This article needs attention from an expert in medicine.(December 2017)
Regressive autism occurs when a child appears to develop typically but then starts to lose speech and social skills, typically between the ages of 15 and 30 months, and is subsequently diagnosed with autism.  Other terms used to describe regression in children with autism are autism with regression, autistic regression, setback-type autism, and acquired autistic syndrome.  There is no standard definition for regression,  and the prevalence of regression varies depending on the definition used.  Some children show a mixture of features, with some early delays and some later losses; and there is evidence of a continuous spectrum of behaviors, rather than, or in addition to, a black-and-white distinction, between autism with and without regression. 
Approximately 25–30% of children with autism spectrum disorders stop speaking after beginning to say words, often before the age of two.  According to Ami Klin, "Most examples of autistic regression... are based upon a child's loss of a handful of words... it's possible that these children were only echoing sounds they heard from their parents"  Some children lose social development instead of language; some lose both. 
After the regression, the child follows the standard pattern of autistic neurological development. The term regressive autism refers to the appearance that 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 not to include such aberrations, with age-appropriate autistic symptoms being clear from birth.
Skill loss may be quite rapid, or may be slow and preceded by a lengthy period of no skill progression; the loss may be accompanied by reduced social play or increased irritability.  The temporarily acquired skills typically amount to a few words of spoken language, and may include some rudimentary social perception. There are several intermediate types of development, which do not neatly fit into either the traditional early onset or the regressive categories, including mixtures of early deficits, failures to progress, subtle diminishments, and obvious losses. If regression is defined strictly to require loss of language, it is less common; if defined more broadly, to include cases where language is preserved but social interaction is diminished, it is more common. 
Regression in autism spectrum disorders is well documented; attribution of regression to environmental stress factors may result in a delay in diagnosis.  The apparent onset of regressive autism is surprising and distressing to parents, who often initially suspect severe hearing loss. A controversy occurred following a fraudulent study linking the MMR vaccine to autism, but since then, studies have shown no connection between autism and the MMR vaccine.   There are also studies being done to test if certain types of regressive autism have an autoimmune basis. 
There are some who believe that regressive autism is simply early-onset autism that was recognized at a later date. Researchers have conducted studies to determine whether regressive autism is a distinct subset of autism spectrum disorders. Over the years, the results of these studies have contradicted one another. Some researchers believe there is still nothing to support a definitive biological difference between early-onset and regressive autism.  However, emerging research shows that males with regressive autism have brains that are six percent larger than anyone with early-onset autism. The brains of females with regressive autism show no difference in brain size. 
Other disorders that involve regression include total blindness from birth, childhood disintegrative disorder, Rett syndrome,  Landau-Kleffner syndrome,  and childhood onset neurodegenerative diseases (e.g. Batten Disease).   "Reversible" regression could anticipate non-pure Tourette syndrome.  
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