Reading disabilities are the inability of an individual to read and/or comprehend a text due to a neurological condition. The three major reading disabilities are dyslexia, alexia (acquired dyslexia), and hyperlexia. These are brain-based conditions, which may be inherited traits, which prevent an individual's ability to read.
Dyslexia is a learning disability in which an individual has difficulty in decoding words, reading comprehension, and/or fluency. It has been estimated that between five and seventeen percent of the population suffers from dyslexia, and the condition has nothing to do with inadequate reading instruction. There are three cognitive subtypes of dyslexia: auditory, visual, and attentive. Even though dyslexia is considered to be both a learning disability and a reading disability, it is not a function of intelligence, and has nothing to do with a person's IQ.
Alexia, or acquired dyslexia, usually occurs after damage to the brain, including areas of the temporal lobe, parietal lobe, or occipital lobe, from a stroke, traumatic brain injury, or progressive illness.
Hyperlexia often occurs in children with autism and is characterized by a precocious ability to read at an early age and is a function of high IQ. However, many of these individuals have trouble understanding speech. It is considered to be the neurological opposite of dyslexia.