All forms of dementia — Alzheimer’s disease included — share overarching diagnostic criteria. According to the fifth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5), a diagnosis of dementia (which they classify as a “major neurocognitive disorder”) requires meeting the following criteria (using a person’s medical history and cognitive assessment):
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An impairment in one or more cognitive domains (e.g., executive function, language, memory.)
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A decline in cognitive ability compared with past levels of functioning
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An impairment in the ability to function independently in daily life
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Symptoms not occurring as a result of delirium
In addition to a dementia diagnosis, conclusively determining whether a person has Alzheimer’s disease requires an autopsy to see if amyloid plaques are present in the brain. This limits diagnosis to after death, which is not ideal. To help diagnose a person while still alive, other types of dementia can be ruled out, thereby increasing the likelihood that Alzheimer’s disease is present. As a result, Alzheimer’s has been referred to as a “disease of exclusion”.
Some other common types of dementia are Lewy body dementia, vascular dementia, and frontotemporal (frontal and temporal lobe) dementia. In general, they vary somewhat in their presentation.
| Type | Pathology | Percentage of all dementia | Characteristics |
|---|---|---|---|
| Alzheimer’s disease | Amyloid plaques and neurofibrillary tangles in the brain | 60–80% | Gradual onset, Progressive course, Multiple cognitive deficits, including memory impairment |
| Vascular dementia | Marker of cardiovascular disease (CVD) event (stroke, blockage in arteries) | 20% | Abrupt onset, fluctuating course, history of CVD event, focal deficits (abnormal gait, muscle weakness, etc.) |
| Lewy body dementia | Alpha-synuclein aggregates in the brain | 5–15% | Fluctuations in cognition, hallucinations, difficulty walking |
| Frontotemporal dementia | Degeneration of frontal and temporal lobes | 3% (people >65 years old) and 10% (people <65 years old) | Difficulty with language (aphasia), executive dysfunction, visuospatial function usually unaffected |
Finally, because Alzheimer’s disease nearly universally features a buildup of amyloid plaques (covered in more detail in the next section), PET scans can be used to differentiate Alzheimer’s disease from other dementia types.[5] Typical radiolabeled compounds are infused into a person’s bloodstream to run these tests. Following infusion, these compounds will make their way into the brain. They have a high affinity for amyloid, meaning the compounds will temporarily accumulate in the brain if more amyloid is present or flow out of the brain if little amyloid is present. When the brain is then imaged using a PET scan, the compounds are visible and serve as a proxy for how much amyloid is present, allowing for a more conclusive diagnosis of Alzheimer’s disease. The use of these PET scans is not widespread, though, due in part to the limited clinical utility (definitive Alzheimer’s diagnosis doesn’t typically alter the treatment plan).