Parkinson's Disease: A High-Yield USMLE Review

Parkinson's disease is a progressive neurodegenerative movement disorder caused by loss of dopaminergic neurons in the substantia nigra pars compacta. It classically presents in older adults with an asymmetric resting tremor, rigidity, and bradykinesia. It is a favorite exam topic both for its clinical tetrad and for the mimics that must be distinguished from it.

Presentation

• Resting tremor (approximately 4–5 Hz) that diminishes with purposeful movement, such as reaching for an object
• Cogwheel rigidity — increased tone with a ratcheting quality on passive movement
• Bradykinesia with reduced arm swing, short shuffling steps, and difficulty turning
• Hypomimia (reduced facial expression, 'flat' or 'masked' face) and soft speech
• Micrographia (progressively smaller handwriting)
• Unilateral/asymmetric onset of symptoms
• Non-motor features that may precede the tremor by years: hyposmia (loss of smell), REM sleep behavior disorder, and constipation
• Postural instability, which typically develops later in the disease course

Diagnosis

• Clinical diagnosis based on the characteristic tetrad (resting tremor, rigidity, bradykinesia, with postural instability developing later); unilateral onset and asymmetry support the diagnosis
• A robust, sustained response to levodopa supports idiopathic Parkinson's disease and helps distinguish it from atypical parkinsonian syndromes
• Recognition of a preceding non-motor prodrome (hyposmia, REM sleep behavior disorder, constipation) aids clinical suspicion

Management

• Carbidopa/levodopa is the most effective therapy and the first-line treatment for motor symptoms
• In younger patients, consider starting a dopamine agonist (pramipexole, ropinirole) to delay levodopa-related motor complications

High-yield

• Pathological hallmark: Lewy bodies containing alpha-synuclein in the substantia nigra pars compacta
• Classic tetrad: resting tremor, rigidity, bradykinesia, and (later) postural instability
• Resting tremor characteristically STOPS when the patient reaches for an object
• Non-motor symptoms — hyposmia, REM sleep behavior disorder, constipation — can precede the motor tremor by years
• Unilateral onset with asymmetry is typical of idiopathic Parkinson's disease

Pitfalls

• Do not mistake progressive supranuclear palsy (PSP) for Parkinson's — red flags include early falls, poor levodopa response, vertical (especially downgaze) gaze palsy, symmetric onset, axial rigidity, and a 'surprised' facial expression
• Distinguish from dementia with Lewy bodies (DLB): dementia developing before or within 1 year of motor symptoms, plus visual hallucinations, fluctuating cognition, and severe neuroleptic sensitivity — in contrast, Parkinson's disease dementia develops years after motor symptoms
• Expecting early postural instability leads to error — in idiopathic Parkinson's it develops later, not at onset