It is possible to have lots of plaques and tangles, but perhaps because you have wonderful vessels in the brain, and the blood perfusion is perfect, you don't have clinical dementia. In which case you don't have Alzheimer's. The brain may meet the criteria for Alzheimer's, but unless you have the clinical syndrome, you can't diagnose Alzheimer's. You can say the brain fulfills neuropathological criteria for Alzheimer's but the patient does not have Alzheimer's disease. We used to say, well, in Alzheimer's disease it goes slowly and progressively.
Vascular dementia goes stepwise.
This is with strokes, with big lesions. But there are these tiny lesions that may happen in the brain, which sometimes may cause catastrophic changes.