Donald Budenz, MD, MPH

Defining OPP

OPP is the force that drives blood through capillaries in the eye.⁷ The primary force for driving blood into the eye is arterial blood pressure at the level of the eye. Mean perfusion pressure simply is mean arterial pressure at the level of the eye minus IOP.⁸

Supportive evidence

Recently, a study by Berisha et al. reported a positive relationship between retinal circulatory abnormalities and retinal nerve fiber layer (RNFL) thinning in patients with early-stage primary open-angle glaucoma (POAG).⁹ These investigators evaluated 12 patients with early open-angle glaucoma and a known maximum untreated IOP >22 mm Hg, and eight age-matched controls, with a blood flow instrument (Laser Doppler, Canon) and optical coherence tomography (Stratus, Carl Zeiss Meditec) for RNFL. They found significantly reduced retinal blood speed (p = 0.009) and flow (p = 0.010) and thinner RNFL (p = 0.002) in the patients with glaucoma.

Several well-known epidemiologic studies have provided evidence regarding OPP and glaucoma prevalence and progression.^10-16 The most recent have been data reported by Leske et al. from the Barbados Eye Study and the Early Manifest Glaucoma Trial (EMGT).^14-16 The Barbados eye study reported that at 4 and 9 years of follow-up, low mean and systolic pressures and DOPP all were significantly associated with increased risk of visual field defect or optic nerve head damage.¹⁵ In the EMGT trial,¹⁶ analysis of patients with high initial IOPs revealed that lower systolic perfusion pressure was associated with an increased risk ratio for the development of glaucoma progression.¹⁶

Clinical considerations

Ophthalmologists should be able to measure blood pressure to calculate and chart perfusion pressure. Calculation of OPP gives us a simple yet powerful technique to examine the role of ocular hemodynamics in the disease process of our patients. In the future, once further data become available, perhaps ophthalmologists will be able to measure blood flow and RNFL as described by Berisha et al.⁹

With an awareness of a patient's OPP, practitioners should be able to manage patient care with a goal of keeping perfusion pressure from dropping lower than pressures associated with greater risk. Lowering IOP will improve mean, and systolic pressures and DOPP.¹⁷ So what can clinicians do to control OPP? Is it possible that blood pressure in some of our patients may be over-controlled?

It is important for ophthalmologists to document concurrent conditions, such as hypertension, and all relevant medications their patients are taking. If a patient falls into a higher risk category, it may be worthwhile to discuss with the patient's internist the implications of excessively low OPP.