What are the key clinical features that differentiate Parkinson's disease from other forms of parkinsonism?

Key clinical features differentiating Parkinson's disease (PD) from other forms of parkinsonism include:

• Asymmetry of motor symptoms: PD typically presents with unilateral onset of tremor, rigidity, and bradykinesia, whereas atypical parkinsonism often shows more symmetric symptoms early on ¹ (Levin et al., 2016).
• Resting tremor: A classic resting tremor is common in PD but less prominent or absent in atypical parkinsonian syndromes and drug-induced parkinsonism ¹ (Jankovic, 2008).
• Response to levodopa: PD patients usually show a good and sustained response to levodopa therapy, while atypical parkinsonism and drug-induced parkinsonism often have a poor or minimal response ¹ (Conn and Jankovic, 2024).
• Progression and clinical features: PD generally has a slower progression with predominant motor symptoms initially, whereas atypical parkinsonism (e.g., progressive supranuclear palsy, multiple system atrophy) often presents with early postural instability, falls, autonomic dysfunction, and poor levodopa response ¹ (Levin et al., 2016).
• Non-motor symptoms and cognitive impairment: Early cognitive impairment and prominent autonomic failure are more suggestive of atypical parkinsonism than PD, where these features tend to appear later ¹ (Jankovic, 2008).
• Drug history: Drug-induced parkinsonism should be suspected if parkinsonian features develop after exposure to dopamine-blocking agents, and symptoms may improve after withdrawal of the offending drug ¹ (Conn and Jankovic, 2024).
• Imaging and diagnostic tests: While structural MRI is not diagnostic for PD, it may help exclude other causes or support atypical parkinsonism diagnosis. Functional imaging such as 123I-FP-CIT SPECT can aid differentiation when clinical features overlap ¹.

In summary, the key differentiators of PD from other parkinsonian syndromes are the asymmetric onset, presence of resting tremor, good levodopa responsiveness, slower progression, and later onset of cognitive and autonomic symptoms. Atypical parkinsonism and drug-induced parkinsonism often lack these features and have additional signs such as early falls, autonomic failure, or a clear drug trigger ¹ (Jankovic, 2008; Levin et al., 2016; Conn and Jankovic, 2024).