In 2005 when I first saw Dr Schwartzman he dx'd cervical plexus irritation as well as brachial plexus - this was first I'd heard of it. But it makes sense if you think of how the pain radiates from the thoracic outlet. It radiates upand across to the shoulder and down the arms to the hand, but also UP, from the scalenes and the spasmed upper back muscles, which is why we get the nasty trigger points and the horrid headaches.
Triggerpoint injections of lidocaine may give you even longer relief, although the added steroid may have given extra benefit. Problem is you don't want to use the steroids too often. But if you can hit the worst trigger points at the traps and lower neck you may find you don't need the upper cervical injections. Have someone carefully feel over the upper 1/2 of your back, especially the inner areas near the shoulder blade borders, and note the most
painful spots, the ones that make you hiss. Will feel as though there is a small pea or rock below the skin, rolling away from your fingers.
These muscles are aggravated because the nerves that provide power to them are compressed in the outlet, so the muscle isn't getting enough power from the source, so it can't function properly - a sick nerve means a sick muscle. (This is what kills me - then they tell you you have to "strengthen" those muscles - easy to SAY!!) But it works like a domino effect - each adjacent muscle served by the same nerve is also sick and aggravated, so the pain and spasms continue along the whole length of the affected nerve.
Best to try and find where it's affected the WORST and treat it there, the relief should be greatest along the entire nerve.
Be sure to dress warmly and wear a scarf around your neck to keep the effects lasting. Even better results may be had when the weather is warmer.
One problem I had was the pooling of the injection meds around the lymph nodes above my collar bones - it was very slow to drain the last few times!
Wish you continued success!