July 2008




Ocular perfusion pressure and the optic nerve

by Tony Realini, M.D. EyeWorld Contributing Editor


Is OPP a new risk factor for glaucoma?

Study evaluates how changes in ocular perfusion pressure impact the optic nerve Source: William Trattler, M.D.

New data from ongoing clinical trials suggests that primary open-angle glaucoma has a new risk factor: low ophthalmic perfusion pressure (OPP). What does this mean for patients with glaucoma and the clinicians who treat them? Several glaucoma experts discussed the new findings at the recent meeting of the American Glaucoma Society in Washington, D.C.

“The pathophysiology of primary open-angle glaucoma remains unknown,” said Farnaz Memarzadeh, M.D., Doheny Eye Center, University of Southern California, Los Angeles. She acknowledged that elevated IOP remains an important risk factor for glaucoma but pointed out that “other factors, particularly those affecting perfusion to the optic nerve, may play a role.”

The mechanism by which elevated IOP leads to optic nerve damage has been debated for decades. Some believe its effect is primarily mechanical, while others hold with the ischemia theory: that elevated IOP reduces blood flow to the optic nerve, thus creating chronic ischemia within the neural tissue. Mean OPP is defined as the mean arterial pressure minus IOP. Systolic and diastolic OPP can also be calculated using systolic and diastolic blood pressure values, respectively, minus IOP.

“Ocular perfusion pressure is a delicate balance between IOP and blood pressure,” said Donald Budenz, M.D., Bascom Palmer Eye Institute, University of Miami. “Lower ocular perfusion pressure is associated with an increased risk of the development of open-angle glaucoma as well as its progression.” He pointed to several major studies, including the Baltimore Eye Survey, the Barbados Eye Study, the Egna-Neumarkt Study, and Projecto Ver, all of which identified low OPP as a risk factor for developing glaucoma. “In the Baltimore Eye Survey, glaucoma was six-fold more common in eyes with low versus high diastolic perfusion pressure.”

“There is broad new evidence that has brought this topic back to the forefront,” said Dr. Budenz. The newest piece of evidence is from the Los Angeles Latino Eye Study (LALES). Dr. Memarzadeh presented data from the ongoing LALES evaluating the relationship between OPP and the risk of having open-angle glaucoma. “This is a cross-sectional, population-based study of 6,130 adult Latinos in Los Angeles,” she explained. All subjects underwent a comprehensive eye examination as well as blood pressure measurement. “Overall, 1,770 subjects had systemic hypertension.”

“We found that low systolic, diastolic, and mean OPP all led to increased risk of having open-angle glaucoma,” she said. Of these, she said, the relationship between glaucoma risk and diastolic perfusion pressure was the strongest. “The risk of glaucoma increases significantly as diastolic perfusion pressure falls,” she said.

These results confirm the Baltimore Eye Survey results and also confirm a recent report from the Barbados Eye Study in which lower mean, systolic, and diastolic OPP were risk factors for incident open-angle glaucoma in this longitudinal survey.

Dr. Memarzadeh reported that in the LALES data, high systemic blood pressure was also related to an increased risk of glaucoma. This is also consistent with previous reports. “It’s possible for both systemic hypertension and systemic hypotension to contribute to the risk of open-angle glaucoma,” she said. “Perhaps this is due to disruption of autoregulation of blood flow.”

Joseph Caprioli, M.D., Jules Stein Eye Institute, David Geffen School of Medicine, University of California, Los Angeles, agreed that low OPP is a risk factor for glaucoma but said that this finding should not come as a surprise. He pointed out that high IOP and low blood pressure are known risk factors for glaucoma. “Ocular perfusion pressure represents a summation of these two known risk factors,” he said, “and the degree of interdependency between them is unknown.”

He also pointed out that OPP as currently measured is only a surrogate for true ocular perfusion. “True ocular perfusion pressure would be calculated using ophthalmic artery pressure, not mean systemic arterial pressure.” He added that even this calculation would not accurately describe perfusion pressure within the optic nerve, where hemodynamics are likely different than in the ophthalmic artery.

“If we think that perfusion pressure is an important aspect of optic nerve head damage in glaucoma,” he said, “then we need real measures of ocular and optic nerve head perfusion pressures. Unfortunately, reliable measures of these entities have been elusive.”

If low OPP is detrimental to patients with glaucoma, what can clinicians do for their patients with low OPP? “First, we have to identify the patients with low blood pressure,” said Dr. Caprioli. “In these patients, we should consider whether they have low blood pressure as a result of overzealous treatment of hypertension.”

In addition, he said, it is worth evaluating changes in blood pressure—perhaps by referring patients for 24-hour blood pressure monitoring—because nocturnal dips in blood pressure may also contribute to glaucoma progression. Salt tablets, or even salty snacks such as potato chips or pretzels, right before bed may help ameliorate nocturnal dips in blood pressure.

“I am relatively selective when referring my patients for 24-hour blood pressure monitoring, but it is a worthwhile consideration in patients who are progressing despite what appears to be adequately controlled IOP,” Dr. Caprioli said.

Editors’ note: Drs. Memarzade, Budenz, and Caprioli have no financial interests related to their comments.

Contact Information

Budenz: dbudenz@med.miami.edu

Caprioli: caprioli@ucla.edu

Memarzadeh: memarzad@usc.edu

Ocular perfusion pressure and the optic nerve Ocular perfusion pressure and the optic nerve
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