From the eye to the brain: Tear fluid as a mirror of neurodegeneration

Tear fluid, traditionally explored in ocular surface research, is emerging as a valuable source of systemic biomarkers, offering a non-invasive, easily accessible medium for diagnostic innovation. In this work, we present a Raman spectroscopy-based protocol tailored for the molecular profiling of human tear fluid, aimed at supporting early diagnosis of neurodegenerative diseases. Our previously published study focused on patients with Amyotrophic Lateral Sclerosis (ALS), analyzing 5μL tear samples dried on BaF2 substrates. As a recent advancement, we extended this protocol to patients with Parkinson's Disease (PD), allowing a direct comparison of spectral fingerprints between ALS and PD. The results reveal condition-specific biochemical patterns, underlining the potential of tear fluid analysis in differential diagnosis. This approach integrates ocular science with systemic diagnostics, demonstrating how optometric research can contribute to broader medical challenges through tear-based molecular phenotyping.