This is what happened: He went for a walk to sober up, but was assaulted and mugged. He panicked, as this triggered memories of childhood trauma for him, and switched to another personality, because he suffers from a yet undiagnosed multiple personality disorder. This is why he now thinks he's a 24-year old med student who goes by his middle name. His disorientation stems from the fact that he has suddenly found himself in a shady neighborhood of a city he doesn't recognize, with a considerable amount of blood alcohol and a major hangover.
But his colleagues at the ER don't know that. For all intents and purposes, he seems to be suffering from some kind of retrogate amnesia. He doesn't act different enough for them to identify it as another persona. They take note of how it seems like a case of Hollywood amnesia more than anything but they figure, there are more things between heaven and earth. Since he's a little banged up from the assault, they suppose it must be a head injury. However, tests show that it isn't. So they think it's drugs, but the only thing that shows up in his blood results is the alcohol. Eventually, they'll suppose it might be a dissociative state and call in a psych consult. I'm clear on how they would check for head trauma, and I know how the psychologist would approach this case. What I can't figure out is what else they would consider. It's a whole ER full of worried, personally invested doctors, and I'm sure that every single one of them would have ideas.
I found this case of memory loss after a mild head injury that didn't show up on a CT. That's pretty awesome intel and my doctors will definitely reference that case.
I know that amnesia can be caused by alcohol consumption and drug abuse. What I can't find out is, can drinking a lot one time cause such a severe case of amnesia? Is there a way of confirming that suspicion? He certainly was really, really drunk that night. Would it be different / more likely a cause if they had a suspicion that he's an alcoholic?
Are there any other possible causes that they might consider and check up on? How would they rule those out? I figure they'd run full tests to cover all their bases, but I'd still like to know which results they'd be looking at to check up on suspicions. It'll be a couple of days until they clue in on the real diagnosis, so I need all the food for speculation I can find.
Also! I'm considering that my protagonist should retain (parts of) his medical expertise in this persona. If it turns out that he still remembers his knowledge from being a doctor, would that tip his colleagues off that they misdiagnosed him? Would it be realistic if they decided it's a really unlikely thing to happen, but here it is happening anyway, "there are so many things we still don't know about the human brain"?
Googled: Any possible variation of amnesia, retrogade amnesia, dissociative state, fugue state, alcohol abuse, drug abuse, recovery and symptoms thereof. Wikipedia entries on the different kinds of amnesia. A lot of stuff on multiple personality disorder, obviously. Also I read the many entries tagged as "amnesia" and "multiple personality disorder" at this comm, and the many links provided in them. Found a lot on the typical symptoms of amnesia, how to diagnose amnesia in a differentiated fashion, also various rants about Hollywood amnesia. Typically though, amnesia seems to be caused by head injury in fiction, and that's just not what's happening here.