Rocket. I'm in the same situation. I have a letter from my doctor of 20 years. You'll want your doctor to state their experience and that they have reviewed your C-file. The doctor has to use the words "as likely as not" to describe a link between the sleep apnea being related to the PTSD. The doctor also has to explain why they believe that. They can mention training, cite medical journals, and scientific papers, etc. I'm about to send my disability papers in and I'm not sure if I should send in my doctor's letter at the beginning or wait for the appeal.