Factors as disparate as ocular blood flow, beta blockers and sleep apnoea may all play roles in the development or progression
of glaucoma. Understanding the role these factors may play in the disease is essential to providing your patients with the
best possible care, said Dr Leo Semes, FAAO, professor of optometry and director of externship programmes at the University
of Alabama at Birmingham (UAB), at the International Vision Expo West.
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."
The complex relationship of blood flow and glaucoma means that patients who have peripheral vascular deregulation — as manifested
by something as simple as cold hands — may be at increased risk of developing glaucoma. It may also mean that the risk factors
for atherosclerosis, such as age, dyslipidemia, cigarette smoking and systemic hypertension, may also be risk factors for
the development of glaucoma.
"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
"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 analogue (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