Negative emg/nerve conduction study. negative mri of head/neck. physical therapy no help. weakness in left arm for 6+ months. what could it be?

Details: Without having more history or context, nor the results of what was done, it is difficult to make an assessment. Suggest you obtain a second opinion, whether it is here on a virtual concierge consult, or with other doctors either in your area or at a tertiary care center like Ohio State.

Find a neurologist or a physiatrist who has experience doing EMG testing of the brachial plexus, as you should be evaluated for Thoracic Outlet Syndrome.