Abstract | Summary | Original Article

Localization of White Matter Volume Increase in Autism and Developmental Language Disorder

Herbert MR, Ziegler DA, Makris N, Filipek PA, Kemper TL, Mormandin JJ, Sanders HA, Kennedy DN and Caviness, Jr., VS

Annals of Neurology 2004; 55: 530-540

Question: Is there a difference in the amount and location of white matter in the brains of children with autism, children with developmental language disorder, and typically developing children?

Background: Over the past several years researchers have noted that children with autism, on average, have a larger head circumference than typically developing children.^1,2,3 In fact, large head size, the only consistent physical characteristic seen among children with autism, was found in about 90% of children examined by one researcher.⁴ The question has been whether the increased head size is due to overall brain enlargement, to volume increases of specific areas of the brain, or to volume increases of one or the other types of brain tissue. There are two types of tissue in the brain: gray matter and white matter. Gray matter is the tissue in which the ?processing? in the brain is done; white matter is the tissue that provides connections between the various gray matter regions in the brain. The white matter contains a lot of nerve fibres insulated with a white fatty substance called myelin. It has been suggested that the head size increase might be due to larger amounts of white matter in the brain.

There is currently no medical test that can make a clear diagnosis of autism spectrum disorders (ASD). This study examined the possibility that abnormal brain structure may be diagnostic of these conditions. The investigators compared the findings of MRI brain scans that measured the volumes of white and gray matter in children with autism to children with developmental language disorder (DLD), a condition in which there is a disturbance of early language acquisition. The disorder is characterized by problems in using language socially, which are similar to the language problems in autism. However, there are differences in the symptoms seen in these two groups of children. Children with autism have language disorders, but they also show difficulties with decision-making, planning, and other, general, cognitive skills. One of the goals of this imaging study was to compare the MRI findings to see if there are structural brain differences between the children with ASD and the children with DLD that could underlie the differences in their behaviour. The MRI results were also compared to those of typically developing children.

Participants: 41 boys and 22 girls participated in this study, divided into three groups. The first group was composed of 13 boys with ASD; the second group was composed of 14 boys and 7 girls with DLD. The third, control, group was composed of 14 boys and 15 girls, all of whom were typically developing. All of the children were aged between 5.7 and 11.3 years, and the children with ASD or DLD all had a nonverbal IQ greater than 80 (low average or above). The children with ASD and DLD were diagnosed by a child psychiatrist using the DSM-III, which was in current use at the time of the study.

Method: All of the children had standardized MRI brain scans done to measure both white matter and gray matter in the brain.

Findings: The children with ASD and DLD, but not the control group children, had enlarged areas of white matter throughout the brain. However, in the children with autism, there was a much more pronounced accumulation of white matter in the prefrontal cerebral cortex, an area associated with problem solving, emotion, and complex thought. (For information, search ?prefrontal cortex? at http://www.driesen.com/glossary_p.htm#PREFRONTAL%20CORTEX)

Conclusions: Compared to typically developing controls, children with ASD and children with DLD had notable increases in the amount of white matter found in their brains. The increased volume was scattered throughout several areas. Compared to children with DLD, children with ASD had a much more marked increase of white matter volume in the prefrontal cortex. No such accumulation of the tissue was seen in typically developing children.

White matter is involved in making connections between different areas of the brain involved in complex thought and behaviour. The brain tissue abnormality described in this study may underlie the range of behavioural and cognitive functioning problems seen in ASD and DLD. The accumulation of white matter may interfere with connections between various gray matter structures involved in processing facial recognition, understanding of emotion, and learning social rules and language use. The researchers speculated that the increase in white matter volume could be due to some abnormality of the myelin ? an ?insulating? covering on the nerve cells.

The investigators in this study suggest that the similarities in white matter abnormalities between the ASD and DLD groups might mean that these two disorders are not entirely distinct. The results suggest that DLD might be considered within the spectrum of autistic disorders.

This study confirms other reports that the increase in white matter volume occurs after birth, and adds to current knowledge with its finding that the tissue accumulates over a period of time. The findings of this study need to be replicated and further explained.

1. Aylward EH, Minshew NJ, Field K, et al. Effects of age on brain volume and head circumference in autism. Neurology 2002; 59: 175-183.

2. Filipek P, Richelme C, Kennedy D, et al. Morphometric analysis of the brain in developmental language disorders and autism. Annals of Neurology 1992; 32: 475.

3. Courchesne E, Carper R, Akahoomoff N. Evidence of brain overgrowth in the first year of life in autism. JAMA 2003; 290:337-344.

4. Courchesne E, Karns CM, Davis HR, et al. Unusual brain growth patterns in early life in patients with autistic disorder: an MRI study. Neurology 2001; 57 :245-254.