A reading disorder can be diagnosed when a person has extreme difficulty in dealing with written materials. This disorder can take several forms but the most common is dyslexia, a problem where letters or numbers appear to move or reverse themselves. These types of difficulties can cause a person to be unable to process written materials due to "decoding" problems; difficulty matching letter shapes and combinations with sounds to make sense out of them.
Some people with reading disorders have to expend so much effort decoding words that they are unable to derive meaning from them. While not a well-understood problem, it is believed to be caused by differences in mental "wiring" and can be hereditary. Fortunately, with modern educational research, there are now methods of coping with reading disorders.