Objective: To determine the diagnostic utility of olfactory testing in patients with neurodegenerative Parkinsonism.

Background: Olfactory dysfunction can be identified in the vast majority of patients with Parkinson’s disease (PD). Less is known about olfactory function in multiple system atrophy (MSA) and progressive supranuclear palsy (PSP) – two atypical Parkinsonian disorders (APD) that may closely mimic PD in early disease stages – but current evidence suggests that olfactory function remains largely intact in these conditions.

Methods: The Sniffin’ Sticks test battery for assessment of odor identification, odor discrimination, and olfactory threshold was applied to two independent cohorts – a screening and a validation cohort. The screening cohort included Parkinson’s disease (PD) patients and healthy controls (HC) and was used to calculate optimal cut-offs for a diagnosis of PD with a sensitivity or specificity exceeding 95%. The validation cohort was used to determine the diagnostic accuracy of the newly established cut-off values in discriminating patients with PD from those with atypical Parkinsonian disorders (APD) including multiple system atrophy (MSA) and progressive supranuclear palsy (PSP).

Results: PD patients (screening cohort n520, validation cohort n547) performed significantly worse in olfactory testing than HC (n541) and patients with MSA (n523) or PSP (n523). Diagnostic performance of the identification subscore was similar to the sum score of the Sniffin’ Sticks test (AUC identification test 0.94, AUC sum score 0.96). In subjects with neurodegenerative Parkinsonism, the specificity cut-off predicted a diagnosis of PD with a sensitivity and specificity of 76.6 and 87.0%, respectively. The discriminative value of this cut-off in separating PD from MSA was 76.7% (sensitivity) and 95.7% (specificity). Optimal cut-offs of the sum score provided a sensitivity and specificity for a diagnosis of PD of 78.7% and 76.1%, respectively. The positive predictive value of olfactory testing adjusted for PD prevalence in the community exceeded 95%.

Conclusions: Our data suggest that assessment of olfactory function, particularly odor identification, can be useful to discriminate PD from APDs, particularly MSA patients.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/olfactory-testing-in-parkinson-plus-syndromes/