Interesting. I have a similar condition, and - as I'm a physio - I'm similarly baffled.

My left pec and long head of tricep have both lost bulk and tone since (I'm embarassed to say) I had treatment from another physio.

I happened to be consulting near a physio who fitted me in to help with some mid-back pain I'd been getting. The mid-back thing, I believe, was unrelated - just simple stiffness. Anyway, towards the end of the session, where he'd been providing good soft tissue and I was feeling good, he mentioned tightness in my posterior shoulder. I said I thought I was probably tight through the capsule and could probably do with some specific work there.

He fully abducted my arm, I don't recall if there was any rotation applied, and then tractioned the joint (i.e. pulled outwards to gap the joint). He sustained that for a few 5-second bursts. I'd never come across treatment like this but thought it probably made sense and let him carry on. There was no pain at the time and it didn't concern me.

Over the next few days at the gym I found my ability to press (standing single arm cable press - like a punch, great whole-body exercise) or to extend my elbow and shoulder (during tricep pressdowns or dumbell kickbacks, which I used to test the realtiv strength of left to right) was severely limited and my control of the movements had just gone - I was wobbling all over the place. (Slow right tricep kickback: 10kg; on the left: 1kg!!!) Freaked me out!

The muscle mass dropped away over following weeks. Control came back over the next month or two and strength has improved somewhat. It's now about 6 months later but my pressing strength on my affected left side remains about 50% that of my right.

My impression is that a few predisposing factors conspired against me: I tend to be stiff generally (45-year old male), have forward head posture (thanks dad and grandpa), and may have some disc bulges and degeneration through my spine as a result of a fairly physical life. So the traction was simply too much and a portion of my brachial plexus (somewhere between C6 and T1) has simply snapped.

With time this may come back. I'm hoping.

Your chiro's comment re prolapsed disc may be correct.

Whatever our conditions bases, it's clearly a neural problem. In my case it's likely I'll just have to wait it out and hope the nerve regrows along its original 'track', assuming traction was the problem; same for you if the sleeping position tractioned or compressed a nerve. If our common problem is a disc bulge/protrusion/rupture, though, we may have a permanent pressure on a nerve. Surgery could be an option. I don't think the electrical stim will help.

The lack of pain suggests our problem is with the motor nerve only.

Is it neuropraxia I'm talking about?, when the nerve cell dies away but the myalin sheath remains intact, providing a course for the nerve to grow back along? And rate of regrowth: what is the rate? I seem to recall something like a millimeter a day...?

Anyone else have some thoughts? (Neuro ain't my thing, clearly)

Regarding your best course of action: A consult with a neurologist is probably your best bet.