August 8, 2004
Norma Devine, Editor
On Wednesday, August 8, 2004, Dr. Rick Wilson, a glaucoma specialist at Wills, and the glaucoma chat group discussed “Target Pressure.”
Moderator: Dr. Wilson, how do you define target pressure?
Dr. Rick Wilson: The target pressure is pressure at which the doctor feels the patient will not get any worse. At first we thought that lowering IOP (intraocular pressure) 20 to 25%, or lowering the patient’s IOP to the normal range, was enough to prevent further damage to the optic nerve. We now know that the IOP needs to be lowered 34 to 40% from the level at which damage was progressing. Optic nerves that have suffered advanced damage need IOPs at 12 mm hg or lower to prevent further damage.
P: Why is an IOP of 21 or 22 mm Hg regarded as the upper limit for glaucoma patients?
Dr. Rick Wilson: The average IOP in America for healthy people is 16 mm Hg. Two standard deviations above 16 is 22 mm Hg, which is why that number was chosen as the upper limit for glaucoma. There is no physiologic basis for the 22 mm Hg level.
P: Recent clinical trials have used various approaches to establishing a target IOP, including a fixed numeric goal, a percent age reduction based upon the untreated baseline IOP, a target range of IOP values, and fixed formulas. There are, however, no data to support an ideal approach. What approach do you use?
Dr. Rick Wilson: For the most part, I use a range. If a patient has been getting worse with IOPs in the abnormal range, but the nerve is only slightly damaged, I will accept IOPs in the 17 to 18 mm Hg range. If there is moderate damage, I want the IOP around 15 mm Hg. For serious damage, the IOP should be 12 mm Hg or lower. Intraocular pressure is interesting in that it varies considerably during the day, is higher in women than in men, and usually increases with age.
P: What does “slightly” damaged mean?
Dr. Rick Wilson: A change in the optic nerve, plus or minus a small visual field defect.
P: What is the maximum IOP for a target pressure?
Dr. Rick Wilson: The numbers I am giving you are my maximums. Lower is perfectly acceptable. Risk factors, such as age, migraines, low blood pressure, cardiac arrhythmias (irregular heart beat), and family history are taken into consideration.
P: If a person does not have glaucoma, what would his or her target pressure be at age 20 and at age 50?
Dr. Rick Wilson: If the nerve is healthy, then an IOP in the 10 to 22 mm Hg range would be acceptable. At the high end of this range, I would watch them carefully. I would treat a 50-year- old person the same. I would watch and worry more about a 70-year old person because of diminished circulation.
P: Why are migraines considered a risk factor?
Dr. Rick Wilson: Migraines are caused by vasospasm of the arteries in the brain. Spasm of the arteries to the eye will reduce blood flow and exacerbate glaucoma.
P: Are frequent and large daily fluctuations in IOP, or high IOP, associated with the greatest risk for loss of vision?
Dr. Rick Wilson: That is still debatable. Two studies suggest that fluctuation of IOP is more dangerous than a constant higher IOP; one study is equivocal. I try for the least fluctuation I can easily obtain.
P: Has it been proven that eyes with thin CCT (central cornea thickness) need more aggressive IOP lowering?
Dr. Rick Wilson: Even if the IOP is adjusted for the CCT, there seems to be an added vulnerability associated with thin corneas.
P: Don’t more appropriate correction tables for corneal thickness need to be established and validated?
Dr. Rick Wilson: Yes, but what would be better would be a technology that measures IOP without being influenced by CCT. Several instruments are being developed.
P: Aren’t pressures in the low 20’s normal for some people?
Dr. Rick Wilson: If the corneas are normal or thin, I am not sure that an IOP in the 20’s is normal for anyone. It may be that we just are not able to detect the slow and subtle damage that is occurring long-term. Some individuals, however, are able to tolerate elevated IOPs for years.
P: Is IOP higher when you are lying down, and is that why IOP is usually higher in the morning?
Dr. Rick Wilson: Having any part of your body higher than your head will cause venous blood pressure around the eye to go up and IOP will rise. Lying down with feet up or doing a headstand are two such positions. Excessive fluid drinking in a short time results in water- loading and a short-term rise in IOP.
P: What activities or circumstances commonly increase or decrease IOP?
Dr. Rick Wilson: Exercise will lower IOP.
P: Are the variations in IOP consistent, such as up in the morning and down in the afternoon? Or does the IOP go up and down all day?
Dr. Rick Wilson: IOP has a diurnal curve; that is, higher in the morning for the majority of individuals. However, IOP also fluctuates a smaller amount all the time.
P: The problem — and not just with target pressure, but with decisions based on IOP generally — is that, as you acknowledge, any given pressure reading is just a snapshot of one point on a curve. How can we have confidence that extrapolating from that snapshot represents what’s actually going on?
Dr. Rick Wilson: We also follow the optic nerve and visual fields. No matter what the IOP is, if the disc or visual field is getting worse, the IOP needs to be 35% to 40% lower to give patients the best chance of avoiding further loss.
P: What would be the average IOP for a healthy female at ages 40, 50, and 60?
Dr. Rick Wilson: Probably 16, 17, and 18 mm Hg, respectively.
P: What would the target IOP be for a baby who was born with IOPs of 40 mm Hg?
Dr. Rick Wilson: Babies normally are born with IOPs in the 8 to 10 mm Hg range. I usually try to keep their IOPs below 15 mm Hg, if I can.
P: What do you think of the Heidelberg retinal flowmeter (HRF)? [Editor’s note: The HRF is a new device for assessing retinal and anterior optic nerve blood flow.]
Dr. Rick Wilson: The HRF can only focus on a small part of the retina to gauge the blood flow. We have not found it as helpful as it sounds.
P: Is the damage to the optic nerve caused by elevated IOP or by poor blood circulation? If an increased IOP is the only change, how does the doctor decide to add another medication or to proceed with surgery?
Dr. Rick Wilson: The doctor should ask whether you are taking your drops regularly. The most likely cause of a medication not working when added to a glaucomatous eye is poor compliance about taking the medication. Second, if a patient’s IOPs have been stable and there are no discernible causes for the increase in IOP, if it is not too high, I will just recheck it in the near future.
P: Is it known whether severe visual field loss in the first eye increases the risk of severe visual field loss in the second eye?
Dr. Rick Wilson: Yes. Both eyes are usually built the same. If one eye is susceptible to damage from elevated IOP, it is likely the other eye is too.
P: I had a trabeculectomy six weeks ago. Before the surgery, my IOP was 38 mm Hg. Now it averages 16 mm Hg, but my doctor would like it to be lower. Can I consider the operation successful, and what is the long-term prognosis?
Dr. Rick Wilson: Sorry, I can’t tell without seeing you, because your target IOP is set by the appearance of your optic nerve, the visual field, systemic susceptibility factors, such as blood pressure, and so on.
P: Are there any foods, herbs, vitamins, or exercise that can help lower IOP?
Dr. Rick Wilson: Exercising 20 minutes, four times a week, is said to be equal to one eye drop in its pressure-lowering effect. No vitamins, and so on, lower IOP. Theoretically, vitamin E should help reduce the harmful effects of elevated IOP.