Overview & Epidemiology

1. What is Parkinson’s Disease?

Parkinson’s disease (PD) is a chronic, progressive neurodegenerative disorder defined by two pathological hallmarks first stitched together by Friedrich Lewy (1912) and Konstantin Tretiakoff (1919):

• Loss of pigmented dopaminergic neurons in the substantia nigra pars compacta (SNc) of the ventral midbrain — producing the classical macroscopic depigmentation visible on gross pathology.
• Lewy bodies & Lewy neurites — eosinophilic intracytoplasmic inclusions composed primarily of misfolded, aggregated α-synuclein, with Ser-129 phosphorylation as the universal post-translational marker.

The clinical syndrome — bradykinesia plus rest tremor or rigidity, with characteristic levodopa response — emerges only after roughly 50–60% of SNc dopamine neuronshave been lost and putaminal dopamine has fallen by ~80% (Fearnley & Lees, Brain 1991; Cheng et al., Ann Neurol 2010). PD is therefore a disease whose clinical onset marks an already-advanced biological process.

2. The Cardinal Features — TRAP

The motor syndrome of PD is captured by the classical mnemonic TRAP:

The 2015 MDS Clinical Diagnostic Criteria (Postuma et al., Mov Disord 2015) require bradykinesia plus either rest tremor or rigidity — postural instability is now treated as a late feature rather than a defining one, precisely because its early appearance suggests parkinsonism-plus.

3. James Parkinson and the Shaking Palsy

In 1817 the London apothecary-surgeon James Parkinson published a 66-page monograph titled An Essay on the Shaking Palsy, describing six cases (only three of whom he had personally examined) of a syndrome he called paralysis agitans:

“Involuntary tremulous motion, with lessened muscular power, in parts not in action and even when supported; with a propensity to bend the trunk forward, and to pass from a walking to a running pace: the senses and intellects being uninjured.”

His description — remarkable for its precision and for the long preclinical observation behind it — would have remained an obscure tract were it not for Jean-Martin Charcot, who at the Salpêtrière half a century later renamed the disorder maladie de Parkinson, distinguished it from multiple sclerosis and other tremors, recognised that not all patients tremor (the rigid form), and introduced the term “bradykinesia”. Charcot also noted — and we have only recently rediscovered — that the “senses and intellects” are not always uninjured: cognitive decline and dementia eventually develop in ~80% of long-survivors.

The path from Parkinson’s clinical sketch to molecular biology runs through Edouard Brissaud (1895, who first localised the lesion to the substantia nigra), Friedrich Lewy (1912, who described the inclusions that bear his name), Konstantin Tretiakoff (1919, who linked SNc loss to the clinical syndrome), Arvid Carlsson (1957, who showed dopamine is a neurotransmitter and that reserpine-induced akinesia is reversed by L-DOPA — Nobel 2000), and George Cotzias (1967, who translated this to a workable oral therapy in patients).

4. Prevalence & Incidence — The Fastest-Growing Neurological Disease

The Global Burden of Disease (GBD) 2019 study established PD as the fastest-growing neurological disorderby both prevalence and disability-adjusted life-years (DALYs): the global PD population grew ~155% between 1990 and 2019, more than any other neurological disease, and is projected to reach ~25 million by 2050 (Dorsey, Bloem and others have called this the “Parkinson Pandemic”). Three drivers compound:

• Population aging — the dominant factor; PD is overwhelmingly a disease of late life.
• Increasing life expectancy of PD patients — better cardiovascular care, better symptomatic dopaminergic therapy.
• Possible environmental contribution — pesticide and solvent exposures (paraquat, rotenone, trichloroethylene; Tanner et al., Environ Health Perspect 2011), industrial-region clusters.

Demographic patterns:

• Age — mean onset ~60 years; ~5–10% are early-onset (<50 yr); juvenile-onset (<21 yr) almost always genetic.
• Sex — men ~1.5× the incidence of women, possibly modulated by oestrogen-mediated dopaminergic neuroprotection.
• Geography — highest age-standardised prevalence in North America, western Europe, and east Asia; rising fastest in middle-income countries with industrialisation.
• Negative association with smoking — remarkably reproducible (RR ~0.5 for ever-smokers; Hernán et al., Ann Neurol 2002), almost certainly causal but mechanism debated (nicotinic neuroprotection vs reverse causation from prodromal hyposmia).
• Negative association with caffeine — ~30% risk reduction in heavy coffee drinkers; ongoing trials of istradefylline-class A2A antagonists.

5. Diagnostic Criteria — from Bradley to MDS 2015

The clinical diagnosis of PD has been progressively refined to improve accuracy against autopsy gold standard:

Despite refinement, clinicopathological accuracy of the clinical diagnosis remains ~80–90% even among movement-disorder specialists; the remainder are mostly MSA, PSP, vascular parkinsonism, or essential tremor. The MDS 2015 criteria are detailed in Part VI (Diagnosis).

6. Hoehn & Yahr Staging

Margaret Hoehn and Melvin Yahr (Neurology 1967, Parkinsonism: onset, progression and mortality) introduced a five-stage clinical scale that, six decades later, remains the most widely used global descriptor of PD severity:

Median time from diagnosis to Hoehn-Yahr 3 is ~7 years in the levodopa era. The modified scale (with 1.5 and 2.5) is preferred for clinical follow-up. The MDS-UPDRS (covered in Part V) gives finer-grained motor and non-motor scoring on top of Hoehn-Yahr.

7. Survival & Mortality

In the pre-levodopa era (Hoehn & Yahr 1967), median survival from diagnosis was ~9 years and the standardised mortality ratio was ~3. The introduction of L-DOPA roughly halved excess mortality for the first decade of disease, and modern estimates put:

• Median survival from diagnosis ~12–15 years; longer in young-onset, shorter in non-tremor / PIGD phenotypes.
• Standardised mortality ratio ~1.5–2.0 vs age-matched controls.
• Cause of death — pneumonia (frequently aspiration; the commonest), cardiovascular disease, and complications of immobility (PE, sepsis).
• Predictors of shorter survival — older age at onset, dementia (PDD), early postural instability, RBD, severe orthostatic hypotension, male sex.
• Predictors of longer survival — tremor-dominant phenotype, younger age at onset, LRRK2 G2019S genotype.

Cognitive trajectory is equally important: the Sydney Multicenter Study of PD (Hely et al., Mov Disord 2008) followed an inception cohort for 20 years and found that ~80% develop dementia(PDD) by year 20, a number that reframed PD as a multisystem proteinopathy in the long view, not a pure motor disease.

8. Cost & Burden

PD imposes a heavy economic burden, dominated by indirect (lost productivity, caregiver) costs:

• USA — total annual cost ~$52 billion (2017, Yang et al., NPJ Parkinsons Dis 2020); per-patient ~$25k/year direct + ~$25k/year indirect.
• Europe — ~€14 billion/year (Gustavsson et al., Eur Neuropsychopharmacol 2011).
• Caregiver burden — spouse-carers spend >30 hours/week on care by Hoehn-Yahr 4; depression rates ~40% in carers.
• Quality-adjusted life expectancy — reduced ~4–6 QALYs from age-matched expectation.

Disability-adjusted life-years (DALYs) from PD globally rose from ~2.5 million in 1990 to ~5.8 million in 2019; the trajectory exceeds that of stroke and dementia in percentage growth, though absolute numbers remain lower (GBD 2019 Neurology Collaborators, Lancet Neurol 2019).

9. Where This Course Goes

The remainder of the course descends into the molecular and clinical detail behind the picture sketched here: