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As researchers look more
carefully at the 5 to 15 percent of children in the United States who have dyslexia, they're
finding there's no single disease
"dyslexia," but rather many dyslexias resulting from unique combinations of genes, mechanisms and changes in the brain. They're also finding that each has an optimal treatment.
Through brain imaging and genetic studies, Children's Hospital Boston researchers are pinning down the
mechanisms involved in different types of dyslexia. Nadine Gaab, PhD, of the Developmental Medicine Center
Laboratory of Cognitive Neuroscience, with Elise Temple of Dartmouth College, found that children with developmental dyslexia, whose reading problems stem from a faulty understanding of the sounds that make up their native language, may lack the proper brain wiring to process fast-changing sounds. While this idea was first proposed in the 1970s, it hadn't been proven using brain imaging in children. According to the theory, infants whose brains can't analyze fast changes within sounds don't properly learn the sounds of language. They miss, for instance, the 40-millisecond pitch sweeps that differentiate "ba" and "da." This leads to a confused sound map in the brain. Later, when these children first try reading, they link the
letters to this confused sound map.
Dr. Gaab documented this problem using functional MRI (fMRI) in 9- to 12-year-olds whose brains responded in a pattern unlike that of normal readers when they listened to sounds that changed quickly in pitch. But the experiment also suggested these children's brains could be "rewired." After eight weeks of
computer exercises, they responded to fast-changing sounds more like typical readers, and their reading improved. It's unclear how long the benefit lasts, since follow-up wasn't done past a few weeks. The study, appearing online in the October 16 Restorative Neurology and Neuro-
science, suggests that other forms of sound training, like musical training, might help children whose primary problem is sound processing and a faulty language map in the brain.
Dr. Gaab's goal is to catch and treat dyslexia before children begin learning to read, sparing them years of frustration and low self-esteem. She's gearing up for a longitudinal study in preschoolers, supported by an fMRI clinic at Children's Waltham campus that can image children as young as 4. The study will look at how soon differences appear between children who develop dyslexia and those who don't, and what marks the difference.
In preschoolers with and without
a family history of dyslexia, her study will evaluate four potential early markers of the disorder: genetic profile, brain
structure, brain function and behavior. Structural studies will examine
gray matter density in reading centers of the brain; functional studies will examine how well brain regions work together
during rhyming, a building block for
reading; and behavioral tests will
evaluate how well the children can
distinguish between fast-changing sounds. Later, when the children begin to read, a battery of tests will indicate whether trouble reading correlates with any of the markers. "My hypothesis is that it's a combination of these four factors," says Dr. Gaab.
Another study, led by Christopher Walsh, MD, PhD, chief of Genetics, looked for clues about dyslexia in adults with the rare genetic disorder periventricular nodular heterotopia, or PNH. People with PNH have misplaced nodules of
gray matter (nerve cells) along the brain's
ventricles, epilepsy and an isolated
problem with reading fluency. Similar
fluency problems have been found in
dyslexic readers without PNH.
Dr. Walsh's team investigated white matter (the wiring connecting nerve cells) in normal readers and PNH patients using diffusion tensor MRI and found striking differences. Normal readers had organized white matter tracts that directly connected brain regions and traveled together in bundles, while those with PNH had
disorganized tracts that took circuitous routes around misplaced gray matter and didn't form proper bundles, which could slow information transmission or leave brain regions poorly connected.
Since smooth reading requires the brain to rapidly link visual information to an inner dictionary of letter sequences and sounds, it may be that reading fluency is the primary problem. "The areas of the brain that are important for reading are not connected efficiently," says study
co-author Bernard Chang, MD, of Beth Israel Deaconess Medical Center.
Studies of white matter in the general population of people with dyslexia have yielded important but sometimes
conflicting results. By correlating reading fluency scores with white matter
disorganization in PNH patients, the researchers found additional evidence
to support the idea that white matter
integrity is likely the basis for reading fluency in the brain. Doctors Walsh and Chang will next use fMRI to examine whether poor white-matter connections actually disrupt the patterns of activation in brain regions that are normally seen during reading.
"As we recognize that dyslexia is many different conditions, it allows us
to develop scientific approaches to
dissect that complexity," says Dr. Walsh. "We need to attack the pieces one at a time, rather than trying to solve the whole puzzle at once."
A video of Dr. Walsh discussing
his research: childrenshospital.org/newsroom.
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