Although typically diagnosed during the second or third grade of school - around age 7 or 8 - a team from Children's Hospital Boston said they could see signs of the disease on brain scans in children as early as 4 and 5, a time when studies show children are best able to respond to interventions.
"We call it the dyslexia paradox," said Nadine Gaab of the Laboratories of Cognitive Neuroscience at Children's, whose study was published in Proceedings of the National Academy of Sciences.
Gaab said most children are not diagnosed until third grade, but interventions work best in younger children, hopefully before they begin to learn to read.
"Often, by the time they get a diagnosis, they usually have experienced three years of peers telling them they are stupid, parents telling them they are lazy.
We know they have reduced self esteem. They are really struggling," Gaab said in a telephone interview.
Her study builds on an emerging understanding of dyslexia as a problem with recognizing and manipulating the individual sounds that form language, which is known as phonological processing.
In order to read, children must map the sounds of spoken language onto specific letters that make up words. Children with dyslexia struggle with this mapping process.
"The beauty is spoken language can present before written language so people can look for symptoms," said Dr. Sally Shaywitz, a director of the Center for Dyslexia and Creativity at Yale University.
Signs of early dyslexia might include difficulty with rhyming, mispronouncing words or confusing similar-sounding words.
"Those are all very early symptoms," Shaywitz said.
Dyslexia affects roughly 5 percent to 17 percent of all children and up to 1 in 2 children with a family history of the disorder will struggle with reading, have poor spelling and experience difficulty decoding words.
In her study, Gaab and colleagues scanned the brains of 36 preschool children while they did a number of tasks, such as trying to decide if two words start with the same sound.
They found that during these tasks, children who had a family history of dyslexia had less brain activity in certain regions of the brain than did children of similar ages, intelligence and socioeconomic status.
Older children and adults with dyslexia have dysfunction in these same areas of the brain, which include the junctions between the occipital and temporal lobes and the temporal and parietal lobes in the back of the brain.
Gaab said the study shows that when children predisposed to dyslexia did these tasks, their brains did not use the area typically used for processing this information. This problem occurred even before the children started learning to read.
"The important point of this paper is it shows the need to look for signs of dyslexia earlier," said April Benasich, director of the Carter Center for Neurocognitive Research at Rutgers, the State University of New Jersey, who was not part of the study.
Benasich studies language processing in even younger children - babies who have a family history of learning disorders.
"There is evidence to suggest that what is thought to be reading failure is there before the kids fail," she said.
Gaab said her study is too small to form the basis of any test for dyslexia but her team has just won a grant from the National Institutes of Health to do a larger study.
Ultimately, she hopes parents will be able to go to their pediatrician and ask for their child to be assessed.
"Families often know that their child has dyslexia as early as kindergarten, but they can't get interventions at their schools," she said in a statement.
"If we can show that we can identify these kids early, schools may be encouraged to develop programs," she said.