Greater exposure to PM2.5 and PM10 was associated with lower corneal nerve measures: the Maastricht study - a cross-sectional study

Abstract

Background: Epidemiological and toxicological studies indicate that increased exposure to air pollutants can lead to neurodegenerative diseases. To further confirm this relationship, we evaluated the association between exposure to ambient air pollutants and corneal nerve measures as a surrogate for neurodegeneration, using corneal confocal microscopy.

Methods: We used population-based observational cross-sectional data from The Maastricht Study including N = 3635 participants (mean age 59.3 years, 51.6% were women, and 19.9% had type 2 diabetes) living in the Maastricht area. Using the Geoscience and hEalth Cohort COnsortium (GECCO) data we linked the yearly average exposure levels of ambient air pollutants at home address-level [particulate matter with diameters of ≤ 2.5 µm (PM2.5), and ≤ 10.0 µm (PM10), nitrogen dioxide (NO2), and elemental carbon (EC)]. We used linear regression analysis to study the associations between Z-score for ambient air pollutants concentrations (PM_2.5, PM₁₀, NO₂, and EC) and Z-score for individual corneal nerve measures (corneal nerve bifurcation density, corneal nerve density, corneal nerve length, and fractal dimension).

Results: After adjustment for potential confounders (age, sex, level of education, glucose metabolism status, corneal confocal microscopy lag time, inclusion year of participants, smoking status, and alcohol consumption), higher Z-scores for PM_2.5 and PM₁₀ were associated with lower Z-scores for corneal nerve bifurcation density, nerve density, nerve length, and nerve fractal dimension [stβ (95% CI): PM_2.5 -0.10 (-0.14; -0.05), -0.04 (-0.09; 0.01), -0.11 (-0.16; -0.06), -0.20 (-0.24; -0.15); and PM₁₀ -0.08 (-0.13; -0.03), -0.04 (-0.09; 0.01), -0.08 (-0.13; -0.04), -0.17 (-0.21; -0.12)], respectively. No associations were found between NO₂ and EC and corneal nerve measures.

Conclusions: Our population-based study demonstrated that exposure to higher levels of PM_2.5 and PM₁₀ are associated with higher levels of corneal neurodegeneration, estimated from lower corneal nerve measures. Our results suggest that air pollution may be a determinant for neurodegeneration assessed in the cornea and may impact the ocular surface health as well.

Keywords: Air pollution; Corneal nerves; Elemental carbon; NO2; Neurodegeneration; Particulate matter.

Fig. 1

Geographical Area of Participant Recruitment: Maastricht Region

Fig. 2

Shows the selection of participants for inclusion in the analyses

Fig. 3

Associations of Z-score for PM_2.5, PM₁₀, NO₂, and EC with Z-score for bifurcation density, corneal nerve density, corneal nerve length, and corneal nerve fractal dimension. Standardized regression coefficients (stβ) represent the differences in corneal nerve measures in SD per SD greater measure of air pollutants. For PM_2.5, PM₁₀, and EC, 1 SD corresponds to 39.9 number of branches/mm² for corneal nerve bifurcation density, 24.3 number of main and branches/mm² for corneal nerve density, 4.4 mm/mm² for corneal nerve length, 0.1 (unit-less) for corneal nerve fractal dimension, 1.4 µg/m³ for PM_2.5, 1.7 µg/m³ for PM₁₀, 3.1 µg/m³ for NO₂, and 0.1 µg/m3 for EC. Variables entered into model: age, sex, level of education (high, medium, low), glucose metabolism status (prediabetes and type 2 diabetes versus normal glucose metabolism), corneal confocal microscopy lag time, inclusion year of participants, smoking status (never, former, current), and alcohol consumption status (none, low, high). Asterisks indicate values that are statistically significant (P < 0.05). Abbreviations: CI, confidence interval; SD, standard deviation