Parkinson Disease

Parkinson disease is a progressive neurodegenerative disorder defined by the loss of dopaminergic neurons in the substantia nigra and the aggregation of alpha-synuclein into Lewy bodies. It produces a characteristic motor syndrome of bradykinesia, rest tremor, and rigidity, together with a wide range of non-motor features such as impaired smell, sleep and autonomic dysfunction, and, in later stages, cognitive decline.

Definition

Parkinson disease is a progressive neurodegenerative disorder characterised pathologically by degeneration of dopaminergic neurons in the substantia nigra pars compacta and by alpha-synuclein-containing Lewy bodies, and clinically by bradykinesia combined with rest tremor and/or rigidity, accompanied by diverse non-motor features.

Scope

This entry covers the core neuropathology of Parkinson disease, the dopaminergic deficit and alpha-synuclein pathology, the motor and non-motor clinical syndrome, the concept of staged spread of pathology, and diagnostic frameworks. It is a reference overview of how Parkinson disease is understood and classified, not a guide to managing any individual patient.

Core questions

• How does loss of nigrostriatal dopamine produce the cardinal motor signs?
• Where does alpha-synuclein pathology begin and how does it spread?
• Why do non-motor features often precede the motor syndrome by years?
• How can the disease be diagnosed accurately and distinguished from other parkinsonisms?

Key concepts

• Nigrostriatal dopaminergic degeneration
• Alpha-synuclein and Lewy bodies
• Bradykinesia, rest tremor, and rigidity
• Non-motor features (hyposmia, REM sleep behaviour disorder, autonomic and cognitive changes)
• Braak staging
• Parkinson disease dementia and the Lewy body disease spectrum
• Differential diagnosis from atypical parkinsonism

Key theories

Braak staging of alpha-synuclein pathology

Braak and colleagues proposed that Lewy pathology spreads in a predictable caudal-to-rostral sequence, beginning in the lower brainstem and olfactory structures and ascending to the substantia nigra and neocortex, offering a framework that links early non-motor symptoms to later motor and cognitive features.

Alpha-synuclein as a propagating proteinopathy

Misfolded alpha-synuclein is thought to template normal protein and spread between connected neurons, framing Parkinson disease as a synucleinopathy in which pathology progresses through neural networks over time.

Mechanisms

The cardinal motor features of Parkinson disease arise from progressive degeneration of dopaminergic neurons in the substantia nigra pars compacta, which depletes dopamine in the striatum and disrupts basal-ganglia motor circuits. The pathological hallmark is the aggregation of alpha-synuclein into Lewy bodies and Lewy neurites. Braak staging describes a stereotyped ascending spread of this pathology that parallels the clinical evolution from early non-motor symptoms (such as loss of smell and REM sleep behaviour disorder) through the motor syndrome to cognitive involvement in later disease, consistent with a propagating synucleinopathy (Bloem et al., 2021; Braak et al., 2003; Dugger & Dickson, 2017).

Clinical relevance

Parkinson disease is a leading movement disorder, and recognising its motor and non-motor features underpins how clinicians distinguish it from other causes of parkinsonism and track its progression. Diagnostic criteria emphasise bradykinesia with tremor or rigidity together with supportive and exclusionary features. This entry describes how the disease is characterised and studied; it is not a basis for individual diagnostic or treatment decisions.

Epidemiology

Parkinson disease is among the most common neurodegenerative disorders and the fastest-growing in terms of prevalence and disability among neurological conditions, with risk rising with age and somewhat higher frequency in men. Its global burden has increased markedly over recent decades, driven largely by population ageing (Bloem et al., 2021).

History

James Parkinson's 1817 essay on the 'shaking palsy' provided the first systematic clinical description, and the disorder was later named in his honour. The twentieth century identified the loss of nigral dopaminergic neurons and the dopamine deficit as central, enabling dopaminergic treatment strategies, while the recognition of alpha-synuclein as the principal component of Lewy bodies reframed the disease as a synucleinopathy. Braak's staging scheme (2003) and updated clinical criteria (Postuma et al., 2015) refined how the disease is conceptualised and diagnosed.

Debates

Where does Parkinson disease pathology originate?

Braak's ascending-spread model and the 'body-first versus brain-first' debate raise the question of whether alpha-synuclein pathology can begin outside the brain (for example in the gut or olfactory system) before reaching the substantia nigra, with implications for early detection.

Is Parkinson disease one disease or several?

Wide variation in age of onset, genetics, motor and non-motor profiles, and progression has prompted debate over whether Parkinson disease is a single entity or a family of overlapping disorders along the Lewy body spectrum.

Key figures

• James Parkinson
• Heiko Braak
• Bastiaan Bloem
• Christine Klein
• Ronald Postuma

Seminal works

• bloem-2021
• braak-2003
• postuma-2015

Frequently asked questions

What are the cardinal motor signs of Parkinson disease?

The core motor features are bradykinesia (slowness of movement), rest tremor, and rigidity; diagnostic criteria require bradykinesia together with rest tremor and/or rigidity, alongside supportive and exclusionary findings.

Does Parkinson disease affect more than movement?

Yes. Many non-motor features, such as reduced sense of smell, REM sleep behaviour disorder, constipation, and mood changes, can precede the motor syndrome, and cognitive impairment and dementia can develop in later stages.