• Dyslexia is a primary reading disorder. It is characterized by difficulty in understanding and using alphabetic and logographic principles to acquire accurate and fluent reading skills
– Results from a written word processing abnormality in the brain
– Not explained by sensory deficits, cognitive deficits, lack of motivation or lack of adequate reading instruction
– Historically termed “word blindness,” but most commonly due to deficit in auditory processing of language (phonological processing) and not visual processing
– Word reversals and skipping words result from linguistic deficiencies rather than visual or perception disorders
• In a population, reading ability and reading disability occur along a continuum, with reading disability representing the lower tail of the distribution
• Depending on definition, 5–17% of children affected; boys more than girls
– Similar rates across languages
• Approximately 80% of people with learning disabilities have dyslexia
• Strongly heritable (54–75%)
– 68% concordance in identical twins and 40% of individuals with affected parent or sibling
– Heritability greater among children whose parents have higher education level
• Polygenic inheritance
– At least 9 associated loci
Early identification and intervention key to improving outcomes
• Disorder within the language system, specifically phonological processing:
– Disruption of left hemisphere posterior brain systems while performing reading tasks
– Increased reliance on ancillary brain regions including frontal lobes and right hemisphere posterior circuits
• Vision problems can interfere with the process of learning.
• However, vision problems are not the cause of primary dyslexia or learning disabilities.
COMMONLY ASSOCIATED CONDITIONS
• Some children may have visual problems that contribute to primary reading or learning dysfunction
– Treatable ocular conditions include:
Convergence and/or focusing deficiencies
• In young children, language delay or not attending to the sounds of words:
– Trouble learning letters of alphabet or nursery rhymes
– Confusing words that sound alike or mispronouncing words
• In older children, slow and laborious reading and writing
• Discrepancy between general intelligence (IQ) and standardized reading test scores
• If a child has suspected learning disabilities, child should be evaluated for medical problems which could affect the child’s ability to learn
• Children with dyslexia should have hearing and visual screenings, according to national standards for all children
– Visual screening with non-letter symbols may be necessary for testing children with dyslexia
• Children with dyslexia have the same visual function and ocular health as other children (1)
Dyslexia is not caused by subtle eye or visual problems including (2)[A]:
Visual perceptual disorders
Jerky eye movements,
– Misaligned or crossed eyes
– Readers with dyslexia may have saccadic eye movements and fixations similar to beginning readers but show normal saccadic eye movements when content is corrected for ability
Saccadic patterns are result of reading disability but not the cause
DIAGNOSTIC TESTS & INTERPRETATION
Currently, there is no indication for imaging in the standard evaluation of dyslexia or other learning disorders.
• Dyslexia as well as other learning disorders should not be diagnosed by physicians but by educators, psychologists, or neuropsychologists with specialized, comprehensive assessments.
• Standardized tests of reading including:
– Comprehensive Test of Phonological Processing in Reading
– Woodcock Johnson Tests for word decoding
– Gray Oral Reading Test for fluency
• In the future, the combination of standardized neurocognitive tests, neuroimaging, genetic and familial information may improve diagnosis and allow for earlier intervention.
• Functional neuroimaging studies have revealed differences in brain function and connectivity that are characteristic of dyslexia: (3)
– Reduced or absent activation of left temporoparietal cortex
– Atypical activation in other regions including:
Left prefrontal regions associated with verbal working memory
Left middle and superior temporal gyri associated with receptive language
Left occipito-temporal regions associated with visual analysis of words and letters.
• Experimental functional neuroimaging studies have demonstrated brain plasticity associated with effective intervention for dyslexia
– Neuroimaging studies have not revealed any differences in the brains of children who do and do not respond to treatment
• Other important reasons for reading failure on the population level include:
– Reduced vocabulary and strategies needed for text comprehension
– Reduced motivation to read
– Both these reasons are often tied to socioeconomic factors at home and at school
• Attention deficit hyperactivity disorder (ADHD)
• Auditory processing disorder
• Less common reasons include visual problems such as strabismus, amblyopia, and refractive errors.
• Other conditions such as convergence insufficiency and poor accommodation (both of which are rare in children under age 10 years) can interfere with the physical act of reading but not decoding and comprehension.
• Multidisciplinary evaluation and management
• Explicit and systematic intensive instruction in small groups in phonological awareness and decoding strategies
– Improvements more likely in younger children (ages 6–8 years) than in older children
• Providing accommodations for older children including extra time for reading, spell check computer programs, tape recorders
• Vision training is not a primary or adjunctive therapy for dyslexia (4)[B]
• Scientific evidence does not support the following alternative therapies for improving the long-term educational performance efficacy:
– Eye exercises
– Behavioral vision therapy
– Special colored or tinted filters or lenses
Issues for Referral
• Patients with suspected learning disabilities should be referred for further educational, psychological, or other appropriate evaluation.
• If vision problem is suspected, children should be referred to an ophthalmologist with experience in assessment and treatment of children.
COMPLEMENTARY & ALTERNATIVE THERAPIES
Scientific evidence does not support alternative therapies aimed at visual training.
Multidisciplinary management by educators
• The International Dyslexia Association (www.interdys.org)
• Persistent, chronic condition, not a transient developmental lag
• A student who fails to read adequately in the 1st grade has a 90% probability of reading poorly in 4th grade and a 75% probability of reading poorly in high school
• For students with dyslexia, early intervention (before the 3rd grade) is key to improving reading ability (5)
1. Helveston EM, Weber JC, Miller K, et al. Visual function and academic performance. Am J Ophthalmol 1985;99(3):346–355.
2. Polatajko HJ. Visual-ocular control of normal and learning disabled children. Dev Med Child Neurol 1987;29(4):477–485.
3. Shaywitz SE, Shaywitz BA, Pugh KR, et al. Functional disruption in the organization of the brain for reading in dyslexia. Proc Natl Acad Sci U S A 1998;95(5):2636–2641.
4. Barrett B. A critical evaluation of the evidence supporting the practice of behavioural vision therapy. Ophthalmic Physiol Opt 2009;29(1):4–25.
5. Schatschneider C, Torgesen JK. Using our current understanding of dyslexia to support early identification and intervention. J Child Neurol 2004;19(10):759–765.