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 the characteristic motor symptoms, response to dopaminergic therapy, and progression pattern.

Parkinson's disease typically presents with a classic triad of motor features: resting tremor, bradykinesia (slowness of movement), and rigidity, often with asymmetrical onset. Postural instability and gait disturbances usually develop later in the disease course. In contrast, atypical parkinsonian syndromes (such as progressive supranuclear palsy, multiple system atrophy, and corticobasal degeneration) often show early postural instability, symmetrical symptoms, poor or absent response to levodopa, and additional neurological signs like early autonomic failure or cerebellar features ¹.

PD patients generally have a good initial and sustained response to levodopa therapy, which improves motor symptoms significantly. Other parkinsonian syndromes tend to have a limited or no response to levodopa, which is a key clinical discriminator ¹.

Non-motor features such as olfactory loss, REM sleep behavior disorder, and gradual cognitive decline are more characteristic of PD, whereas rapid cognitive decline and early dementia are more suggestive of atypical parkinsonism (Levin et al., 2016). Drug-induced parkinsonism, another differential, often presents with symmetrical symptoms and lacks the classic resting tremor; it usually improves with withdrawal of the offending drug (Conn and Jankovic, 2024).

Imaging techniques like 123I-FP-CIT SPECT can aid differentiation when clinical features overlap, showing reduced dopaminergic uptake in PD and atypical parkinsonism but normal scans in drug-induced parkinsonism ¹. Structural MRI is not diagnostic for PD but may help exclude other causes or support diagnosis of atypical syndromes ¹.

In summary, the key clinical differentiators of PD from other parkinsonian disorders are asymmetrical onset, classic motor features with good levodopa responsiveness, and a characteristic progression pattern, supported by clinical history and specialist assessment ¹ (Jankovic, 2008; Levin et al., 2016; Conn and Jankovic, 2024).