Slow reading in dyslexia tied to disorganized brain tracts
Circuitous connections may keep some patients from reading fluently
December 3, 2007
Dyslexia marked by poor reading fluency -- slow and choppy reading -- may be caused by disorganized, meandering tracts of nerve fibers in the brain, according to researchers at Children's Hospital Boston, Beth Israel Deaconess Medical Center (BIDMC), and the Harvard Graduate School of Education (HGSE). The study, using the latest imaging methods, gives researchers a glimpse of what may go wrong in the structure of some dyslexic readers' brains, making it difficult to integrate the information needed for rapid, "automatic" reading.
Christopher Walsh, MD, PhDThe study was led by Christopher Walsh, MD, PhD, chief of the Division of Genetics at Children's Hospital Boston, Bernard Chang, MD, a neurologist at BIDMC, and Tami Katzir, PhD, an assistant professor at HGSE (now at the University of Haifa in Israel). Findings will appear in the journal Neurology on December 4.
"We looked at dyslexia caused by a particular genetic disorder, but what we found could have implications for understanding the causes of dyslexia in other populations as well," says Walsh, who is also a Howard Hughes Medical Institute investigator at BIDMC.
Dyslexia, which affects 5 to 15 percent of all children, has different forms. Subjects in the study had reading problems caused by a rare genetic disorder known as periventricular nodular heterotopia, or PNH. Although their intelligence is normal, people with PNH have trouble reading fluently, or smoothly, lacking the rapid processing necessary for this aspect of reading.
The genetic mutation that causes PNH disrupts brain structure. In a normal brain, much of the gray matter (consisting mostly of nerve cells) appears on the brain's surface, while white matter (consisting mostly of nerve fibers, or "wiring" interconnecting areas of gray matter) runs deeper in the brain. In PNH, portions of gray matter sit deep in the brain's core, in the white matter, having failed to migrate out to the surface as the brain was developing.
|In a normal brain (left), white matter (light gray) is in the interior, and gray matter (dark gray) is mostly on the surface. In patients with periventricular nodular heterotopia (right), clumps of gray matter, called nodules (red arrows), appear deep within the brain, instead of on the surface. Image courtesy of Bernard Chang, MD, Beth Israel Deaconess Medical Center|
To learn more about how these developmental changes in the brain might lead to reading problems, the researchers tested cognitive skills needed for reading in 10 patients with PNH, 10 individuals with dyslexia without neurological problems, and 10 normal readers. They used a specialized form of MRI called diffusion tensor imaging to look at white matter in the brain. In normal readers, white matter fibers were organized so they directly connected brain regions and traveled together in bundles. Walsh compares this to electrical wires, bundled into cables, running between rooms in a house. "If you were wiring your house, you would organize the wires this way in order to be efficient," says Walsh. "It seems that nature has decided that's also the most efficient way to wire the brain."
In PNH patients, white matter fibers were organized inefficiently. They took circuitous routes around the misplaced gray matter, and in some cases, didn't form proper bundles, which could slow information transmission, or leave brain regions poorly connected. Importantly, the more disorganized the PNH patients' white matter, the less fluent their reading.
|This image, created by a specialized form of MRI called diffusion tensor imaging, shows white matter tracts (colored lines), in one corner of the brain. White matter tracts connect brain regions so they can communicate. Tracts appear in this image only if they are organized. In a normal brain (left), tracts run in an organized, uninterrupted fashion between points in the brain (tracts in white box). In patients with periventricular nodular heterotopia (right), tracts are disrupted by nodules of gray matter (red arrow), leaving areas without organized fiber tracts (lack of tracts in white box), which might lead to poor connections between parts of the brain. Image courtesy of Bernard Chang, MD, Beth Israel Deaconess Medical Center|
While other studies have found disorganized white matter in the general population of people with dyslexia, these individuals often struggle with several aspects of reading, making it "hard to know exactly what the role of white-matter integrity is in isolation," says Chang. By demonstrating white-matter problems in PNH patients, who have an isolated reading fluency problem, and correlating that with reading fluency scores, the researchers were able to conclude that white-matter integrity and organization may be the structural basis in the brain for reading fluency.
"This makes sense," says Chang. "When we read, we need to take in information visually, hook it up with our inner dictionary of what letters and words mean, and when we're reading aloud, connect that with the region that gives us our ability to speak." For smooth, automatic reading, "the white matter is there to connect different regions of gray matter and allow them to function seamlessly." When reading fluency is the primary problem, "it may be that the areas of the brain that are important for reading are not connected efficiently," says Chang.
Most people with dyslexia who have trouble reading fluently don't have misplaced gray matter or PNH. But Walsh and Chang believe they do have disorganized white matter, which could similarly alter brain patterns in both groups. Walsh and Chang's next study will examine how disorganized white matter alters brain patterns during reading in PNH patients and in dyslexic readers with poor fluency, who do not have PNH.
Pinpointing the brain structures responsible for fluent reading may eventually help researchers and educational specialists develop and use remediation techniques that improve the automatic nature of reading in children and adults with these kinds of difficulties, the researchers note.
"You can probably get to dyslexia by having many different changes in the brain," says Walsh. "That probably means there are treatments that work better for some children than for others. The more we understand about the differences between children, the better we can optimize their treatment."
"Tying genetics to dyslexia allows us to identify a potential problem at birth, so that allows us the earliest possible intervention," Walsh adds.
Children's Hospital Boston
Beth Israel Deaconess Medical Center
Children's Hospital Boston is home to the world's largest research enterprise based at a pediatric medical center, where its discoveries have benefited both children and adults since 1869. More than 500 scientists, including eight members of the National Academy of Sciences, 11 members of the Institute of Medicine and 12 members of the Howard Hughes Medical Institute comprise Children's research community. Founded as a 20-bed hospital for children, Children's Hospital Boston today is a 377-bed comprehensive center for pediatric and adolescent health care grounded in the values of excellence in patient care and sensitivity to the complex needs and diversity of children and families. Children's also is the primary pediatric teaching affiliate of Harvard Medical School. For more information about the hospital and its research visit: www.childrenshospital.org/newsroom.
Beth Israel Deaconess Medical Center is a patient care, teaching and research affiliate of Harvard Medical School, and consistently ranks in the top four in National Institutes of Health funding among independent hospitals nationwide. BIDMC is clinically affiliated with the Joslin Diabetes Center and is a research partner of the Dana-Farber/Harvard Cancer Center. BIDMC is the official hospital of the Boston Red Sox. For more information, visit: www.bidmc.harvard.edu.
Christopher Walsh, MD, PhD