In a study that appeared on Monday in Biological Psychiatry, scientists from the Universities of California and North Carolina, with several other universities in the United States and Canada, report a strong correlation between abnormal distribution of cerebrospinal fluid in infants and later development of autistic symptoms.
"The more extra-axial CSF present at six months, the more severe the autism symptoms when the kids were diagnosed at 24 months of age," said first author Dr. Mark Shen, a postdoctoral fellow at the University of North Carolina.
The study covered 343 children examined aged six months, twelve months, and twenty-four months, 221 of whom had older siblings with autism.  Children with higher than usual volumes of cerebrospinal fluid in the subarachnoid space — the area just around the brain — were more likely to be diagnosed with autism later in life, with a strong correlation in the high-risk group.  Ultimately, cerebrospinal volume was able to provide an early diagnosis of probably ASD in high-risk children with 70% accuracy.  The six-month-old babies who later went on to a diagnosis of autism had an average of 18% more CSF by volume than those who were not so diagnosed.  This built on the findings of a 2013 study that covered only 55 children.
Researchers said it is not clear whether a large amount of cerebrospinal fluid actually causes autism or not.  While studies have shown that cerebrospinal fluid, once thought to act solely as a cushioner and shock absorber for the brain, can influence the way neurons grow, Shen speculated that the large amount of fluid may itself be a symptom: "We believe that extra-axial CSF is an early sign that CSF is not filtering and draining when it should. The result is that there could be a buildup of neuro-inflammation that isn't being washed [a]way."
Currently, coauthor David Amaral said, children are not diagnosed with autism spectrum disorders until they are old enough for their behavior to change, usually at two or three years old.  Researchers said these findings could be used to develop an early diagnostic system usable when the patient is as young as six months old.

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Scientists say excess cerebrospinal fluid may serve as early sign of autism