Myoclonus-How to tell if it's drug-induced or hypoxic brain injury?

My husband has had startle myoclonus (recently diagnosed) since 2005. His records bounce back and forth from saying he had a brain injury, or questioning if it was actually just myoclonus. He did experience mild cognitive problems in the beginning, but they are all gone now. I believe that he might have had a hypoxic brain injury that has mostly healed, but before it could they put him on too much medication. He was on 7+ medications at once for 3.5 years. He had tremors, seizures, ataxia, etc. And all of these symptoms fluctuated with his medicines. Now that he's off of them, he only has the abnormal startle response. My question is: How can you tell if it was drug induced or from hypoxia? And please, can you tell me which neurotransmitters go wrong with myoclonus? Thank you