HNPP can also share many symptoms with thoracic outlet syndrome (TOS) for short..
If you have done a lot of repetitive or desk work,or any prior whiplash / sports injury, TOS might be a possibility. Or if you have had other RSI injuries ? (carpal tunnel/ golfers elbow, etc) Those can set you up for TOS/TOS like conditions.
Useful TOS forum sticky thread-
http://neurotalk.psychcentral.com/thread84.html