<P> Dyslexia is thought to have two types of cause, one related to language processing and another to visual processing . It is considered a cognitive disorder, not a problem with intelligence . However, emotional problems often arise because of it . Some published definitions are purely descriptive, whereas others propose causes . The latter usually cover a variety of reading skills and deficits, and difficulties with distinct causes rather than a single condition . The National Institute of Neurological Disorders and Stroke definition describes dyslexia as "difficulty with phonological processing (the manipulation of sounds), spelling, and / or rapid visual - verbal responding". The British Dyslexia Association definition describes dyslexia as "a learning difficulty that primarily affects the skills involved in accurate and fluent word reading and spelling" and is characterized by "difficulties in phonological awareness, verbal memory and verbal processing speed". </P> <P> Acquired dyslexia or alexia may be caused by brain damage due to stroke or atrophy . Forms of alexia include pure alexia, surface dyslexia, semantic dyslexia, phonological dyslexia, and deep dyslexia . </P> <P> There is some variability in the definition of dyslexia . Some sources, such as the U.S. National Institutes of Health, define it specifically as a learning disorder . Other sources, however, define it simply as an inability to read in the context of normal intelligence, and distinguish between developmental dyslexia (a learning disorder) and acquired dyslexia (loss of the ability to read caused by brain damage). ICD 10, the manual of medical diagnosis used in much of the world, includes separate diagnoses for "developmental dyslexia" (81.0) and for "dyslexia and alexia" (48.0). DSM 5, the manual of psychiatric diagnosis used in the United States, does not specifically define dyslexia, justifying this decision by stating that "the many definitions of dyslexia and dyscalculia meant those terms would not be useful as disorder names or in the diagnostic criteria". Instead it includes dyslexia in a category called specific learning disorders . </P> <P> In early childhood, symptoms that correlate with a later diagnosis of dyslexia include delayed onset of speech, difficulty distinguishing left from right, difficulty with direction, and a lack of phonological awareness, as well as being easily distracted by background noise . A common myth closely associates dyslexia with mirror writing and reading letters or words backwards . These behaviors are seen in many children as they learn to read and write, and are not considered to be defining characteristics of dyslexia . </P>

When was dyslexia first recognised as a disability