Differentiating Parkinson’s disease from drug-induced Parkinsonism

The process of differentiating between drug-induced Parkinsonism (DIP) and Parkinson’s disease (PD) in psychotic illness is difficult but crucial to ongoing management. Myocardial scintigraphy for sympathetic denervation has practical limitations, as it requires withdrawal of psychotropic medication in vulnerable patients for up to 7 days. The radiopharmaceuticals for dopamine transporter positron emission tomography (PET) have recently become more readily available in Australia. These techniques differentiate these disorders with a high sensitivity and specificity, without requiring medication withdrawal, proving clinically useful (Brigo et al., 2014).

Mr A is a 61-year-old man with a schizoaffective illness who has been under psychiatric care since 1995. He had been managed for 20 years on a combination of lithium 900 mg, olanzapine 10 mg and venlafaxine 300 mg. His previous antipsychotic exposure included risperidone and quetiapine. He presented with a gradual onset of predominantly left-sided tremor affecting rest, posture and action. He showed bradykinesia and rigidity. He had a shuffling walk and was prone to falls. He showed clinical features that suggested PD including anosmia, asymmetrical tremor and absence of tardive dyskinesia. Attempts to reduce antipsychotic medication failed because it significantly increased psychotic symptoms.

Fluorodopa (F18-DOPA) PET imaging was selected, as it could be performed without antipsychotic medication withdrawal (see Figure 1). Visual and semi-quantitative assessment demonstrated asymmetric reduced striatal binding particularly in the putamen, being more marked on the right (consistent with the contralateral symptom deficit). As presynaptic dopaminergic function is preserved in DIP, this supported the diagnosis of PD unmasked by postsynaptic D2 blockade.

Figure 1.

Transaxial tomographic image through striatum.

Asymmetric right-sided reduction in dopaminergic function was more severe in the putamen. Putamen occipital cortex ratios were as follows: right, 1.83; left, 2.00 (normal, 2.10–3.30) (Eshuis et al., 2009).

Levodopa 100 mg and benserazide 25 mg were slowly started and tremor, rigidity and bradykinesia all decreased. These changes were associated with a significant functional improvement.

In summary, imaging of the dopaminergic pathway is proving to be an effective, clinically useful way of differentiating between PD and DIP.