Combined Olfactory Testing and Substantia Nigra Hyperechogenicity for Diagnostic Differentiation of Parkinson's...

Olfactory impairment and substantia nigra (SN) hyperechogenicity are well-recognized features of Parkinson's disease (PD). Their complementary diagnostic value and demographic influences warrant further clarification. This study aimed to assess the relationship and combined diagnostic value of SN hyperechogenicity and olfactory dysfunction in patients with clinically diagnosed PD.

We conducted a cross-sectional study including 89 PD patients and 113 healthy controls. Olfactory function was evaluated using the 16-item Sniffin' Sticks test, and SN echogenicity was measured by transcranial sonography. Participants were stratified by age (≤60 and >60 years). The diagnostic performance of each marker, individually and in combination, was analyzed.

PD patients showed significantly lower olfactory scores and higher SN echogenicity than controls (both p < 0.001). Both markers were significantly more prevalent in PD. In controls, olfactory function declined with age (r = -0.489, p < 0.001), whereas SN echogenicity remained stable. No overall correlation was observed between the two markers in PD. Sex differences in olfaction appeared in controls but not in PD. Combining hyposmia and SN hyperechogenicity yielded 83.1% sensitivity, 95.6% specificity, and 90.1% accuracy. In participants aged ≤60 years, higher specificity and positive predictive value were observed, although this subgroup included a relatively small number of patients.

Olfactory dysfunction and SN hyperechogenicity are significantly more prevalent in PD than in controls. When used together, these markers provide complementary diagnostic information and high specificity for diagnostic differentiation between PD and healthy controls. Age-related effects on olfactory function highlight the importance of demographic adjustment when interpreting olfactory test results.