Neurodegenerative disease (alzeihmer) or dementia or depression in my grandparent?

Can be hard to tell: Alzheimer's & other neurodegenerative diseases often cause dementia, a clinical syndrome of chronic cognitive impairment. So the real question is whether dementia is present, & if so, why. It could be due to alzheimers, strokes, toxins, etc. Depression in the elderly can look like dementia, & in fact is sometimes termed "pseudo-dementia." a careful exam (or testing) can tell them apart.

Without knowing : Your grandparent, i can't say. All dementias are neurodegenerative illnesses in one way or another. And, yes, depression in older patients can present with memory problems. Have your grandparent evaluated by a geriatric psychiatrist to help you figure this out.