Parkinson's Disease

Overview

Pathobiology

Etiology

• Environmental risks: Pesticides, heavy metals, traumatic brain injury; the neurotoxin MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) directly models the disease by inhibiting complex I of the mitochondrial electron transport chain, causing selective dopaminergic neuron death via oxidative stress.
• Genetic causes (~10% of cases):
  • Autosomal dominant: SNCA (α-synuclein gene - point mutations, duplications/triplications), LRRK2 (the most common cause of familial PD), VPS35
  • Autosomal recessive (early-onset): Parkin (PARK2), PINK1, DJ-1
  • Risk factor: Glucocerebrosidase (GBA) heterozygous mutations (Gaucher disease gene) - associated with faster progression and higher dementia risk
• Prion-like spread: Cell-to-cell transmission of misfolded α-synuclein via "permissive templating" propagates neurodegeneration through the nervous system.
• Goldman-Cecil Medicine, p. 3962; Robbins & Kumar Basic Pathology, p. 854; Adams and Victor's Principles of Neurology

Pathology

• Loss of pigmented dopaminergic neurons in substantia nigra pars compacta with gliosis
• Lewy bodies - the pathological hallmark: round, eosinophilic cytoplasmic inclusions consisting of fine filaments of α-synuclein, neurofilaments, and ubiquitin
• Lewy neurites - dystrophic neuritic processes also containing aggregated α-synuclein

• Brain stem nuclei (dorsal motor nucleus of vagus - early involvement causing sleep/autonomic features)
• Olfactory system (early anosmia)
• Enteric nervous system
• Cortical regions (leading to dementia)

Clinical Features

Prodromal / Premotor Phase

• REM sleep behavior disorder (RBD) - often earliest manifestation
• Anosmia (loss of smell)
• Autonomic dysfunction (constipation, orthostatic hypotension)
• Depression / anxiety
• Cardiac denervation (reduced MIBG uptake)

Cardinal Motor Features (TRAP)

Non-Motor Features

• Cognitive: Dementia (common in advanced disease); if within 1 year of motor onset → Lewy body dementia
• Psychiatric: Visual hallucinations (especially drug-related), depression, anxiety, impulse control disorders (with dopamine agonists), paranoid psychosis
• Autonomic: Orthostatic hypotension, constipation, urinary dysfunction, weight loss
• Sleep: RBD, excessive daytime sleepiness
• Pain and sensory symptoms (some levodopa-responsive)
• Dysphagia: Prevalence 35-82% depending on objective vs. subjective measures; involves oral, pharyngeal, and esophageal phases; aspiration in 15-56%, silent aspiration in 15-33%
• Goldman-Cecil Medicine (Table 378-2); Bradley and Daroff's Neurology

Diagnosis

• Core feature: Motor parkinsonism (bradykinesia + rigidity ± tremor)
• Supportive criteria (increase confidence): Dramatic response to levodopa, levodopa-induced dyskinesia, rest tremor of a limb, anosmia, cardiac sympathetic denervation
• Absolute exclusion criteria
• Red flags (must be counterbalanced by supportive criteria)

Differential Diagnosis: Atypical Parkinsonisms ("Parkinson-plus")

Treatment

When to Start

Medical Therapy

• Levodopa is a dopamine precursor that crosses the blood-brain barrier; carbidopa is a peripheral decarboxylase inhibitor that prevents conversion outside the CNS, reducing GI and cardiovascular side effects and increasing CNS availability.
• Motor complications develop in ~50% within 2-5 years:
  • Wearing off - end-of-dose deterioration; managed by more frequent dosing, controlled-release formulations, COMT or MAO-B inhibition
  • Dyskinesias (peak-dose: chorea/athetosis; diphasic; off-period dystonia) - earlier in young-onset PD
  • On-off fluctuations - unpredictable; may benefit from continuous enteral infusion or subcutaneous levodopa prodrugs

• Preferred as initial therapy in younger (<65), cognitively intact patients to delay motor complications
• Less effective than levodopa; risk of hallucinations, impulse control disorders, excessive daytime sleepiness
• All patients eventually require levodopa addition

• Prevent dopamine breakdown; useful early (mild symptoms) or as adjunct to reduce wearing-off
• Rasagiline used early when there is little functional impairment

• Block peripheral dopamine metabolism; used adjunctively to smooth levodopa action and reduce wearing-off

• Amantadine: Anticholinergic + antiglutamatergic + pro-dopaminergic; useful for dyskinesia
• Anticholinergics: For tremor-predominant disease in younger patients (cognitive risk limits use)
• Neuroanatomy through Clinical Cases, p. 792; Goldman-Cecil Medicine, p. 3963; Textbook of Family Medicine

Surgical Treatment: Deep Brain Stimulation (DBS)

• Subthalamic nucleus (STN): Larger motor benefit in "off" state; allows larger medication dose reduction; slightly higher neuropsychiatric risk
• Globus pallidus internus (GPi): Better dyskinesia suppression; more flexible long-term; safer neuropsychiatric profile; preferred for "brittle" dyskinesias

Non-Pharmacological

• Physical therapy: Gait and balance training, fall prevention
• Occupational therapy: Activities of daily living
• Speech therapy: For hypophonia and dysphagia (LSVT LOUD)
• Aerobic exercise: Evidence for positive disease-modifying effect
• Swallowing evaluation: Videofluoroscopy or FEES recommended given high rate of silent aspiration

Prognosis

Parkinson's Disease - Pathogenesis

1. α-Synuclein Misfolding and Aggregation

• Under conditions of increased concentration, mutation, or impaired clearance, α-synuclein misfolds into oligomers (protofibrils) - the most cytotoxic form
• Protofibrils polymerize into insoluble fibrils
• Fibrils accumulate in neuronal cytoplasm as Lewy bodies (round eosinophilic inclusions with a dense core and pale halo) and Lewy neurites
• Lewy bodies contain: α-synuclein, ubiquitin, hyperphosphorylated neurofilament proteins, lipids, and iron

• SNCA point mutations (A53T, A30P) - promote oligomerization and decrease protein stability
• SNCA duplications and triplications - cause PD by overproducing wild-type protein alone; triplication carriers are more severely affected than duplication carriers
• Impaired proteasomal/lysosomal clearance (see below)
• Interaction with dopamine - α-synuclein forms dopamine adducts that stabilize the toxic protofibrillar form

• Adams and Victor's Principles of Neurology (12th ed.); Robbins, Cotran & Kumar Pathologic Basis of Disease

2. Impaired Protein Clearance

A. Ubiquitin-Proteasome System (UPS)

• Normal pathway: Proteins tagged with ubiquitin (via E1 → E2 → E3 ligase cascade) are degraded by the 26S proteasome
• Parkin (PARK2) is an E3 ubiquitin ligase; parkin mutations impair ubiquitination and proteasomal disposal of α-synuclein and other substrates, causing their accumulation
• UCHL-1 (PARK5): A ubiquitin carboxy-terminal hydrolase; mutations impair the recycling of ubiquitin monomers, compromising the entire UPS
• Importantly, Lewy bodies are not found in most parkin-mutation PD cases, suggesting the Lewy body forms downstream of failed clearance

B. Autophagy-Lysosome Pathway (ALP)

• α-Synuclein aggregates are primarily cleared via macroautophagy and chaperone-mediated autophagy (CMA)
• GBA1 (glucocerebrosidase) heterozygous mutations impair lysosomal function → reduced clearance of α-synuclein → accumulation. The reciprocal is also true: accumulated α-synuclein inhibits GCase by interfering with ER-to-Golgi trafficking → vicious cycle
• GBA1 mutations account for ~5% of PD cases and are associated with faster progression and higher dementia risk
• This lysosomal impairment is not limited to GBA1-mutation carriers - GCase levels are also reduced in sporadic PD
• Robbins, Cotran & Kumar, p. 1183-1184; Adams and Victor's, p. 1090; Harrison's Principles of Internal Medicine 22e

3. Mitochondrial Dysfunction

• MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) selectively destroys dopaminergic neurons via its active metabolite MPP⁺, a potent Complex I inhibitor of the mitochondrial electron transport chain - directly causing oxidative stress and ATP depletion
• Complex I activity is reduced in the SNc and platelets of sporadic PD patients
• Pesticides (rotenone, paraquat) - also Complex I inhibitors - are epidemiologically linked to increased PD risk

• PINK1 (PTEN-induced kinase 1): Localizes to the outer membrane of dysfunctional (depolarized) mitochondria; phosphorylates Parkin to recruit it to damaged mitochondria
• Parkin + PINK1 together execute mitophagy - the autophagic clearance of damaged mitochondria. Mutations in either gene impair mitophagy → accumulation of defective, ROS-generating mitochondria
• DJ-1 (PARK7): A transcriptional regulator that translocates to mitochondria under oxidative stress and has cytoprotective effects; DJ-1 mutations impair this response and increase α-synuclein aggregation
• α-Synuclein itself directly interacts with mitochondrial membranes and inhibits Complex I in a dose-dependent manner
• Bradley and Daroff's Neurology in Clinical Practice; Robbins, Cotran & Kumar, p. 1184

4. Oxidative Stress

• Dopamine metabolism (by MAO and auto-oxidation) generates hydrogen peroxide and reactive oxygen species (ROS)
• High iron content of the SNc - iron catalyzes the Fenton reaction, generating hydroxyl radicals
• Reduced glutathione (GSH) levels found early in PD
• Mitochondrial Complex I inhibition further generates superoxide

5. Braak Staging - Spatiotemporal Progression

6. Prion-Like Propagation ("Body-First" vs "Brain-First" Hypothesis)

• Misfolded α-synuclein can be released from one neuron and taken up by a neighboring cell
• Once internalized, misfolded protein acts as a template, inducing misfolding of native α-synuclein ("permissive templating")
• Pathology spreads in a stereotyped, trans-neuronal pattern consistent with Braak staging

7. Neuroinflammation

• Activated microglia are found in the SNc of PD patients and in MPTP animal models
• Pro-inflammatory cytokines (TNF-α, IL-1β, IL-6) contribute to dopaminergic neuron death
• α-Synuclein aggregates can act as DAMPs (damage-associated molecular patterns), activating microglial and astrocytic inflammatory responses
• T-cell infiltration has been documented in the SNc of PD patients
• Maladaptive immune responses, possibly initiated in the gut, are now recognized as accelerating PD pathogenesis - per the 2024 Lancet review by Morris et al. (PMID: 38245249)

8. Basal Ganglia Circuit Dysfunction

• Direct pathway (striatum → GPi/SNr, inhibitory): Normally dopamine activates D1 receptors, facilitating movement. With dopamine loss, this pathway is underactive → less inhibition of GPi → more GPi output
• Indirect pathway (striatum → GPe → STN → GPi): Normally dopamine inhibits D2 receptors here, dampening the pathway. With dopamine loss, this pathway is overactive → more STN excitation of GPi → more GPi output
• Net result: Excessive inhibitory output from GPi/SNr → over-suppression of thalamocortical circuits → hypokinesia, bradykinesia, rigidity

9. Key Genetic Causes Summary