The brains of autistic children undergo an abnormal and dramatic growth spurt in the first year of their lives, according to a new study. The growth makes their heads markedly larger than those of normal infants.
This difference emerges long before any behavioural symptoms and researchers say their finding suggests a way to make earlier, better diagnoses. This would allow earlier treatment, which is known to be beneficial.
The head-size difference is 鈥渢he first known neurobiological early-warning sign of autism,鈥� says Eric Courchesne, who conducted the study with colleagues at the University of California, San Diego.
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Because the aberrant brain enlargement occurs in infancy, he says, it also serves as evidence against popular theories that autism is caused by vaccinations, allergies, or exposure to toxins during early childhood.
Close correlation
Paediatricians routinely measure the circumference of children鈥檚 heads during examinations. When the authors of the new study looked back at the medical records of 48 normal and autistic children aged two to five, they found that 60 per cent of the autistic children had head circumferences far above the norm when they were six to 14 months of age.
By contrast, only six per cent of the normal children had enlarged heads. The severity of the autistic children鈥檚 symptoms was also correlated closely with the size of their heads during infancy.
During normal development, the brain grows slowly, and synapses, the connections among nerve cells, are retained or eliminated in response to sensory stimulation or motor activity.
Courchesne proposes that the unchecked brain growth seen in the study may undermine this synaptic strengthening and pruning process, leading to the social isolation characteristic of the disorder. 鈥淭hey鈥檙e developing synapses so rapidly that experience doesn鈥檛 have a chance to determine which are maintained and which are eliminated,鈥� he told 快猫短视频.
Rigid criterion
It has been known for 60 years that some autistic individuals, both children and adults, have unusually large heads. About 20 per cent are formally diagnosed with 鈥渕acrocephaly,鈥� which requires that a patient鈥檚 head circumference measurement be in the highest two per cent for their age group.
Courchesne believes that the rigidity of this diagnostic criterion may lead doctors to overlook cases in which infants鈥� heads are abnormally large but do not surpass the threshold for macrocephaly.
Janet Lainhart of the University of Utah told 快猫短视频 that the new study sheds important new light on the developmental origins of autism, but she cautions that head size measurements alone cannot be used to screen children for the disorder: 鈥淵ou certainly wouldn鈥檛 want to be taking head circumference measures and telling parents, 鈥榊our child is at risk for autism.'鈥�
Courchesne notes that the heads of infants who later develop autism are actually smaller at birth than their peers, before undergoing the growth spurt. He says this pattern will be a good marker for autism.
Journal reference: Journal of the American Medical Association (vol 290, p 337)