Dyslexia facts for kids

Kids Encyclopedia Facts

Dyslexia is a very common problem and has an effect on the way our brain understands words. The most common signs of dyslexia are reading and writing problems. Estimates are that in the United States between five and nine percent of schoolchildren have dyslexia, though estimates go as high as fifty percent.

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 phonological processing (the manipulation of sounds), spelling, and/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".

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 dyscalculia 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. A common myth closely associates dyslexia with mirror writing and reading letters or words backwards. These behaviors are seen in many children as they learn to read and write, and are not considered to be defining characteristics of dyslexia.

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 dysorthographia or dysgraphia, 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.

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. Logographic 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 disabilities include:

  • Dysgraphia – 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, 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.
  • 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

Inferior parietal lobule - superior view animation
Inferior parietal lobule (superior view). Some dyslexics demonstrate less electrical activation in this area.

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.

Neuroanatomy

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). fMRIs 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 automatization 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 ectopias, more rarely, vascular micro-malformations, and microgyrus. 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 DCDC2 and KIAA0319 on chromosome 6, and DYX1C1 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 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. 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. Neurophysiological and imaging procedures are being used to ascertain phenotypic characteristics in dyslexics, thus identifying the effects of certain genes.

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 is done for children at a young age can be successful in reducing reading failure.

There is some evidence that the use of specially-tailored fonts may help with dyslexia. These fonts, which include Dyslexie, OpenDyslexic, and Lexia 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. The benefits, however, might simply be due to the added spacing between words.

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.

There is no evidence demonstrating that the use of music education is effective in improving dyslexic adolescents' reading skills.


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