In this study, we investigated the relationship between olfactory dysfunction measured by the UPSIT test, with other motor and non-motor symptoms separately in three motor subtypes of PD, including tremor dominant (TD), postural instability, and gait difficulty (PIGD), indeterminate and healthy subjects.

The olfactory dysfunction in patients with Parkinson’s disease (PD) is one of the most prevalent non-motor symptoms (NMS) in patients with PD. Numerous studies have evaluated the association between motor and non-motor symptoms and olfactory dysfunction in PD.

A total of 487 early-stage PD patients [43 PIGD, 406 TD, and 38 Indeterminate] and healthy controls (HCs) (n= 197) were obtained from the Parkinson Progression Markers Initiative (PPMI). All participants completed motor and non-motor tests at baseline visit and after four years of follow-up. Subjects underwent common PD scaling tests.

Olfactory dysfunction was significantly correlated with declined motor and non-motor functions in the TD subtype. Also, significant correlations were noticed between olfactory dysfunction and speed-attention processing and executive function in the HCs as well. Finally, no significant or meaningful association was observed in the PIGD and indeterminate subtype. Anosmia and hyposmia subjects in the TD group had the worse motor and non-motor scores compared to normosmia subjects after four years.

Olfactory dysfunction was significantly correlated with declined motor and non-motor functions in the TD subtype. This is indicating that olfactory dysfunction may be an early motor and non-motor biomarker only in the TD subtype. However, it is possible that the involvement of olfactory function in other subtypes is not strong enough to make it a useful marker of diseases progression.