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Dyslexia
The most current definition of dyslexia is now codified in U.S. Federal law (First Step Act of 2018, PL: 115–391): “The term dyslexia means an unexpected difficulty in reading for an individual who has the intelligence to be a much better reader, most commonly caused by a difficulty in the phonological processing (the appreciation of the individual sounds of spoken language), which affects the ability of an individual to speak, read, and spell.” In typical readers, development of reading and intelligence quotient (IQ) are dynamically linked over time. In dyslexic readers, however, a developmental uncoupling occurs between reading and IQ ( Fig. 50.1 ), such that reading achievement is significantly below what would be expected given the individual's IQ. The discrepancy between reading achievement and IQ provides the long-sought empirical evidence for the seeming paradox between cognition and reading in individuals with developmental dyslexia, and this discrepancy is now recognized in the Federal definition as unexpected difficulty in reading.
Uncoupling of reading and IQ over time: empirical evidence for a definition of dyslexia.
Left , In typical readers, reading and IQ development are dynamically linked over time. Right, In contrast, reading and IQ development are dissociated in dyslexic readers, and one does not influence the other.
(Data adapted from Ferrer E, Shaywitz BA, Holahan JM, et al: Uncoupling of reading and IQ over time: empirical evidence for a definition of dyslexia, Psychol Sci 21(1):93–101, 2010.)
Etiology
Dyslexia is familial, occurring in 50% of children who have a parent with dyslexia, in 50% of the siblings of dyslexic persons, and in 50% of the parents of dyslexic persons. Such observations have naturally led to a search for genes responsible for dyslexia, and at one point there was hope that heritability would be related to a small number of genes. Genome-wide association studies (GWAS), however, have demonstrated that a large number of genes are involved, each producing a small effect. Advances in genetics have confirmed what the GWAS suggested, that complex traits such as reading are the work of thousands of genetic variants, working in concert (see Chapter 99 ). Thus, pediatricians should be wary of recommending any genetic test to their patients that purports to diagnose dyslexia in infancy or before language and reading have even emerged. It is unlikely that a single gene or even a few genes will reliably identify people with dyslexia. Rather, dyslexia is best explained by multiple genes , each contributing a small amount toward the expression of dyslexia.
Epidemiology
Dyslexia is the most common and most comprehensively studied of the learning disabilities, affecting 80% of children identified as having a learning disability. Dyslexia may be the most common neurobehavioral disorder affecting children, with prevalence rates ranging from 20% in unselected population-based samples to much lower rates in school-identified samples. The low prevalence rate in school-identified samples may reflect the reluctance of schools to identify dyslexia. Dyslexia occurs with equal frequency in boys and girls in survey samples in which all children are assessed. Despite such well-documented findings, schools continue to identify more boys than girls, probably reflecting the more rambunctious behavior of boys who come to the teacher's attention because of misbehavior, while girls with reading difficulty, who are less likely to be misbehaving, are also less likely to be identified by the schools. Dyslexia fits a dimensional model in which reading ability and disability occur along a continuum, with dyslexia representing the lower tail of a normal distribution of reading ability.
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