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

Reflecting the underlying phonologic weakness, dyslexics manifest problems in both spoken and written language. Spoken language difficulties are typically manifest by mispronunciations, lack of glibness, speech that lacks fluency with many pauses or hesitations and “ums” heard, word-finding difficulties with the need for time to summon an oral response and the inability to come up with a verbal response quickly when questioned; these reflect sound-based, and not semantic or knowledge-based difficulties.

Struggles in decoding and word recognition can vary according to age and developmental level. The cardinal signs of dyslexia observed in school-aged children and adults are a labored, effortful approach to decoding, word recognition, and text reading. Listening comprehension is typically robust. Older children have been found to improve reading accuracy over time, albeit without commensurate gains in reading fluency; they remain slow readers. Difficulties in spelling typically reflect the phonologically based difficulties observed in oral reading. Handwriting is often affected as well.

A parental history often identifies early subtle language difficulties in dyslexic children. During the preschool and kindergarten years, at-risk children display difficulties playing rhyming games and learning the names for letters and numbers. Kindergarten assessments of these language skills can help identify children at risk for dyslexia. Although a dyslexic child enjoys and benefits from being read to, he or she might avoid reading aloud to the parent or reading independently.

Anxiety is often present and increases over time. Dyslexia may co-occur with attention-deficit/hyperactivity disorder (Chapter 30); this comorbidity has been documented in both referred samples (40% comorbidity) and nonreferred samples (15% comorbidity).

Diagnosis

Dyslexia is a clinical diagnosis, and history is especially critical. The clinician seeks to determine through history, observation, and psychometric assessment, if there are unexpected difficulties in reading (based on the person’s cognitive capacity as shown by age, intelligence, or level of education or professional status) and associated linguistic problems at the level of phonologic processing. There is no single test score that is pathognomonic of dyslexia. The diagnosis of dyslexia should reflect a thoughtful synthesis of all clinical data available.

Dyslexia is distinguished from other disorders that can prominently feature reading difficulties by the unique, circumscribed nature of the phonologic deficit, one that does not intrude into other linguistic or cognitive domains. Family history, teacher and classroom observation, and tests of language (particularly phonology), reading including fluency, and spelling represent a core assessment for the diagnosis of dyslexia in children; additional tests of intellectual ability, attention, memory, general language skills, and mathematics may be administered as part of a more comprehensive evaluation of cognitive, linguistic, and academic function. Once a diagnosis has been made, dyslexia is a permanent diagnosis and need not be reconfirmed by new assessments.

For informal screening, in addition to a careful history, the primary care physician in an office setting can listen to the child read aloud from his or her own grade-level reader. Keeping a set of graded readers available in the office serves the same purpose and eliminates the need for the child to bring in schoolbooks. Oral reading is a sensitive measure of reading accuracy and fluency. The most consistent and telling sign of a reading disability in an accomplished young adult is slow and laborious reading and writing.

It must be emphasized that the failure either to recognize or to measure the lack of fluency in reading is perhaps the most common error in the diagnosis of dyslexia in older children and accomplished young adults. Simple word identification tasks will not detect dyslexia in a person who is accomplished enough to be in honors high school classes or to graduate from college or obtain a graduate degree. Tests relying on the accuracy of word identification alone are inappropriate to use to diagnose dyslexia because they show little to nothing of the struggle to read. It is important to recognize that because they assess reading accuracy but not automaticity (speed), the kinds of reading tests commonly used for school-aged children might provide misleading data on bright adolescents and young adults. The most critical tests are those that are timed; they are the most sensitive in detecting dyslexia in a bright adult. There are few standardized tests for young adult readers that are administered under timed and untimed conditions; the Nelson-Denny Reading Test is an exception. The helpful Test of Word Reading Efficiency (TOWRE) examines simple word reading under timed conditions. Any scores obtained on testing must be considered relative to peers with the same degree of education or professional training.

Management

The management of dyslexia demands a life-span perspective. Early on, the focus is on remediation of the reading problem. Application of knowledge of the importance of early language and phonologic skills leads to significant improvements in children’s reading accuracy, even in predisposed children. As a child matures and enters the more time-demanding setting of secondary school, the emphasis shifts to the important role of providing accommodations. Based on the work of the National Reading Panel, evidence-based reading intervention methods and programs are identified. Effective intervention programs provide systematic instruction in 5 key areas: phonemic awareness, phonics, fluency, vocabulary, and comprehension strategies. These programs also provide ample opportunities for writing, reading, and discussing literature.

Taking each component of the reading process in turn, effective interventions improve phonemic awareness: the ability to focus on and manipulate phonemes (speech sounds) in spoken syllables and words. The elements found to be most effective in enhancing phonemic awareness, reading, and spelling skills include teaching children to manipulate phonemes with letters; focusing the instruction on 1 or 2 types of phoneme manipulations rather than multiple types; and teaching children in small groups. Providing instruction in phonemic awareness is necessary but not sufficient to teach children to read. Effective intervention programs include teaching phonics, or making sure that the beginning reader understands how letters are linked to sounds (phonemes) to form letter-sound correspondences and spelling patterns. The instruction should be explicit and systematic; phonics instruction enhances children’s success in learning to read, and systematic phonics instruction is more effective than instruction that teaches little or no phonics or teaches phonics casually or haphazardly.

Fluency is of critical importance because it allows the automatic, rapid recognition of words. Although it is generally recognized that fluency is an important component of skilled reading, it is often neglected in the classroom. The most effective method to build reading fluency is a procedure referred to as guided repeated oral reading: the teacher models reading a passage aloud, and the student rereads the passage repeatedly to the teacher, another adult, or a peer, receiving feedback until he or she is able to read the passage correctly. Evidence indicates that guided repeated oral reading has some positive effect on word recognition, fluency, and comprehension at a variety of grade levels. The evidence is less clear for programs for struggling readers that encourage large amounts of independent reading, that is, silent reading without any feedback to the student. Thus, even though independent silent reading is intuitively appealing, at this time, the evidence is insufficient to support the notion that in struggling readers, reading fluency improves. In contrast to teaching phonemic awareness, phonics, and fluency, interventions for vocabulary development and reading comprehension are not as well established. The most effective methods to teach reading comprehension involve teaching vocabulary and strategies that encourage active interaction between the reader and the text.

For those in high school, college, and graduate school, provision of accommodation rather than remediation most often represents the most effective approach to dyslexia. Imaging studies now provide neurobiologic evidence for the need for extra time for dyslexic students; accordingly, college students with a childhood history of dyslexia require extra time in reading and writing assignments as well as examinations. Many adolescent and adult students have been able to improve their reading accuracy but without commensurate gains in reading speed. Other helpful accommodations include the use of laptop computers with spelling checkers, use of recorded books, access to lecture notes, tutorial services, alternatives to multiple-choice tests, and a separate quiet room for taking tests. In addition, the impact of the primary phonologic weakness mandates special consideration during oral examinations so that students are not graded on their lack of glibness or speech hesitancies but on their content knowledge. Unfortunately, often speech hesitancies or difficulties in word retrieval are wrongly confused with insecure content knowledge. Thus, such “performance” tests are inappropriate for children and adults who are dyslexic.

Prognosis

Application of evidence-based methods to young children (kindergarten to grade 3), provided with sufficient intensity and duration, can result in improvements in reading accuracy and, to a much lesser extent, fluency. In older children and adults, interventions result in improved accuracy but not fluency. Accommodations are critical in allowing the dyslexic child to demonstrate his or her knowledge. Parents should be informed that with proper support, dyslexic children can succeed in a range of future occupations that might seem out of the reach of dyslexic children including medicine, law, journalism, and writing.