Current Concepts

DYSLEXIA

SALLY E. SHAYWITZ, M.D.

DEVELOPMENTAL dyslexia is characterizedby an unexpected difficulty in reading in chil-dren and adults who otherwise possess the

intelligence, motivation, and schooling considerednecessary for accurate and fluent reading. Dyslexia (orspecific reading disability) is the most common andmost carefully studied of the learning disabilities, af-fecting 80 percent of all those identified as learning-disabled.! Although the diagnosis and implications ofdyslexiawere once quite uncertain, recent advances inour knowledge of its epidemiology, neurobiology,and genetics, as well as of the cognitive influences onthis disorder, now allow physicians to approach dys-lexia within the framework of a traditional medicalmodel. This article reviews these advances and theirimplications for the approach to patients presentingwith a possible reading disability.

EPIDEMIOLOGY OF DYSLEXIA

Recent epidemiologic data indicate that, like hy-pertension and obesity, dyslexia fits a dimensionalmodel. In other words, within the population, read-ing ability and reading disability occur along a con-tinuum, with reading disability representing the lowertail of a normal distribution of reading ability.2,3Dys-lexia is perhaps the most common neurobehavioraldisorder. affecting children, with prevalence ratesrangingftom5 to 10 petcent to 17.5 percent.4,5Pre-viously, it was believed that dyslexia affected boysprimarily6; however, more recent data7-9indicate sim-ilar numbers of affected boys and girls. Longitudinalstudies, both prospectivelO,ll and retrospective,12-14indicate that dyslexia is a persistent, chronic condi-tion; it does not represent a transient "developmentallag." Over time, poor readers and good readers tendto maintain their relative positions along the spec-trum of reading ability.lO

NEUROBIOLOGIC INFLUENCES

Heritability

Dyslexia is both familial and heritable.!5 Familyhistory is one of the most important risk factors,with 23 percent to as much as 65 percent of children

From the Department of Pediatrics, YaleUniversity School of Medicine,333 Cedar St., New Haven, cr 06510, where reprint requests should beaddressed to Dr. Shaywitz.

@1998, Massachusetts Medical Society.

who have a parent with dyslexia reported to have thedisorder.!6 A rate among siblings of affected personsof approximately 40 percent15 and among parentsranging from 27 to 49 percent15 provides opportu-nities for early identification of affected siblings andoften for delayed but helpful identification of affect-ed adults. Linkage studies implicate loci on chromo,somes 6 and 15 in reading disability.17-19

NeurobiologicStudies

A range of neurobiologic investigations using post-mortem brain specimens,2°brain morphometry, func-tional brain imaging, and electrophysiology suggeststhat there are differences in the temporo-parieto-occipital brain regions between people with dyslexiaand those who are not reading-impaired.21,22Somestudies suggest differences in the striate or extrastri-ate cortex,23 findings that coincide with those in alarge body of literature describing anatomical lesionsin posterior brain regions in acquired alexia, mostprominently centered on the angular gyruS.24

COGNITIVE INFLUENCES

Reading - the process of extracting meaningfrom print - involves. both visual-perceptual andlinguistic processes. Theories of dyslexia have beenproposed that are based on the visual system,25thelanguage system,26and other factors, such as tempo-ralprocessing of stimuli within these systems.27,28Whatever the contributions of other systems andprocesses, there is now a strong consensus amonginvestigators in the field that the centtaldifficulty indy~!~~are,flectsadeficieneywithin a specificC'ompo-netit of the language system, the phonologk'mod-ule, which is engaged in processing the soundS' ofspeech.29,3OAccording to the phonologic-deficit hy-pothesis, people with dyslexia have difficulty devel-oping an awareness that words, both written andspoken, can be broken down into smaller units ofsound and that, in fact, the letters constituting theprinted word represent the sounds heard in the spo-ken word.

The Phonologic-DeficitHypothesis

The language system can be conceptualized as ahierarchical series of components. At the higher lev-els are neural systems engaged in processing, for ex-ample, semantics, syntax, and discourse; at the low-est level is the phonologic module, dedicated toprocessing the distinctive sound elements that con-stitute language. The functional unit of the phono-logic module is the phoneme, defined as the smallestdiscernible segment of speech; for example, theword "bat" consists of three phonemes: Ibl laelIt I (buh, aah, tuh). To speak a word, th,e speakerretrieves the word's phonemic constituents from thelexicon, assembles the phonemes, and then uttersthe word. Conversely, to read a word, the reader

Reprinted ITam The New England Journal of Medicine338:307-312 (January 29), 1'998

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The New England Journal of Medicine

must first segment the word into its underlying pho-nologic elements. There is abundant evidence that adeficit in phonologic analysis is related to difficultiesin learning to read: phonologic measures predict lat-er reading achievement31-33;deficits in phonologicawareness (i.e., awareness that words can be brokendown into smaller segments of sound) consistentlydistinguish children with dyslexia from those whoare not reading-impaired34,35; phonologic deficitspersist into adulthood13,14;and instruction in phono-logic awareness promotes the acquisition of readingskills.31,36-39Additional findings of the strong herita-bility of phonologic awareness suggest "that it maybe the main proximal cause of most genetically-based deficits in word recognition, and thus it maybe the most appropriate focus for diagnosis and re-mediation."40

IMPLICATIo.NSo.F THE PHo.No.Lo.GICMo.DEL o.F DYSLEXIA

Basically, reading comprises two main processes- decoding and comprehension. In dyslexia, a def-icit at the level of the phonologic module impairsthe ability to segment the written word into its un-derlying phonologic elements. As a result, the readerexperiences difficulty, first in decoding the word andthen in identifying it. The phonologic deficit is do-main-specific; that is, it is independent of other,nonphonologic, abilities. In particular, the higher-order cognitive and linguistic functions involved incomprehension, such as general intelligence and rea-soning, vocabulary,3Oand syntax,41.are generally in-tact. This pattern - a deficit in phonologic analysiscontrasted with intact higher-order cognitive abili-ties - offers an explanation for the paradox of oth-erwise intelligent people who experience great diffi-culty in reading.42

According to the model, a circumscribed deficit ina lower-order linguistic (phonologic) function blocksaccess to higher-order processes and to the ability todraw meaning from text. The problem is that theperson cannot use his or her higher-order linguisticskills to access the meaning until the printed wordhas first been decoded and identified. Suppose, forexample, that a man knows the precise meaning of thespoken word "apparition"; however, until he can de~code and identify the printed word on the page, hewill not be able to use his knowledge of the meaningof the word and it will appear that he does not knowthe word's meaning.

APPRo.ACH TO. THE DIAGNo.STICEVALUATIo.N

Guided by knowledge of the presumed underlyingpathophysiology, the clinician seeks to determinethrough history, observation, and psychometric as-sessment, first, whether there are difficulties in read-ing that are unexpected, given the person's age, inc

308 . January 29, 1998

telligence, or level of education, and second, whetherthere are associated linguistic problems at the levelof phonologic processing43 (Table 1). The way read-ing and language are assessed depends on the ageand educational level of the patient.

EVALUATIo.N o.F SCHo.OL-AGE CHILDREN

Presenting problems most commonly center onschool performance ("she's not doing well inschool"), and parents (and teachers) often do notrealize that the cause is a difficulty in reading. Thus,an evaluation for dyslexia should be considered forall children presenting with learning problems, evenif reading difficulty is not explicitly reported.

Assessment of ReadingAbility

Reading ability is assessed by the measurement ofdecoding skills and comprehension. In the school-age child, the most important element of the psy-chometric evaluation is how accurately the child candecode words - that is, read single words in isola-tion. Reading passages allows bright children withdyslexia to use the context to guess the meaning ofa word they might otherwise have trouble decoding.As a result, readers with dyslexiaoften perform betteron measures of comprehension and worse onmeas-ures of the ability to decode isolated single words. Inpractice, the' reliance on context makes such tests asmultiple-choice examinations, which typically pro¥idescanty context, especially burdensome fOf'readerswith dyslexia.

Assessment of Intelligence

The role of tests ofintelligence.MJk ~<Jgnosis,of

dyslexia is controversial.45,46'];'1'<J- ..'-'... .' ..e. cqn-cept of dyslexia as an unexpeCted dfflj,e141i5:'inreadlnghas been interpreted as UI1der~9hiev.Rl111ej'ltinr,ea<;iingrelative to ability (IQ) - 'Watis, ..a.di.sCl"epancybetween the level of rea<;iingachievement pr@Q.jctedon the basis of IQ and the .a:~Wa-1,Jevel-Qf£eildingachievement. Consequently, IQ testsaregenet;lllyused to assess dyslexia in school-age children, and infact, eligibility for special-education programs inpublic schools is usually based on the demonstrationof an ability-achievement discrepancy. More recently,many have questioned the requirement of such adiscrepancy.34,35The issue is complex. In certain re-spects, children with dyslexia identified on the basisof an ability-achievement discrepancy do not seemdifferent from those of average intelligence whosedyslexia is identified solely on the basis oflow read-ing achievementfor chronologic age; . both have adeficit in phonologic processing34,35and follow thesame developmental trajectory in reading.ll At thesame time, it should be recognized that the use of anapproach based on such a discrepancy in the diagno-.sis of dyslexia is important for the identification ofvery bright children who have dyslexia.

CURRENT CONCEPTS

Approximately 75 percent of children meeting thecriteria for a discrepancy between ability and achieve-ment also have low reading achievement47; however,the remaining 25 percent of children meeting dis-crepancy criteria - most of whom are extremelybright and also manifest a phonologic deficit - donot meet low-achievement criteria and would be ex-cluded from special-education services if the low-achievement criterion were the only one employed.Thus, a consensus is developing that in school-agechildren the criterion of "unexpected" reading diffi-culties may be met by children of at least average in-telligence who meet either discrepancy criteria rela-tive to IQ or low"achievement criteria relative tochronologic age.46,48

SPECIAL CONSIDERATIONS IN YOUNGERAND OLDER AGE GROUPS

Assessment at SchoolEntry

Currently, most children's reading disability is notdiagnosed until they are in the third grade, or aboutnine years 01d.49The application of what has beenlearned about the acquisition of reading and theavailability of tests of phonologic skills now make itpossible, first, to identifY children with dyslexia evenbefore they fail in reading, 50and then, to provideappropriate early interventions (see "Management,"below). A history oflanguage delay or of not attend-ingto the sounds of words (trouble playing rhyminggames with words or confusing words that soundalike), along with a family history, are important riskfactors for dyslexia,l6The most helpful measures inpredicting difficulties are those often referred to asreading-readiness tests16(Table 2).

Assessment of BrightYoungAdults

The developmental course of dyslexia has nowbeen characterized. First, dyslexia is persistent; itdoes not go away,lO,llOn a practical level, this meansthat once a person is given a diagnosis of dyslexiathere is no need for reexamination after high schoolto confirm the diagnosis. Second, over the course ofdevelopment, the ability to decode words becomesmore accurate and automatic in skilled readers; theydo not need to rely on context for word identifica-tion. The skills of readers with dyslexia, too, becomemore accurate over time, but they do not becomeautomatic. Residua of the phonologic deficit per-sist,13,14so that reading remains effortful, even forthe brightest people with childhood histories of dys-lexia.51The failure either to recognize or to measurethe lack of automaticity in reading is, perhaps, themost common error in the diagnosis of dyslexia inaccomplished young adults. It is often not under-stood that tests measuring word accuracy may be in-adequate for the diagnosis of dyslexia in youngadults at the level of college or graduate or profes-sional school, and that for these people, timed meas-

TABLE 1. CLUES TO DYSLEXlAIN SCHOOL-AGECHlLDREN. *

History

Delayed languageProblems with the sounds of words (trouble rhyming words,

confusion of words that sound alike)Expressive language difficulties (mispronunciations, hesita-

tions, word-finding difficulties)Difficulty naming (difficulty learning the letters of the alpha-

bet and the names of numbers)Difficulty learning to associate sounds with lettersHistory of reading and spelling difficulties in parents and sib-

lings

Reading

Difficulty decoding single wordsParticular difficulty reading nonsense or unfumiliar wordsInaccurate and labored oral readingSlow readingComprehension often superior to isolated decoding skillsPoor spelling

Language

Relatively poor performance on tests of word retrieval (namethe pictured item)

Relatively superior performance on tests of word recognition(point to the pictureditem) .

Poor performance on tests of phonologic awareness

Clues most specific to young children at risk for dyslexiaDifficulty with tests assessing knowledge of the names oftet-

ters, the ability to associate sounds with letters, and pho-nologic awareness

Clues most specific to bright young adults with dyslexia

Childhood history ofreading and spelling difficultiesAccurate but not automatic readingSlow performance on timed reading tests (e.g., Nelson-Den-

ny Reading Test")Penalized by multiple-choice tests

'Clues are based on history, observations, testing, or acombination of the three.

TABLE2. TYPES OF TESTS USEFUL IN IDENTIFYlNGCHlLDREN AT RISK FOR DYSLEXlAAT THE TIME

OF SCHOOL ENTRY.

Letter identification (naming letters of the alphabet)Letter-sound association (e.g., identitying words that begin

with the same letter from a list: doll, dog, boat)Phonologic awareness (e.g., identitying the word that would

remain if a particular sound was removed: if the /k/sound was taken away from "cat")

Verbal memory (e.g., recalling a sentence or a story that wasjust told)

Rapid naming (rapidly naming a continuous series offamiliarobjects, digits, letters, or colors)

Expressive vocabulary, or word retrieval (e.g., naming singlepictured objects)

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tires of reading must be used to make the diagnosis.However, there are very few standardized tests foradult readers that are administered under timed anduntimed conditions; the Nelson-Denny ReadingTest44 is an exception. The reading measures52,53commonly used for school-age children may providemisleading data on some adolescents and youngadults, since they assess reading accuracy but not au-tomaticity (speed).

DIAGNOSIS

For bright young adults especially, the history isperhaps the most sensitive and accurate indicator ofdyslexia. A history of phonologically based lan-guage difficulties (e.g., mispronouncing words, speechpunctuated by hesitations and dysfluencies), oftrou-ble reading new or unfamiliar words, of spelling dif-ficulties, and of requiring additional time for'readingand taking tests relative to the level of educationachieved represents the distinct diagnostic signatureof dyslexia.

Tests of reading, spelling, language, and cognitive

TABLE3. SOME DISORDERS THAT MAy PRESENTWITH READING DIFFICULTIES.

Developmental dyslexia

PhonologiC'deficit primaryReading impairment at the level of single-word decodingOther components oflanguage system intact (e.g., syntax,

semantics)Intelligence not affected and may be in superior or gifted

range

Language-learning disability": the primary deficit involvesall aspects of language, both phonologic and semantic-syntactic

Reading difficulty at the level of both decoding and compre-hension

Prominent language difficultiesMeasures of verbal intelligence significantly affected by lan-

guage deficit; may be in subaverage range

Acquired alexiaf: the loss or diminution of reading abilityResult of trauma, tumor, or stroke (e.g., occlusion of poste-

rior cerebral artery)Several forms reflecting specific loci of neuroanatomicalle-

sions (e.g., alexia with or without agraphia)May be accompanied by other features reflecting locus and

extent of the lesion

Hyperlexia:j:

Word-recognition ability substantially better than readingcotuprehension

Early intense interest in words and lettersExceptional word-recognition ability, apparent by the age of

five yearsVery poor reading comprehensionDisordered language development, especially affecting aural

comprehensionDeficits in reasoning and abstract problem-solvingBehavioral atypicalities affecting interpersonal relationships

"Discussed in Catts and Kamhi.54

f Discussed in Damasio and Damasio.55

:j:Discussed in Aram and Healy.56

310 . January 29, 1998

abilities (for school-age children) represent a corebattery for the diagnosis of dyslexia; additional testsof academic achievement (in mathematics, for exam-ple), language, or memory may be administered aspart of a more comprehensive evaluation of academ-ic, linguistic, or cognitive function. There is no sin-gle test score that is pathognomonic of dyslexia. Aswith any other medical diagnosis, the diagnosis ofdyslexia should reflect a thoughtful synthesis of allthe clinical data available, including the history, ob-servations, and testing data (TabJe 1). What the cli-nician is seeking is converging evidence of a phono-logically based reading disability, as indicated by adisparity between the person's reading and phono-logic skills and his or her intellectual capabilities,age, or level of education. Dyslexia is distinguishedfrom other disorders that may feature reading diffi-culties prominently by the unique, circumscribednature of the phonologic deficit, one that does notintrude into other linguistic or cognitive domains(Table 3). Primary sensory impairments should beruled out, particularly in young children; other lab-orat9ry measures, such as imaging studies, electro-encephalography, or genetic tests, are ordered onlyif there are specific clinical indications. Althoughthere have been important advances in the use ofimaging to study cognitive function, such technolo-gy is still reserved for investigational pUrposes. At-tention deIi,cit-hyperactivity disorder may also affectlearning in both children and adults. It is an entirelydifferent disorder from dyslexia; they differ in theirproposed mechanisms, symptoms, assessments, andinterventions. 57A proportion of patients with dys-lexia (12 to 24 percent)5 will also have attentiOn dd-icit-hyperactivity disorder, and if there is any sugges-tion that inattention may be a problem, the patientshould be examined for specific symptoms that meetthe criteria for attention deficit-hyperactivity disor-der of the fourth edition of the Diagnostic and Sta-tistical Manual of Mental Disorders.58

MANAGEMENT

The management of dyslexia demands a life-spanperspective; early on, the focus is on remediation. Asa child matures and enters the more time-demand-ing setting of secondary school, the emphasis shiftsto the important role of providing accommodations.Since physicians are frequently asked about variousreading programs for dyslexia, they should under-stand the key elements of an effective training pro-gram. To learn to read, all children must discoverthat spoken words can be broken down into smallerunits of sound, that letters on the page representthese sounds, and that written words have th<;:samenumber and sequence of sounds heard in the spokenword. When children have made these associations,usually by the end of first grade, they have discov-ered the "alphabetic principle" and have essentially

CURRENT CONCEPTS

broken the reading code. Children with dyslexia donot easily acquire the basic phonologic skills thatserve as a prerequisite to reading; consequently, con-cepts such as phoneme awareness must be taught ex-plicitly. Operationally, this is accomplished with sys-tematic and highly structured training exercises,such as identifying rhyming and nonrhyming wordpairs, blending isolated sounds to form a word, orconversely, segmenting a spoken word into itsindi-vidual sounds. Furthermore, it is now recognizedthat although awareness of phonemes is necessaryfor learning to read, it is not sufficient.59In additionto learning that words can be segmented into small-er units of sound (phoneme awareness) and thatthese sounds are linked with specific letters and let-ter patterns (phonics), children with dyslexia requirepractice in reading stories, both to allow them to ap-ply their newly acquired decoding skills to readingwords in context and to experience reading formeaning.60A number of protocols differing in meth-od, format, intensity, and duration of the reading in-tervention are now being tested in large-scale stud-ies; data from these trials should provide importantinformation to clarify further which specific pro-grams are most effective for particular groups ofchildren with dyslexia.6OLarge-scale studies to datehave focused on younger children; as yet, there arefew or no data available on the effect of these train-ing programs on older children. People with dyslexiaand their families frequently consult their physiciansabout unconventional approaches to the remedia-tion of reading difficulties; in general, there are veryfew credible data to support the claims made forthese treatments (e.g., optometric training, medica-tion for vestibular dysfunction, chiropractic manip-ulation, and dietary supplementation).61

The management of dyslexiain students in second-ary school, and especially college and graduateschool, is based on accommodation rather than reme-diation. College students with a history of childhooddyslexia often present a paradoxical picture; they aresimilar to their unimpaired peers on measures of wordrecognition yet continue to suffer from the phono-logic deficit that malcesreading less automatic, moreeffortful, and sloW)3,14,51For these readers with dys-lexia the provision of extra time is an essential accom-modation; it allows them the time to decode eachword and to apply their unimpaired higher-order cog-nitive and linguistic skills to the surrounding contextto get at the meaning of words that they cannot en-tirely or rapidly decode. Although providing extratime for reading is by far the most common accom-modation for people with dyslexia, other helpful ac-commodations include allowing the use of lap-topcomputers with spelling checkers, tape recorders inthe classroom, and recorded books (materials areavailable from Recording for the Blind and Dyslexic,telephone 800-221-4792) and providing access to

syllabiand lecture notes, tutors to "talk through" andreview the content of reading material, alternatives tomultiple-choice tests (e.g., reports or orally adminis-tered tests), and a separate, quiet room for talcingtests. With such accommodations, many studentswith dyslexia are now successfullycompleting studiesin a range of disciplines, including medicine.

Supported by grants (POI HD-21888 and P50 HD-25802) ITom theNational Instimte of Child Health and Human Development.

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