Axial-tremor imbalance as a clinical marker for distinguishing progressive supranuclear palsy from Parkinson's disease.

BACKGROUND: Distinguishing Parkinson's disease (PD) from progressive supranuclear palsy (PSP) remains challenging, particularly in early disease stages. Although axial motor impairment is typically more prominent in PSP and tremor in PD, the relative distribution of these motor domains has not been systematically quantified. OBJECTIVE: To evaluate whether the imbalance between axial and tremor-related motor features may aid in distinguishing PSP from PD. METHODS: We conducted a retrospective cross-sectional study including 533 patients with PD and 48 patients with PSP evaluated at a tertiary movement disorders center. Motor symptoms were assessed using the MDS-UPDRS part III in the OFF-medication state. Axial motor impairment was quantified using an axial core score, and tremor burden using rest tremor and total tremor scores. Derived ratios, particularly the axial/rest tremor ratio, were calculated. A PD cohort matched for age and disease duration was constructed, and receiver operating characteristic (ROC) analyses were performed. RESULTS: Across the overall, matched, and short-disease-duration cohorts, PSP patients showed consistently greater axial motor impairment and lower tremor burden than PD patients. In the matched cohort, the axial/rest tremor ratio showed the numerically highest AUC among the evaluated motor measures (AUC 0.909, 95% CI 0.869-0.944). A cut-off of ≥ 4.57 yielded 89.4% sensitivity and 79.0% specificity. Sensitivity analyses using alternative pseudocounts yielded similar AUCs, although the optimal numerical threshold varied. CONCLUSIONS: The axial/rest tremor ratio, derived from routine MDS-UPDRS part III examination, may serve as a simple and clinically accessible adjunctive marker for distinguishing PSP from PD.