Unraveling the multiple mysteries of ocular perfusion pressure in open angle glaucoma

Atlanta-Evidence is mounting that ocular perfusion pressure (OPP)-arterial pressure minus intraocular pressure (IOP)-is an important risk factor for open-angle glaucoma. However, this growing consensus still leaves several questions, such as how reliabile are OPP measurements, and how do you translate findings from epidemiologic studies into the clinical practice.

"Is taking blood pressure important in the diagnosis and management of glaucoma?" Dr.Marrelli asked. "I'm going to come down on the side that yes, it is important, although it's a little complex."

Vascular risk factors such as blood pressure and OPP may be more important than previously thought, because therapeutic intervention can alter their course, Dr. Marrelli said.

Autoregulation enables the eye to maintain a relatively constant blood flow over a fairly wide range of OPP, she explained. When OPP is either very high or very low, however, autoregulation may fail and the blood flow may not be consistent. When this occurs, the risk of developing glaucoma increases.

"Numerous studies over many different populations-in North America, in the Caribbean, in Europe, in Africa-have found that low diastolic OPP (less than 50-55mm Hg) has been associated with glaucoma as well as with glaucoma progression," Dr. Marrelli said.

Dippers big and little

She added, however, that interpretating this data is complicated. Reliable clinical measures of optic nerve head perfusion have been elusive, and using systemic blood pressure as a surrogate does not always accurately represent blood pressure in the ophthalmic artery.

Nocturnal blood pressure changes also complicate the picture, Dr. Marrelli said. Most people have a small but measurable (10% to 15%) drop in overnight blood pressure ("physiological dippers"), and a few have minimal or no change ("non-dippers"). A small group have a greater than 20% drop in overnight blood pressure ("extreme dippers").

If an extreme dipper's exceptionally low overnight blood pressure coincides with a peak IOP, OPP could drop to a level that places the individual at risk for development or progression of glaucoma, Dr. Marrelli said.

What does it mean?

Despite some variability, one predominant finding in glaucoma studies is that extreme dippers have a higher incidence of glaucoma and a higher risk of progression. Some studies have even shown that significant nocturnal fluctuation in blood pressure also may be a risk factor for glaucoma, she noted.

Determining what to do with these findings is problematic, Dr. Marrelli said. "We have pretty good evidence that low OPP and low blood pressure may be risk factors for glaucoma and glaucoma progression," she said.

"But what we don't have is evidence that if we raise the blood pressure, we will have any affect on the disease process, or even if we change OPP by doing things other than changing the IOP that we will change the course of the disease."