<Tr> <Th> Frequency </Th> <Td> 3--7% </Td> </Tr> <Tr> <Td_colspan="2"> (edit on Wikidata) </Td> </Tr> <P> Dyslexia, also known as reading disorder, is characterized by trouble with reading despite normal intelligence . Different people are affected to varying degrees . Problems may include difficulties in spelling words, reading quickly, writing words, "sounding out" words in the head, pronouncing words when reading aloud and understanding what one reads . Often these difficulties are first noticed at school . When someone who previously could read loses their ability, it is known as alexia . The difficulties are involuntary and people with this disorder have a normal desire to learn . </P> <P> Dyslexia is believed to be caused by both genetic and environmental factors . Some cases run in families . It often occurs in people with attention deficit hyperactivity disorder (ADHD) and is associated with similar difficulties with numbers . It may begin in adulthood as the result of a traumatic brain injury, stroke, or dementia . The underlying mechanisms of dyslexia are problems within the brain's language processing . Dyslexia is diagnosed through a series of tests of memory, spelling, vision, and reading skills . Dyslexia is separate from reading difficulties caused by hearing or vision problems or by insufficient teaching . </P>

Is not being able to spell a form of dyslexia