Dyslexia, or developmental reading disorder, is characterised by difficulty with learning to read fluently and with accurate comprehension despite normal or above-average intelligence. This includes difficulty with phonological awareness, phonological decoding, processing speed, orthographic coding, auditory short-term memory, language skills/verbal comprehension, and/or rapid naming.
Dyslexia is the most common learning difficulty and most recognised reading disorder. There are other reading difficulties that are unrelated to dyslexia.
Some see dyslexia as distinct from reading difficulties resulting from other causes, such as a non-neurological deficiency with vision or hearing, or poor or inadequate reading instruction. There are three proposed cognitive subtypes of dyslexia (auditory, visual and attentional), although individual cases of dyslexia are better explained by specific underlying neuropsychological deficits (e.g. an auditory processing disorder, an attention deficit hyperactivity disorder, a visual processing disorder) and co-occurring learning difficulties (e.g. dyscalculia and dysgraphia). Although it is considered to be a receptive (afferent) language-based learning disability, dyslexia also affects one's expressive (efferent) language skills. Researchers at MIT found that people with dyslexia exhibited impaired voice-recognition abilities. A study published online (and in the July issue of the American Journal of Human Genetics), reported a possible genetic origin to the disorder, and other learning disabilities, that could help lead in some cases to earlier diagnoses and more successful interventions.
Signs and Symptoms
In early childhood, early symptoms that correlate with a later diagnosis of dyslexia include delays in speech, letter reversal or mirror writing, difficulty knowing left from right and directions, and being easily distracted by background noise. This pattern of early distractibility is occasionally partially explained by the co-occurrence of dyslexia and attention-deficit/hyperactivity disorder. Although this disorder occurs in approximately 5% of children, 25–40% of children with either dyslexia or ADHD meet criteria for the other disorder.
Dyslexic children of school age can have various symptoms. The symptoms may include:
- Difficulty identifying or generating rhyming words, or counting syllables in words (phonological awareness).
- A difficulty segmenting words into individual sounds, or blending sounds to make words.
- A difficulty with word retrieval or naming problems (anomic aphasia), commonly very poor spelling, which has been called dysorthographia or dysgraphia (orthographic coding), whole-word guesses.
- Tendencies to omit or add letters or words when writing and reading are considered tell-tale signs.
Signs persist into adolescence and adulthood and may be accompanied by trouble with summarising a story, memorising, reading aloud, and learning a foreign language. Adult dyslexics can read with good comprehension, although they tend to read more slowly than non-dyslexics and perform more poorly at spelling and nonsense word reading, a measure of phonological awareness.
A common misconception about dyslexia assumes that dyslexic readers all write words backwards or move letters around when reading. In fact, this only occurs in a very small population of dyslexic readers. Individuals with dyslexia are better identified by measuring reading accuracy, fluency, and writing skills and trying to match these measurements to their level of intelligence as determined from prior observations.
Several learning disabilities often occur with dyslexia, but it is unclear whether these learning disabilities share underlying neurological causes with dyslexia.
These disabilities include:
- Dysgraphia – a disorder which expresses itself primarily through writing or typing, although in some cases it may also affect eye–hand coordination, direction- or sequence-oriented processes such as tying knots or carrying out a repetitive task. In dyslexia, dysgraphia is often multifactorial, due to impaired letter writing automaticity, finger motor sequencing challenges, organisational and elaborative difficulties, and impaired visual word form which makes it more difficult to retrieve the visual picture of words required for spelling. Dysgraphia is distinct from developmental coordination disorder in that developmental coordination disorder is simply related to motor sequence impairment.
- Attention deficit disorder – A significant degree of co-morbidity has been reported between ADD/ADHD and dyslexia/reading disorders, it occurs in between 12% and 24% of those with dyslexia. Research studying the impact of interference on adults with and without dyslexia has revealed large differences in terms of attention deficits for adults with dyslexia, and has implications for teaching reading and writing to dyslexics in the future.
- Auditory processing disorder – A condition that affects the ability to process auditory information. Auditory processing disorder is a listening disability. It can lead to problems with auditory memory and auditory sequencing. Many people with dyslexia have auditory processing problems and may develop their own logographic cues to compensate for this type of deficit. Auditory processing disorder is recognised as one of the major causes of dyslexia.
- Developmental coordination disorder – A neurological condition characterised by a marked difficulty in carrying out routine tasks involving balance, fine-motor control, kinesthetic coordination, difficulty in the use of speech sounds, problems with short-term memory and organization are typical of dyspraxics.
Experience of speech acquisition delays, and speech and language problems, can be due to problems processing and decoding auditory input prior to reproducing their own version of speech, and may be observed as stuttering, cluttering or hesitant speech.
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