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P. Mitchell, T. Y. Wong, G. Liew, E. Rochtchina, A. Wainwright, J. Wang, Blue Mountains Eye Study; Fractal Dimension of the Retinal Vasculature: Associations, Applications and Limitations of Applying this Technique in an older Population. Invest. Ophthalmol. Vis. Sci. 2009;50(13):3901.
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Fractal dimension is an overall measure of the density (‘fullness’ or ‘sparseness’) of a branching structure. Applying this objective measurement to the retinal vasculature promises improved understanding of the functioning of microvascular beds, and could potentially provide additive information to other retinal vessel imaging techniques. We aimed to explore associations, potential applications and limitations of applying the fractal technique to retinal images taken in a large population study.
The Blue Mountains Eye Study (BMES) examined 3654 participants in baseline exams (1992-4). Using the IRIS FRACTAL computer program, fractal dimension (Df) was measured from the R eyes of 3,354 participants (91.8% of examined), using Field 1 images. This produces a skeletonized image of the retinal vasculature. The age-gender distribution of Df and relationship with anthropometric, ocular and systemic and medical history variables was assessed.
Df was narrowly distributed with a mean of 1.441 (SD 0.024), and skew to the left ('sparse' vascular bed), minimum 1.324, maximum 1.506. Df declined with age, mean 1.450 (ages <60) to 1.420 (ages 80+), a trend more apparent in women, although there was no gender difference after adjustment (age-height adjusted mean Df =1.436 in both women and men). Fractal measures appear to be significantly influenced by contrast: darker iris colour resulted in slightly higher Df and significant cataract decreased it. Df was positively associated with both wider retinal arteriolar and venular caliber. Factors reducing Df included the presence and severity of hypertension, increasing blood glucose or diabetes, longer axial length and presence of glaucomatous optic neuropathy. Factors increasing Df included smoking and higher hematocrit or hemoglobin. The relationship between Df and and incident coronary heart disease (CHD) mortality was ‘U’-shaped, multivariate-adjusted hazard ratio (HR) for CHD death 1.5 (confidence interval, CI 1.2-2.0) and HR 1.5 (CI 1.1-1.9) for the 1st and 4th quartile of Df, respectively (2nd and 3rd quartiles as referent). A similar U-shaped relationship was observed for the relationship of Df to retinopathy in people without diabetes.
This finding suggests that Df could represent an optimal configuration of the retinal microvasculature. Its measurement may permit physiological and pathological associations of small vessel disease in the population to be objectively and rapidly measured.
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