Dementia with Lewy bodies is a synucleinopathy: α-synuclein, the same misfolded protein behind Parkinson disease, aggregates in cortical and brainstem neurons. It accounts for roughly 5–10% of dementia. Two features distinguish it at the bedside, early visual hallucinations and fluctuating cognition, and one treatment caution makes recognition important.
Core clinical features
The hallucinations are visual, well-formed, and early: vivid people or animals in the room, unlike the fragmented hallucinations of delirium or the auditory hallucinations of schizophrenia. They arise because Lewy body deposition in visual association cortex disrupts normal visual processing, so the brain generates images the patient perceives as real.
Cognition also fluctuates markedly: lucid in the morning and profoundly confused by afternoon, with good days and bad days. During episodes the patient stares into space, seems disconnected, and responds poorly. This fluctuation reflects unstable cholinergic and dopaminergic transmission, and it distinguishes Lewy body dementia from the steady decline of Alzheimer disease and the stepwise drops of vascular dementia.
Supporting features
Other features support the diagnosis and each traces back to the same pathology spreading through the brain. Parkinsonism, the movement syndrome of rigidity and slowness, is present but usually mild: rigidity of neck and arms with slowness of movement, caused by Lewy body damage to nigral dopamine production. REM sleep behaviour disorder, in which patients act out dreams, sometimes violently, can precede the dementia by years; the brainstem circuits that normally paralyse muscle during REM sleep are damaged. Autonomic dysfunction such as orthostatic hypotension, urinary problems, and constipation reflects spread to autonomic pathways.
Lewy body dementia against its mimics
Several conditions share parts of this picture. The table sets Lewy body dementia beside Parkinson disease dementia, which has the same pathology but a different timing, and Alzheimer disease.
| Feature | Lewy body dementia | Parkinson disease dementia | Alzheimer disease |
|---|---|---|---|
| Onset | Insidious | Insidious | Insidious |
| Hallucinations | Visual, early | Visual, later | Often absent |
| Fluctuation | Prominent | Less prominent | Minimal |
| Parkinsonism | Within about a year of dementia | Years before dementia | Absent early |
| Memory | Preserved early | Preserved early | Lost early |
The hardest confusion is with delirium, since both fluctuate. Onset settles it: insidious and progressive points to Lewy body dementia, acute and illness-triggered to delirium.
Imaging clues
Imaging can support the clinical picture. Dopamine transporter SPECT (DaTscan, a scan of dopamine terminals in the striatum) shows reduced striatal dopaminergic uptake, which separates Lewy body dementia from Alzheimer disease, where dopaminergic imaging is normal.
FDG-PET, which maps regional glucose metabolism, shows occipital hypometabolism, the mirror image of Alzheimer disease, which spares primary visual cortex; relative sparing of the posterior cingulate within that pattern is sometimes called the cingulate island sign. MRI is less specific but may show less atrophy than the degree of cognitive impairment would suggest in Alzheimer disease.
Treatment balance
Three problems compete: cognitive decline, Parkinsonism, and psychosis, and each treatment can worsen another. Cholinesterase inhibitors, particularly rivastigmine, have the strongest support; the cholinergic deficit is even deeper than in Alzheimer disease, and response can include clearer thinking and fewer hallucinations. Levodopa helps Parkinsonism but is used cautiously because it can worsen hallucinations and psychosis.
The antipsychotic caution
Psychosis is the problem where the wrong drug does the most harm. Sensitivity to antipsychotics can be severe and even fatal. Because dopaminergic neurons are already depleted, dopamine-blocking drugs can precipitate profound rigidity, autonomic instability, and a neuroleptic malignant-like reaction. Typical antipsychotics should be avoided; when behavioural symptoms force the issue despite cholinesterase inhibitor treatment, agents with low dopamine D2 affinity are used with extreme caution under specialist supervision.