Lewy bodies are tiny, abnormal clumps of protein that build up inside nerve cells in the brain. They are the hallmark of several serious neurological conditions, including Parkinson’s disease and a form of dementia called dementia with Lewy bodies (DLB). These protein deposits, typically 8 to 30 micrometers across (far too small to see without a microscope), gradually interfere with how brain cells function and eventually cause those cells to die.
What Lewy Bodies Are Made Of
The primary ingredient in a Lewy body is a protein called alpha-synuclein. In a healthy brain, alpha-synuclein helps nerve cells communicate with each other. But when this protein misfolds and clumps together, it forms the dense, rounded deposits we call Lewy bodies. Along with alpha-synuclein, these clumps contain ubiquitin (a protein the cell uses to tag damaged material for disposal), fragments of the cell’s internal scaffolding, and bits of mitochondria, the tiny structures that produce energy for the cell.
Under a microscope, a classical Lewy body looks like a small sphere with a dense core surrounded by a pale halo of radiating fibers. These are the type most commonly found in the brainstem. Lewy bodies that form in the outer layers of the brain (the cortex) look different: they lack the halo, appear more angular and disorganized, and are harder to spot. A neurologist named Friedrich Lewy first described these inclusions in 1912 while studying the brains of people who had died with Parkinson’s disease, finding them initially in the brainstem’s vagus nerve region.
How Lewy Bodies Damage Brain Cells
Lewy bodies don’t just sit passively inside neurons. They actively disrupt the cell’s ability to survive, though the exact mechanism differs depending on where in the brain they form.
In the cortex, where higher-order thinking happens, Lewy bodies appear to pull in microtubules, the tiny internal highways that transport nutrients and signals throughout a neuron. As these highways get sucked into the growing protein clump, the cell’s mitochondria (its power generators) get dragged along with them. Eventually, the neuron loses most of its energy-producing machinery. Without enough fuel, the cell can no longer maintain itself or communicate with neighboring cells, and it dies.
In the substantia nigra, the deep brain region responsible for movement, the damage takes a somewhat different path. There, the alpha-synuclein clumps interact with and degrade the cell’s nucleus, where DNA is stored and gene instructions are read. This nuclear breakdown appears to be the primary way these neurons are destroyed in Parkinson’s disease. The combined result of both mechanisms is depleted energy, damaged DNA, and collapsed transport systems, all leading to progressive nerve cell loss.
Where Lewy Bodies Spread in the Brain
Lewy body disease doesn’t strike the entire brain at once. The protein deposits appear to spread in a roughly predictable pattern over years or even decades. Current staging systems describe five general phases. The earliest deposits typically appear in the olfactory bulb, the brain’s smell-processing center. This is why a diminished sense of smell is one of the earliest warning signs of both Parkinson’s disease and Lewy body dementia, often showing up years before other symptoms.
From there, Lewy bodies spread to either the brainstem (which controls basic functions like sleep, heart rate, and movement) or the limbic system (which governs emotion and memory). In later stages, both regions are affected. In the most advanced stage, the deposits reach the neocortex, the brain’s outer layer responsible for complex thought, language, and perception. Which symptoms a person develops, and in what order, depends largely on where the deposits are concentrated at any given time.
Conditions Caused by Lewy Bodies
Lewy bodies are the defining pathology behind two major diagnoses: Parkinson’s disease and dementia with Lewy bodies. Both involve the same abnormal protein, but they’re distinguished clinically by which symptoms appear first.
In Parkinson’s disease, movement problems come first: tremor, stiffness, slowness, and balance difficulty. If cognitive decline develops, it typically arrives at least a year after motor symptoms began, at which point the diagnosis becomes Parkinson’s disease dementia (PDD). In dementia with Lewy bodies, the cognitive problems either come first or appear at roughly the same time as movement symptoms. This “one-year rule” is the main clinical distinction between the two.
DLB has four core features that help doctors identify it:
- Visual hallucinations that are vivid, detailed, and recurrent, often involving people or animals
- Cognitive fluctuations with unpredictable shifts in alertness and attention, sometimes from hour to hour
- Parkinsonism with spontaneous movement symptoms like stiffness or shuffling gait
- REM sleep behavior disorder, where a person physically acts out dreams, sometimes violently, during sleep
A probable DLB diagnosis requires at least two of these four features. REM sleep behavior disorder is particularly notable because it can appear 10 or more years before any cognitive or movement symptoms, making it one of the earliest detectable signs that Lewy body pathology may be developing.
Another important feature of DLB is severe sensitivity to certain antipsychotic medications. People with Lewy body pathology can have dangerous reactions to drugs commonly prescribed for hallucinations or agitation, making accurate diagnosis critical for safe treatment.
How Lewy Bodies Are Detected
For most of medical history, the only way to confirm Lewy bodies existed in someone’s brain was through autopsy. That’s changing. A newer type of test called a seed amplification assay (SAA) can detect misfolded alpha-synuclein in body fluids and tissues while a person is still alive. The test works by exposing a sample to conditions that encourage any misfolded protein present to replicate its shape, amplifying trace amounts to detectable levels.
The most reliable version uses cerebrospinal fluid (collected through a spinal tap), achieving about 89% sensitivity and 93% specificity in identifying Parkinson’s disease. Blood-based versions are nearly as accurate, detecting the abnormal protein with roughly 90% sensitivity and 91% specificity. Skin biopsies perform similarly well. Even saliva-based testing shows promise, though with somewhat lower sensitivity at around 77%.
These tests represent a major shift in how Lewy body diseases can be diagnosed. Rather than relying solely on clinical symptoms, which can overlap with Alzheimer’s disease and other conditions, doctors can now look for the underlying protein pathology directly. This is especially valuable in the early stages of disease, when symptoms may be subtle or ambiguous, and in distinguishing Lewy body dementia from Alzheimer’s, which requires different management approaches.
Why Lewy Bodies Matter for Patients and Families
Understanding that Lewy bodies are the root cause of these conditions helps explain some of their most confusing features. The day-to-day fluctuations in alertness that people with DLB experience, where someone seems sharp one hour and confused the next, reflect the way these protein deposits disrupt neural circuits unevenly. The vivid hallucinations aren’t a psychiatric problem but a direct consequence of Lewy body deposits in the brain’s visual processing areas. The sleep disturbances, constipation, blood pressure drops, and other autonomic symptoms that often accompany these diseases all trace back to Lewy bodies forming in the brainstem and peripheral nervous system, regions that regulate involuntary body functions.
Because symptoms can vary so widely from person to person, Lewy body diseases are frequently misdiagnosed as Alzheimer’s, psychiatric illness, or simple aging. The average time to an accurate DLB diagnosis spans multiple doctor visits over months or years. Recognizing the constellation of symptoms, particularly the combination of cognitive changes, visual hallucinations, movement problems, and sleep disturbances, is key to getting the right diagnosis and avoiding potentially harmful medications.