Dyslexia: a hundred years onBMJ 1996; 313 doi: http://dx.doi.org/10.1136/bmj.313.7065.1096 (Published 02 November 1996) Cite this as: BMJ 1996;313:1096
A verbal not a visual disorder, which responds to early intervention
The first case of developmental dyslexia was reported by Pringle-Morgan in the BMJ on 7 November 1896.1 Pringle-Morgan, a general practitioner, and Hinshelwood, an ophthalmologist also writing at the turn of the century,2 speculated that such difficulties with reading and writing were due to “congenital word blindness,” and for many years the dominant view was that dyslexia was caused by visual processing deficiencies. There is still continuing interest in the role of visual factors in the aetiology of dyslexia, especially in low level impairments of the visual system.3 4 However, most research suggests that these are not its cause. The most widely accepted view today is that dyslexia is a verbal deficit5 and can be considered part of the continuum of language disorders. Indeed, converging evidence supports a specific theory, that dyslexic readers have phonological (speech) processing deficits.
Dyslexia tends to runs in families, and the relatives of dyslexic readers often have other language problems. Dyslexia is more common in boys than girls, and there is strong evidence that it is heritable; the probability of a boy becoming dyslexic if his father is dyslexic can be as high as 50%; it is somewhat lower for a girl. Results of large scale twin studies suggest that sensitivity to speech sounds (as measured by tests of rhyming skill) shares genetic variance with phonological reading skills.6 Gene markers on chromosomes 1 and 15 have been identified in dyslexic families, and linkage on chromosome 6 in the region of the human leucocyte complex may explain an often reported association between dyslexia and autoimmune disease.7 There have also been advances in the neurobiology of dyslexia. Evidence on brain abnormalities has implicated the perisylvanian regions, and postmortem studies have revealed unusual symmetry of the plenum temperale with cortical dysplasia and scarring.8
Dyslexia is a developmental disorder that affects people of all ages, but its symptom profile changes with age. Studies of children at genetic risk of dyslexia have reported difficulties in speech production and grammatical expression at 30 months, followed by slower vocabulary acquisition during the preschool years, culminating in deficits in phonological awareness and alphabet knowledge in young schoolchildren.9 Parental reports of delayed speech and language among children with reading difficulties have been common in epidemiological studies.
The most comprehensive picture of dyslexia available is in children of school age.10 Although in most cases speech perceptual abilities are intact, dyslexic children have difficulty in reflecting on the sound structure of spoken words. Such phonological problems make it difficult to learn how the letters and sounds of printed words are related. Most dyslexic children have difficulty using a phonic approach to reading, and their spelling often fails to represent the sound structure of target words. Although dyslexic children overcome many of their difficulties, in adulthood they experience subtle problems with phonological awareness and reading and writing skills. Functional brain imaging is beginning to elucidate why this is so; it has been shown that, when dyslexic adults perform rhyme judgment and verbal short term memory tasks, they activate only a subset of the brain regions usually involved. Plausibly, their phonological difficulties may be due to weak connectivity between anterior and posterior language areas of the left hemisphere.11
Knowledge of the predictors of reading achievement and of dyslexia has led to innovations in methods of intervention. A pioneering study in Oxford showed that children who performed poorly on a phonological processing task before they went to school benefited significantly from a training programme in sound categorisation using rhyme and alliteration activities, particularly when it was combined with teaching of letter sounds.12 Subsequently, it has been shown that training in phonological awareness combined with a structured reading intervention is an effective form of treatment for poor readers and produces greater gains than training in either reading or phonological awareness alone.13
Although the term dyslexia remains debated, there is good evidence that unexpected reading problems in children are caused by language deficiencies within the phonological domain. Children with reading difficulties report a variety of psychosomatic problems: complaints of headaches and difficulties in seeing are common. A detailed case and family history may uncover dyslexic difficulties, and the routine assessment of preschool children can usefully incorporate a test of knowledge of nursery rhymes and letters.14 Clinical experience shows that, with regard to dyslexia, it is a fallacy to “wait and see how the child develops.” A delay at the start of learning to read can quickly develop into a considerable reading disorder if unattended.