When an older adult develops tremor, rigidity, and gait disturbance, the first question for many clinicians is: Is this Parkinson’s disease or drug-induced Parkinsonism? The distinction is critical, because the treatment, prognosis, and counseling differ greatly.

Case Vignette

A 70-year-old man on risperidone for behavioral disturbances develops a bilateral hand tremor and difficulty walking. His family worries it may be Parkinson’s disease. On examination, the tremor is symmetrical, there is no clear resting tremor, and symptoms appeared within three months of starting medication. The picture suggests drug-induced Parkinsonism rather than idiopathic Parkinson’s.

Understanding the Difference

1. Onset and Progression

• Parkinson’s disease: insidious, progressive, typically starting on one side.

• Drug-induced Parkinsonism: relatively sudden, often within weeks to months of starting a dopamine-blocking drug.

2. Symmetry of Symptoms

• Parkinson’s disease: usually asymmetrical at onset (right vs. left dominance).

• Drug-induced Parkinsonism: typically symmetrical from the beginning.

3. Tremor Pattern

• Parkinson’s disease: resting tremor, “pill-rolling” quality.

• Drug-induced Parkinsonism: more often postural or action tremor, less classic in appearance.

4. Associated Features

• Parkinson’s disease: non-motor symptoms such as constipation, loss of smell (anosmia), REM sleep behavior disorder, and gradual cognitive decline.

• Drug-induced Parkinsonism: usually lacks prodromal features; symptoms link closely to drug exposure.

5. Course After Stopping the Drug

• Parkinson’s disease: symptoms continue and worsen over time.

• Drug-induced Parkinsonism: often improves once the drug is reduced or stopped, though in some cases it may unmask underlying Parkinson’s disease.

Common Culprit Medications

Drugs most often linked to drug-induced Parkinsonism include:

• Antipsychotics: haloperidol, risperidone, olanzapine.

• Antiemetics: metoclopramide, prochlorperazine.

• Calcium channel blockers: cinnarizine, flunarizine.

Role of Imaging

Advanced imaging such as 99mTc-TRODAT SPECT can help distinguish the two:

• Parkinson’s disease: shows presynaptic dopaminergic dysfunction (reduced uptake).

• Drug-induced Parkinsonism: typically shows normal uptake, as the underlying neurons remain intact.

Why the Distinction Matters

• Mislabeling a drug side effect as Parkinson’s can lead to unnecessary lifelong treatment.

• Missing early Parkinson’s disease delays proper initiation of dopaminergic therapy.

• Accurate diagnosis allows clinicians to balance psychiatric needs and neurological health.

Clinical Pearls

• Always review medication history in new Parkinsonian symptoms.

• Look for asymmetry and non-motor features to favor idiopathic Parkinson’s disease.

• Consider TRODAT imaging when in doubt.

• Collaborate across neurology and psychiatry for best patient outcomes.

✅ Dr. Srinivas Rajkumar T, MD (Psychiatry)
Consultant Psychiatrist, Apollo Clinic Velachery, Chennai
📞 8595155808