WASHINGTON - Abnormal brain growth starting at four months of age occurs in a type of autism in which toddlers lose language and social skills they once had, according to a US study published on Monday.

The brains of boys with regressive autism grew 6 percent larger than typically developing counterparts and toddlers who showed signs of autism early in life, a form called early onset autism.

The research, involving 180 subjects and described as the "largest study of brain development in preschoolers with autism to date", also found no evidence of a brain growth spurt in girls with autism.

"This adds to the growing evidence that there are multiple biological subtypes of autism, with different neurobiological underpinnings," said co-author David Amaral, research director of the MIND Institute at University of California, Davis.

Autism includes a wide spectrum of developmental differences and may range from mild social awkwardness to complete inability to communicate, repetitive movements, sensitivity to certain lights and sounds, and behavioral problems.

As many as one in 110 children is diagnosed with autism, though its cause remains a mystery. The disorder is more common in boys than girls by a factor of four to one.

Previous studies have suggested that clinical signs of autism tend to coincide with a period of abnormal brain and head growth that becomes apparent between the ninth and 18th month of life.

However this study, published in the Proceedings of the National Academy of Sciences issue of Nov 29, is the first to show a difference in brain size between toddler boys with regressive versus early onset autism.

"The finding that boys with regressive autism show a different form of neuropathology than boys with early onset autism is novel," said lead author Christine Wu Nordahl, a researcher at the MIND Institute.

"Moreover, when we evaluated girls with autism separately from boys, we found that no girls - regardless of whether they had early onset or regressive autism - had abnormal brain growth."

The findings showed that boys with regressive autism had a "pronounced increase" in head circumference beginning as early as four months of age and lasting through 19 months.

The data was based on head circumference measurements taken from pediatric well-baby visits from birth through 18 months, and magnetic resonance imaging (MRI) scans done on all 180 participants at age 3.

The authors noted that the study was limited because it relied upon parental reports of when the children's autism began to appear.

Previous research has "pointed out significant complexities in defining and measuring the onset of autism symptoms", with as many as 45 percent of children in one study showing signs of autism on video that were not reported by parents.

"The major finding of this study is that a subset of boys with regressive autism have normal head circumference at birth, which diverges from normality around four to six months of age, well before any loss of skills were documented," said the study, calling for more research.

"Thus, rapid head growth beginning around four-six months of age may be a risk factor for future loss of skills."

Agence France-Presse