Autism spectrum disorders (ASDs) are a group of severe neurodevelopmental disorders, in which patients show deficits in social interaction, impaired communication, repetitive behavior and restricted interests and activities. The prevalence of ASDs is rising, and it is estimated that 60 per 10,000 individuals currently have a form of ASD and 13 per 10,000 individuals have autism. ASDs include several clinically defined conditions, of which pervasive developmental disorder (not otherwise specified) and autistic disorder (‘classic’ autism) are the most common. Less frequent are Asperger syndrome, Rett syndrome, and childhood disintegrative disorder. Rett syndrome typically occurs in females and is characterized by loss of speech and purposeful hand use, microcephaly, seizures, ataxia, and stereotypic hand movements. Several atypical variants of Rett syndrome have been reported, including one that has an infantile onset of encephalopathy and spasms, followed by severe global developmental delay and intellectual impairment. Autism often appears to be associated with either macrocephaly or microcephaly. It is reported that 83% of patients with autism have a head circumference greater than the 50th percentile, and 24% have a head circumference above the 98th percentile.