Factors as disparate as ocular blood flow, beta blockers, and sleep apnea all may play roles in the development and progression of glaucoma.
According to Dr. Semes, ocular blood flow as relevant to glaucoma cannot universally be evaluated with any single blood flow imaging device. Therefore, with the current state of the art, assessing perfusion to the optic nerve and inner retina is done indirectly. A comprehensive approach using multiple imaging technologies is required for meaningful insight.
"It's important to remember that the eye has multiple vascular beds and a segmental blood supply," Dr. Semes said. "Although the majority of blood flow to the optic nerve head is derived from the choroidal plexus, you can't just look at retinal blood flow, you can't just look at choroidal blood flow, and you can't just look at blood flow to the optic nerve head."
"It may be that approaches that reduce blood pressure or increase blood flow, such as exercise, are equally good for the eyes as they are for the heart," Dr. Semes said.
Dr. Semes added that it's important to look at perfusion pressure to the optic nerve head, especially in patients in whom glaucoma is progressing even though their intraocular pressure (IOP) is at the target level. To determine the diastolic ocular perfusion pressure (DOPP), subtract the patient's mean IOP from his or her mean systemic diastolic blood pressure. A DOPP above 55 generally means a lower risk of ocular nerve damage.1
"This is something we're still getting our arms around," Dr. Semes said. "Measuring perfusion pressure isn't as entrenched as measuring IOP, but it's an important emerging area."
Evaluate beta blocker use
Dr. Semes said that maintaining an adequate ocular perfusion pressure is why he urges caution when using topical beta blockers at night.
"Low blood pressure at night, coupled with high IOP in the supine position, can compromise ocular perfusion pressure," he said. "Patients who are using topical beta blockers at night may be reducing their IOP, but they also may be reducing perfusion to the optic nerve head. This is because systemic absorption of the drops, from the blood vessels on the surface of the eye, may be depressing blood pressure systemically."
Instead of using beta blockers twice a day, Dr. Semes recommends having patients use the beta blocker in the morning and a prostaglandin analog (PGA) at night when the PGA is insufficient. "To minimize nocturnal hypotension, avoid IOP meds that lower systemic blood pressure at night, such as beta blockers and alpha agonists," he said. This advice has been suggested previously, and is particularly important in normal tension glaucoma.2