I have what is called Neuralgic Amyothrophy (aka Parsonage Turner Syndrome). It can have many possible causes including trauma from close by surgery, but is now thought to be a autoimmune disease if there is no trauma. There is also a hereditary version. The more common attack point would be the brachial plexus, impacting the shoulders. The most common visible symptom would be winged scapula. Both of mine are winged although the right is worse than left. I also have TOS of my right shoulder due to this. The theory as far as I understand it is that the collapse of the scapula pulls the trapezius which is attached near the first rib. This puts upward pressure on trap pulling and compressing the bundle of nerves and blood vessels against the collar bone.