Etiology

There are numerous theories regarding the etiology of dyslexia, including those implicating deficits in the temporal processing of auditory and visual stimuli and those that hypothesize language-specific impairments. The latter category posits that at a cognitive-linguistic level, dyslexia reflects deficits within a specific component of the language system, the phonologic module, which is engaged in processing the sounds of speech. As predicted by this model, dyslexic persons have difficulty developing an awareness that words, both spoken and written, can be segmented into smaller elemental units of sound (phonemes)—an essential ability given that reading an alphabetic language (English) requires that the reader map or link printed symbols to sound. The linguistic abilities related to learning to read involve phonology, and deficits in phonologic awareness are a strong predictor of dyslexia. There is some evidence that other cognitive processes are involved in reading, including attentional mechanisms, the disruption of which can play a causal role in reading difficulties.

Dyslexia is both familial and heritable. Family history is one of the most important risk factors; approximately 50% of children who have a parent with dyslexia, 50% of the siblings of dyslexic persons, and 50% of the parents of dyslexics may have the disorder. Dyslexia reflects a multifactorial model of the interaction between genetic and environmental factors. Multiple genes can influence the disorder, with each gene individually contributing a small amount of variance and with a single etiologic factor insufficient to cause or explain dyslexia. The neural systems are the final common pathway for multiple influences, and it is unlikely that a single gene or even several genes cause or explain dyslexia.

Epidemiology

Dyslexia may be the most common neurobehavioral disorder affecting children, with prevalence rates ranging from 5-10% in clinic- and school-identified samples to 17.5% in unselected population-based samples in the U.S. and other countries. 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. Dyslexia affects both boys and girls; epidemiologic samples indicate slightly more boys. Dyslexia is a persistent, chronic condition rather than a transient developmental lag. Although dyslexic and poor readers maintain their relative positions along the distribution of reading ability, approaches using focused, early, and intensive intervention provide indications that these trends possibly can be modified. There are no reports of closure of the gap in reading fluency between typical and dyslexic readers, and this goal appears elusive at this time. Furthermore, longitudinal data indicate that in typical readers, intelligence and reading track together and are dynamically linked. In contrast, in dyslexic readers, intelligence and reading are quite separate and are not dynamically linked, providing empirical evidence for the “unexpected” nature of the reading difficulty in dyslexia.

Pathogenesis

A range of neurobiologic investigations using primarily functional brain imaging suggest that there are differences in the left temporo-parieto-occipital brain regions between dyslexic and nonimpaired readers. Functional brain imaging in both children with dyslexia and adult dyslexic readers demonstrates a failure of the left hemisphere posterior brain systems to function properly during reading, with increased activation in the frontal regions, a pattern referred to as the neural signature of dyslexia. Thus, functional brain imaging has for the first time made visible what has always been a hidden disability. These data suggest that rather than the smoothly functioning and integrated reading systems observed in nonimpaired children (Fig. 31-1), inefficient functioning of the posterior reading systems results in dyslexic children’s attempting to compensate by shifting to other, ancillary systems, for example, anterior sites, such as the inferior frontal gyrus. In dyslexic readers, inefficient functioning of the posterior reading systems underlies the failure of skilled reading to develop, whereas a shift to ancillary systems supports accurate, but not automatic word reading.

Figure 31-1 Left lateral image of the brain indicating the 3 major reading systems, including 1 anterior (inferior frontal gyrus) and 2 posterior (parietotemporal and occipitotemporal systems), also called the word-form area.

(From Shaywitz SE: Overcoming dyslexia: a new and complete science-based program for reading problems at any level, New York, 2003, Alfred A. Knopf.)

Clinical Manifestations