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Classification and external resources
Dyslexic words.jpg
Ten variations of the word "Teapot" as written by dyslexics[citation needed]
ICD-10 R48.0
ICD-9 315.02
OMIM 127700
DiseasesDB 4016
MedlinePlus 001406
Patient UK Dyslexia
MeSH D004410








Signs and symptoms[edit]

The news journal The Economist reported in September 2014 that the world-wide incidence of dyslexia ranges between 5% to 15% of the general population depending on the precise definition of dyslexia being employed.[27] In early childhood, early symptoms that correlate with a later diagnosis of dyslexia include delays in speech,[28] letter reversal or mirror writing, difficulty knowing left from right and directions,[29][30] and being easily distracted by background noise.[31] 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.[32][33]

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),[34] a difficulty segmenting words into individual sounds, or blending sounds to make words (phonemic awareness[35]),[36] a difficulty with word retrieval or naming problems (see anomic aphasia),[37][38][39] commonly very poor spelling,[40] which has been called dysorthographia or dysgraphia (orthographic coding), whole-word guesses, and 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 summarizing a story, memorizing, reading aloud, and learning a foreign language.[41] 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.[42][43]

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.[44] 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.[citation needed]


The complexity of a language's orthography (i.e., its conventional spelling system, see orthographic depth) has a direct impact upon how difficult it is to learn to read that language. English has a comparatively deep orthography within the Latin alphabet writing system, with a complex orthographic structure that employs spelling patterns at several levels: principally, letter-sound correspondences, syllables, and morphemes. Other languages, such as Spanish, have mostly alphabetic orthographies that employ letter-sound correspondences, so-called shallow orthographies. It is relatively easy to learn to read languages like Spanish; it is much more difficult to learn to read languages with more complex orthographies such as English.[45] Logographic writing systems, notably Japanese and Chinese characters, have graphemes that are not linked directly to their pronunciation, which pose a different orthographically-specific type[46] of dyslexic difficulty.[15][47][48][49] Whereas English uses an alphabet of 26 letters, the Chinese orthography uses a system of almost three thousand orthographic symbols for reading and writing.[50]


Associated conditions[edit]

Several learning disabilities often occur with dyslexia, but it is unclear whether these learning disabilities share underlying neurological causes with dyslexia.[51] These disabilities include:

  • Attention deficit disorder[28][52][53] – A significant degree of co-morbidity has been reported between ADD/ADHD and dyslexia/reading disorders,[54] it occurs in between 12% and 24% of those with dyslexia.[55] 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.[56]
  • Auditory processing disorder – A condition that affects the ability to process auditory information. Auditory processing disorder is a listening disability.[57] It can lead to problems with auditory memory and auditory sequencing. Many people with dyslexia have auditory processing problems[58] and may develop their own logographic cues to compensate for this type of deficit. Auditory processing disorder is recognized as one of the major causes of dyslexia.[59][60][61]

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,[62][63] and may be observed as stuttering, cluttering or hesitant speech.[21]


Main article: Theories of dyslexia

Researchers have been trying to find a biological basis of dyslexia since it was first identified by Oswald Berkhan in 1881[64] and the term dyslexia coined in 1887 by Rudolf Berlin.[65][66] The theories of the etiology of dyslexia have been and are evolving.



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,[68] with some implication that the observed neural manifestation of developmental dyslexia is task-specific (i.e., functional rather than structural).[69] fMRI's in dyslexics have provided important data supporting the interactive role of the cerebellum and cerebral cortex as well as other brain structures.[70][71][72]


Genetic research into dyslexia and its inheritance has its roots in the examination of post-autopsy brains of people with dyslexia.[73][74] 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. These studies and those of Cohen et al. 1989[75] suggested abnormal cortical development which was presumed to occur before or during the sixth month of fetal brain development.[76]

Abnormal cell formations in dyslexics found on autopsy have also been reported in non-language cerebral and subcortical brain structures.[74][77] MRI data have conﬁrmed a cerebellar role in dyslexia.[78]

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,[79] and teacher quality[80] have determined that genetics have greater influence in supportive, rather than less optimal environments.[81] 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.[82]


Central dyslexias[edit]

Central dyslexias include surface dyslexia, semantic dyslexia, phonological dyslexia, and deep dyslexia.[84][85][86][87] 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. The ICD-10 classification is presently titled as: R48.0 Dyslexia and other symbolic dysfunctions.

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.[88] Difficulty distinguishing homophones is diagnostic of some forms of surface dyslexia.[88] This disorder is usually accompanied by (surface) agraphia and fluent aphasia.[88]

Surface dyslexia is associated with lesions in the temporo-parietal region of the left hemisphere, especially the superior temporal gyrus.[citation needed] Some research shows that the most dramatic cases of surface dyslexia are caused by lesions in the inferolateral left temporal regions, which are considered to involve semantic processing.[citation needed]

Phonological dyslexia[edit]

Main article: Phonological dyslexia

In phonological dyslexia, patients can read familiar words but have difficulty reading unfamiliar words (such as invented pseudo-words). 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. Research has pointed towards the theory that phonological dyslexia is a development of deep dyslexia.[citation needed]

A treatment for phonological dyslexia is the Lindamood Phoneme Sequencing Program (LiPS). 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.[89][90]

Deep dyslexia[edit]

See also: Deep dyslexia

Patients with deep dyslexia experience semantic paralexia (para-dyslexia), which happens when the patient reads a word, and says a related meaning instead of the denoted meaning.[88] Deep alexia is more recently seen as a severe version of phonological dyslexia.

Deep dyslexia is caused by lesions that are often widespread and include much of the left frontal lobe.[88] Research suggests that damage to the left perisylvian region of the frontal lobe causes deep dyslexia, as both the phonological and lexical routes of language are impaired.[citation needed]

Peripheral dyslexias[edit]

Peripheral dyslexias have been described by Bub as "impairment to processes that convert letters on the page into an abstract representation of visual word forms".[84] These include hemianopic dyslexia, neglect dyslexia, attentional dyslexia, and pure dyslexia (also known as dyslexia without agraphia).[84][85][87]

Pure dyslexia[edit]

Main article: Pure alexia

Pure dyslexia, 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.[85] It is 'pure' because it is not accompanied by other (significant) language-related impairments. Pure dyslexia does not include speech, hand writing style, language, or comprehension impairments.[91]

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.[88]

Multiple oral re-reading (MOR) is a treatment for pure dyslexia. 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 and/or accuracy improves a predetermined amount. The idea behind MOR is to learn how to use context, syntax, and semantics of the text to process written information rather than using bottom-up processing techniques in which letter by letter (LBL) reading is necessary. The theory that the MOR technique only uses top-down processing has been questioned and some studies have shown that in fact, bottom-up processing is in part responsible for reading improvement. This has been proven by reading tests that are engineered to use as few of the same words as possible that are used in training texts during MOR treatment. In these studies, patients did not significantly improve in reading speed or accuracy when reading untrained passages. Untrained passages are defined by having differing vocabulary from the texts used in reading practice. This supports the findings that MOR also has bottom-up processing components.[91] In addition to tactile and kinesthetic reading techniques, other therapies that are aimed to improve letter-level reading include: timed semantic and lexical association tasks and limited-time single word identification.[91]

Hemianopic dyslexia[edit]

Commonly considered to derive from visual field loss due to damage to the primary visual cortex. 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 the (possibility of) effective treatment.[citation needed]

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. This alexia is associated with right parietal lesions.

Use of prism glasses in treatment has been demonstrated to produce substantial benefit.[92]

Attentional dyslexia[edit]

Patients with attentional dyslexia complain of letter crowding or migration, sometimes blending elements of two words into one. The lesion usually affects the left parietal lobe (Warrington et al., 1993). Patients 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 should reduce the effects of flanking interference from nearby words; however, no trials of this or indeed any other therapy for left parietal syndromes have been published.

Dual-route cascaded model of reading[edit]

The dual-route theory of reading aloud was first described in the early 1970s.[93] 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.[93][24][94] 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.[93][95] The other mechanism is the nonlexical or sublexical 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.[93][95][96]


Through compensation strategies and therapy, dyslexic individuals can learn to read and write with educational support.[97] There are techniques and technical aids that can manage or even conceal symptoms of the disorder.[98] Removing stress and anxiety alone can sometimes improve written comprehension.[99]


There is some evidence that the use of specially tailored fonts may provide some measure of assistance for people who have dyslexia.[102][103] Among these fonts are Dyslexie and OpenDyslexic, which were created with the notion that many of the letters in the Latin alphabet are visually similar and therefore confusing for people with dyslexia.[104] Dyslexie, along with OpenDyslexic, put emphasis on making each letter more unique to assist in reading.[104]


The prevalence of dyslexia is unknown; with estimations varying greatly between 1% to 33% of the population.[105] It is often estimated that the prevalence of dyslexia is around 5–10 percent of a given population although there have been no studies to indicate an accurate percentage.[10][55][106] Internationally, there are differing definitions of dyslexia, but despite the significant differences between the writing systems, Italian, German and English speaking populations suffer similarly from dyslexia.[107] Dyslexia is not limited to difficulty in converting letters into sounds, but Chinese dyslexics have difficulty in extracting shapes of Chinese characters into meanings.[108]


Dyslexia was identified by Oswald Berkhan in 1881,[109] but the term dyslexia was coined in 1887 by Rudolf Berlin, who was an ophthalmologist in Stuttgart.[65] He used the term to refer to a case of a young boy who had a severe impairment in learning to read and write in spite of showing typical intellectual and physical abilities in all other respects.


Castles and Coltheart describe phonological and surface types of developmental dyslexia (dysphonetic and dyseidetic, respectively) to classical subtypes of alexia which are classified according to the rate of errors in reading non-words.[21][76][76][111][112] The surface/phonological distinction is only descriptive, and devoid of any etiological assumption as to the underlying brain mechanisms.[113] Studies have, however, alluded to potential differential underlying brain mechanisms in these populations given performance differences.[114][115][116] The dysphonetic/dyseidetic distinction refers to two different mechanisms; one that relates to a speech discrimination deficit, and another that relates to a visual perception impairment.


Main article: Dyslexia research

The majority of currently available dyslexia research relates to the alphabetic writing system, and especially to languages of European origin. However, substantial research is also available regarding dyslexia for speakers of Arabic, Chinese, and Hebrew.[113][117][118][119][120][121]

See also[edit]


  17. ^ Human Voice Recognition Science Magazine July 2011
  18. ^ "All About Dyslexia". 
  20. ^ Michael Rice Dr Michael Rice (University of Cambridge, formerly Senior Research Officer, NRDC Institute of Education) with Greg Brooks Research Director, NRDC Sheffield, and Professor of Education, University of Sheffield (1 May 2004). "Developmental dyslexia in adults: a research review". National Research and Development Centre for Adult Literacy and Numeracy. pp. *133–147. Retrieved 13 May 2009. 
  27. ^ http://www.economist.com/blogs/analects/2014/09/dyslexia-chinese
  35. ^ "Why Is Phonemic Awareness An Effective Remedial Strategy For Dyslexia?". http://taarezameenke.blogspot.in/. Retrieved 7 December 2014. 
  84. ^ Cite error: The named reference Bub_2003 was invoked but never defined (see the help page).
  85. ^ Cite error: The named reference Cherney_2004 was invoked but never defined (see the help page).
  88. ^ a b c d e f Friedman, Rhonda B.; Hadley, Jeffrey A. "Letter-by-letter surface alexia". Cognitive Neuropsychology 9 (3 year=1992): 185–208. doi:10.1080/02643299208252058. 
  89. ^ Kendall, Diane; Conway, Tim; Rosenbek, John; Gonzalez‐Rothi, Leslie (2003). "Case study Phonological rehabilitation of acquired phonologic alexia". Aphasiology 17 (11): 1073–1095. doi:10.1080/02687030344000355. 
  90. ^ Beeson, PM.; Rising, K.; Kim, ES.; Rapcsak, SZ. (Apr 2010). "A treatment sequence for phonological alexia/agraphia.". J Speech Lang Hear Res 53 (2): 450–68. doi:10.1044/1092-4388(2009/08-0229). PMC 3522177. PMID 20360466. 
  91. ^ a b c Starrfelt, R.; Olafsdóttir, RR.; Arendt, IM. (2013). "Rehabilitation of pure dyslexia: a review.". Neuropsychol Rehabil 23 (5): 755–79. doi:10.1080/09602011.2013.809661. PMC 3805423. PMID 23808895. 
  92. ^ Angeli, V; Benassi, MG; Ladavas, E (2004). "Recovery of oculo-motor bias in neglect patients after prism adaptation". Neuropsychologia 42 (9): 1223–34. doi:10.1016/j.neuropsychologia.2004.01.007. 
  94. ^ Yamada J, Imai H, Ikebe Y (July 1990). "The use of the orthographic lexicon in reading kana words". J Gen Psychol 117 (3): 311–23. PMID 2213002. 
  95. ^ a b Zorzi, Marco; Houghton, George; Butterworth, Brian (1998). "Two routes or one in reading aloud? A connectionist dual-process model.". Journal of Experimental Psychology: Human Perception and Performance 24 (4): 1131–1161. doi:10.1037/0096-1523.24.4.1131. ISSN 1939-1277. 
  96. ^ Coltheart, Max (2005). Margaret J Snowling; Charles Hulme, ed. Word recognition processes in reading. Modeling reading : the dual-route approach. The science of reading : a handbook (Malden, MA: Blackwell Pub.,). pp. 6–23. ISBN 9781405114882. OCLC 57579252. 

External links[edit]

Dyslexia Associations
Dyslexia Organizations