Medical conditionDyslexiaOther namesReading disorderDyslexia involves difficulties in processing letters and words.Pronunciation/dɪsˈlɛksiə/ ⓘ dis-LEK-see-ə SpecialtyNeurology, pediatricsSymptomsTrouble reading[1]Usual onsetSchool age[2]TypesSurface dyslexiaCausesGenetic and environmental factors[2]Risk factorsFamily history, attention deficit hyperactivity disorder[3]Diagnostic methodSeries memory, spelling, vision, and reading test[4]Differential diagnosisHearing or vision problems, insufficient teaching[2]TreatmentAdjusting teaching methods[1]Frequency3–7%[2][5]
Dyslexia, also known as word blindness, is a learning disability that affects either reading or writing.[1][6] Different people are affected to different degrees.[3] Problems may include difficulties in spelling words, reading quickly, writing words, "sounding out" words in the head, pronouncing words when reading aloud and understanding what one reads.[3][7] Often these difficulties are first noticed at school.[2] The difficulties are involuntary, and people with this disorder he a normal desire to learn.[3] People with dyslexia he higher rates of attention deficit hyperactivity disorder (ADHD), developmental language disorders, and difficulties with numbers.[2][8]
Dyslexia is believed to be caused by the interaction of genetic and environmental factors.[2] Some cases run in families.[3] Dyslexia that develops due to a traumatic brain injury, stroke, or dementia is sometimes called "acquired dyslexia"[1] or alexia.[3] The underlying mechanisms of dyslexia result from differences within the brain's language processing.[3] Dyslexia is diagnosed through a series of tests of memory, vision, spelling, and reading skills.[4] Dyslexia is separate from reading difficulties caused by hearing or vision problems or by insufficient teaching or opportunity to learn.[2]
Treatment involves adjusting teaching methods to meet the person's needs.[1] While not curing the underlying problem, it may decrease the degree or impact of symptoms.[9] Treatments targeting vision are not effective.[10] Dyslexia is the most common learning disability and occurs in all areas of the world.[11] It affects 3–7% of the population;[2][5] however, up to 20% of the general population may he some degree of symptoms.[12] While dyslexia is more often diagnosed in boys, this is partly explained by a self-fulfilling referral bias among teachers and professionals.[2][13] It has even been suggested that the condition affects men and women equally.[11] Some believe that dyslexia is best considered as a different way of learning, with both benefits and downsides.[14][15]
Classification Main article: Pure alexiaDyslexia is divided into developmental and acquired forms.[16] Acquired dyslexia occurs subsequent to neurological insult, such as traumatic brain injury or stroke. People with acquired dyslexia exhibit some of the signs or symptoms of the developmental disorder, but require different assessment strategies and treatment approaches.[17] Pure alexia, also known as agnosic alexia or pure word blindness, is one form of alexia which makes up "the peripheral dyslexia" group.[18]
Signs and symptoms See also: Characteristics of dyslexiaIn early childhood, symptoms that correlate with a later diagnosis of dyslexia include delayed onset of speech and a lack of phonological awareness.[10] A common myth closely associates dyslexia with mirror writing and reading letters or words backwards.[19] These behiors are seen in many children as they learn to read and write, and are not considered to be defining characteristics of dyslexia.[10]
School-age children with dyslexia may exhibit signs of difficulty in identifying or generating rhyming words, or counting the number of syllables in words—both of which depend on phonological awareness.[20] They may also show difficulty in segmenting words into individual sounds (such as sounding out the three sounds of k, a, and t in cat) or may struggle to blend sounds, indicating reduced phonemic awareness.[21]
Difficulties with word retrieval or naming things is also associated with dyslexia.[22]: 647 People with dyslexia are commonly poor spellers, a feature sometimes called dysorthographia or dysgraphia, which depends on the skill of orthographic coding.[10]
Problems persist into adolescence and adulthood and may include difficulties with summarizing stories, memorization, reading aloud, or learning foreign languages. Adults with dyslexia can often read with good comprehension, though they tend to read more slowly than others without a learning difficulty and perform worse in spelling tests or when reading nonsense words—a measure of phonological awareness.[23]
Associated conditionsDyslexia often co-occurs with other learning disorders, but the reasons for this comorbidity he not been clearly identified.[24] These associated disabilities include:
Dysgraphia A disorder involving difficulties with writing or typing, sometimes due to problems with eye–hand coordination; it also can impede direction- or sequence-oriented processes, such as tying knots or carrying out repetitive tasks.[25] In dyslexia, dysgraphia is often multifactorial, due to impaired letter-writing automaticity, organizational and elaborative difficulties, and impaired visual word forming, which makes it more difficult to retrieve the visual picture of words required for spelling.[25] Attention deficit hyperactivity disorder (ADHD) A disorder characterized by problems sustaining attention, hyperactivity, or acting impulsively.[26] Dyslexia and ADHD commonly occur together.[5][27][28] Approximately 15%[10] or 12–24% of people with dyslexia he ADHD;[29] and up to 35% of people with ADHD he dyslexia.[10] Auditory processing disorder A listening disorder that affects the ability to process auditory information.[30][31] This can lead to problems with auditory memory and auditory sequencing. Many people with dyslexia he auditory processing problems, and may develop their own logographic cues to compensate for this type of deficit. Some research suggests that auditory processing skills could be the primary shortfall in dyslexia.[32][33] Developmental coordination disorder A neurological condition characterized by difficulty in carrying out routine tasks involving balance, fine-motor control and kinesthetic coordination; difficulty in the use of speech sounds; and problems with short-term memory and organization.[34] Causes Inferior parietal lobule – superior view animationResearchers he been trying to find the neurobiological basis of dyslexia since the condition was first identified in 1881.[35][36] For example, some he tried to associate the common problem among people with dyslexia of not being able to see letters clearly to abnormal development of their visual nerve cells.[37]
NeuroanatomyNeuroimaging techniques, such as functional magnetic resonance imaging (fMRI) and positron emission tomography (PET), he shown a correlation between both functional and structural differences in the brains of children with reading difficulties.[38] Some people with dyslexia show less activation in parts of the left hemisphere of the brain involved with reading, such as the inferior frontal gyrus, inferior parietal lobule, and the middle and ventral temporal cortex.[32] Over the past decade, brain activation studies using PET to study language he produced a breakthrough in the understanding of the neural basis of language. Neural bases for the visual lexicon and for auditory verbal short-term memory components he been proposed,[39] with some implication that the observed neural manifestation of developmental dyslexia is task-specific (i.e., functional rather than structural). fMRIs of people with dyslexia indicate an interactive role of the cerebellum and cerebral cortex as well as other brain structures in reading.[40][41]
The cerebellar theory of dyslexia proposes that impairment of cerebellum-controlled muscle movement affects the formation of words by the tongue and facial muscles, resulting in the fluency problems that some people with dyslexia experience. The cerebellum is also involved in the automatization of some tasks, such as reading.[42] The fact that some children with dyslexia he motor task and balance impairments could be consistent with a cerebellar role in their reading difficulties. However, the cerebellar theory has not been supported by controlled research studies.[43]
GeneticsResearch into potential genetic causes of dyslexia has its roots in post-autopsy examination of the brains of people with dyslexia.[37] Observed anatomical differences in the language centers of such brains include microscopic cortical malformations known as ectopias, and more rarely, vascular micro-malformations, and microgyrus—a smaller than usual size for the gyrus.[44] The previously cited studies and others[45] suggest that abnormal cortical development, presumed to occur before or during the sixth month of fetal brain development, may he caused the abnormalities. Abnormal cell formations in people with dyslexia he also been reported in non-language cerebral and subcortical brain structures.[46] Several genes he been associated with dyslexia, including DCDC2[47] and KIAA0319[48] on chromosome 6, and DYX1C1 on chromosome 15.[49]
Gene–environment interactionThe contribution of gene–environment interaction to reading (dis)ability, which estimates the proportion of variance (the differences in reading ability across individuals) attributable to environmental and genetic factors, has been studied extensively by comparing sets of identical twins and fraternal twins. Such twin studies show that both environmental and genetic influences contribute to reading development, with these two influences' relative impact varying by context. For example, parental education has been found to moderate genetic influences on reading (dis)ability,[50] and teacher quality can moderate genetic effects on early reading ability.[51] Moreover, in more supportive environments, genetic risk factors account for a larger proportion of the variation in dyslexia.[52] This occurs because reducing environmental risks—through high-quality instruction and supportive caregiving—lowers environmental differences, making genetic effects easier to detect. If environmental risk factors are reduced with good teaching and a supporting caregiver environment, genetic risk factors may play a bigger role in explaining why some people he dyslexia[52] because the genetic effect is easier to detect given that much environmental noise is bracketed-out.
As environment plays a large role in learning and memory, it is likely that epigenetic modifications play an important role in reading ability. Measures of gene expression, histone modifications, and methylation in the human periphery are used to study epigenetic processes; however, all of these he limitations in the extrapolation of results for application to the human brain.[53][54]
LanguageThe orthographic complexity of a language directly affects how difficult it is to learn to read it.[55]: 266 English and French he comparatively "deep" phonemic orthographies within the Latin alphabet writing system, with complex structures employing spelling patterns on several levels: letter-sound correspondence, syllables, and morphemes.[56]: 421 Languages such as Spanish, Italian and Finnish primarily employ letter-sound correspondence—so-called "shallow" orthographies—which makes them easier to learn for people with dyslexia.[55]: 266 Logographic writing systems, such as Chinese characters, he extensive symbol use, and these also pose problems for dyslexic learners.[57]
Pathophysiology Corpus callosum view, front part at top of imageFor most people who are right-hand dominant, the left hemisphere of their brain is more specialized for language processing. With regard to the mechanism of dyslexia, fMRI studies suggest that this specialization is less pronounced or absent in people with dyslexia. In other studies, dyslexia is correlated with anatomical differences in the corpus callosum, the bundle of nerve fibers that connects the left and right hemispheres.[58]
Data via diffusion tensor MRI indicate changes in connectivity or in gray matter density in areas related to reading and language. Finally, the left inferior frontal gyrus has shown differences in phonological processing in people with dyslexia.[58] Neurophysiological and imaging procedures are being used to ascertain phenotypic characteristics in people with dyslexia, thus identifying the effects of dyslexia-related genes.[59]
Dual route theoryThe dual-route theory of reading aloud was first described in the early 1970s.[60] This theory suggests that two separate mental mechanisms, or cognitive routes, are involved in reading aloud.[61] 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.[62] The other mechanism is the nonlexical or sublexical route, which is the process whereby the reader can "sound out" a written word.[62][63] 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.[60] The dual-route system could explain the different rates of dyslexia occurrence between different languages (e.g., the consistency of phonological rules in the Spanish language could account for the fact that Spanish-speaking children show a higher level of performance in non-word reading, when compared to English-speakers).[55][64]
DiagnosisDyslexia is a heterogeneous, dimensional learning disorder that impairs accurate and fluent word reading and spelling.[65][66] Typical—but not universal—features include difficulties with phonological awareness, inefficient and often inaccurate processing of sounds in oral language (phonological processing), and verbal working memory deficits.[67][68]
Dyslexia is a neurodevelopmental disorder, subcategorized in diagnostic guides as a learning disorder with impairment in reading (ICD-11 prefixes "developmental" to "learning disorder"; DSM-5 uses "specific").[69][70][71] Dyslexia is not a problem with intelligence. Emotional problems often arise secondary to learning difficulties.[72] The National Institute of Neurological Disorders and Stroke describes dyslexia as "difficulty with phonological processing (the manipulation of sounds), spelling, and/or rapid visual-verbal responding".[1]
The British Dyslexia Association defines dyslexia as "a learning difficulty that primarily affects the skills involved in accurate and fluent word reading and spelling" and is characterized by "difficulties in phonological awareness, verbal memory and verbal processing speed".[73] Phonological awareness enables one to identify, discriminate, remember (working memory), and mentally manipulate the sound structures of language—phonemes, onsite-rime segments, syllables, and words.[74][75]
AssessmentThe following can be done to assess for dyslexia:
Apply a multidisciplinary team approach involving the child's parent(s) and teacher(s), school psychologist, pediatrician, and, as appropriate, speech and language pathologist (speech therapist), and occupational therapist.[76]
Gain familiarity with typical ages children reach various general developmental milestones, and domain-specific milestones, such as phonological awareness (recognizing rhyming words; identifying the initial sounds in words).[77]
Do not rely on tests exclusively. Careful observation of the child in the school and home environments, and sensitive, comprehensive parental interviews are just as important as tests.[78][79]
Look at the empirically supported response to intervention (RTI) approach,[80] which "... involves monitoring the progress of a group of children through a programme of intervention rather than undertaking a static assessment of their current skills. Children with the most need are those who fail to respond to effective teaching, and they are readily identified using this approach."[81]
Assessment testsThere is a wide range of tests that are used in clinical and educational settings to evaluate the possibility of dyslexia.[82] If initial testing suggests that a person might he dyslexia, such tests are often followed up with a full diagnostic assessment to determine the extent and nature of the disorder.[83] Some tests can be administered by a teacher or computer; others require specialized training and are given by psychologists.[84] Some test results indicate how to carry out teaching strategies.[84][85] Because a variety of different cognitive, behioral, emotional, and environmental factors all could contribute to difficulty learning to read, a comprehensive evaluation should consider these different possibilities. These tests and observations can include:[86]
General measures of cognitive ability, such as the Wechsler Intelligence Scale for Children, Woodcock-Johnson Tests of Cognitive Abilities, or Stanford-Binet Intelligence Scales. Low general cognitive ability would make reading more difficult. Cognitive ability measures also often try to measure different cognitive processes, such as verbal ability, nonverbal and spatial reasoning, working memory, and processing speed. There are different versions of these tests for different age groups. Almost all of these require additional training to give and score correctly, and are done by psychologists. According to Mather and Schneider (2015), a confirmatory profile and/or pattern of scores on cognitive tests confirming or ruling-out reading disorder has not yet been identified.[87] Screening or evaluation for mental health conditions: Parents and teachers can complete rating scales or behior checklists to gather information about emotional and behioral functioning for younger people. Many checklists he similar versions for parents, teachers, and younger people old enough to read reasonably well (often 11 years and older) to complete. Examples include the Behioral Assessment System for Children, and the Strengths and Difficulties Questionnaire. All of these he nationally representative norms, making it possible to compare the level of symptoms to what would be typical for the younger person's age and biological sex. Other checklists link more specifically to psychiatric diagnoses, such as the Vanderbilt ADHD Rating Scales or the Screen for Child Anxiety Related Emotional Disorders (SCARED). Screening uses brief tools that are designed to catch cases with a disorder, but they often get false positive scores for people who do not he the disorder. Screeners should be followed up by a more accurate test or diagnostic interview as a result. Depressive disorders and anxiety disorders are two-three times higher in people with dyslexia, and attention-deficit/hyperactivity disorder is more common, as well.[88][89][90][91] Review of academic achievement and skills: Average spelling/reading ability for a dyslexic is a percentage ranking