A new Pressure attenuation index to evaluate retinal circulation : A link to protective factors in diabetic retinopathy

Background: Low ocular perfusion pressure (two thirds of mean arterial pressure minus intraocular pressure) and myopia have been associated with protection of the retina from clinical diabetic retinopathy. This prompts the question as to whether myopia's protective role could also be a pressure effect, given that pressure could be dissipated in the longer arteriole tree of the myopic eye. Methods: We combined the Ohm, Poiseuille, and Murray laws to derive the following new formulation: the pressure attenuation along a vessel varies directly with its length and inversely with its diameter. mean pressure attenuation index was calculated for 22 healthy control subjects, 25 patients with axial myopia, and 6 patients with retinitis pigmentosa using digitized fundus images. Results: The myopic arteriolar tree would produce a 16% greater pressure attenuation than that of controls (P = .002), with a linear relationship between mean pressure attentuation index and axial length (r= 0.93). Mean pressure attentuation index of the group with retinitis pigmentosa is increased 67% above that of controls, which is calculated to contribute an additional 10 mm Hg of pressure dissipation along their retinal arteriolar system. Conclusions: Pressure attenuation in retinal arterioles is directly proportional to the length and inversely proportional to the diameter of the arteriole segment being measured. Clinical Relevance: A pressure attenuation index may be important in light of the entities known or presumed to protect the retina from diabetic retinopathy. The results support the hypothesis that low-end arteriolar pressure is a common denominator for many protective conditions in diabetic retinopathy.