Blood-Derived Inflammatory Indices Across Essential Tremor, Parkinson’s Disease, and Progressive Supranuclear Palsy: An Exploratory Retrospective Analysis

Background/Objectives: Evidence suggests that inflammation contributes to the pathogenesis of neurodegenerative disorders. The utility of blood-derived inflammatory biomarkers in differentiating neurodegenerative disorders remains incompletely understood. The aim of this study was to compare peripheral inflammatory markers in patients with essential tremor (ET), Parkinson’s disease (PD), progressive supranuclear palsy (PSP), and control participants. Methods: This retrospective study included 44 patients with ET, 47 with PD, 44 with PSP, and 45 control participants. The neutrophil-to-lymphocyte ratio (NLR), platelet-to-lymphocyte ratio (PLR), monocyte-to-lymphocyte ratio (MLR), systemic immune-inflammation index (SII), systemic inflammation response index (SIRI), and aggregate index of systemic inflammation (AISI) were calculated from routine complete blood counts. Between-group comparisons were performed using the Kruskal–Wallis test, with a Holm correction across the six indices. The effect sizes were also estimated. Results: The PLR was the only inflammatory marker that significantly differentiated the analyzed groups (p = 0.045), with the highest value observed in PSP patients and the lowest in ET patients. A post hoc analysis indicated different PLR values in PSP than in ET patients (Cliff’s delta = 0.336, small-to-moderate effect). However, the overall association did not remain statistically significant after Holm correction across the six indices (adjusted p = 0.268), and the diagnostic group was not independently associated with PLR after adjustment for age and sex. No statistically significant differences were observed for the NLR, MLR, SII, SIRI, or AISI. Nevertheless, PSP patients consistently exhibited the highest median values of the NLR, SII, and AISI, whereas ET patients generally showed lower inflammatory marker levels. Conclusions: PLR was the only inflammatory marker that significantly differentiated the analyzed groups and was the highest among patients with PSP. The observed trends suggest a tendency toward greater peripheral immune activation in PSP compared with PD and ET. Larger prospective studies incorporating both inflammatory and neurodegenerative biomarkers are warranted to validate these findings.