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Abstract
Developmental dyslexia, or specific reading disability, is defined as an unexpected, specific, and persistent failure to acquire efficient reading skills despite conventional instruction, adequate intelligence, and sociocultural opportunity. Although still a matter of debate, this exclusionary definition was adopted by the diagnostic and statistical manual of mental disorders and the international classification of disorders.
Over the years a large amount research on dyslexia has been undertaken but how these findings fit together to form an overall picture still remains elusive. Research evidence has revealed the existence of several subtypes of dyslexia; however, the research strongly suggests that the underlying causes of dyslexia is most probably phonological deficits. Phonological awareness has also been found to be strongly predictive of reading and spelling acquisition, where a large number of studies have shown that good phonological awareness skills characterise good readers, whereas poor phonological awareness skills characterise poor readers .
The magnocellular theory has received much attention, and arose from observations of impaired visual processing in the magnocellular pathway. Patients with dyslexia showed poor thresholds for stimuli with low contrasts, low spatial or high temporal frequencies, and poor sensitivity to visual motion. The magnocellular system is important for directing visual attention, control of eye movements, and visual search—three skills that have a role in reading ability.
However, the magnocellular hypothesis has been criticised because findings are not reproducible or specific.
The role of the cerebellum in the pathogenesis of dyslexia stems from the conceptualisation of dyslexia as a learning disorder, in which failure to acquire and automatise reading and writing skills might be the most prominent but not a unique symptom,as it was reported that children with dyslexia sometimes have balance and motor-coordination disorders in attention-demanding circumstances. This observation suggests that motor tasks in these children are undertaken at the expense of attentional resources . Impairment of automatisation and time-evaluation deficits in dyslexia have been linked to cerebellar dysfunction; this association could also account for disorders of handwriting and articulatory skills, the latter possibly affecting phonological awareness .However,cerebellar signs are not always reported in dyslexia. Although compensatory mechanisms could account for negative findings
A range of neurobiological investigations using postmortem brain specimens and, more recently, brain morphometry and diffusion tensor magnetic resonance imaging (MRI) suggests that there are differences between dyslexic and nonimpaired readers in the back of the brain, specifically in the temporoparietooccipital brain regions. Functional brain imaging studies also show a failure of left hemisphere posterior brain systems to function properly in adult dyslexic readers while they perform reading tasks.
Dyslexia is both familial and heritable. Interestingly, a higher heritability for dyslexia has been reported in children with higher IQs .
Genetic transmission is complex, with both recessive and dominant transmission observed in different cases, with at least 50% or more of the variance explained by genetic factors and the remainder attributed to environmental influences.
Linkage studies have implicated genes on four chromosomes—2, 6, 15, and 18—in dyslexia . At least nine loci have been reported to be associated with the disorder. Much attention has recently centered on DCDC2, located on the short arm (p) of chromosome 6 in band 22 (6p22), and its association with dyslexia has been reported . These findings of a strong genetic influence have educational implications; If a child has a parent or sibling who is dyslexic, that child should be considered at risk and observed carefully for signs of a reading difficulty. It is also important to emphasize that a genetic etiology does not constrain a positive response to reading intervention ; once identified, dyslexic children deserve and will benefit from evidence-based interventions.
Dyslexia is a clinical diagnosis, best made by an experienced clinician who has taken a careful history, observed the child or young adult reading, and administered a battery of tests that assess the child’s cognitive ability, academic skills including reading accuracy, fluency, and comprehension, spelling, and mathematics (an area in which skills are often high), and language skills, particularly phonological processing . The uneven peaks and valleys of both cognitive and academic functioning contribute to the clinical picture of dyslexia: a weakness in phonologically based skills in the context of often stronger cognitive and academic skills in nonreading-related areas.
As children mature, compensation often occurs that results in relatively accurate, but not fluent, reading. Awareness of this developmental pattern is critically important for the diagnosis in older children, young adults, and beyond. The consequence is that such dyslexic older children may appear to perform reasonably well on a test of word reading or decoding; on these tests, credit is given irrespective of how long it takes the individual to respond or if initial errors in reading are later corrected. Accordingly, tests of reading fluency—how quickly and accurately individual words and passages are read—and tests assessing reading rate are keystones of an assessment for, and an accurate diagnosis of, dyslexia.
The different subtypes of dyslexia are likely to exhibit different responses to treatment that focus on training different cognitive skills, thus the purpose of assessing strengths and weaknesses in reading subskills is to help educators to develop an appropriate educational plan tailored to the child’s individualised needs. A first attempt to this would entail well-balanced and individualised remedial intervention that would build upon a child’s existing knowledge base. The evidence suggests that a child’s response to this type of intervention would provide guidance as to their long-term instructional needs, regardless of the origin of their reading difficulties. It has been shown that instruction in phonological awareness and phonics at an early age reduces the prevalence of dyslexia compared to individuals who did not receive training. However, with appropriate instruction, older children still exhibit gains in reading accuracy, but a fluency gap still exists and they tend to require longer and more intensive instruction.
There are many reported interventions available that claim to tackle dyslexia from teaching reading(Earobics, Fast forword, Orton-gillingham method, Barton method , Project Read, The Wilson Reading System, lips, and many others...) To programs that focus on balance and coordination(The Dore system,INPP), through to taking dietary supplements such as omega-3
Dyslexic individuals often show symptoms of related neurodevelopmental behavioural disorders, e.g., deficits in oral language acquisition, motor coordination deficits, visuospatial impairment, and attentional abilities. There exists frequent comorbidity between dyslexia and speech sound disorder (SSD), specific language impairment (SLI), Meares-Irlene syndrome ,attention deficit hyperactivity disorder (ADHD), dysgraphia (difficulties in writing), dyspraxia (motor coordination deficits), and dyscalculia (difficulties in mathematics). All these behavioural disorders manifest in learning problems at an early age, despite normal intelligence, that can affect the cognitive and emotional development of a child
A recent concept emerged lately denoting that these many comorbidities are in fact are several faces of a single coin ,this coin is called : atypical brain development or ABD, ABD is a concept that can be used to address the full range of developmental disorders that are found to be overlapping much of the time in any sample of children studied. It is also a term that appropriately links neurology to learning and behaviour .