Dyslexia and Deafness
Since sound is such a vital piece to reading, it is a tall task for a deaf person to learn to read. Braille is an alternate form of print but not an alternate language. American Sign Language (ASL) is a completely different language and does not follow the same pattern or rules as the English language. A deaf person must learn sign language first then a variant sign language used to teach the English language in order to read English. Deaf children of deaf parents are more likely to learn to read as they are more likely to be more fluent in ASL. Knowing a language makes it easier to learn to read a language because there is a foundation from which to build. Not many deaf people are able to read above a 6th grade level but those that can don't rely on phonetics when reading and therefore can't be dyslexic.
If you are questioning dyslexia for yourself or a loved one, speak with your eye doctor about specialized vision testing. It is important to rule out a treatable vision disorder.
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 also known as decoding. It's essentially a reading disorder not a vision disorder.
The first step of reading is to be able to clearly and accurately view the word as well as control eye movement to stay still on it. The next step is the decoding, or understanding the letters, and assigning their particular sounds. At this point, the concept of inner speech, also known as cortical vocalization, 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. Inner speech is a necessary component of the reading process.
It is at this step of decoding print (even braille) that a disruption occurs at a specific region in the dyslexic brain. Sight words are typically safe but when dyslexics need phonetics to help sound out the word, a breakdown occurs. The visual representation of the word and the phonetics of the word don't match and poor comprehension ensues.
Dyslexia & Visual Impairment
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. When a visually impaired person reads braille, the sensory information from their fingers ends up reaching the same centers in the brain as the sensory information from the eyes of a typically seeing person. This information must match up with the particular sound which is said in your head then understood or comprehended. If there is an error in the match or an error in the comprehension of the content due to a wrong word, then the signal is resent. This means the eyes or fingers back up and the word or sentence is reread for a second attempt at decoding and understanding. For a dyslexic person, reading means a lot of repeating which is frustrating. Coping mechanisms include throwing in a word that is close enough and moving on in hopes to gain comprehension from the rest of the sentence or story.
Dyslexia is a very complex diagnosis. From a visual standpoint, there exists a multitude of diversions along the visual path before the dyslexic brain center is reached. It is vital that the visual system functions efficiently so visual information can properly reach the decoding brain center intact to have the best chance at correct comprehension.
Christenson, G., et al., Helping Children Overcome Dyslexic Reading and Writing Problems, Optometric Extension Program Foundation, 2014.
Clark, P. How do deaf children learn to read?, ASL University, http://www.braillesc.org/blog/..., April, 2003. Accessed April, 2016.
Braille and Dyslexia, Braille, http://www.braillesc.org/blog/..., November, 2010. Accessed April, 2016.