Neuroinflammation as a Link in Parkinson's and Alzheimer's Diseases: A Systematic Review and Meta-Analysis

Neuroinflammation plays a critical role in Alzheimer's (AD) and Parkinson's diseases (PD) onset, pathophysiology, and progression. The aim of our meta-analysis was to review the available literature to assess the role of neuroinflammation in the pathogenesis of the two most common neurological diseases: Parkinson's disease and Alzheimer's disease. Two medical databases were searched: Web of Science and PubMed in the period from 2009-2023, where a total of 37 publications that met the inclusion criteria were selected for further evaluation. Both patients with AD and with PD showed statistically significantly higher levels of interleukin IL-6 compared to the control group: p-value of 0.0034 for AD (SMD, 1.17; 95% CI, 0.39-1.96) and p-value of 0.0487 for PD (SMD 0.29 95% Cl 0.00-0.59). In AD patients, statistical significance (for random effect) was also observed for IL-1β, where higher values of this cytokine were recorded in patients compared to controls (p-value <0.001). In turn, in patients with PD, apart from IL-6, statistical significance was also observed for tumor necrosis factor-α (TNF-α) (p= 0.0431, SMD 0.52 95%Cl 0.02-1.02). Significant heterogeneity was also recorded (Q =85.48; P < 0.01; I² = 87%). In both study groups, significant differences in common effect were observed for the anti-inflammatory cytokine IL-10, which could suggest a protective effect of this cytokine in patients with neurodegenerative diseases. The obtained results reinforce the existing clinical evidence that Alzheimer's and Parkinson's diseases are accompanied by an inflammatory response, with considerably higher blood levels observed for pro-inflammatory cytokines: IL-6, TNF-α and IL-1β.