WikiJournal of Medicine/Dyslexia: Difference between revisions

Article information

Abstract

It is considered a cognitive disorder, not a problem with intelligence. However, emotional problems often arise secondary to learning difficulties.^[13] Some published definitions propose causes, which usually cover a variety of reading skills and deficits, and difficulties with distinct causes rather than a single condition.^[14] The National Institute of Neurological Disorders and Stroke definition describes dyslexia as "difficulty with phonological processing (the manipulation of sounds), spelling, and/or rapid visual-verbal responding".^[1]

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".^[15] Phonological awareness enables one to identify, discriminate, remember, and manipulate the sound structures of language—phonemes, onsite-rime segments, syllables, and words.^[16][17]

Acquired dyslexia, or alexia, may be caused by brain damage due to traumatic brain injury, stroke or atrophy.^[18][19] Forms of alexia include pure alexia, surface dyslexia, semantic dyslexia, phonological dyslexia, and deep dyslexia.^[20]

Clinical and educational diagnosis uses standardized testing; there is no neuroimaging test that can diagnose dyslexia at present. There are a variety of different classification models for specific learning disabilities. Three that have recently been the focus of research include:

• Concordance-Discordance Method:^[21] The Concordance-Discordance method compares pairs of scores, testing whether the difference is larger than could be attributed to imprecision of the scores. The model tests at least three pairs of scores within a person, looking for a discrepancy between two cognitive scores, a difference between the higher cognitive score (the cognitive “strength”) and the reading achievement score (with the reading score being significantly lower) no difference between the reading score and the lower cognitive score.^[21][22] This model is more conservative than the “simple difference” method, where a reading achievement score one standard deviation lower (15 points on most tests) would be considered indicative of a reading disability. The simple difference method ignores the fact that neither component test is perfectly reliable.^[23]

• Discrepancy/Consistency Method:^[24] This model compares the reading ability to general cognitive ability, and tests whether the difference between scores is large enough to be statistically reliable. Some experts argue for using tests where the cognitive and achievement tests were normed in the same sample, as this gives a more accurate estimate of the degree of correlation between scores, as well as avoiding artifacts such as sampling differences or the Flynn effect when scales from different test batteries are compared.^[24]

• Cross Battery Assessment:^[25][26] The cross-battery model also looks for patterns of strengths and weaknesses in cognitive and achievement scores as a way of detecting learning disability. The “cross-battery” aspect combines subtests from multiple cognitive ability measures to so that there are usually at least four different scales converging to measure an aspect of functioning. For example, most versions of the Wechsler include two subtests for working memory. In a cross-battery approach, a psychologist might add two other working memory subtests from different measures to create an estimate of working memory based on four scores. Unlike the other discrepancy methods, cross-battery assessment also considers whether the reading ability is below average for the youth’s age, defined as a standard score of <85.^[25]

All of these approaches conceptually are testing whether the reading performance is below what would be expected based on the person’s general cognitive functioning. Limitations of these approaches include (a) poor agreement about which cases have a learning disability,^[22][23] (b) low reliability both in simulation studies^[27] and when testing is repeated with real youths,^[28][27] and (c) inconsistent evidence of validity of the definitions when predicting grades, graduation, or other criteria of interest.^[29][23]

In early childhood, symptoms that correlate with a later diagnosis of dyslexia include delayed onset of speech and a lack of phonological awareness, as well as being easily distracted by background noise.^[9] A common myth closely associates dyslexia with mirror writing and reading letters or words backwards.^[30] These behaviors are seen in many children as they learn to read and write, and are not considered to be defining characteristics of dyslexia.^[9]

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.^[31] They may also show difficulty in segmenting words into individual sounds or may blend sounds when producing words, indicating reduced phonemic awareness.^[32] Difficulties with word retrieval or naming things is also associated with dyslexia.^[33]:647 People with dyslexia are commonly poor spellers, a feature sometimes called dysorthographia or dysgraphia, which depends on orthographic coding.^[9]

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.^[34]

Dyslexia is often accompanied by several learning disabilities, but it is unclear whether they share underlying neurological causes.^[35] 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.^[36] 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.^[36]
• Attention deficit hyperactivity disorder (ADHD) – A disorder characterized by problems sustaining attention, hyperactivity, or acting impulsively.^[37] Dyslexia and ADHD commonly occur together.^[11][38][39] Roughly 15%^[9] or 12–24% of people with dyslexia have ADHD;^[40] and up to 35% of people with ADHD have dyslexia.^[9]
• Auditory processing disorder – A listening disorder that affects the ability to process auditory information.^[41][42] 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 suggests that auditory processing skills could be the primary shortfall in dyslexia.^[43][44]
• Developmental coordination disorder – A neurological condition characterized by 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.^[45]

Researchers have been trying to find the neurobiological basis of dyslexia since the condition was first identified in 1881.^[46][47] 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.^[48]

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.^[49] 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.^[43] 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,^[50] 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.^[51][52]

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.^[53] The fact that some dyslexic children have 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.^[54]

Research into potential genetic causes of dyslexia has its roots in post-autopsy examination of the brains of people with dyslexia.^[48] 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.^[55] The previously cited studies and others^[56] suggest that abnormal cortical development, presumed to occur before or during the sixth month of fetal brain development, may have caused the abnormalities. Abnormal cell formations in dyslexics have also been reported in non-language cerebral and subcortical brain structures.^[57] Several genes have been associated with dyslexia, including DCDC2 and KIAA0319 on chromosome 6,^[58] and DYX1C1 on chromosome 15.^[59]

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. Both environmental and genetic factors appear to contribute to reading development. Studies examining the influence of environmental factors such as parental education^[60] and teaching quality^[61] have determined that genetics have greater influence in supportive, rather than less optimal, environments.^[62] 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.^[62]

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 have limitations in the extrapolation of results for application to the human brain.^[63][64]

The orthographic complexity of a language directly affects how difficult it is to learn to read it.^[65]:266 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.^[66]:421 Languages such as Spanish, Italian and Finnish have mostly alphabetic orthographies, which primarily employ letter-sound correspondence – so-called "shallow" orthographies – which makes them easier to learn for people with dyslexia.^[65]:266 Logographic writing systems, such as Chinese characters, have extensive symbol use; and these also pose problems for dyslexic learners.^[67]

Most people who are right-hand dominant have the left hemisphere of their brain specialize more in language processing. In terms of the mechanism of dyslexia, fMRI studies suggest that this specialization may be less pronounced or even absent in cases with dyslexia. Additionally, anatomical differences in the corpus callosum, the bundle of nerve fibers that connects the left and right hemispheres, have been linked to dyslexia via different studies.^[68] Data via diffusion tensor MRI indicate changes in connectivity or in gray matter density in areas related to reading/language. Finally, the left inferior frontal gyrus has shown differences in phonological processing in dyslexics.^[68]

Neurophysiological and imaging procedures are being used to ascertain phenotypic characteristics in dyslexics, thus identifying the effects of certain genes.^[69]

The dual-route theory of reading aloud was first described in the early 1970s.^[70] This theory suggests that two separate mental mechanisms, or cognitive routes, are involved in reading aloud.^[71] 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.^[72] The other mechanism is the nonlexical or sublexical route, which is the process whereby the reader can "sound out" a written word.^[72][73] 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.^[70] 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).^[65][74]

There are a wide range of tests that are used in clinical and educational settings to evaluate the possibility that a person might have dyslexia.^[75] If initial testing suggests that a person might have dyslexia, such tests are often followed up with a full diagnostic assessment to determine the extent and nature of the disorder.^[76] Some tests can be administered by a teacher or computer; others require specialized training and are given by psychologists.^[77] Some test results indicate how to carry out teaching strategies.^[77][78] Because a variety of different cognitive, behavioral, emotional, and environmental factors all could contribute to difficultly learning to read, a comprehensive evaluation should consider these different possibilities. These tests and observations can include:^[79]

• 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 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.^[80]
• Screening or evaluation for mental health conditions: Parents and teachers can complete rating scales or behavior checklists to gather information about emotional and behavioral functioning for the youth. Many checklists have similar versions for parents, teachers, and youths old enough to read reasonably well (often 11 years and older) to complete. Examples include the Behavioral Assessment System for Children, and the Strengths and Difficulties Questionnaire. All of these have nationally representative norms, making it possible to compare the level of symptoms to what would be typical for the youth'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 have 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 dyslexics, and attention-deficit/hyperactivity disorder is more common, as well.^{[81][82][83][84]}
• Review of academic achievement and skills: Average spelling/reading ability for a dyslexic is a percentage ranking <16, well below normal. In additional to reviewing grades and teacher notes, standardized test results are helpful in evaluating progress. These include group administered tests, such as the Iowa Tests of Educational Development, that a teacher may give to a group or whole classroom of youths at the same time. They also could include individually administered tests of achievement, such as the Wide Range Achievement Test, or the Woodcock-Johnson (which also includes a set of achievement tests). The individually administered tests again require more specialized training.^[85][86][87]

The condition of dyslexia is defined simply as an inability to read in the context of normal intelligence. Sometimes it is helpful to distinguish between developmental dyslexia (a learning disorder) and acquired dyslexia (loss of the ability to read caused by brain damage). The International Classification of Diseases (ICD 10), the World Health Organization's manual of medical diagnosis used in most of the world, includes separate diagnoses for "developmental dyslexia" (81.0)^[88] and for "dyslexia and alexia" (48.0).^[89] The new update to the ICD, version 11, retains these. The Diagnostic and Statistical Manual of the American Psychiatric Association, DSM 5, the manual of psychiatric diagnosis more widely used in the United States, does not specifically define dyslexia. The DSM-5 authors justified this decision by stating that "the many definitions of dyslexia and dyscalculia meant those terms would not be useful as disorder names or in the diagnostic criteria". Instead DSM-5 subsume dyslexia in a category called specific learning disorders.^[90]

A high-level distinction can be made between developmental dyslexias (also referred to as congenital dyslexias or specific developmental dyslexias) versus acquired dyslexia (also referred to as traumatic dyslexia).^[91] Although it is likely that subtypes of dyslexia exist, their implications for treatment planning are not yet well understood, and interventions that claim to work for specific subtypes have not accrued clear support yet.^[92]

Central dyslexias include the following: surface dyslexia, semantic dyslexia, phonological dyslexia, and deep dyslexia.^[18][93]

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.^[94] Difficulty distinguishing homophones is a diagnostic feature used to help identify some forms of surface dyslexia. Surface dyslexia is usually accompanied by surface agraphia and fluent aphasia.^[95] Acquired surface dyslexia arises when a previously literate person experiences brain damage, which results in pronunciation errors that indicate impairment of the lexical route.^[18][96][97]

In phonological dyslexia, sufferers can read familiar words but have difficulty with unfamiliar words, such as invented pseudo-words.^[99] 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. Dyslexics often compensate by overusing a front-brain region called Broca's area, which is associated with aspects of language and speech.^[100] The Lindamood Phoneme Sequencing Program (LiPS) is designed to treat phonological dyslexia.^[101] It uses 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 LiPS.^[102]

Human syntax--the arrangement of words and phrases--is established in the posterior region of Broca's area, via white matter links to the human temporal cortex. This neurological circuit is important in human language, particularly as it arranges words and phrases.^[103]

Wernicke's area is located in the posterior left sylvian fissure, anatomically speaking. Functional neuroimaging methods indicate that Wernicke's area is important in phonologic retrieval in normal-functioning individuals.^[104] During rhyming tasks, Wernicke's area was not activated while Broca's was activated among cases with dyslexia.^[105]

Individuals with deep dyslexia experience both semantic paralexia (para-dyslexia) and phonological dyslexia, causing the person to read a word and then say a related meaning instead of the denoted meaning.^[106] Deep dyslexia is associated with clear phonological processing impairments.^[18] Deep dyslexia can be caused by widespread damage to the brain that often includes the left hemisphere.^[107] The "continuum" hypothesis claims that deep dyslexia develops from phonological dyslexia.^[108]

Peripheral dyslexias have been described as affecting the visual analysis of letters as a result of brain injury.^[109] Hemianopsia, a visual field loss on the left/right side of the vertical midline, is associated with this condition.^[110][111]

Pure, or phonologically-based,^[112] dyslexia, also known as agnosic dyslexia, dyslexia without agraphia, 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.^[113] 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 splenium. It is usually accompanied by a homonymous hemianopsia in the right side of the visual field.^[109] Multiple oral re-reading (MOR) is a treatment for pure dyslexia.^[114] 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.^[115]

Hemianopic dyslexia is commonly considered to derive from visual field loss due to damage to the primary visual cortex.^[116] 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.^[117]

In neglect dyslexia, some letters, most commonly those at the beginning or left side of a word, are skipped or misread during reading.^[118] This alexia is associated with right parietal lesions. The use of prism glasses has been shown to mitigate this condition substantially.^[119]

Through the use of compensation strategies, therapy and educational support, dyslexic individuals can learn to read and write.^[120] There are techniques and technical aids that help to manage or conceal symptoms of the disorder.^[121] Reducing stress and anxiety can sometimes improve written comprehension.^[122] 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. Reinforced collateral training focused on reading and spelling may yield longer-lasting gains than oral phonological training alone.^[123] Early intervention can be successful in reducing reading failure.^[124]

There is some evidence that the use of specially-tailored fonts may help with dyslexia.^[125] These fonts, which include Dyslexie, OpenDyslexic, and Lexie Readable, were created based on the idea that many of the letters of the Latin alphabet are visually similar and may, therefore, confuse people with dyslexia. Dyslexie and OpenDyslexic both put emphasis on making each letter more distinctive in order to be more easily identified.^[126] The benefits, however, might largely be due to the added spacing between words.^[127]

There have been many studies conducted regarding intervention in dyslexia. A meta-analysis found that there was functional activation as a result.^[128] There is no evidence showing that music education significantly improves dyslexic adolescents' reading skills.^[129]

Dyslexic children require special instruction for word analysis and spelling from an early age.^[130] While there are fonts that may help people with dyslexia better understand writing, this might simply be due to the added spacing between words.^[125][127] The prognosis, generally speaking, is positive for individuals who are identified in childhood and receive support from friends and family.^[1] The New York educational system (NYED) indicates "a daily uninterrupted 90 minute block of instruction in reading", furthermore "instruction in phonemic awareness, phonics, vocabulary development, reading fluency" so as to improve the individuals reading ability^[131].

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.^[132] While it is diagnosed more often in males,^[6] 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.^[133] Dyslexia is not limited to difficulty in converting letters to sounds, and Chinese dyslexics may have difficulty converting Chinese characters into their meanings.^[134][135] The Chinese vocabulary uses logographic, monographic, non-alphabet writing where one character can represent an individual phoneme.^[136]

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.^[137]

Dyslexia was clinically described by Oswald Berkhan in 1881,^[46] but the term dyslexia was coined in 1887 by Rudolf Berlin, an ophthalmologist in Stuttgart.^[138] He used the term to refer to the case of a young boy who had severe difficulty learning to read and write, despite showing typical intelligence and physical abilities in all other respects.^[139] 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".^[140] The distinction between phonological versus 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.^[141]

Most currently available dyslexia research relates to alphabetic writing systems, and especially to European languages.^[142] However, substantial research is also available regarding dyslexics who speak Arabic, Chinese, Hebrew, or other languages.^[143] The outward expression of individuals with reading disability and regular poor readers is the same in some respects^[144].

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 neurological disability. As a result, society often misjudges those with the disorder.^[122] There is also sometimes a workplace stigma and negative attitude towards those with dyslexia.^[145] 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.^[146]