Blood Flow and Glaucoma
November 19, 2003
Norma Devine, Editor
On Wednesday, November 19, 2003, Dr. Rick Wilson, a glaucoma specialist at Wills, and the glaucoma chat group discussed “Blood Flow and Glaucoma.”
Moderator: Dr. Wilson, tonight our topic is blood flow. Are there many blood vessels leading to the eye or just one large one?
Dr. Rick Wilson: That depends upon what level of the eye you are talking about. The central retinal artery supplies the retina. The short posterior ciliary arteries supply the choroid, or vascular layer of the eye. There may be two, to as many as seven, short posterior ciliary arteries. They supply the optic nerve, and come most into play in glaucoma.
P: Are the posterior ciliary arteries branches from the carotid artery or are they supplied internally from the Circle of Willis (major artery center of the brain)?
Dr. Rick Wilson: The carotids provide a major part of the blood flow. After that, I am not sure.
P: Where does the ciliary artery originate? In the carotids or the Circle of Willis?
Dr. Rick Wilson: The ciliary artery gives rise to the long and short posterior ciliaries that provide the blood supply to the root of the iris and ciliary body.
P: If the posterior ciliaries feed the blood supply to the ciliary body, is there any relationship between blood flow and production of aqueous humor?
Dr. Rick Wilson: It is the anterior ciliary arteries that supply the ciliary body; but, yes, blood flow has a significant effect on the production of aqueous humor. If the blood pressure is low, less aqueous is produced.
P: How do sitting, standing, and lying down affect intraocular pressure (IOP)?
Dr. Rick Wilson: Most people have their IOP measured when they are sitting, and are therefore upright. Lying down may increase the pressure by two or more millimeters. Elevating the legs may increase the pressure a third or more.
P: Does blood pressure have to be abnormally low to reduce aqueous production?
Dr. Rick Wilson: In the normal person, as the blood-pressure drops in the ciliary body, the vessels enlarge to keep the blood flow through the ciliary body the same. Since the blood flow is the same, there is no significant change in aqueous production.
P: Does blood pressure correlate with eye pressure, or is the concern about low blood pressure that already damaged optic nerves will suffer more damage from lack of oxygen?
Dr. Rick Wilson: If blood pressure suddenly increases, the eye pressure will also increase. However, the auto regulation of the vessels in the eye soon allows the eye pressure to return to normal. If the blood pressure stays up for two or more years, it seems that the eye pressure will start to rise. Elevated blood pressure, soon after it starts rising, helps blood perfusion. After blood pressure has been elevated for some time, however, changes in the vessels tend to restrict perfusion.
P: How can you tell if damage to the optic nerve is from poor blood supply?
Dr. Rick Wilson: Certain types of nerve injury appear to be associated with problems with blood flow. A pale nerve, or one with hemorrhages around it, suggest that the blood supply has been compromised. Retinal blood flow and gross blood flow to the optic nerve can be measured.
P: How important is good blood flow?
Dr. Rick Wilson: After intraocular pressure, good blood flow seems to be the next most important variable in determining who gets glaucoma and how severe the disease will be. I am sure there are many other factors involved, but we still don’t know what they are.
P: Has it been shown that the amount of abnormal blood flow to the optic nerve in glaucoma is related to the amount of damage to the optic nerve?
Dr. Rick Wilson: Many studies have linked abnormal blood flow to glaucoma. Low systemic blood pressure is one of the biggest risk factors.
P: What are the visual changes to the optic nerve caused by decreased blood flow from hypotension or atherosclerosis?
Dr. Rick Wilson: If a small vessel in the eye is occluded, the loss occurs in the top or bottom half of the vision. If the occlusion of a blood vessel occurs in the back of the brain, where vision is located, either the right or left half of the vision is lost. If the occlusion occurs in the nerves from the eye, the vision loss is quite dependent upon the location of the occlusion.
P: Ginseng is supposed to increase blood flow to the brain. Could it also cause an increase in IOP?
Dr. Rick Wilson: I don’t think anyone knows that for sure.
P: What effect, if any, do blood disorders such as hemachromotosis or leukemia have on blood flow to the optic nerve?
Dr. Rick Wilson: If there is only low blood pressure or poor perfusion to the optic nerve, the loss of visual field will be indistinguishable from glaucomatous loss. If the level of red blood cells carrying oxygen to the optic nerve is decreased significantly, or there are increased proteins or other materials in the blood that make the blood so thick it has trouble moving through the vessels, an optic neuropathy that looks just like glaucoma occurs.
P: What is perfusion?
Dr. Rick Wilson: Perfusion is the blood pressure into the eye, minus the pressure in the eye that inhibits the flow of blood into the eye.
P: How does low blood pressure affect glaucoma?
Dr. Rick Wilson: As mentionedabove, low blood pressure is a serious risk factor for glaucoma. It may increase the risk as much as six times. If the pressure in the eye is high, or the pressure of the blood being pushed into the eye is low, the optic nerve may not get enough blood supply.
P: Are allergy eye drops bad for glaucoma patients because they constrict the blood vessels?
Dr. Rick Wilson: It does not seem that enough of the allergy eye drop gets to the back of the eye to affect blood flow to the eye.
P: During an earlier chat, you mentioned the potential benefits of the regular use of baby aspirin. A few months ago, I saw on the the web site of the Review of Optometry a little news blurb associating regular aspirin use with an increased incidence of disc hemorrhage. Will this cause the risk of aspirin to be reconsidered, and can you extrapolate this risk to other classes of blood thinners, such as ginkgo biloba?
Dr. Rick Wilson: Aspirin does increase the risk of bleeding. If your family is prone to strokes caused by bleeding into the brain, aspirin is not for you. On the other hand, if your family is prone to strokes caused by blockages of blood vessels in the brain, then aspirin is for you. The same analogy holds true for the eye. For most people, making platelets less sticky helps to improve circulation throughout the nerve and retina. However, attendant in this is a slightly increased risk of bleeding especially, from macular degeneration.
P: You said that certain nerve injuries appear to be associated with a problem in ocular blood flow. Can you speak a little more about those injuries? Also, what other things seem to restrict blood flow to the optic nerve?
Dr. Rick Wilson: Migraines seem to be caused by inflammation in the brain that may cause spasm of the blood vessels there. Spasm of the vessels to the optic nerve would be a serious risk factor for glaucoma. Diseases that cause severe anemia or overly thick blood also cause problems with blood flow to the nerve. Tumors that press on the eye will obviously cause problems to the optic nerve. Hyperthyroidism often causes thickening and tightening of the muscles around the eye, which can raise the intraocular pressure, especially when the eye tries to move against a thickened and tightening muscle.
P: When the optic neuropathy that looks like glaucoma occurs, is it permanent?
Dr. Rick Wilson: Some of the secondary glaucomas are more prone to improvement than open-angle glaucoma if the cause is discovered and treated. Most optic neuropathies, however, cause injury in which the improvement is limited.
P: How much does diastolic pressure decrease during sleep? What level would be of concern?
Dr. Rick Wilson: Two-thirds of the population are what we refer to as “dippers.” Their blood pressure drops more than 10% when they are asleep. That decrease can be exacerbated by some medications used to treat systemic hypertension. Blood pressures that drop to 80 over 50 mm Hg, or lower, concern me. Clearly, that risk depends on the health and age of the individual.
P: Who can evaluate blood flow to the optic nerve? I’m sure my ophthalmologist cannot do that. Should glaucoma patients have that checked?
Dr. Rick Wilson: Only specialists at major medical centers have the capability to check blood flow on a routine basis. That is usually not needed unless the patient seems to be progressing with good intraocular pressures.
P: Before eye surgery, are patients with arthritis asked to stop using NSAID’s (non-steroidal anti-inflammatory drugs) for a week or two?
Dr. Rick Wilson: Yes, a week is usually sufficient. The same holds true for the use of aspirin and other blood thinners.
Moderator: Is less aqueous humor bad?
Dr. Rick Wilson: If intraocular pressure is high because there is a blockage to outflow, then less aqueous is good. If there is normal outflow, less aqueous would be a sign of a less healthy ciliary body.
P: Are afferent pupillary defects seen more often in normal-tension glaucoma than in POAG (primary open-angle glaucoma)?
Dr. Rick Wilson: Not necessarily. They are more likely to be seen in patients with pseudoexfoliation, where the disease is often asymmetric. It is the asymmetry of nerve damage that causes afferent pupillary defects.
P: I am concerned about the risks that body position and anesthesia/anesthetic adjuncts (e.g., Anectine) can present during surgery in general, especially for persons with glaucoma. Is it possible (or would it be useful) to monitor blood flow to the optic nerve during surgery?
Dr. Rick Wilson: It would be difficult to monitor blood flow to the optic nerve during anesthesia. However, the anesthetist should be warned to avoid too low systemic blood pressure.
P: In the medical literature the term “microcirculation” is used in reference to clinically significant aspects of blood flow to the optic nerve. To what does “microcirculation” — as opposed to simply “circulation” or “blood flow” — refer?
Dr. Rick Wilson: Microcirculation refers to the blood flow through small vessels. The vessels that supply the optic nerve would fit into that category.
Moderator: Thank you, Dr. Wilson.
Dr. Rick Wilson: Good night all. Have a good week