How is Alzheimer’s Disease Different from Dementia?

A common question we get, both in the clinical and research settings, is, “What is the difference between dementia and Alzheimer’s disease?” The terms and language around these disorders are confusing. 

Dementia is a clinical diagnosis. It means that someone is having cognitive difficulties, in at least two domains of thinking, that are bad enough to cause the person to have trouble successfully doing the things they need to do in their daily life. “Domains” of cognition include memory, visuospatial ability, executive functions, and language abilities. Cognitive or “neuropsychological” testing is often used to assess performance in different cognitive domains. Trouble successfully doing daily activities can manifest as trouble paying bills, forgetting appointments, remembering to take medications, and getting lost. Since dementia is a clinical diagnosis, it is based on the clinical presentation of the patient, often supplemented by information from a loved one and cognitive testing. 

The diagnosis of dementia is in contrast with the diagnosis of “Mild Cognitive Impairment” in which someone has cognitive symptoms, often in just one domain, which do not significantly impair daily functioning. All patients with dementia pass through a phase of Mild Cognitive Impairment prior to developing dementia, but the converse is not necessarily true: while many do, some patients with Mild Cognitive Impairment do not progress to dementia.

When someone has dementia, the next logical question is, “What illness is causing the dementia”? Alzheimer’s disease is a diagnosis of what we believe is going on in the brain to cause the cognitive difficulties we observe clinically. The definitive diagnosis of Alzheimer’s disease can only be made after death through a neuropathological examination of the brain. Alzheimer’s disease is defined by the presence of a certain amount of amyloid plaques and tau tangles in the brain on neuropathological examination. 

Alzheimer’s disease is the most common cause of dementia, but it is not the only cause. Other causes of dementia include Lewy Body Disease, Frontotemporal dementia, and Vascular Dementia. While Alzheimer’s disease usually presents with memory symptoms, some of these other causes of dementia can present with symptoms primarily in other cognitive domains. We have also learned that atypical cases of Alzheimer’s disease can present with symptoms in domains other than memory, such as language.

We often utilize biological measures, including brain MRI and PET scans, to help us diagnose Alzheimer’s disease in living patients. Since dementia is a clinical diagnosis and Alzheimer’s disease is the diagnosis of a brain illness, dissociations between these diagnoses can occur. For example, because the brain changes of Alzheimer’s disease start before clinical symptoms manifest, there may be a situation in which elevated amyloid plaques and tau tangles are detected in a person who is cognitively normal.  This person would have the biological changes of Alzheimer’s disease, but without dementia at that time. Although this person would be at high risk for developing dementia later.