Is it possible to become UNenlightened? Is it possible for i.e. an Arhat to become unenlightened after he reached the state of Nirvana?

What if an enlightened being gets a severe brain injury by some cause leaving him with amnesia.
Could it affect one's enlightenment in any way?
No! Once you attain Nibbana, the defilements are annihilated without the possibility of return. Just like uprooting a tree instead of chopping it down or trimming it. So it can never grow back from roots. Nibbana does not depend on the condition of the body or any other thing. It is uncaused, unborn and permanent. Venerable Moggalana was beaten up by thugs until his bones were pulverized. Still his mind was at perfect peace. One Arahath thero in Sri Lanka was thrown into a cauldron of boiling oil by the order of a King. That even caused a Tsunami in sri Lanka. But he was still at perfect peace when his life ended.

Take a Sothapanna person as an example. He can die 6 times before attaining Arhathship. If death itself is not going to affect his attainment, how can brain damage affect it?