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Regressive autism is a manner of the neurological development of an autistic child in which they appear to develop typically in a certain area, such as speech, and then start to lose those skills at about the age of 18 months. They thereafter following the standard pattern of autistic neurological development. The term refers to the appearance that their neurological development has reversed; it is actually only the affected developmental skills, rather than the neurology as a whole, that regresses. It is more usual for autistic neurological development to not include such aberrations, with age-appropriate autistic symptoms being clear from birth, in which case the term 'early infantile autism' is used.

The temporarily acquired skills typically amount to a few words of spoken language, and may include some rudimentary social perception. Skill loss may be quite rapid, or may be slow and preceded by a lengthy period of no skill progression. In some cases a child will go through a period of learning new words at the cost of losing previously acquired words.

The apparent onset of regressive autism is surprising and distressing to parents, who often initially suspect severe hearing loss. The pattern of symptoms naturally leads many to suspect that postnatal environmental factors trigger autism. In particular, because obvious symptoms start just after children receive multiple vaccinations, such as MMR plus varicella (chickenpox), some people perceive a causal link between vaccination and autism, especially if accompanied by a fever, rash, and rapid skill loss. Although some controversy remains, the prevailing understanding is that regular autism is caused by prenatal biochemistry, and that there may be a link to vaccines with regard to regressive autism.

• Landau-Kleffner syndrome