Dyslexia is considered to be a learning disability. It manifests primarily as a difficulty with written language, particularly with reading and spelling. It is separate and distinct from reading difficulties resulting from other causes, such as a non-neurological deficiency with vision or hearing, or from poor or inadequate reading instruction. Evidence also suggests that dyslexia results from differences in how the brain processes written and/or spoken language. Although dyslexia is allegedly the result of a neurological difference, it is not an intellectual disability. Dyslexia has been diagnosed in people possessing all levels of intelligence.

The word dyslexia comes from the Greek words dys- ("impaired") and lexis ("word").

Overview

Dyslexia is most commonly characterized by difficulties with learning how to decode words, to spell, and to read accurately and fluently. Dyslexia can also lead to difficulties in mathematics due to challenges in comprehension and sequencing. Dyslexic individuals often have difficulty "breaking the code" of sound-letter association (the alphabetic principle), and they may also reverse or transpose letters when writing or confuse letters such as b, d, p, q, especially in childhood. However, dyslexia is not a visual problem that involves reading letters or words backwards or upside down, nor are such reversals a defining characteristic of dyslexia.

Many individuals with dyslexic symptoms involving reading, writing, and spelling also exhibit symptoms in other domains such as poor short-term memory skills, poor personal organizational skills and problems processing spoken language. These symptoms may coexist with or overlap with characteristics among others of attention-deficit hyperactivity disorder, auditory processing disorder, developmental dyspraxia, dyscalculia, and/or dysgraphia. However, dyslexia and attention deficit/hyperactivity disorder are not correlated developmental problems.

Evidence that dyslexia is of neurological origin is substantial. Acquired dyslexias occur due to brain damage in the left hemisphere's key language areas, which may be a clue when it comes to developmental types. Research also suggests an association with biochemical and genetic markers. However, experts disagree over the precise definition and criteria for diagnosis, and some advocate that the term dyslexia be dropped altogether and replaced with the term reading disorder or reading disability (RD). 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 two to five percent with the most severe reading deficits.