text: dyslexia dyslexia , also known as reading disorder , is characterized by trouble with reading despite normal intelligence . different people are affected to varying degrees . 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 . often these difficulties are first noticed at school . when someone who previously could read loses their ability , it is known as alexia . the difficulties are involuntary and people with this disorder have a normal desire to learn . the cause of dyslexia is believed to involve both genetic and environmental factors . some cases run in families . it often occurs in people with attention deficit hyperactivity disorder ( adhd ) and is associated with similar difficulties with numbers . it may begin in adulthood as the result of a traumatic brain injury , stroke , or dementia . the underlying mechanisms are problems within the brain 's language processing . dyslexia is diagnosed through a series of tests of memory , spelling , vision , and reading skills . dyslexia is separate from reading difficulties caused by insufficient teaching ; or either hearing or vision problems . treatment involves adjusting teaching methods to meet the person 's needs . while not curing the underlying problem , it may decrease the degree of symptoms . treatments targeting vision are not effective . dyslexia is the most common learning disability , affecting 3 – 7 % of the population ; however , up to 20 % may have some degree of symptoms . while dyslexia is more often diagnosed in men , it has been suggested that it affects men and women equally . dyslexia occurs in all areas of the world . some believe that dyslexia should be best considered as a different way of learning , with both benefits and downsides . classification dyslexia is thought to have two types of cause , one related to language processing and another to visual processing . it is considered a cognitive disorder , not a problem with intelligence . however , emotional problems often arise because of it . some published definitions are purely descriptive , whereas others propose causes . the latter usually cover a variety of reading skills and deficits , and difficulties with distinct causes rather than a single condition . 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 " . 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 " and is characterized by " difficulties in phonological awareness , verbal memory and verbal processing speed " . acquired dyslexia or alexia may be caused by brain damage due to stroke or atrophy . forms of alexia include pure alexia , surface dyslexia , semantic dyslexia , phonological dyslexia , and deep dyslexia . definition there is some variability in the definition of dyslexia . some sources , such as the u.s. national institutes of health , define it specifically as a learning disorder . other sources , however , define it simply as an inability to read in the context of normal intelligence , and distinguish between developmental dyslexia ( a learning disorder ) and acquired dyslexia ( loss of the ability to read caused by brain damage ) . icd 10 , the manual of medical diagnosis used in much of the world , includes separate diagnoses for " developmental dyslexia " ( 81 . 0 ) and for " dyslexia and alexia " ( 48 . 0 ) . dsm 5 , the manual of psychiatric diagnosis used in the united states , does not specifically define dyslexia , justifying this decision by stating that " the many definitions of dyslexia and meant those terms would not be useful as disorder names or in the diagnostic criteria " . instead it includes dyslexia in a category called specific learning disorders . signs and symptoms 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 , and a lack of phonological awareness , as well as being easily distracted by background noise . the reversal of letters or words and mirror writing are sometimes seen in people with dyslexia , but are not considered to be defining characteristics of the disorder . 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 . they may also show difficulty in segmenting words into individual sounds or may blend sounds when producing words , indicating reduced phonemic awareness . difficulties with word retrieval or naming things is also associated with dyslexia . people with dyslexia are commonly poor spellers , a feature sometimes called or , which depends on orthographic coding . problems persist into adolescence and adulthood and may accompany 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 . a common myth about dyslexia is that its defining feature is reading or writing letters or words backwards , but this is true of many children as they learn to read and write . language the orthographic complexity of a language directly impacts how difficult learning to read the language is . english and french have 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 . languages such as spanish , italian and finnish have mostly alphabetic orthographies , which primarily employ letter - sound correspondence – so - called shallow orthographies – which for dyslexics makes them easier to learn . writing systems , such as chinese characters , have extensive symbol use , and pose problems for dyslexic learners . associated conditions dyslexia is often accompanied by several learning disabilities , but it is unclear whether they share underlying neurological causes . these associated , or comorbid , disabilities include : – a disorder which primarily expresses itself through difficulties with writing or typing , but in some cases through difficulties associated with eye – hand coordination and direction or sequence - oriented processes such as tying knots or carrying out repetitive tasks . in dyslexia , is often multifactorial , due to impaired letter - writing , organizational and difficulties , and impaired visual word forming which makes it more difficult to retrieve the visual picture of words required for spelling . attention deficit hyperactivity disorder ( adhd ) – a disorder characterized by problems paying attention , excessive activity , or taking action without forethought . dyslexia and adhd commonly occur together . either 15 % or 12 – 24 % of people with dyslexia have adhd . 35 % of people with adhd have dyslexia . auditory processing disorder – a listening disability that affects the ability to process auditory information . this 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 indicates that auditory processing skills could be the primary shortfall in dyslexia . developmental coordination disorder – a neurological condition characterized by 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 . causes researchers have been trying to find the neurobiological basis of dyslexia since the condition was first identified in 1881 . for example , some have tried to associate the common problem among dyslexics of not being able to see letters clearly to abnormal development of their visual nerve cells . modern neuroimaging techniques such as functional magnetic resonance imaging ( fmri ) and positron emission tomography ( pet ) have shown a correlation between both functional and structural differences in the brains of children with reading difficulties . some dyslexics show less electrical 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 . over the past decade , brain activation studies using pet to study language have 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 have been proposed , with some implication that the observed neural manifestation of developmental dyslexia is task - specific ( i.e. functional rather than structural ) . in dyslexics have provided important data which point to the interactive role of the cerebellum and cerebral cortex as well as other brain structures . 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 are characteristic of some dyslexics . the cerebellum is also involved in the of some tasks , such as reading . the fact that some dyslexic children have motor task and balance impairments has been used as evidence for a cerebellar role in their reading difficulties . however , the cerebellar theory is not supported by controlled research studies . genetics research into potential genetic causes of dyslexia has its roots in post - autopsy examination of the brains of people with dyslexia . observed anatomical differences in the language centers of such brains include microscopic cortical malformations known as , more rarely , vascular micro - malformations , and . the previously cited studies and others suggest that abnormal cortical development presumed to occur before or during the sixth month of fetal brain development was the cause of the abnormalities . abnormal cell formations in dyslexics have also been reported in non - language cerebral and subcortical brain structures . several genes have been associated with dyslexia , including and on chromosome 6 , and on chromosome 15 . gene – environment interaction the contribution of gene – environment interaction to reading disability has been intensely studied using twin studies , which estimate the proportion of variance associated with a person 's environment and the proportion associated with their genes . studies examining the influence of environmental factors such as parental education and teacher quality have determined that genetics have greater influence in supportive , rather than less optimal , environments . however , more optimal conditions may just allow those genetic risk factors to account for more of the variance in outcome because the environmental risk factors have been minimized . as 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 ; however , both types of study have many limitations in the extrapolation of results for application to the human brain . mechanisms the dual - route theory of reading aloud was first described in the early 1970s . this theory suggests that two separate mental mechanisms , or cognitive routes , are involved in reading aloud . 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 . the other mechanism is the or route , which is the process whereby the reader can " sound out " a written word . 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 . the dual - route system could explain the different rates of dyslexia occurrence between different languages ( e.g. the spanish language dependence on phonological rules accounts for the fact that spanish - speaking children show a higher level of performance in non - word reading , when compared to english - speakers ) . dyslexia disorder is not caused by mutation in one gene ; in fact , it appears to involve the combined effects of several genes . studying the cognitive problems associated with other disorders helps to better understand the genotype - phenotype link of dyslexia . and imaging procedures are being used to ascertain phenotypic characteristics in dyslexics , thus identifying the effects of certain genes . diagnosis there are tests that can indicate with high probability whether a person is a dyslexic . if diagnostic testing indicates that a person may be dyslexic , such tests are often followed up with a full diagnostic assessment to determine the extent and nature of the disorder . tests can be administered by a teacher or computer . some test results indicate how to carry out teaching strategies . central dyslexias central dyslexias include surface dyslexia , semantic dyslexia , phonological dyslexia , and deep dyslexia . 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 the terms are applied to developmental dyslexia and inherited dyslexia along with developmental aphasia and inherited alexia , which are considered synonymous . surface dyslexia in surface dyslexia , words with regular pronunciations ( highly consistent with their spelling , e.g. mint ) are read more accurately than words with irregular pronunciation , such as colonel . difficulty distinguishing homophones is a diagnostic used for some forms of surface dyslexia . this disorder is usually accompanied by surface and fluent aphasia . acquired surface dyslexia arises when a previously literate person experiences brain damage , which results in pronunciation errors that indicate impairment of the lexical route . phonological dyslexia in phonological dyslexia , sufferers can read familiar words but have difficulty with unfamiliar words , such as invented pseudo - words . phonological dyslexia is associated with lesions in the parts of the brain supplied with blood by the middle cerebral artery . the superior temporal lobe is often also involved . furthermore , dyslexics compensate by overusing a front - brain region called broca 's area , which is associated with aspects of language and speech . the sequencing program ( ) is used to treat phonological dyslexia . this system is based on a three - way sensory feedback process , using auditory , visual , and oral skills to learn to recognize words and word patterns . case studies with a total of three patients found a significant improvement in spelling and reading ability after using . deep dyslexia individuals with deep dyslexia experience both semantic ( para - dyslexia ) and phonological dyslexia , which causes the person to read a word and then say a related meaning instead of the denoted meaning . deep alexia is associated with clear phonological processing impairments . deep dyslexia is caused by widespread damage to the brain that often includes the left hemisphere . the " continuum " hypothesis claims that deep dyslexia develops from phonological dyslexia . peripheral dyslexias peripheral dyslexias have been described as affecting the visual analysis of letters as a result of brain injury . , a visual field loss on the left / right side of the vertical midline , is associated with this condition . pure dyslexia pure , or phonologically - based , dyslexia , also known as dyslexia , dyslexia without , and pure word blindness , is dyslexia due to difficulty in recognizing written sequences of letters ( such as words ) , or sometimes even letters . it is considered ' " pure " because it is not accompanied by other significant language - related impairments . pure dyslexia does not affect speech , handwriting style , language or comprehension impairments . 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 or the . it is usually accompanied by a homonymous hemianopsia in the right side of the visual field . multiple oral re - reading ( mor ) is a treatment for pure dyslexia . it is considered a top - down processing technique in which affected individuals read and reread texts a predetermined number of times or until reading speed or accuracy improves a predetermined amount . dyslexia dyslexia is commonly considered to derive from visual field loss due to damage to the primary visual cortex . sufferers may complain of abnormally 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 . neglect dyslexia in neglect dyslexia , some letters , most commonly those at the beginning or left side of a word , are skipped or misread during reading . this alexia is associated with right parietal lesions . the use of prism glasses has been shown to substantially mitigate this condition . dyslexia people with attentional dyslexia complain of letter - crowding or migration , sometimes blending elements of two words into one . sufferers read better when words are presented in isolation rather than flanked by other words and letters . using a large magnifying glass may help mitigate this condition by reducing 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 . management through the use of compensation strategies , therapy and educational support , dyslexic individuals can learn to read and write . there are techniques and technical aids which help to manage or conceal symptoms of the disorder . removing stress and anxiety alone can sometimes improve written comprehension . 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 how sounds blend into words . it has been found that reinforced collateral training focused on reading and spelling yields longer - lasting gains than oral phonological training alone . early intervention – that done while the language areas of the brain are still developing – is the most successful in reducing the long - term impacts of dyslexia . there is some evidence that the use of specially - tailored fonts may mitigate the effects of dyslexia . these fonts , which include , , and , were created based on the idea that many of the letters of the latin alphabet are visually similar and may therefore confuse dyslexics . and both put emphasis on making each letter more distinctive in order to be more easily identified . font design can have an effect on reading , reading time , and the perception of legibility of all readers , not only those with dyslexia . there have been many studies conducted regarding intervention in dyslexia . among these studies one meta - analysis found that there was functional activation as a result . prognosis children require special instruction for word analysis and spelling from an early age . however , there are fonts that can help dyslexics better understand writing . the prognosis , generally speaking , is positive for individuals who are identified in childhood and receive support from friends and family . epidemiology the percentage of people with dyslexia is unknown , but it has been estimated to be as low as 5 % and as high as 17 % of the population . while it is diagnosed more often in males , some believe that it affects males and females equally . there are different definitions of dyslexia used throughout the world , but despite significant differences in writing systems , dyslexia occurs in different populations . dyslexia is not limited to difficulty in converting letters to sounds , and chinese dyslexics may have difficulty converting chinese characters into their meanings . the chinese vocabulary uses logographic , monographic , non - alphabet writing where one character can represent an individual phoneme . the phonological - processing hypothesis attempts to explain why dyslexia occurs in a wide variety of languages . furthermore , the relationship between phonological capacity and reading appears to be influenced by orthography . history dyslexia was identified by oswald in 1881 , but the term dyslexia was coined in 1887 by rudolf berlin , an ophthalmologist in stuttgart . he used the term to refer to the 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 . 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 " . the distinction between phonological and surface types of dyslexia is only descriptive , and without any etiological assumption as to the underlying brain mechanisms . however , studies have alluded to potential differences due to variation in performance . research and society the majority of currently available dyslexia research relates to alphabetic writing systems , and especially to european languages . however , substantial research is also available regarding dyslexics who speak arabic , chinese , hebrew or other languages . as is the case with any disorder , society often makes an assessment based on incomplete information . before the 1980s , dyslexia was thought to be a consequence of education , rather than a basic disability . as a result , society often those with the disorder . there is also sometimes a workplace stigma and negative attitude towards those with dyslexia . if a dyslexic 's instructors lack the necessary training to support a child with the condition , there is often a negative effect on the student 's learning participation . there is no evidence demonstrating that the use of music education is effective in improving dyslexic adolescents ' reading skills .