It’s almost like Alzheimer’s, but at a youthful age..............
I read long ago that the earliest recorded case of Alzheimers was 27.
They can't diagnose a patient while they are alive. They do a post mortem after death.
They do a battery of tests to eliminate other causes of dementia. If they can't find another cause, they determine it's Alzheimers.
It is a tragic, devastating disease for the patient as well as the caretaker. My family has first hand experience.