Permanent brain damage can cause someone to loose their Arihathness? Brain is directory connected to mind. Examining the differences of wave patterns that emit from brain when someone is mad or meditating explains there is a direct connection. What if someone achieved nirvana or a lower state of the path to nirvana, and met in an accident and lost all his memories or had permanent brain damage due to a disease .  As an example in alzheimer’s disease brain shrinks and it losses its memories and abilities to think . So in a situation like this , can someone loose his state in the path to nirvana ?
Brain damage might reduce mental capacity.

An Arahat is free from the unwholesome roots, i.e., they have stopped reacting to sensual stimuli. Brain damage or not unwholesome roots do not arise again. 

So they do not lose their attainment.