Commonly, most people think of dyslexia as a problem with reversals(b and d, p and q) or transpositions(12 for 21). While this is a form of dyslexia, called motoric dyslexia, it is the most mild of the variants and the most uncommon compared to the visual and auditory types. Dyslexia in a nutshell is a problem of visual and auditory matching.
In the first step of seeing, it is assumed that the ability to clearly see the word is accurate as well as the control of eye movement to stay still on the word. Then the decoding or understanding the letters and assigning their particular sounds. At this point, the concept of inner speech is becomes relevant to the reading process. Once the sounds are assigned to the letters of the word, the word is 'said' in your head. In order to silently read, you are actually reading out loud in your head. Interestingly, even though your eyes move left to right as you read, they don't pause on every word instead they tend to scan over small words(the, an, of, to, was) and pause on larger content words. Even more interesting, your inner speech says these words even though your visual system doesn't register them.
It is at this step of decoding print (even braille) that is disrupted at a specific region in the dyslexic brain. Sight words are typically safe but when dyslexics need phonetics to help sound out the word breakdown occurs. The visual representation of the word and the phonetics of the word don't match and poor comprehension ensues.
True dyslexia assumes a perfectly functioning visual and auditory system. Visually impaired people who read braille can be dyslexic as braille is just an alternate form of print. Deaf persons must learn sign language first then a variant sign language used to teach the English language in order to read English. Few read above a 6th grade level but those that can don't rely on phonetics when reading therefore they can't ever be dyslexic.