Definition and Continuum of Dyslexia

Dyslexia is a language-based reading disability. Dyslexia manifests on a continuum from mild to moderate to severe.


The terms “dyslexia” and “specific reading disability” are often used interchangeably. The following definition, adopted by the International Dyslexia Association in 2002, is the definition recognized in Fairfax County Public Schools (FCPS). Key words in the definition are underlined and further described in the text that follows. In addition, the Appendix contains a glossary of terms.


Dyslexia is a specific learning disability that is neurobiological in origin. It is characterized by difficulties with accurate and/or fluent word recognition and by poor spelling and decoding abilities.

These difficulties typically result from a deficit in the phonological component of language that is often unexpected in relation to other cognitive abilities and the provision of effective classroom instruction. Secondary consequences may include problems in reading comprehension and reduced reading experience that can impede growth of vocabulary and background knowledge” (International Dyslexia Association, 2002).

Dyslexia is…

Specific learning disability: Dyslexia stems from a problem in a narrow area of development, rather than a global problem in overall development.

Neurobiological: Dyslexia is a brain-based disorder. Specific genes account for almost 80% of the differences found in the brains of students who demonstrate characteristics of dyslexia (Powers, et al., 2013).  Brain imaging for students with dyslexia reveal that the centers for specific aspects of reading and specific pathways between these centers are unlike those of typically developing readers.

Students with dyslexia develop a less efficient pathway in the brain for word recognition, which causes the student to process written text differently. Due to these neurobiological differences, decoding is inefficient, fluency is compromised, and meaning can be lost (Shaywitz, 2003).

Characterized by: Students with dyslexia typically struggle to sound out new words when reading (decoding), and they typically struggle to represent sounds in words accurately when spelling (encoding). Some students with dyslexia may be accurate readers, particularly if they have had an intervention, but they still struggle with reading fluency, reading quickly and accurately. For people with severe dyslexia, even if oral language comprehension skills are intact, reading comprehension may be compromised.

See the section entitled, Characteristics and Signs of Dyslexia for more information.

Phonological component of language: Dyslexia is not characterized as a visual problem, but as a language problem. It can be seen in the way words are pronounced, read, and/or spelled. Although a phonological deficit is typically present, weaknesses may also exist in other cognitive and linguistic abilities, such as rapid automatized naming.

Unexpected: Dyslexia is unexpected relative to the student’s cognitive strengths and the type of instruction that has been provided. Sally Shaywitz (2003) refers to these skills as a “sea of strengths” that surrounds the reading and spelling deficit. Strengths such as problem solving, critical thinking, vocabulary and background knowledge can help students develop compensatory strategies which can then mask their difficulty with decoding.

Secondary consequences: While dyslexia is a targeted difficulty in the phonological component of language, it can impact the overall development of literacy skills. Because of difficulties with word reading and spelling, students can also struggle with reading comprehension and writing composition. Students can also experience social and emotional impacts, such as lack of motivation in reading and writing, anxiety, depression and/or a pattern of avoiding reading or writing tasks.


Dyslexia exists on a continuum of severity and can occur in students at all levels of intelligence. For these reasons, it is not always immediately evident when an individual is actually presenting with characteristics of dyslexia.  Due to strong cognitive abilities, some students are able to develop strategies to accurately identify words using other abilities to compensate for their poor word reading or slow reading rate.  Some students may demonstrate less severe characteristics of dyslexia, causing them less difficulty when initially learning decoding skills. These students do not always require targeted reading remediation. However, those students with more severe forms of dyslexia do require targeted interventions focusing on phonemic awareness, word identification, and/or fluency.

Dyslexia affects some people more than others, meaning that the impact of dyslexia can differ from one student to the next. Because it manifests differently on the mild end of the continuum than it does on the severe end of the continuum, signs and symptoms can vary. There are a variety of factors that can affect the severity of dyslexia. This continuum from mild to severe helps to explain why dyslexia is not always easy to identify, why students with dyslexia need different levels of intensity of intervention, and why they may respond differently to these interventions.

Although phonological processing deficits are the most characteristic difficulty related to dyslexia, not all students with dyslexia have a phonological deficit. Students with dyslexia may have weaknesses in rapid automatized naming, or the ability to name objects and symbols quickly and accurately; orthographic mapping, establishing the connections between the speech sounds and the written letters; processing speed, visually scanning symbols quickly; and/or working memory, apprehending and then reorganizing information mentally. Students with weaknesses in more than one of these areas tend to have more severe dyslexia.


Neurologically, dyslexia is characterized as inefficient and/or inaccurate processing of the sounds in words when speaking, reading or writing. Positive environmental factors, such as access to a rich oral language early literacy environment and strong early literacy instruction or intervention, can lessen the impact of dyslexia on learning to read. These early experiences can create more efficient pathways in the brain needed for automatic processing of the sounds in the written word (Aylward et al., 2003).

Dyslexia may be mild if the source of dyslexia is a lack of automaticity in retrieving information from long term memory. Dyslexia may also be mild if the related factors, such as phonological and/or rapid automatized naming deficits, are mild.

Finally, dyslexia may appear to be mild because of the cognitive strengths a given individual with dyslexia may have. Strengths in vocabulary, reasoning, problem solving, or oral language comprehension (described by Shaywitz as a “sea of strengths”) can mediate the severity of dyslexia.

Students with strong oral language skills can often develop average reading skills which can then make their dyslexia more difficult to identify. They can also better compensate for their weaknesses, thus keeping the dyslexia “hidden.”


Shaywitz’s “sea of strengths” model can help to explain why mild dyslexia may be overlooked by families and educators (Shaywitz, 2003). However, it is important to note that not all students with dyslexia will have these strengths, nor will all students with dyslexia present with above average cognitive skills. Dyslexia occurs evenly across the population, and is not unique to people with average or above intelligence. Average or above intelligence, however, can help facilitate the process of learning to read, as well as the student’s rate of response to intervention. 

Dyslexia is often experienced with moderate impact on reading achievement when phonological challenges are paired with rapid naming deficits, referred to as the “double deficit” hypothesis (Wolf & Bowers, 1999). Students with a “double-deficit” often have more difficulty learning to read. Once reading accuracy is established, often through a structured multisensory phonics approach, interventions must be used to increase reading fluency and rate.

Co-existing conditions, such as attention-deficit/hyperactivity disorder (ADHD) or dysgraphia, can also increase dyslexia’s impact on behavior or academic performance, and can lessen the positive impact of early intervention... Co-existing conditions may lead to a more moderate (vs. mild) expression of dyslexia. The section entitled, Common Co-existing Conditions with Dyslexia explains more about the most common disorders that accompany dyslexia.


Many students with dyslexia have phonological processing deficits. Students who have additional challenges or other cognitive and linguistic weaknesses (e.g., orthographic mapping, processing speed, rapid automatized naming, working memory, and/or executive functioning) will often experience a more severe form of dyslexia.

Dyslexia impacts accurate and fluent word recognition. While mild or moderate dyslexia often impacts automatic word recognition, severe dyslexia impacts automatic word recognition to such a degree that comprehension of text is often compromised as well. In much the same way, while mild or moderate dyslexia often impacts spelling, severe dyslexia impacts spelling to such a degree that, even with accommodations, students may have difficulty expressing their thoughts in writing. 

Finally, students who demonstrate more severe dyslexia often have family histories of language, reading, and writing difficulties. Genes account for at least 80% of individuals with this reading disorder (Powers et al., 2013).


Aylward E.H., Richards T.L., Berninger V.W., Nagy W.E., Field K.M., Grimme A.C., Richards A.L., Thomson J.B., & Cramer S.C. (2003). Instructional treatment associated with changes in brain activation in children with dyslexia. Neurology, 61, 212–219.

Powers N, Eicher J, Butter F, Kong, Y., Miller, L., Ring, S., Mann, M., & Gruen, J. (2013). Alleles of a Polymorphic ETV6 Binding Site in DCDC2 Confer Risk of Reading and Language Impairment. The American Journal of Human Genetics, 93, 19–28; July 11, 2013.

Shaywitz, S. (2003). Overcoming Dyslexia: A new and complete science-based program for reading problems at any level. New York, NY: Knopf.

Wolf, M. & Bowers, P. (1999). The double-deficit hypothesis for the developmental dyslexias. Journal of Educational Psychology, 91, 415-438.

The FCPS online dyslexia handbook provides information and resources to FCPS schools and parents alike to support students with dyslexia.

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