Autism and related pervasive developmental disorders (PDD)  are early-onset neurobiological conditions that share fundamental impairments in social reciprocity, pragmatic and semantic communication, reactions to environmental stimuli, and the nature of preferred interests and activities.  Although there is a broad range of cognitive, linguistic, and adaptive functioning across the autism spectrum, impairments in social understanding and communication are universally present.

It is this impairment in social reciprocity and communication that distinguishes autism spectrum disorders (ASD or PDD) from other conditions, such as mental retardation, developmental language disorders, specific learning disabilities, among others.

For example, whereas mental retardation is characterized by a pervasive developmental delay, autism is characterized by a distinctive impairment in the understanding of all things social, including complex emotions, intricate interpersonal relationships, social intuition, insight and judgment.

Although some of the manifestations of autism may improve over time, the condition persists throughout life.  In the vast majority of cases, autism results from abnormal brain development beginning early in gestation.  As is the case for other developmental conditions, autism may not be recognized and diagnosed until the child is of several years of age.  However, subtle manifestations are typically present during the first year of life, and can be identified by the assessment of joint attention, affective reciprocity, and somewhat later, metacognition.

Some families report a regression in language and social development between 18 and 24 months of age; however, it is unlikely that this reflects a new insult of some kind, but rather the progression of a genetically predetermined pathophysiological process.  It is important to distinguish this modest regression from the marked and dramatic loss of developmental capabilities in childhood disintegrative disorder, which becomes manifest following a completely normal 3 to 4 year period of development.

There are no established biologic markers for autism, although recent investigations have implicated several chromosomal loci as conferring a genetic predisposition to the development of this condition.  However, it is relatively clear that there are at least 20 autism related genes.

During the past decade, there has been a dramatic increase in the prevalence of autism (total number of existing cases).  Recent reviews indicate that as many as 1/250 to 1/500 individuals may have some type of autism spectrum disorder.  However, it is unclear whether the incidence of autism is increasing (the number of new cases).   Potential contributors to the rise in prevalence include improved diagnostic techniques, earlier identification, recognition of a broader range of severity than typified by classic Kanner autism, and less reluctance to make a formal diagnosis (because of the recent identification of effective educational and treatment interventions).

Current evidence suggests that autism may be a disorder of neuronal connectivity (e.g., orbitofrontal-limbic, amygdaloid-temporal association cortex tracts) and that deficits in social cognition and communication in autism may be related to dysfunction in the amygdala, hippocampus, and related limbic and cortical structures. Other neuroanatomical structures, such as the cerebellum, also may form part of a distributed neuronal network responsible for social cognition and communication.

Genetics play a major role in autism, but what is inherited and how broad the inheritable phenotype is remain unclear. At a neurochemical level, a number of neurotransmitter systems have been implicated in autism, including serotonin, dopamine, and several neuropeptides. Seizures and epileptiform discharges are relatively common in autism, present in 20% to 25% of cases, with onset during adolescence in a sizeable proportion of cases.  Active research programs dedicated to elucidating the cellular and molecular mechanisms underlying autism are ongoing.

Currently, there are several clinical subtypes of ASD/PDD recognized within official systems of diagnosis (i.e., DSM-IV-TR, ICD-10), including:

• Autistic disorder
  
• Asperger’s syndrome
  
• Rett’s disorder
  
• Childhood disintegrative disorder
  
• PDD, not otherwise specified (PDD-NOS).