An estimated 8 to 10 percent of children in the U.S. have some form of learning disability involving difficulties in the acquisition and use of listening, speaking, reading, writing, reasoning, or mathematical abilities. These problems often occur in people with normal or even high intelligence.
Dyslexia, or specific reading disability, is the most common and most carefully studied of the learning disabilities. It affects 80 percent of all those identified as learning disabled, or as many as 15 to 20 percent of Americans. Dyslexia is characterized by an unexpected difficulty in speaking and reading in children and adults who otherwise possess the intelligence, motivation, and schooling considered necessary for accurate and fluent reading. Studies indicate that although there can be improvement, dyslexia is a persistent, chronic condition.
There is now a strong consensus that the central difficulty in most forms of dyslexia reflects a deficit within the language system — more specifically, in a component of the language system called phonology. This deficit results in difficulty in both oral language and reading. There may be mispronunciations of words, lack of fluency in speech, hesitations before responding, and word retrieval difficulties. As a result, a person with dyslexia often needs time to summon a verbal response when questioned.
As children approach adolescence, one manifestation of dyslexia may be a very slow reading rate. Children may learn to read words accurately, but their reading will not be fluent or automatic, reflecting the lingering effects of a phonologic deficit. Because they can read words accurately — albeit very slowly — dyslexic adolescents and young adults may mistakenly be assumed to have “outgrown” their dyslexia. The ability to read aloud accurately, rapidly, and with good expression, as well as facility with spelling, may be most useful clinically in distinguishing students who are average from those who are poor readers. In some languages that are more consistent in the relationship between letters and sounds, such as Finnish and Italian, slow reading may be the only manifestation of dyslexia at any age.
A range of investigations indicates that there are differences in brain regions between dyslexic and nonimpaired readers involving three important left hemisphere neural systems, two posteriorly (parieto-temporal, occipito-temporal) and one anteriorly around the left inferior frontal region (Broca’s area). Converging evidence derived from functional brain imaging indicates that dyslexic readers demonstrate a functional inefficiency in an extensive neural system in the posterior portion of the brain. The brain images that result from these studies are referred to as the neural signature of dyslexia.
It is clear that dyslexia runs in families, but initial hopes that dyslexia would be explained by one or just a few genes have not been realized. Genome-wide association studies (GWAS) in dyslexia have so far identified genetic variants that account for only a very small percentage of the risk — less than 1 percent — making it unlikely that a single gene or even a few genes will identify people with dyslexia. Current evidence suggests that dyslexia is best conceptualized within a multifactorial model, with multiple genetic and environmental risk and protective factors leading to dyslexia.
Interventions to help children with dyslexia focus on teaching the child that words can be segmented into smaller units of sound and that these sounds are linked with specific letter patterns. In addition, children with dyslexia require practice in reading stories, both to allow them to apply their newly acquired decoding skills to reading words in context and to experience reading for meaning and enjoyment.