Age-related changes in the fractal dimension of the retinal microvasculature, effects of cardiovascular risk factors and smoking behaviour

Purpose This cross-sectional study investigates the association between retinal vessel complexity and age and studies the effects of cardiovascular health determinants. Methods Retinal vessel complexity was assessed by calculating the box-counting fractal dimension (D-f) from digital fundus photographs of 850 subjects (3-97 years). All photographs were labelled as 'non-pathological' by the treating ophthalmologist. Results Statistical models showed a significantly decreasing relationship between age and D-f (linear: R-squared = 0.1897, p < 0.0001; quadratic: R-squared = 0.2343, p < 0.0001; cubic: R-squared = 0.2721, p < 0.0001), with the cubic regression model offering the best compromise between accuracy and model simplicity. Multivariate cubic regression showed that age, spherical equivalent and smoking behaviour have an effect (p < 0.0001) on D-f. A significantly increasing effect of the number of pack-years on D-f was observed (effect: 0.0004, p = 0.0017), as well as a significantly decreasing effect of years since tobacco abstinence (effect: -0.0149, p < 0.0001). Conclusion We propose using a cubic trend with age, refractive error and smoking behaviour when interpreting retinal vessel complexity.