Autism spectrum disorders (ASD) are a heterogeneous group of biologically based neurodevelopmental disorders of brain function. They are a behaviorally defined group of disorders that are characterized by impairments in two areas: (1) deficits in social communication and social interactions and (2) restricted and repetitive patterns of behavior, interests, and activities. The term autism (originating from the Greek word autos meaning self) was adopted by Leo Kanner in 1943 to describe children whose behavior was “governed rigidly and consistently by the powerful desire for aloneness and sameness.”

The ASDs have been defined in the Diagnostic and Statistical Manual of Mental Disorders (DSM) of the American Psychiatric Association under many different names including the umbrella term pervasive developmental disorder or PDD, causing much confusion for both families and professionals. With the most recent 2013 revision, Diagnostic and Statistical Manual of Mental Disorders, 5th edition (DSM-5), ASD now subsumes what were previously separate diagnostic categories of autistic disorder (also referred to as classic autism or early infantile autism), pervasive developmental disordernot otherwise specified (PDD-NOS), and Asperger syndrome. These changes are based on research results which have failed to document these categories as separate biologic entities.

Under the DSM-5, a diagnosis of ASD requires an individual to exhibit three deficits in social communication and at least two symptoms in the category of restricted range of activities/repetitive behaviors. The prior version of the diagnostic manual also included deficits in language expression as a separate criterion, but this is no longer the case, as not all children with ASD have specific language disorders. Pragmatic language skills, however, are incorporated into the social domain, as all individuals with ASD have deficits in this domain of language. Within the second category, a new symptom is included: hyper- or hyporeactivity to sensory input or unusual interests in sensory aspects of the environment. Deficits in social communication and interactions include those in social reciprocity; nonverbal communication; and skills in developing, maintaining, and understanding relationships. Symptoms must be present in early development but need not be shown until social demands exceed the individual’s capacity. Furthermore, DSM-5 specifies three levels of severity (mild, moderate, severe) rated separately for social communication and restricted, repetitive behaviors based on what level of support the individual requires. In addition to the diagnosis, individuals are also described in terms of any known genetic cause (e.g., fragile X syndrome, Rett syndrome), level of language and intellectual disability, and presence of medical conditions such as seizures, psychiatric disorders (e.g., anxiety, depression), and/or gastrointestinal disorders. The variability in severity and range of disabilities seen in children with ASD cannot be overemphasized. A full description of the DSM-5 criteria for ASD can be found in the American Psychiatric Association, Diagnostic and Statistical Manual of Mental Disorders, 5th edition.

There has been a significant increase in the prevalence of ASD in the United States, particularly since the late 1990s when the estimated frequency was about 1 per 1,000 for autism and 2.8 per 1,000 for all ASD subtypes (autistic disorder, Asperger syndrome, and PDD-NOS). The estimated prevalence is much higher now. The Autism and Developmental Disabilities Monitoring Network (2010, 2014) that identifies ASD through screening and review of health and education records that document behaviors associated with ASD in 11 sites in the United States most recently reported a prevalence rate of 14.7 per 1,000 (1 in 68) among 8-year-olds, with prevalence estimates varying from 5.7 to 21.9 per 1,000 in the different sites. Non-Hispanic white children were approximately 30% more likely to be identified with ASD than non-Hispanic black children and almost 50% more than Hispanic children. ASDs are four times more likely in males than in females.

Factors such as increased awareness among parents and professionals, broadening of the diagnosis with emphasis on the spectrum aspect of the disorder, including mildly affected individuals, change in referral patterns, and using the diagnosis as a basis for intervention services may account for increased prevalence rates. The theory that vaccines, in particular the measles, mumps, rubella (MMR) vaccine, play any role in the increased prevalence of autism has been completely discredited.

Social communication dysfunction is the hallmark of ASDs. Deficits in sociability are variable but an essential and defining feature. Early signs include failure to respond to name, abnormal eye gaze (gaze aversion), paucity of gestures, and impairment of
joint attention (the ability to draw another person’s attention to an object of interest through the use of eye gaze and gestures, such as pointing). Children appear aloof with a lack of interest in others and mainly interact with others when they need something rather than to share their interests, enjoyment, and achievements. As children age, a lack of empathy becomes apparent along with difficulty interpreting and reciprocating the social and emotional behaviors of others. Pragmatic language skills are always impaired, that is, the social rules for communicating are not well understood. These individuals have a hard time taking turns in conversations, talk only about topics of their interest or knowledge, talk at people without checking to see if anyone is listening or interested, fail to maintain appropriate interpersonal distance, and are inappropriately intrusive or persistent. There is an impaired ability to read facial expressions, body language, and tone of voice of others, which contributes to these deficits. They are often described as “odd” or “annoying,” as their social approaches are peculiar.

Another cardinal feature of ASD is a restricted range of behaviors, interests, and activities that are often repetitive or stereotyped. These include motor stereotypies such as hand flapping or licking, finger flicking, spinning, and body rocking. They may have unusual preoccupations and interests and play skills are impaired. These children can watch the same video for hours, play with unusual objects (e.g., a piece of string), or line up toys instead of playing with them imaginatively. Some children have routines or rituals that need to be followed and many such children have difficulty making transitions. Overall, there is a tolerance for monotony and a resistance to change.

Although no longer a criterion for a diagnosis of ASD, verbal communication disturbances are commonly found. The most common presenting complaint is a delay in language development. Despite this delay, there is a lack of compensation through nonverbal modes of communication, such as gesture and imitation. Spoken language is often echolalic with repetitive and stereotyped phrases. Some children will recite scripts from television or videos. Some children fail to process language and despite normal hearing, appear deaf (verbal auditory agnosia). Abnormalities in syntax (word order), semantics (word meaning), and most prominently pragmatics are present in higher functioning individuals. These children are often extremely literal and concrete and have difficulties understanding jokes, humor, or sarcasm. Speech prosody (melody of language) is impaired, as evidenced by mechanical, monotonous, excessively rapid, high-pitched, and high-volume speech. There can be a fascination with letters and numbers in the early toddler years and some children may learn to read without instruction, e.g. hyperlexia, although comprehension is limited.

Intelligence is not a defining feature but strongly influences prognosis. Intellectual disability (IQ <70) occurs in approximately 55% of individuals with ASD, although many have average or even superior intellectual ability. Prodigious or savant skills can be seen in visuospatial ability, memory, calculation, or music ability. IQ is a key predictor of long-term outcome in autism, especially for those with an IQ of less than 50 who generally fare poorly. Average to superior IQ is a positive prognostic sign. Having a higher verbal IQ at 2 to 3 years of age appears to predict better outcome, particularly if intervention is delivered early.

Developmental regression or loss of previously established skills during the first 1 to 3 years of life has been estimated as occurring in approximately one-third of children with ASD. There is debate as to whether children with ASD and regression develop better, worse, or the same on IQ, language, and social functioning than those with no regression, but good recovery in children with regressive symptoms is rare.