According to what I've read, he would have suffered from Alzheimers disease and Lewy body dementia. The latter is the same that Robin Williams had had but never got a diagnosis for.

Today I learned in a roundabout way that my competent cancer doctor, that I used to see to follow up my case isn't working in that clinic any more, and instead I have my previous doctor -- who is a totally worthless idiot piece of sh*t who had done absolutely nothing a few years ago when I had clear symptoms of cancer.
That guy should have had his doctor's title taken away from him a long time ago.

the loss of Walter Koenig.

I didn't even know Patrick Stewart was that old.

I knew that Stewart was getting up there, but I had no idea that Koenig was in his 40s for TOS.