This is one of the most interesting—and still evolving—stories in neurology. The short answer is that Parkinson’s disease and Lewy body dementia were discovered from opposite directions. They were initially defined by different symptoms rather than by their shared biology.
Here’s how that happened.
The beginning: Parkinson’s disease came first
James Parkinson published An Essay on the Shaking Palsy in 1817. He described people with:
- Tremor
- Stooped posture
- Slow movement
- Difficulty walking
He didn’t know what caused it—he simply described the syndrome. For almost a century, Parkinson’s disease was considered purely a movement disorder.
The discovery of Lewy bodies
Around 1912, Friedrich Lewy was examining brains from people with Parkinson’s disease and noticed strange protein clumps inside nerve cells.
Those clumps eventually became known as Lewy bodies.
At the time, no one knew what they meant. They were simply considered a pathological hallmark of Parkinson’s disease.
A surprising observation
Over the next several decades, doctors noticed something odd.
Some patients with Parkinson’s:
- developed dementia many years later
while others:
- became confused,
- hallucinated,
- lost attention,
- fluctuated dramatically,
sometimes before their movement symptoms became severe.
These patients didn’t fit neatly into “Parkinson’s disease.”
The birth of Dementia with Lewy Bodies
In the 1980s and 1990s, neuropathologists began examining brains from these patients.
They found…
The exact same Lewy bodies.
The difference wasn’t the protein.
The difference was where it accumulated first.
In classic Parkinson’s disease:
- Lewy pathology usually begins in the lower brainstem.
- Motor circuits are affected first.
- Dementia often develops years later—if it develops at all.
In Dementia with Lewy Bodies:
- Lewy pathology reaches the cortex much earlier.
- Thinking, attention, visual processing, and hallucinations appear early.
- Parkinsonism develops alongside or after cognitive symptoms.
Same protein.
Different distribution.
Different timing.
The “one-year rule”
Clinicians still needed a practical way to distinguish the diagnoses.
They adopted what’s called the one-year rule.
If dementia begins:
- before, or within one year of, Parkinsonian motor symptoms → Dementia with Lewy Bodies (DLB)
If dementia develops:
- more than one year after established Parkinson’s disease → Parkinson’s disease dementia (PDD)
Here’s the remarkable part:
That one-year cutoff is not based on biology.
It’s a clinical convention so doctors can communicate consistently.
Many researchers acknowledge it’s somewhat arbitrary.
Modern understanding
Today, many neurologists view these conditions as a spectrum.
You’ll often hear the term:
Lewy Body Disease
with several clinical presentations:
- Parkinson’s disease
- Parkinson’s disease dementia
- Dementia with Lewy Bodies
Rather than completely separate diseases.
Think of it like different points along the same continuum.
Why are they still separated?
There are good practical reasons.
Different specialists often see the patients first:
- A movement disorders neurologist sees tremor and stiffness → Parkinson’s.
- A cognitive neurologist sees hallucinations and memory problems → DLB.
The expected progression differs.
Medication choices differ.
For example:
- DLB patients are especially sensitive to many antipsychotic medications.
- Parkinson’s patients often need more aggressive treatment of motor symptoms.
Clinical trials also need clearly defined patient groups.
So even though the underlying pathology overlaps, keeping the diagnoses separate still has value.
The future
The field is moving toward diagnosing people by biology rather than by symptom timing.
Researchers are developing tests that detect abnormal alpha-synuclein, the protein that forms Lewy bodies, in spinal fluid, skin biopsies, and other tissues. These biomarkers may eventually allow doctors to diagnose Lewy body disease before significant symptoms appear and may replace today’s symptom-based categories.
In fact, many experts now describe them as different clinical expressions of the same alpha-synuclein disease, with the major differences being which brain networks are affected first and how the illness unfolds over time, rather than a fundamentally different disease process.
Uncle Lewy 