Parkinson's Disease

Overview

Parkinson's disease (PD) is the second most common neurodegenerative disorder after Alzheimer's disease. It results from progressive loss of dopaminergic neurons in the substantia nigra pars compacta, leading to dopamine depletion in the striatum and the characteristic motor and non-motor features.

Pathophysiology

The hallmark pathological features are:

Loss of dopaminergic neurons in the substantia nigra, causing the macroscopic finding of depigmentation of the midbrain
Lewy bodies — intraneuronal cytoplasmic inclusions composed of aggregated alpha-synuclein protein
Lewy neurites — thread-like alpha-synuclein deposits in axonal processes

The Braak staging hypothesis suggests pathology begins in the olfactory bulb and lower brainstem before ascending to the substantia nigra and cortex, which explains the early non-motor symptoms.

Parkinson's disease neuropathology showing substantia nigra depigmentation (macroscopic) and Lewy bodies/Lewy neurites on alpha-synuclein immunohistochemistry (microscopic)

Neuropathology of Parkinson's disease: macroscopic depigmentation of the substantia nigra (top) and microscopic alpha-synuclein aggregates — classical Lewy bodies (rounded inclusions) and Lewy neurites (thread-like processes) in both brainstem and cortex (bottom).

Etiology & Risk Factors

Factor	Details
Age	Most important risk factor; incidence rises sharply after age 60
Sex	Men affected ~1.5× more than women
Genetics	LRRK2, SNCA, Parkin, PINK1, DJ-1 mutations; ~10–15% familial
Environmental	Pesticide/herbicide exposure (rotenone, paraquat), well water
Protective	Smoking (controversial), caffeine, physical activity

Clinical Features

Motor Symptoms (Cardinal Features)

The diagnosis rests on clinical criteria (MDS 2015 criteria):

Feature	Description
Bradykinesia	Slowness and decrement of movement amplitude — required for diagnosis
Rest tremor	4–6 Hz "pill-rolling" tremor, suppressed by movement
Rigidity	"Lead-pipe" or "cogwheel" (if tremor superimposed)
Postural instability	Late feature; leads to falls

Classic presentation: TRAP — Tremor, Rigidity, Akinesia/bradykinesia, Postural instability

Non-Motor Symptoms (Often Precede Motor Features)

Olfactory loss (hyposmia) — one of the earliest signs
REM sleep behavior disorder (RBD)
Constipation and autonomic dysfunction
Depression and anxiety
Cognitive impairment / Parkinson's disease dementia (late)
Orthostatic hypotension

Diagnosis

Diagnosis is clinical, based on MDS criteria:

Parkinsonism must be present (bradykinesia + rest tremor and/or rigidity)
Absence of absolute exclusion criteria (e.g., cerebellar signs, supranuclear gaze palsy, rapid progression)
Supportive criteria: unilateral onset, clear levodopa responsiveness, rest tremor, olfactory loss

Imaging: DaTscan (dopamine transporter SPECT) can support diagnosis in ambiguous cases but is not required. MRI is mainly used to exclude structural causes.

Differentials to exclude (Parkinson's Disease, p. 8):

Essential tremor
Drug-induced parkinsonism (neuroleptics, metoclopramide)
Normal pressure hydrocephalus
Vascular parkinsonism
Atypical parkinsonian syndromes: PSP, MSA, CBD, DLB

Management

(Parkinson's Disease, p. 3)

Pharmacological

Drug Class	Examples	Role
Levodopa/Carbidopa	Sinemet, Rytary	Most effective; first-line, can be used early
Dopamine agonists	Pramipexole, ropinirole, rotigotine	Often used in younger patients to delay levodopa
MAO-B inhibitors	Selegiline, rasagiline, safinamide	Mild symptomatic benefit; possible neuroprotective debate
COMT inhibitors	Entacapone, opicapone	Adjunct to levodopa to reduce "off" time
Amantadine	—	Anti-dyskinesia; mild dopaminergic effects
Anticholinergics	Trihexyphenidyl	For tremor, especially in younger patients

Levodopa remains the gold standard. A diagnostic trial of levodopa is useful when diagnosis is uncertain — a robust response supports idiopathic PD.

Device-Based Therapies (Advanced Disease)

Deep brain stimulation (DBS) — subthalamic nucleus or globus pallidus interna; highly effective for motor fluctuations and dyskinesias
Levodopa-carbidopa intestinal gel (LCIG/Duopa) — continuous jejunal infusion; reduces "off" time
Focused ultrasound — thalamotomy for tremor-dominant disease

Non-Pharmacological

Exercise: proven benefit when started early; neuroprotective effects suspected
Physiotherapy: gait, balance, fall prevention
Speech therapy: hypophonia, dysphagia (Lee Silverman Voice Treatment)
Occupational therapy: ADL maintenance
Nutritional counseling: protein redistribution to minimize levodopa interactions

Impulse Control Disorders

Dopaminergic therapy — especially dopamine agonists — can cause hypersexuality, pathological gambling, binge eating, and compulsive shopping at any disease stage. Patients should be counseled proactively.

Complications & Prognosis

Complication	Notes
Motor fluctuations	"Wearing off," "on-off" phenomenon — manage with dose adjustments, COMT inhibitors, MAO-B inhibitors
Dyskinesias	Peak-dose choreiform movements; amantadine, DBS
Dementia	~80% lifetime risk; associated with Lewy body spread to cortex
Dysphagia	Aspiration pneumonia is a leading cause of death
Falls	Major source of morbidity; postural instability + freezing of gait
Psychosis	Hallucinations; use quetiapine or clozapine (avoid dopamine blockers); pimavanserin is approved

PD is not directly fatal but significantly reduces quality of life and life expectancy. Median survival from diagnosis is approximately 10–15 years, though this varies widely.

Summary

Parkinson's disease is a progressive alpha-synucleinopathy driven by dopaminergic neurodegeneration. Diagnosis is clinical; levodopa responsiveness is both diagnostic and therapeutic. Management is multimodal — pharmacological, device-based, and rehabilitative. Early exercise and proactive management of non-motor symptoms significantly improve outcomes.

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