I'm in HR. We've suffered our share of turmoil in my organization due to some difficult personalities. Here is my advice. Do not mention the medical situation, It is adequate to say, "As you can see I worked at prior employer for x years. A new manager was brought in, and it wasn't a good personality fit. 8 out of ten senior staff resigned or were terminated within six months. I also chose to leave.
I'm proud of having worked with many different people of all personality types in my ten-plus years (or whatever) and I haven't experienced a situation where I chose to leave due to a personality mis-match before. I do have an excellent reference from (others in the former org who will give you a reference).
Believe me, if you are interviewing for a senior level position, the people hiring you will have seen enough sh*t shows to get what you are saying: "guy was an ass".