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Dyslexia

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Dyslexia
Classification and external resources
ICD-10 R48.0
ICD-9 315.02
OMIM 127700
DiseasesDB 4016
MedlinePlus 001406
Patient UK Dyslexia
MeSH D004410

Dyslexia, also known as reading disorder or alexia,[1] is a learning difficulty characterised by trouble reading despite a normal intelligence.[2] Different people are affected to different degrees.[1] Problems may include sounding out words, spelling words, reading quickly, writing words, pronouncing words when reading aloud, and understanding what was read.[1][3] Often these difficulties are first noticed at school.[4] The difficulties are not voluntary and people with this disorder have a normal desire to learn.[1]

The cause of dyslexia is believed to involve both genetic and environmental factors.[4] Some cases run in families.[1] It occurs more often in people with attention deficit hyperactivity disorder (ADHD) and is associated with problems with math.[4] When the condition begins in adults it may be the result of a traumatic brain injury, stroke, or dementia.[2] The underlying mechanism involves problems with the brain's processing of language.[1] Diagnosis is by a series of tests of a person's memory, spelling, ability to see, and reading skills.[5] It is separate from reading difficulties due to poor teaching, or hearing or vision problems.[4]

Treatment usually involves adjusting teaching methods to meet the persons needs.[2] While this does not cure the underlying problem, difficulties can be lessened.[6] Treatments aimed at vision are not effective.[7] Dyslexia is the most common learning disability.[8] It affects about 3% to 7% of people.[4][9] While it is diagnosed more often in males,[4] some believed it affects males and females equally.[8] Up to 20% of the population may have some degree of symptoms.[10] Dyslexia occurs in all areas of the world.[4]

Classification[edit]

Dyslexia is thought to have two kinds of causes, one related to language processing, and the other, to visual processing.[11] It is considered a cognitive disorder, not a problem with intelligence; there are often emotional problems that arise because of it.[11] There are many published definitions which are purely descriptive or involve proposed causes that encompass a variety of reading skills, deficits, and difficulties with distinct causes rather than a single condition.[12] The National Institute of Neurological Disorders and Stroke definition describes dyslexia as "difficulty with spelling, phonological processing (the manipulation of sounds), or rapid visual-verbal responding."[2] The British Dyslexia Association definition describes dyslexia as "a learning difficulty that primarily affects the skills involved in accurate and fluent word reading and spelling."[13] Characterized by "difficulties in phonological awareness, verbal memory and verbal processing speed."[13]

Acquired dyslexia or alexia may be caused by brain damage due to a stroke or atrophy.[14] Forms of alexia include pure alexia, surface dyslexia, semantic dyslexia, phonological dyslexia, and deep dyslexia.[15]

Signs and symptoms[edit]

In early childhood, symptoms that correlate with a later diagnosis of dyslexia include delayed onset of speech, difficulty distinguishing left from right, difficulty with direction,[16] as well as being easily distracted by background noise.[7] While they do occur in people with dyslexia, reversal of letters or words, or mirror writing, is not included in the definition of dyslexia,[7] and its relationship with dyslexia is controversial.[17]

Dyslexia and attention deficit hyperactivity disorder (ADHD) commonly occur together;[9] about 15% of people with dyslexia also have ADHD and 35% of those with ADHD have dyslexia.[7]

School-age dyslexic children may exhibit signs of difficulty identifying or generating rhyming words, or when counting syllables in words – both of which depend on phonological awareness.[18] They may also show difficulty when segmenting words into individual sounds or may blending sounds when producing words – indicating reduced phonemic awareness.[19] Difficulties with word retrieval or when naming things also feature in dyslexia.[20]:647 Dyslexics are commonly poor spellers, a feature sometimes called dysorthographia or dysgraphia, which depends on orthographic coding.[7]

Problems persist into adolescence and adulthood and may accompany difficulties with summarizing stories, memorization, reading aloud, or learning foreign languages. Adult dyslexics can often read with good comprehension, although they tend to read more slowly than non-dyslexics and perform worse in tests of spelling or when reading nonsense words – a measure of phonological awareness.[21]

A common myth about dyslexia is that its defining feature is reading or writing letters or words backwards; however, this is true of many children as they learn to read and write.[22]

Language[edit]

The orthographic complexity of a language directly impacts how difficult learning to read the language is.[23]:266 English and French have comparatively deep orthographies within the Latin alphabet writing system with complex structures employing spelling patterns of several levels: letter-sound correspondence, syllables, and morphemes.[24]:421 Other languages, such as Spanish, Italian and Finnish have mostly alphabetic orthographies which employ primarily letter-sound correspondence, so-called shallow orthographies, making them easier to learn for people with dyslexia.[23]:266 Logographic writing systems such as Chinese characters have graphemes that are in part unlinked to pronunciation, pose problems to dyslexic learners.[25]

Associated conditions[edit]

Several learning disabilities often occur together with dyslexia, but it is unclear whether they share underlying neurological causes.[26] These disabilities may 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.[27] In dyslexia, dysgraphia is often multifactorial, due to impaired letter writing automaticity, finger motor sequencing challenges, organizational and elaborative difficulties, and impaired visual word form which makes it more difficult to retrieve the visual picture of words required for spelling.[28]
  • Attention deficit disorder – A significant degree of comorbidity has been reported between ADHD and dyslexia or reading disorders,[29][30] it occurs in 12–24% of all individuals with dyslexia.[31]
  • Auditory processing disorder – A condition that affects the ability to process auditory information. Auditory processing disorder is a listening disability.[32][33] 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. Some research could indicate auditory processing skills could be the primary shortfall in Dyslexia.[34][35]
  • Developmental coordination disorder – A neurological condition characterized 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.[36]

Causes[edit]

Inferior parietal lobule - superior view animation – dyslexia neuroanatomy (frontal gyrus, temporal cortex not shown) neurological[37]
Main article: Theories of dyslexia

Researchers have been trying to find the neurobiological basis of dyslexia since it was first identified in 1881.[38][39] An example of one of the problems dyslexics experience would be seeing letters clearly, which may be due to abnormal development of their visual nerve cells.[40]

Neuroanatomy[edit]

In the area of neurological research into dyslexia, modern neuroimaging techniques such as functional magnetic resonance imaging (fMRI) and positron emission tomography (PET) have shown a correlation between functional and structural differences in the brains of children with reading difficulties.[41] Some individuals with dyslexia show less electrical activation in parts of the left hemisphere of the brain involved in reading, which includes the inferior frontal gyrus, inferior parietal lobule, and middle and ventral temporal cortex.[42] Brain activation studies using PET to study language have produced a breakthrough in understanding of the neural basis of language over the past decade. A neural basis for the visual lexicon and for auditory verbal short-term memory components have been proposed,[43] with some implication that the observed neural manifestation of developmental dyslexia is task-specific (i.e., functional rather than structural). fMRIs in dyslexics have provided important data supporting the interactive role, of the cerebellum and cerebral cortex as well as other brain structures.[44][45]

The cerebellar theory of dyslexia is based on the association of dyslexia with balance, coordination, and time estimation. Research has identified impairment on both sides of the cerebellum, displaying lower blood flow in the areas in question when active. Furthermore, in the early stages of human development such as crawling then walking may be delayed. It should be noted that, there is no current consensus in regards to the cerebellar/Dyslexia connection.[46][47]

Genetics[edit]

Genetic research into dyslexia and its inheritance has its roots in the examination of post-autopsy brains of people with dyslexia.[40] When they observed anatomical differences in the language center in a dyslexic brain, they showed microscopic cortical malformations known as ectopias and more rarely vascular micro-malformations, and in some instances these cortical malformations appeared as a microgyrus.[48] These studies and others[49] suggested abnormal cortical development which was presumed to occur before or during the sixth month of fetal brain development. Abnormal cell formations in dyslexics found on autopsy have also been reported in non-language cerebral and subcortical brain structures.[50] Several genes have been associated with dyslexia, including DCDC2 and KIAA0319 on chromosome 6,[51] and DYX1C1 on chromosome 15.[52]

Gene–environment interaction[edit]

Research has examined gene–environment interactions in reading disability through twin studies, which estimate the proportion of variance associated with environment and the proportion associated with heritability. Studies examining the influence of environmental factors such as parental education[53] and teacher quality[54] have determined that genetics has greater influence in supportive, rather than less optimal, environments.[55] Instead, it may just allow those genetic risk factors to account for more of the variance in outcome, because environmental risk factors that affect that outcome have been minimized.[55] As the environment plays a large role in learning and memory, it is likely that epigenetic modifications play an important role in reading ability. Animal experiments and measures of gene expression and methylation in the human periphery are used to study epigenetic processes, both of which have many limitations in extrapolating results for application to the human brain.[56]

Mechanisms[edit]

Dual route hypothesis to reading[57]

The dual-route theory of reading aloud was first described in the early 1970s.[57] This theory suggests that two separate mental mechanisms, or cognitive routes, are involved in reading aloud, with output of both mechanisms contributing to the pronunciation of a written stimulus.[58] One mechanism is the lexical route, which is the process whereby skilled readers can recognize known words by sight alone, through a “dictionary” lookup procedure.[59] The other mechanism is the nonlexical or sublexical route, which is the process whereby the reader can “sound out” a written word.[59][60] This is done by identifying the word's constituent parts (letters, phonemes, graphemes) and applying knowledge of how these parts are associated with each other, for example how a string of neighboring letters sound together.[57] The dual-route system can provide an explanation for the differences in dyslexia rates between different languages (e.g. the Spanish language dependence on phonological rules account for the fact that Spanish-speaking children show a higher level of performance in non-word reading,when compared to English-speakers).[23][61]

Diagnosis[edit]

There are tests that can indicate with high probability if the person is dyslexic.[62] This is often follow up with of a full diagnostic assessment to determine the extent and nature of the disorder,[63] tests can be administered by a teacher or computer.

Central dyslexias[edit]

Central dyslexias include surface dyslexia, semantic dyslexia, phonological dyslexia, and deep dyslexia.[64][65] ICD-10 reclassified the previous distinction between dyslexia (315.02 in ICD-9) and alexia (315.01 in ICD-9) into a single classification as R48.0. The terms are applied for developmental dyslexia and inherited dyslexia along with developmental aphasia and inherited alexia, which are now read as cognates in meaning and synonymous.[66]

Surface dyslexia[edit]

Main article: Surface dyslexia

In surface dyslexia, words whose pronunciations are 'regular' (highly consistent with their spelling e.g., mint) are read more accurately than words with irregular pronunciation, such as colonel.[67] Difficulty distinguishing homophones is diagnostic of some forms of surface dyslexia. This disorder is usually accompanied by (surface) agraphia and fluent aphasia.[68] Acquired surface dyslexia arises after brain damage in a previously literate person and results in pronunciation errors that indicate impairment of the lexical route.[14][69][70]

Phonological dyslexia[edit]

Broca's area - lateral view[71]
Main article: Phonological dyslexia

In phonological dyslexia, patients can read familiar words but have difficulty reading unfamiliar words (such as invented pseudo-words).[72] It is thought that they can recognize words by accessing lexical memory orthographically but cannot 'sound out' novel words. Phonological dyslexia is associated with lesions in varied locations within the territory of the middle cerebral artery. The superior temporal lobe is often also involved. Furthermore, dyslexics compensate by overusing a front-brain section, called Broca's area, associated with aspects of language and speech.[73] Research has pointed towards the theory that phonological dyslexia is a development of deep dyslexia. A treatment for phonological dyslexia is the Lindamood Phoneme Sequencing Program (LiPS).[74] This program is based on a three-way sensory feedback process. The subject uses their auditory, visual, and oral skills to learn to recognize words and word patterns. This is considered letter-by-letter reading using a bottom-up processing technique. Case studies with a total of three patients found a significant improvement in spelling and reading ability after using LiPS.[75]

Deep dyslexia[edit]

See also: Deep dyslexia

Patients with deep dyslexia experience semantic paralexia (para-dyslexia) and phonological dyslexia, which happens when the patient reads a word, and says a related meaning instead of the denoted meaning.[76] Deep alexia is associated with clear phonological processing impairments.[14] Deep dyslexia is caused by lesions that are often widespread and include much of the left frontal lobe; specifically, research suggests that damage to the left perisylvian region of the frontal lobe causes deep dyslexia.[77]

Peripheral dyslexias[edit]

Peripheral dyslexias have been described as a type of acquired dyslexia which is marked by problems in processing the visual factors of terms and, not like other dyslexias, originates from an injury to the system of visual analysis.[78] Hemianopsia is associated with this condition and is a visual field loss on the left/right side of the vertical midline.[79]

Pure dyslexia[edit]

Main article: Pure alexia

Pure dyslexia (phonologically based),[80] also known as agnosic dyslexia, dyslexia without agraphia, and pure word blindness, is dyslexia due to difficulty recognizing written sequences of letters (such as words), or sometimes even letters. It is 'pure' because it is not accompanied by other (significant) language-related impairments. Pure dyslexia does not include speech, handwriting style, language, or comprehension impairments.[81] Pure dyslexia is caused by lesions on the visual word form area (VWFA). The VWFA is composed of the left lateral occipital sulcus and is activated during reading. A lesion in the VWFA stops transmission between the visual cortex and the left angular gyrus. It can also be caused by a lesion involving the left occipital lobe and the splenium of the corpus callosum. It is usually accompanied by a homonymous hemianopsia in the right side of the visual field.[79] Multiple oral re-reading (MOR) is a treatment for pure dyslexia.[82] It is considered a top-down processing technique in which patients read and re-read texts a predetermined number of times or until reading speed or accuracy improves a predetermined amount.[83]

Hemianopic dyslexia[edit]

Commonly considered to derive from visual field loss due to damage to the primary visual cortex.[84] Sufferers may complain of slow reading but are able to read individual words normally. This is the most common form of peripheral alexia, and the form with the best evidence of effective treatments.[85]

Neglect dyslexia[edit]

In neglect dyslexia, some letters are neglected (skipped or misread) during reading – most commonly the letters at the beginning or left side of words.[86] This alexia is associated with right parietal lesions. Use of prism glasses in treatment has been demonstrated to produce substantial benefit.[87]

Attentional dyslexia[edit]

People with attentional dyslexia complain of letter crowding or migration, sometimes blending elements of two words into one.[88] Sufferers perform better when word stimuli are presented in isolation rather than flanked by other words and letters. Using a large magnifying glass may help as this may reduce the effects of flanking from nearby words; however, no trials of this or indeed any other therapy for left parietal syndromes have been published as of 2014.[89]

Management and testing[edit]

An example of OpenDyslexic typeface.[90]

Through use of compensation strategies, therapy, and educational support dyslexic individuals can learn to read and write.[91] There are techniques and technical aids that help manage or conceal symptoms of the disorder.[92] Removing stress and anxiety alone can sometimes improve written comprehension.[93] For dyslexia intervention with alphabet writing systems, the fundamental aim is to increase a child's awareness of correspondences between graphemes (letters) and phonemes (sounds), and to relate these to reading and spelling by teaching blending of the sounds into words. It has been found that reinforced collateral training focused towards visual language (reading) and orthography (spelling) yields longer-lasting gains than only oral phonological training.[94] Early intervention – when language areas of the brain are developing is most successful in reducing long-term impacts of dyslexia. There is some evidence that use of specially tailored fonts may be helpful.[90] Among these fonts are Dyslexie, OpenDyslexic and Lexia Readable which were created with the notion that many of the letters in the Latin alphabet are visually similar and may therefore be confusing for people with dyslexia. Dyslexie, along with OpenDyslexic, puts emphasis on making each letter more unique in order to be identified easier.[95] Font design can have an effect on reading, reading time and the perception of legibility, this is true for all readers, including those who have dyslexia and those who do not.[96]

There have been many studies done regarding intervention, as a result one such meta-analysis found that there was functional activation as a result.[97] Some test results indicate how to carry out teaching strategies.[98][99]

Prognosis[edit]

From an early age, children as they age into adults with dyslexia, require instruction for word analysis and spelling,[100] however, there are fonts which can help the reader better comprehend the material before them.[90] The prognosis, generally speaking is positive for individuals who are identified in childhood, but who also have support from friends and family.[101]

Epidemiology[edit]

The percentage of people with dyslexia is unknown but it is estimated to vary between 1–33% of the population.[102] An average estimate is 3% to 4% of a given population.[9] Internationally there are different definitions of dyslexia, but despite significant differences in writing systems different populations suffer similarly from dyslexia.[103] Dyslexia is not limited to difficulty in converting letters to sounds, and Chinese dyslexics have difficulty in extracting shapes of Chinese characters into meanings.[104][105]

History[edit]

Map showing predominant forms of writing systems by country and what their characters represent: [106]
  Alphabet (consonants and vowels)
  Abjad (only consonants)
  Abugida (family-related syllables)
  Syllabary (isolated syllables)

Dyslexia was identified by Oswald Berkhan in 1881,[38] but the term dyslexia was coined in 1887 by Rudolf Berlin, an ophthalmologist in Stuttgart.[107] He used the term to refer to a case of a young boy who had a severe impairment in learning to read and write despite showing typical intelligence and physical abilities in all other respects.[108] In 1896, W. Pringle Morgan, a British physician from Seaford, East Sussex published a description of a reading-specific learning disorder in a report to the British Medical Journal titled "Congenital Word Blindness".[109] The distinction between phonological and surface types of Dyslexia is only descriptive, and devoid of any etiological assumption as to the underlying brain mechanisms. However, studies have alluded to potential differences due to variation in performance.[110]

Research and society[edit]

Main article: Dyslexia research

The majority of currently available dyslexia research relates to alphabetic writing systems, and especially to European languages.[111] However, substantial research is also available regarding dyslexia in speakers of Arabic, Chinese, Hebrew, as well as other languages.[112]

As is the case with any disorder, society often makes an assessment based on incomplete information. At the end of the 20th century (1980's) dyslexia was thought a consequence of education, rather than a basic disability. As a result, society often misjudges those afflicted with the disorder.[113] Sometimes there is a lack of positive attitude in the work environment towards certain people with dyslexia.[114]

See also[edit]

References[edit]

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Further reading[edit]

External links[edit]

Dyslexia associations