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.