Severe Right Shoulder Pain. Nerve Conduction Test Shows C8 Nerve

Severe right shoulder pain. Nerve conduction test shows c8 nerve compression, however a ct scan shows no problems in neck. Referred to shoulder specialist . Debilitating pain, not able to work as a mailman, and thus far doctors are not saying it is work related? See shoulder doctor in 2 weeks, looking for what the possibilities as to cause, currently taking pain meds, but would like to know possible causes rather than keep dealing with side effects.