Elsevier

The Lancet

Volume 379, Issue 9830, 26 Mayā€“1 June 2012, Pages 1997-2007
The Lancet

Seminar
Developmental dyslexia

https://doi.org/10.1016/S0140-6736(12)60198-6Get rights and content

Summary

Dyslexia is a neurodevelopmental disorder that is characterised by slow and inaccurate word recognition. Dyslexia has been reported in every culture studied, and mounting evidence draws attention to cross-linguistic similarity in its neurobiological and neurocognitive bases. Much progress has been made across research specialties spanning the behavioural, neuropsychological, neurobiological, and causal levels of analysis in the past 5 years. From a neuropsychological perspective, the phonological theory remains the most compelling, although phonological problems also interact with other cognitive risk factors. Work confirms that, neurobiologically, dyslexia is characterised by dysfunction of the normal left hemisphere language network and also implicates abnormal white matter development. Studies accounting for reading experience demonstrate that many recorded neural differences show causes rather than effects of dyslexia. Six predisposing candidate genes have been identified, and evidence shows gene by environment interaction.

Section snippets

Definition

Individuals with developmental dyslexia have difficulties with accurate or fluent word recognition and spelling despite adequate instruction and intelligence and intact sensory abilities.1 The ultimate goal of reading is comprehension. Dyslexia is defined by difficulties with decoding, whereas by comparison comprehension is more intact. So-called poor comprehenders show the opposite profile of adequate decoding but poor understanding of what is read.2 Although some previous nosologies have

Prevalence

Prevalence estimates depend on the definition of dyslexia. A common definition sets the cutoff for reading achievement 1Ā·5 standard deviations below the mean for age and identifies 7% of the population as dyslexic; a similar IQā€“achievement discrepancy definition identifies a similar proportion.9 A small but significant male predominance exists (1Ā·5ā€“3:1);10 however, the sex difference in referred samples is higher (3ā€“6:1).11 Boys with dyslexia come to clinical attention more often than girls,

Neuropsychology

Early theories of dyslexia postulated a basic deficit in visual processing and focused on the reversal errors commonly made by individuals with the disorderā€”such as writing ā€˜bā€™ for ā€˜dā€™ or ā€˜wasā€™ for ā€˜sawā€™.31 Vellutino32 reported that such reversal errors in dyslexia were restricted to print in a person's own language, and thus were linguistic rather than visual in nature. Since then, much research has made clear that dyslexia is a language-based disorder whose primary underlying deficit involves

Functional findings

Because reading is a linguistic skill, we would expect it to involve activation of brain structures used in oral-language processing and some additional structures associated with visual-object processing and establishment of visualā€“linguistic mappings. A large number of functional imaging studies have shown aberrant activation patterns in these regions in dyslexia. The most common findings, as described in several qualitative reviews, encompass abnormalities of a distributed left hemisphere

Causes

Like all behaviourally defined disorders, the cause of dyslexia is multifactorial and is associated with multiple genes and environmental risk factors (panel 5). Dyslexia is familial and moderately heritable109 and has been linked to nine risk loci (DYX1ā€“DYX9) through replicated linkage studies,110, 111 although not every study has replicated these results.112, 113

The main advance in the genetics of dyslexia since the previous Lancet Seminar75 has been the identification of six candidate genes (

Treatment

The development of evidence-based treatments for dyslexia has benefited from our understanding of the neuropsychology of the disorder, and the best interventions provide intensive, explicit instruction in phonological awareness, the alphabetic principle and phonics, word analysis, reading fluency, and reading comprehension.118, 119 Much more is known about effective remediation of reading problems in younger than in older children. Accuracy difficulties seem to be easier to treat than issues

Conclusion

Of all the neurodevelopmental disorders, dyslexia has been the most studied and is the best understood. The specialty continues to benefit from reciprocal relations between basic and clinical neuroscience, and there has been considerable progress over the past 5 years in understanding dyslexia's cross-cultural manifestation, causes, neuropsychology, and neurobiology. Much of the most exciting work includes an interdisciplinary focus across these different specialties. This research has helped

Search strategy and selection criteria

We searched PubMed under the medical subject heading ā€œdyslexiaā€ and also searched PsycInfo with the terms ā€œdyslexiaā€ and ā€œreading disabilityā€ in conjunction with the terms ā€œdevelopmentalā€, ā€œgeneticsā€, ā€œtreatmentā€, ā€œbrain imagingā€, and ā€œcross-linguisticā€. We largely selected publications published in the past 5 years, but did not exclude commonly referenced and highly regarded older publications. We also searched the reference lists of articles identified by this search strategy and selected any

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