Dyslexia is also known as reading disorder. It is a learning disability characterized by reading trouble and 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. The cause of dyslexia is believed to involve 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 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 insufficient teaching, or either hearing or vision problems. The symptoms that correlate with a later diagnosis of dyslexia include delayed onset of speech, difficulty distinguishing left from right, difficulty with direction, as well as being easily distracted by background noise. The reversal of letters or words and mirror writing are behaviors sometimes seen in people with dyslexia, but are not considered to be defining characteristics of the disorder. Problems persist into adolescence and adulthood and may accompany difficulties with summarizing stories, memorization, reading aloud, or learning foreign languages. Adult dyslexics can often read with good comprehension, though they tend to read more slowly than non-dyslexics and perform worse in spelling tests or when reading nonsense words – a measure of phonological awareness.
The percentage of people with dyslexia is unknown, but it has been estimated to be as low as 5% and as high as 17% of the population. While it is diagnosed more often in males,some believe that it affects males and females equally. There are different definitions of dyslexia used throughout the world, but despite significant differences in writing systems, dyslexia occurs in different populations. Dyslexia is not limited to difficulty in converting letters to sounds, and Chinese dyslexics may have difficulty converting Chinese characters into their meanings. The Chinese vocabulary uses logographic, monographic, non-alphabet writing where one character can represent an individual phoneme. The phonological-processing hypothesis attempts to explain why dyslexia occurs in a wide variety of languages. Furthermore, the relationship between phonological capacity and reading appears to be influenced by orthography Dyslexia and attention deficit hyperactivity disorder (ADHD) commonly occur together, about 15% of people with dyslexia also have ADHD and 35% of those with ADHD have dyslexia.