Lewy Body Dementia or Parkinson's Dementia? The Timing Rule Doctors Use
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A woman in her late sixties begins telling her daughter about children playing in the living room each evening. They are detailed, and she describes what they are wearing. Some days she is sharp and handles her own bills. Other days she stares and cannot follow a conversation. A few months later, she starts to shuffle when she walks.
Her family wonders about Parkinson's disease, or Alzheimer's, or something psychiatric. A specialist raises another possibility, Lewy body dementia, and asks a question that changes the conversation: which symptoms came first, the thinking changes or the movement changes? This story is a composite of conversations we have had with families.
What Lewy Body Dementia Is
The National Institute of Neurological Disorders and Stroke (NINDS) explains that Lewy body dementia is associated with abnormal deposits of a protein called alpha-synuclein in the brain. It has two forms: dementia with Lewy bodies and Parkinson's disease dementia. NINDS says that although they have some differences, the two forms are related.
It is a common condition. NINDS describes Lewy body dementia as one of the most common causes of dementia, after Alzheimer's disease and vascular disease, and says it typically begins at age 50 or older.
NINDS also notes that dementia with Lewy bodies is often hard to diagnose, because early symptoms can resemble Alzheimer's disease or a psychiatric illness. That is why families sometimes spend months or years with a different label first. The five clues below describe how clinicians tell the two forms apart and what points toward Lewy body dementia in the first place. None of them is a test on its own.
Clue 1: Which Came First
The first clue is timing. The 2017 consensus report from the international DLB Consortium, published in the journal Neurology, says that dementia with Lewy bodies should be diagnosed when dementia occurs before, or at the same time as, parkinsonism. Parkinsonism means slowness, stiffness, or tremor.
NINDS describes the other side of the rule this way: if dementia develops more than a year after movement symptoms appear, Parkinson's disease dementia may be diagnosed.
The consortium recommends this one-year cutoff for research studies that need to separate the two. In everyday care, the label matters less than getting the right treatment and safety plan. It helps to write down, with dates, when the movement changes began and when the memory and thinking changes began.
Clue 2: Good Days and Bad Days
The second clue is fluctuating thinking. The consortium lists it as a core feature, described as pronounced variations in attention and alertness. NINDS describes unpredictable changes in concentration, attention, alertness, and wakefulness.
Families often describe it as someone who is clear in the morning and drifting by afternoon, or sharp for a day and then staring for hours. A record of the pattern, including what time of day it happens and what the person was doing, gives a specialist something concrete to work with.
Clue 3: Visual Hallucinations
The third clue is visual hallucinations. The consortium describes them as recurrent and typically well formed and detailed, such as people or animals. NINDS says visual hallucinations occur in most people with Lewy body dementia, often early in the disease.
Hallucinations can also appear as a side effect of levodopa, the main Parkinson's medication, and NINDS says so directly. That is a reason to tell the prescriber about hallucinations promptly, rather than waiting for the next scheduled visit. We have a full video on how to respond to hallucinations calmly: How to Respond to Hallucinations.
Clue 4: Acting Out Dreams
The fourth clue is REM sleep behavior disorder, in which a person physically acts out dreams. They may shout, flail, or fall out of bed. The consortium lists it as a core feature of dementia with Lewy bodies and notes that it may precede cognitive decline. NINDS describes the same sleep change among the symptoms of Lewy body dementia.
If a family has noticed dream enactment for years before any memory change, that history is worth telling the specialist.
Clue 5: Medication Sensitivity
The fifth clue is a safety issue as much as a diagnostic one. The consortium lists severe sensitivity to antipsychotic medications as a supportive feature. NINDS says people with Lewy body dementia may have severe reactions or side effects from antipsychotics, and that typical antipsychotics such as haloperidol generally should not be prescribed.
NINDS adds that many Lewy body dementia experts prefer quetiapine or clozapine when a behavioral symptom needs medication. For thinking symptoms, cholinesterase inhibitors may reduce hallucinations and other psychiatric symptoms. Very low doses of clonazepam are often effective for REM sleep behavior disorder. This is information for conversations with the care team and is not a prescribing recommendation.
Any medication decision belongs to the prescriber. If someone has Lewy body dementia, or it is being considered, tell every clinician, emergency room, and care facility before a new medication is started.
The Honest Caveat
There is no single test that confirms Lewy body dementia. The consortium lists supportive tests, including a dopamine transporter scan, a heart scan called MIBG scintigraphy, and a sleep study showing REM sleep without muscle paralysis. Diagnosis still rests on the history and examination.
NINDS says the two forms are related, and that dementia with Lewy bodies symptoms may worsen more quickly than Parkinson's disease dementia. It also states there is currently no cure, though symptoms can be treated.
A hallucination or a bad day does not mean Lewy body dementia. These clues exist to help a family ask better questions at an evaluation.
What to Bring and What to Ask
- A one-page timeline with dates for movement changes, memory and thinking changes, hallucinations, and any dream enactment
- A list of every medication with start dates
- A short phone video of a good day and a bad day, if possible
Then ask the specialist three questions. Do you see features of Lewy body dementia here? Which medications should we avoid? And what should every other clinician and the emergency room know about this diagnosis?
This article is educational and does not replace medical advice. Do not start, stop, or change any medication without talking with the prescriber. The opening story is a composite of conversations with families.
Sources: National Institute of Neurological Disorders and Stroke, Lewy Body Dementia. McKeith, I.G., et al. (2017). Diagnosis and management of dementia with Lewy bodies: Fourth consensus report of the DLB Consortium. Neurology, 89(1), 88-100.