After being denied for Obstructive Sleep Apnea (secondary to my service connected PTSD) I contacted Dr. Drew for a Nexus letter describing how my OSA was aggravated by my PTSD. He provided a nexus letter that included scientific statements and his professional opinion that my OSA was indeed aggravated by PTSD.
I submitted my claim along with his Nexus letter and was approved for secondary connection at 50%!