Lewy body dementia is caused by abnormal clumps of a protein called alpha-synuclein building up inside nerve cells in the brain. These clumps, known as Lewy bodies, damage and eventually kill neurons in regions that control thinking, movement, behavior, and mood. The disease typically begins around age 57 to 59, and while scientists have identified several genetic and environmental factors that raise the risk, the full picture of why some people develop it remains incomplete.
How Protein Buildup Damages the Brain
Alpha-synuclein is a normal, abundant protein found in healthy neurons. Under typical conditions, it exists as a loosely structured molecule floating freely inside cells. Problems start when the protein changes shape. When alpha-synuclein comes into contact with the fatty membranes inside cells, it folds into a more rigid structure that makes individual molecules stick together, first forming pairs, then small clusters called oligomers. These oligomers are thought to be the most toxic form of the protein, directly poisoning nerve cells.
Over time, these small clusters continue to pile up into larger, insoluble masses that become Lewy bodies. Chemical modifications to the protein, such as the addition of phosphate groups or other molecular tags, accelerate the clumping process and increase its toxicity. The brain cannot clear these deposits effectively, so they accumulate and interfere with normal cell function. Lewy bodies appear in the brainstem as well as across the frontal and temporal lobes of the cerebral cortex, producing a wide range of symptoms depending on which regions are most affected.
The Role of Brain Chemistry
As Lewy bodies destroy neurons, they disrupt the brain’s chemical messaging systems. Two neurotransmitters take the hardest hit: dopamine and acetylcholine.
Dopamine loss in areas connecting to the frontal brain impairs the ability to adapt to changing situations and contributes to the slowness, stiffness, and tremor that resemble Parkinson’s disease. But the cognitive decline in Lewy body dementia appears to be driven more by a severe loss of acetylcholine, the chemical messenger critical for attention, memory, and clear thinking. Research in Neurology found that people with Lewy body dementia show dramatic acetylcholine depletion across the outer layers of the brain. This dual chemical disruption is what gives the disease its distinctive combination of movement problems and fluctuating mental clarity.
Genetic Risk Factors
Most cases of Lewy body dementia are not directly inherited, but genetics play a meaningful role in who develops it. The clearest genetic links involve four genes:
- SNCA and SNCB: These genes carry the instructions for making alpha-synuclein and a related protein. Mutations in either gene can directly cause Lewy body dementia, inherited in a pattern where a single copy of the altered gene is enough to trigger the disease. These cases are rare.
- GBA1: Variants in this gene, which provides instructions for an enzyme involved in cell cleanup, increase the risk of developing Lewy body dementia but don’t guarantee it. Having one altered copy raises susceptibility rather than serving as a direct cause.
- APOE e4: The same version of this gene that raises Alzheimer’s risk also increases the likelihood of Lewy body dementia. Again, carrying one copy raises risk but does not make the disease inevitable.
For most people who develop Lewy body dementia, no single gene is responsible. The disease more likely results from a combination of genetic susceptibility and other factors accumulated over a lifetime.
Environmental Triggers
A growing body of research points to environmental exposures that may set the stage for Lewy body formation. Heavy metals, particularly mercury, can trigger alpha-synuclein to clump together and have been linked to cognitive damage in the brain. Certain pesticides have repeatedly been identified as risk factors for neurodegenerative diseases involving alpha-synuclein, though not all pesticides carry this risk.
Air pollution exposure may act as a triggering factor, and infections with certain microorganisms appear capable of initiating the disease process in some people. Researchers have also noted that environmental exposures and adverse climate conditions can fuel the kind of chronic brain inflammation that drives neurodegeneration, with some studies reporting increased incidence of Lewy body dementia in connection with climate-related environmental changes. These are considered modifiable risk factors, meaning they could theoretically be reduced through public health measures, unlike genetic predisposition.
Overlap With Alzheimer’s Pathology
Lewy body dementia rarely exists in isolation. At autopsy, more than 50% of people diagnosed with the condition also have the amyloid plaques and tau tangles characteristic of Alzheimer’s disease. This overlap is not random: it increases dramatically with age. Among people with Lewy body dementia around age 55, roughly 60% have no detectable Alzheimer’s pathology. By age 85, that figure drops to about 20%, meaning the vast majority of older patients are dealing with both disease processes simultaneously.
This mixed pathology has real consequences. Amyloid buildup appears to push the onset of dementia earlier in the disease course, while tau pathology may delay or reduce the severity of movement symptoms. The interplay between these two disease processes helps explain why Lewy body dementia can look so different from one person to the next, and why it is frequently misdiagnosed as Alzheimer’s.
How It Differs From Parkinson’s Disease
Lewy body dementia and Parkinson’s disease are closely related. Both involve alpha-synuclein deposits, dopamine loss, and overlapping symptoms. The clinical distinction comes down to timing: if cognitive decline develops before movement symptoms or within one year of their onset, the diagnosis is dementia with Lewy bodies. If movement symptoms come first and dementia follows at least a year later, it is classified as Parkinson’s disease dementia. Together, these two conditions make up the spectrum of Lewy body dementia, one of the most common forms of neurodegenerative dementia.
Recognizing the Pattern
The 2017 diagnostic criteria identify four core features that distinguish Lewy body dementia from other forms of cognitive decline. Fluctuating cognition, with noticeable swings in attention and alertness, is one of the earliest and most characteristic signs. Recurrent visual hallucinations, often vivid and detailed (seeing people or animals that aren’t there), are another hallmark. A sleep disorder called REM sleep behavior disorder, in which people physically act out their dreams by kicking, punching, or shouting during sleep, frequently appears years before any cognitive symptoms. Movement changes resembling Parkinson’s disease, including slowness, rigidity, and tremor, round out the core features.
Beyond these, a range of supportive symptoms help paint the full picture: repeated falls, fainting episodes, severe constipation, blood pressure drops upon standing, urinary incontinence, excessive daytime sleepiness, loss of smell, depression, anxiety, and apathy. One notable feature is a dangerous sensitivity to antipsychotic medications, which can cause severe, sometimes life-threatening reactions in people with Lewy body dementia. Early in the disease, prominent memory loss may not be present. Instead, trouble with attention, planning, and the ability to judge spatial relationships tends to stand out, which is part of why the condition is so often missed or confused with other diagnoses.