I am not familiar with the process of testing when someone requires special accommodations, however, I do know that you need to call the testing center first and let them know you need special accommodations. Ask them what sort of letter is required after you explain your particular situation.
I believe that your doctor needs to document the hearing loss as well as the loss of feeling in your fingertips, and sign the letter. The testing center should know exactly what type of letter is needed and inform you when you contact them to schedule your test.