Kussmal first described a case of acquired word-blindness in an adult alexic patient with a parietal lobe lesion in 1877.^2,3 Hinshelwood, an ophthalmologist from Scotland, studied and described an adult with word-blindness in 1895.^2,4 In 1903, an autopsy of this patient showed abnormalities in the left angular gyrus immediately posterior to the Wenicke area.⁴ Morgan, a general practitioner from England, published the first case of a child with congenital word-blindness in 1896.^2,5 Subsequently, Hinshelwood turned his attention to both congenital and acquired word-blindness. He credited the term dyslexia to Berlin.⁶ In 1917, he highlighted the potentially inherited aspect of reading disability. Hinshelwood postulated that 1 in 1,000 students in elementary schools might have “word-blindness” and that the primary disability was in visual memory for words and letters. He strongly advocated intensive, individualized personal instruction.^2,4

Beginning in the 1920s, Samuel Orton, a neuropsychiatrist, studied children with reading disabilities and also demonstrated a hereditary component. Orton’s definition of reading disabilities was much broader than Hinshelwood’s and included a graded series of all degrees of severity of disability. This more liberal definition increased the presumed prevalence to more than10% of school children. IQ testing revealed that these children scored near average or above. In 1925 Orton attributed dyslexia to a problem in the visual system, suggesting that an apparent dysfunction from “mixed cerebral dominance” caused problems in visual perception and visual memory, characterized by perception of letter and words in reverse.^2,7,8

The theory that visual dysfunction caused dyslexia led to a proliferation of training programs developed for visual perceptual and/or visual-motor disabilities. In the 1960s, those prominent in developing and promoting these programs included Kephart, Frostig, Getman, Barsch, Dorman, and Delacato. Research into the programs found that, although these programs were sometimes effective in improving perceptual and/or perceptual-motor development, they were ineffective in improving academic performance.^9,10,11,12 Although the use of perceptual and perceptual-motor training by educators persisted for a time, by the mid-1980s its use had waned considerably.

We now know that defects in the visual system do not cause dyslexia. Improved understanding began with a series of related studies that systematically evaluated traditional and widely accepted etiologic conceptualizations such as Orton’s optical reversibility theory,⁷ Hermann’s spatial confusion theory,¹³ and other theories that implicated deficits in visual processes such as visualization, visual sequencing, and visual memory as basic causes of reading difficulties.^14,15

Although Orton attributed dyslexia to visual dysfunction, he was the first to advocate intensive phonics instruction, sound blending, and multisensory training.^2,8 Orton’s work served as the stimulus for Gillingham and Stillman¹⁶ who also emphasized multisensory training. Subsequently, the Orton-Gillingham phonics techniques have served as the basis for many remediation programs. The International Dyslexia Society, formerly the Orton Dyslexia Society, provides information and resources to professionals and parents regarding reading disabilities.

Research has confirmed that dyslexia is a specific learning disability that is neurobiologic in origin. It is a primary reading disorder that is separate from secondary forms of reading difficulties caused by visual or hearing disorders, mental retardation, and experiential or instructional deficits.^14,17,18,19 Dyslexia is characterized by difficulties with decoding, accurate and/or fluent word recognition, and/or reading comprehension skills. These difficulties typically result from a deficit in the phonologic component of language that is often unexpected in relation to the child’s other cognitive abilities despite effective classroom instruction. Secondary consequences may include problems in reading comprehension and reduced reading experience that can impede growth of vocabulary, written expression, and background knowledge.²⁰ Reversing letters or words, and mirror writing, occur in early readers and writers. Children with dyslexia are not unusually prone to reversals. So, though they occur, reversing letters or words, or mirror writing, is not included in the definition of dyslexia.^14,21,22

The terms specific reading disability, reading disability, reading disorder, and dyslexia are often used interchangeably in the literature.¹⁴ Approximately 80% of individuals with learning disabilities have dyslexia, making it the most common learning disability.^{17,18,23,24,25,26,27,28} Depending on the definition chosen, the prevalence of reading disability is approximately 5% to 20% of school-aged children in the United States.^17,24,27,29 Reading ability and reading disability occur along a continuum, with reading disability representing the lower tail of a normal bell-shaped distribution of reading ability.²⁹ Dyslexia can vary from mild to severe, though most children with reading disabilities have relatively mild reading disabilities and a smaller number have extreme reading disabilities.^23,29 Because reading skills occur on a continuum with no clear distinction between typical readers and dyslexic readers, some experts assert that the term dyslexia should be reserved for the 2% to 5% with the most severe reading deficits.³⁰ Dyslexia occurs at all levels of intelligence and is a persistent problem that does not represent a transient developmental lag.^{14,17,18,19,20,21,22,23,24,27,28,31,32,33,34,35,36,37} Dyslexia is identified in some people early in their lives, but in others is not diagnosed until much later, when more complex reading and writing skills are required. Over time, poor readers and good readers tend to maintain their relative positions along the spectrum of reading ability. Many people with dyslexia learn to read accurately, but they continue to read slowly and not automatically throughout their lives.³⁸ Readers with dyslexia must expend more attention, concentration, and energy on the task, which makes reading much more tiring.³⁸

Reading disabilities appear to affect males more than females.^39,40,41 But, schools do not identify girls as readily as boys.²⁷ Both environmental and genetic influences affect the expression of dyslexia.³⁸ Dyslexia has been identified as having a strong genetic basis.^{14,17,18,19,23,24,42,43} Recent genetic linkage studies have identified many loci where dyslexia-related genes are encoded. Approximately 40% of siblings, children, or parents of an affected individual will have dyslexia. Though dyslexia is often inherited, it may exist in the absence of a family history. People with dyslexia may be very creative and bright. Their high-level thinking is unaffected and they may be gifted in math, science, the arts, or even in unexpected areas such as writing.²¹ Untreated, learning difficulties may lead to frustration, low self-confidence, and poor self-esteem and substantially increase the risk of developing psychological and emotional problems.^23,44 Approximately 15% of students with reading disability also have attention deficit disorder/hyperactivity disorder whereas approximately 35% of students with disorders of attention also have reading disability.^17,23,44,45 However, the two disorders are distinct and separable.

Oral language development has been found to play a critical role in learning to read.^{1,28,46,47,48} Oral language acquisition is preprogrammed into human development. Writing is an artificially designed use of abstract symbols to represent language.²⁷ English uses an alphabetic system in which each letter is a symbol that is an abstract building block of that language’s phonemes (sounds). English is a phonemically complex language in which the 26 letters of the alphabet create 44 sounds or phonemes in approximately 70 letter combinations.^25,26,49 The phonemic complexity of a language corresponds to the prevalence of dyslexia, pointing to the linguistic origin of dyslexia.^14,22 Manifestations of dyslexia are often worse in English because of the greater number of inconsistencies and exceptions within the English language, but dyslexia is confined neither to the United States nor to English speakers.¹⁴

Reading and writing require active learning. Reading is more difficult than speaking, because children must be aware of the sound structure in spoken language (phonologic awareness), understand that words are broken into smaller pieces of sound (phonemic awareness), and then break the alphabetic code to acquire the sound/symbol connection. Unless a child can convert the written symbols into the phonetic code, these letters remain lines and circles totally devoid of linguistic meaning.²⁷ Most people with dyslexia have a neurobiologic deficit in the processing of the sound structure of language;alled a phonemic deficit, which impairs decoding, preventing word identification.^{1,14,15,17,18,19,23,24,25,26,27,28,30,31,34,35,36,37,38,49,50,51,52,53,54} The hallmark of dyslexia is the presence of a phonologic deficit despite relatively intact overall language abilities.^{15,17,18,23,24,25,26,27,28}

Children with more severe forms of dyslexia may have a second deficit of slow naming of letters, numbers, words, and pictures, creating a double deficit.^{14,24,55,56,57,58} Other children may have problems with their short-term working memory, attention, or comprehension.⁵⁹ Some children with reading difficulties also experience a deficit in orthographic skills, which are defined as difficulties with letter/number orientation recognition and memory, though this may improve with development.^14,60

Reading comprises many processes including attention, perception, decoding, fluency, and comprehension. A child must first accurately decode a word before it can be read fluently. An inexperienced reader will use the phonetic method to sound out most words and consequently will read slowly. No fluent reader uses phonics routinely. Experienced readers use the whole word method and will quickly recognize most words as individual units. Average readers require 4 to 14 exposures to a word before it becomes a sight word.^25,26 Fluent readers read with speed, accuracy, proper expression, and good comprehension and are free from word identification difficulties. The hallmark of fluent reading is the ability to decode and comprehend at the same time. Fluency then forms the bridge between decoding and comprehension. Comprehension is dependent on attention, memory, knowledge, vocabulary, and cultural influences. If reading is slow and labored because of decoding difficulties, children do not have enough attentional capacity and cognitive energy to remember what they have read, much less relate the ideas to their own background knowledge.^25,26,27,53 Current theory maintains that the deficit in lower-order phonologic linguistic decoding function blocks access to the usually intact higher-order cognitive and linguistic functions and applying them in the reading process.^{15,17,18,23,24,25,26,27,28}

There is strong scientific evidence supporting the neurobiologic basis for the phonologic coding deficit theory of reading disabilities.^{14,17,18,23,24,25,26,27,28,30,34,36,37,38,42,55,61,62,63,64,65,66,67,68,69,70,71,72,73} Both anatomic and brain imagery studies show differences in the way the brain of a dyslexic person develops and functions. Neuroanatomic changes, microarchitectural distortion, and MRI findings in the language-related areas have been observed in the brains of dyslexic patients including the absence of the normal asymmetry in the language areas of the brain and similar volume in the left and right planum temporale; normally the left is larger.^74,75 Functional MRI (fMRI) and positron emission tomography (PET scans) measure changes in metabolic activity and blood flow during cognitive tasks in specific brain regions. In typical readers, fMRI and PET scan studies have shown that reading takes place predominantly in left hemisphere sites including the inferior frontal (Broca area), which is associated with articulation, naming, and silent reading; two areas in the posterior brain regions—the parietal temporal region serving word analysis and the left occipitotemporal area involved in word form and fluent reading; and the posterior inferior temporal cortex associated with lexical retrieval. Children with dyslexia, on the other hand, use different areas of the brain when reading.^{14,17,18,23,24,27,34,36,37,38,42,55,61,62,63,64,65,66,67,68,69,70,71,72,73} People with dyslexia have demonstrated a dysfunction in the left hemisphere posterior reading systems and have shown compensatory use of the inferior frontal gyri of both hemispheres and the right occipitotemporal word-form area.^{17,18,23,24,27,34,36,37,42,55,61,62,63,64,65,66,67,68,69,70,71,72,73} These studies have demonstrated that dyslexia is an abnormality in the word analysis pathways of the brain that interferes with its ability to convert written words into spoken words. It is postulated that this is causal, not a result of poor reading experience. Functional MRI studies have also shown that the dyslexia-specific brain activation profile improves following successful evidence-based phonologic remedial intervention.^36,68,73

Dyslexia is a developmental disorder that affects people of all ages, but its symptom profile changes with age.^69,76 Since dyslexia is both familial and heritable, affected siblings can often be identified earlier. Families with a history of dyslexia should observe their children for early language difficulties. An early history of delay or difficulty in developing speech and language, learning rhymes, or recognizing letters and sound/symbol connections, may be an early indication of dyslexia.^{14,17,27,28,47,50,69} Parents or teachers may detect early warning signs of learning difficulties in preschool children; however, in many cases, learning disabilities are not discovered until children experience academic difficulties in elementary school.^17,18,27,69 Children with learning disabilities may experience difficulty with reading, spelling, handwriting, remembering words, or doing mathematical computation. Parents or teachers who suspect that a child has a learning disability may request a formal educational evaluation by the school district. The Individuals with Disabilities Education Act (IDEA), section 504 of the Rehabilitation Act, and the Americans with Disabilities Act (ADA) define the rights of students with dyslexia and other specific learning disabilities.^77,78,79 IDEA allows parents to request a formal educational evaluation by the school district to determine eligibility for special education.

Assessments for difficulties with alphabet recognition in kindergarten^46,80 and difficulties with phonemic awareness and rapid naming in kindergarten and first grade can predict many of those who will have difficulty learning to read.^{1,14,17,23,25,26,27,28,30,31,47,49,51,53,54,80} Underachievement is not synonymous with specific learning disability.⁸¹ Some children enter school with experiential deficits in oral language skills and general knowledge as well as delayed phonologic skills.²⁸ As early reading difficulties may be caused primarily by experiential and instructional deficits, there are two approaches that can be used in the young underachieving child.¹⁴ The first method is called the response to intervention method. In the response to intervention method, the child will be placed directly in an educational intervention program when he or she first experiences academic difficulties. Only the children who do not show significant improvement with the first-tier group intervention program and the second-tier targeted intense individual intervention program will undergo a full diagnostic educational assessment.^14,82,83 Ideally, this approach will allow earlier and more effective identification of learning disabilities than the traditional method by which the child must show persistent poor academic achievement prior to referral, assessment, and remediation. A “wait to fail” situation can occur when an ability/achievement discrepancy formula is used to determine whether a student qualifies for a formal diagnostic assessment for a learning disability.^{28,30,54,82,83}

At all ages, dyslexia is a clinical diagnosis.⁶⁹ A formal evidence-based evaluation is needed to discover whether a child has a learning disability. Educational psychologists and neuropsychologists diagnose learning disabilities by performing appropriate testing as part of an educational assessment of the child’s abilities and disabilities. These results are used to develop a treatment plan.^{17,27,49,51,52,69} Since remediation is more effective during the early years, prompt diagnosis is important.^{1,14,17,18,19,23,24,25,26,27,28,31,49,50,53,54,84}

Children with learning disabilities should undergo assessments of their health, development, hearing, and vision to evaluate for secondary causes. Detected conditions should undergo medical and psychological interventions as appropriate.⁸⁵ The diagnosis and treatment of learning disabilities depend on the collaboration of a team that may include educators; educational remediation specialists; audiologists; speech, physical, and occupational therapists; teachers for the visually impaired; psychologists; and physicians.

Educational therapists or educators with specialized training in learning disabilities develop and implement treatment plans for children with learning disabilities and dyslexia. Because dyslexia is a language-based disorder, treatment should be directed at this etiology.^{1,14,17,18,23,24,25,26,27,28,30,49,51,52,53,54,69,84} Treatment plans include evidence-based educational remediations, accommodations, and modifications.^{17,23,26,27,49,51,52,69} Remediation programs should include specific instruction in decoding, fluency training, vocabulary, and comprehension.^{1,14,25,26,27,28,30,49,51,52,53,54,69} Most students with dyslexia require highly structured intensive individualized instruction by an educational therapist or skilled teacher specially trained explicitly in teaching phonemic awareness and the application of phonics.^{1,14,17,18,23,24,25,26,27,28,49,51,52,53,54} Longitudinal data indicate that systematic phonics instruction results in more favorable outcomes for readers with disabilities than does a context-emphasis (whole language) approach.^{1,14,23,27,49,52,54,69,84} Comprehension is gained through fluency training, vocabulary instruction, and active reading comprehension.^27,28 Techniques enhancing active reading comprehension include prediction, summarization, visualization, clarification, critical thinking, making inferences, and drawing conclusions.^{14,17,18,26,27,28,49,52,54} The critical elements for effective intervention include individualization, feedback and guidance, ongoing assessment, and regular ongoing practice.²⁷ Children with dyslexia should read aloud to their parents. Parents should help with practice and reinforcement at home in a supportive and nurturing environment with adequate opportunity for their child to participate in other activities where he or she excels.

Because people with dyslexia continue to have slower reading throughout their lives, accommodations and modifications may be necessary in addition to remediation.^17,18,27,69 Accommodations are alterations of the educational environment. Examples include extra time, shortened assignments, a separate quiet room for taking tests; bypassing reading by using tape recorders, lecture notes, recorded books, or testing alternatives; or extra assistance using computers, spellcheckers, a line guide, or tutors.^{17,18,27,35,69}