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 pres sure minus intraocular pressure) and myopia have been associated with prote ction of the retina from clinical diabetic retinopathy. This prompts the qu estion as to whether myopia's protective role could also be a pressure effe ct, given that pressure could be dissipated in the longer arteriole tree of the myopic eye. Methods: We combined the Ohm, Poiseuille, and Murra) laws to derive the fol lowing new formulation: the pressure attenuation along a vessel varies dire ctly with its length and inversely with its diameter. A mean pressure atten uation index was calculated for 22 healthy control subjects, 25 patients wi th axial myopia, and 6 patients with retinitis pigmentosa using digitized f undus images. Results: The myopic arteriolar tree would produce a 16% greater pressure at tenuation than that of controls (P =.002), with a linear relationship betwe en mean pressure attentuation index and axial length (r = 0.93). Mean press ure 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 proport ional to the length and inversely proportional to the diameter of the arter iole 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 retin opathy. The results support the hypothesis that low-end arteriolar pressure is a common denominator for many protective conditions in diabetic retinop athy.