The Retinal Vasculature as a Fractal: Methodology, Reliability, and Relationship to Blood Pressure

Objective: Fractals represent a type of derived geometric pattern that permits the characterization of the branching pattern of retinal vessels. We examined a new semiautomated method to measure retinal vessel fractals. Design: Methodology study. Participants: Three hundred randomly selected participants from the population-based Blue Mountains Eye Study. Methods: We developed a semiautomated computer program to measure the fractal dimension (D_f) of the retinal vessels from digitized images of disk-centered retinal photographs. Two trained graders masked to participant characteristics measured D_f of right eye images of participants. Reliability was determined by repeat grading of the images from 60 participants, and association with systolic and diastolic blood pressure was examined in all 300 participants. Main Outcome Measure: D_f of the retinal vessels. Results: Mean D_f was 1.437 with a standard deviation of 0.025. Intragrader and intergrader reliability estimates were high with intraclass correlation ranging from 0.93 to 0.95. D_f was inversely correlated with age (r = -0.42, P = 0.001) and systolic blood pressure (r = -0.29, P<0.0001). After adjustment for age and sex, mean D_f was significantly lower in participants with than without hypertension (D_f difference 0.01, P = 0.02). Conclusions: The D_f of the retinal vessels can be reliably measured from photographs and shows a strong inverse correlation with blood pressure. These data suggest that the D_f may be a measure of early microvascular alterations from elevated blood pressure. Further studies to examine the systemic and ocular correlates of the D_f of the retinal vessels are needed. Financial Disclosure(s): The authors have no proprietary or commercial interest in any materials discussed in this article.