There is no set definition for a rapidly progressive dementia or RPD. I usually use the definition of a person going from normal cognition to dementia in less than two years. Usually, it’s a matter of weeks to months from normal cognitive function to dementia.
I define dementia as somebody who is impaired in one or more cognitive domains — memory, language, visual-spatial function, executive function, organizing, planning, multitasking — that they can no longer function the way they could prior to the onset of the condition. There has to be functional impairment due to cognitive dysfunction and usually it occurs over weeks to months. But I extend it up to two years.
Although the survival of most of the patients with prion disease is less than a year from onset, let alone onset to dementia which is much shorter — we do have patients who sometimes have a slower course of prion disease and they may not develop full dementia until up to two years or sometimes even longer. But the vast majority of RPDs, we’re usually talking about somebody who has less than a year from onset of symptoms to development of functional impairment due to cognitive dysfunction.