonash UniversityClaytonAustralia