Parkinson's Disease (PD) is a progressive neurodegenerative disorder characterized by the progressive loss of dopaminergic neurons in the substantia nigra pars compacta and the presence of Lewy bodies—intracellular inclusions composed primarily of alpha-synuclein. It is the second most common neurodegenerative disease after Alzheimer's Disease, affecting approximately 1-2% of the population over 65 years of age and up to 4% of those over 85.
First described by James Parkinson in his 1817 essay "An Essay on the Shaking Palsy" and later characterized in detail by Jean-Martin Charcot, Parkinson's disease is characterized clinically by resting tremor, bradykinesia, rigidity, and postural instability—collectively known as the cardinal motor symptoms.
- Prevalence: Approximately 10 million people worldwide
- Age: Affects approximately 1% of those over 60 and 3-5% of those over 85
- Gender: Men are 1.5 times more likely to develop PD than women
The aggregation of alpha-synuclein into soluble oligomers and insoluble fibrils is central to PD pathogenesis. This process is thought to be toxic to neurons through multiple mechanisms:
- Loss of normal function: Alpha-synuclein normally regulates synaptic vesicle trafficking and neurotransmitter release
- Gain of toxic function: Oligomers and fibrils disrupt cellular membranes, impair mitochondria, and activate inflammatory pathways
- Prion-like propagation: Pathological alpha-synuclein may spread between neurons
While most PD cases are sporadic, approximately 10-15% have a familial pattern. Key genetic forms include:
| Gene |
Inheritance |
Onset |
Mechanism |
| SNCA |
Autosomal dominant |
30-50 years |
Alpha-synuclein aggregation |
| LRRK2 |
Autosomal dominant |
50-70 years |
Kinase hyperactivity |
| GBA |
Risk factor |
Variable |
Lysosomal dysfunction |
| PARK2 (Parkin) |
Autosomal recessive |
<40 years |
Impaired mitophagy |
| PINK1 |
Autosomal recessive |
20-40 years |
Mitochondrial QC failure |
Mitochondrial impairment is a key pathological feature:
- Complex I deficiency observed in substantia nigra of PD patients
- Toxins that inhibit complex I (MPTP, rotenone) induce parkinsonian features
- PINK1 and Parkin function in mitochondrial quality control (mitophagy)
Microglial activation and chronic neuroinflammation contribute to neurodegeneration:
- Post-mortem studies show elevated inflammatory markers in PD brains
- Microglia surround Lewy bodies
- The gut-brain axis may propagate alpha-synuclein pathology from the enteric nervous system
- Resting tremor: 4-6 Hz tremor in the hands ("pill-rolling"), typically asymmetric at onset
- Bradykinesia: Slowness of movement, including reduced blink rate, hypomimia, micrographia
- Rigidity: Increased muscle tone, lead-pipe or cogwheel rigidity
- Postural instability: Impaired balance and falls, typically developing later
Non-motor symptoms can precede motor symptoms by years or decades:
- Sleep disorders: REM sleep behavior disorder (RBD), insomnia
- Autonomic dysfunction: Orthostatic hypotension, constipation, urinary dysfunction
- Neuropsychiatric symptoms: Depression, anxiety, apathy, visual hallucinations
- Cognitive impairment: Executive dysfunction, memory problems, eventually dementia
- Sensory symptoms: Hyposmia (loss of smell), pain, paresthesias
PD progresses over 15-25 years, with motor complications developing in most patients after long-term levodopa therapy:
- Motor fluctuations: "Wearing off" phenomenon, on-off fluctuations
- Dyskinesias: Involuntary movements, typically choreiform
Diagnosis remains clinical, based on UK Parkinson's Disease Society Brain Bank criteria:
- Presence of bradykinesia plus at least one other cardinal symptom
- Asymmetric onset
- Exclusion of alternative causes
- Response to levodopa or dopamine agonists
- Presence of hyposmia
- REM sleep behavior disorder
- Dopamine transporter SPECT imaging (DaTscan)
- Imaging: DaT SPECT, MRI, PET
- CSF: Alpha-synuclein seeding assays, neurofilament light chain (NfL)
- Blood: NfL, phosphorylated alpha-synuclein
Dopamine Precursors:
- Levodopa/Carbidopa: Gold standard, converted to dopamine in the brain
Dopamine Agonists:
- Pramipexole, ropinirole, rotigotine (transdermal)
MAO-B Inhibitors:
- Selegiline, rasagiline, safinamide
COMT Inhibitors:
- Entacapone, opicapone, tolcapone
- Deep Brain Stimulation (DBS): Effective for advanced PD with motor complications
- Focused ultrasound: For tremor-dominant PD
- Exercise and physical therapy
- Speech therapy for dysarthria
- Occupational therapy
- Neuropsychiatric treatment
No approved disease-modifying therapies exist, but numerous approaches are in development:
- Alpha-synuclein targeting: Immunotherapies, small molecules
- GBA modulators
- Mitochondrial protectants
- Neurotrophic factors
- What triggers alpha-synuclein misfolding in sporadic PD?
- Why do Lewy bodies spread in a predictable pattern?
- What determines clinical heterogeneity?
- Can we develop sensitive preclinical detection methods?
- Gut-Brain Axis: Role of the microbiome and enteric nervous system
- Immune system involvement: Both neuroinflammation and peripheral immune changes
- Metabolic factors: Impaired glucose metabolism and mitochondrial defects