Lewy body dementia is the third most common dementia after Alzheimer's and vascular dementia. The pathology — alpha-synuclein aggregates (Lewy bodies) in cortex and subcortical structures — is shared with Parkinson's disease, with the distinction lying in timing of motor versus cognitive onset.
The four core clinical features (McKeith criteria, 2017):
Fluctuating cognition with marked day-to-day variation in attention and alertness. A patient may be conversational and clear one day, confused and somnolent the next. The fluctuation is striking enough that families often describe it without prompting.
Recurrent visual hallucinations that are typically well-formed and detailed. Often children, animals, or familiar people. Some patients have insight (recognize the hallucinations as not real); others do not. Distinguished from delirium hallucinations by the relatively preserved alertness during their occurrence.
REM sleep behavior disorder — acting out dreams during sleep. Often pre-dates dementia by years to decades; the strongest prodromal marker of synucleinopathy. Bed partners report being kicked, punched, or hearing the patient shout during sleep.
Parkinsonism — bradykinesia, rigidity, sometimes tremor. Usually less prominent than in Parkinson's disease and emerging in the context of dementia. The "1-year rule" distinguishes LBD (motor and cognitive symptoms within 1 year of each other) from Parkinson's disease dementia (motor symptoms preceding cognitive by more than 1 year).
Two or more core features = probable LBD.
Additional supportive features: severe autonomic dysfunction (orthostatic hypotension, urinary incontinence), excessive daytime sleepiness, hyposmia, hallucinations in other modalities, severe sensitivity to antipsychotic medications.
Neuroleptic sensitivity is hallmark and dangerous. Standard doses of antipsychotics can produce severe parkinsonism, autonomic instability, marked sedation, and rare but devastating neuroleptic malignant syndrome. Avoid haloperidol, risperidone, olanzapine in suspected LBD. When antipsychotic needed: quetiapine at very low dose (12.5-25 mg) or pimavanserin (FDA-approved for Parkinson's disease psychosis, used off-label in LBD — no dopamine receptor effects, safest option).
Treatment of cognitive symptoms: cholinesterase inhibitors often particularly helpful in LBD — rivastigmine has best evidence and is FDA-approved for Parkinson's disease dementia. May help both cognition and visual hallucinations.
Treatment of behavioral symptoms: address modifiable triggers first (pain, infection, constipation, medication review). For severe agitation, pimavanserin or low-dose quetiapine cautiously. Avoid benzodiazepines (worsen confusion, fall risk).
Treatment of parkinsonism: levodopa may help motor symptoms but can worsen visual hallucinations and confusion. Lower doses typically used than in Parkinson's disease. Risk-benefit individualized.
When you encounter a patient with dementia plus fluctuating cognition, visual hallucinations, RBD history, and parkinsonism, LBD is the diagnosis. The antipsychotic caution is essential — knowing the diagnosis prevents iatrogenic harm.