most cases resolve within 2 years as most arise from blunt trauma causing neuropraxia:
sudden depression of the shoulder girdle
unusual twisting of the neck and shoulder
repetitive movements such a hedge clipping, car washing, digging
prolonged stretch of the nerve - lying on bed with arms abducted and propping up the head to read
local pressure from enlarged and inflamed bursae - subcoracoid or subscapular bursae
iatrogenic - chiropractic manipulation, use of a single axillary crutch, surgery
electric shock