Objective: In this study, we evaluated detailed differential features of olfactory function in PD and DIP.

Background: DIP is common but it is challenging to differentiate from PD without a dopamine transporter image. Because hyposmia is common even in early PD, the olfactory function test would be a helpful tool for the differentiation of PD from DIP, although olfactory function is affected by normal aging.

Method: For this study, demographic and clinical data from 127 patients were collected, who was diagnosed as drug-naïve PD or DIP. ¹⁸F-FP-CIT PET was done in all participants to confirm the diagnosis. Patients with the medical history – dementia, brain trauma, and nasal surgery – which can influence olfactory function were excluded. The YSK olfactory function (YOF) test which detects olfactory TDI domain was conducted in all patients.

Results: Total 83 patients with PD and 31 patients with DIP were enrolled. The DIP group was older than the PD group (73.35 years vs 65.73 years, P < 0.001). In DIP group, female predominated significantly (female, 84.4% in DIP vs 50.6% in PD, P = 0.001). There were no differences between the groups in disease duration. Patients with DIP were less educated and had poorer global cognition than patients with PD. Olfactory dysfunction (YOF score ≤21) was over 70% in each group and was not different in prevalence (P = 0.499). There were no differences in total TDI and threshold score (P > 0.05) between groups. Meanwhile, the discrimination score was lower in the PD group compared to the DIP group (4.34 vs 5.48, P = 0.029). That difference was statistically significant even after being adjusted by age, sex, disease duration, and cognitive function (P = 0.010). In the detail of the discrimination test, PD patients specifically had trouble distinguishing scents between styrallyl acetate and para-cresol methyl ether (19.3% vs 50.0%, P = 0.003). No significant differences were found in identification test scores (7.97 vs 7.12, P = 0.242), but PD patients particularly failed to get the right answer in scorched rice even though this smell is very familiar to Korean (67.5% vs 92.9%, P = 0.008).

Conclusion: In this study, although olfactory dysfunction can be observed in DIP, we found that impairment in olfactory discrimination is a distinguishing feature in PD, unlike DIP. The underlying pathomechanism of this feature needs to be evaluated further.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/olfactory-discrimination-differentiate-drug-induced-parkinsonism-and-parkinsons-disease/