Understanding Intraocular Pressure
November 23, 2005
Norma Devine, Editor
On Wednesday, November 23, 2005, Dr. Rick Wilson. a glaucoma specialist at Wills, and the glaucoma chat group discussed “Understanding Intraocular Pressure.”
Moderator: Will you please begin by explaining what intraocular pressure (IOP) is, and how it is related to glaucoma?
Dr. Rick Wilson: Intraocular pressure is the pressure of the fluid inside the sclera, the white outer coat of the eye. It seems that IOP is the most important risk factor we know for glaucoma. Lowering IOP in a person who is getting worse with an IOP of 14 mm Hg usually slows or stops progressive glaucoma damage similarly to when the IOP is 34 mm Hg. Why that is so is not understood.
P: How has the understanding of IOP in relation to glaucoma changed over the past 15 or 20 years?
Dr. Rick Wilson: Over the past 20 years, the target (desired) levels of IOP have been dropping, as we became more informed about the disease. When I was a resident and a patient came in with an IOP of 30 mm Hg, we were happy to get the IOP in the high-normal range. Then we learned that patients were still getting worse, and we started to lower IOP 20 to 25% from the level at which damage was occurring. Often that wasn’t enough.
We moved the target to a 30 to 40% drop, depending upon the amount of glaucoma damage present. Suddenly, we started to see visual fields, not only stop progressing, but also, in a few cases, actually improving slightly.
P: So instead of setting target pressures according to the level at which damage is occurring and lowering it a certain percentage, how do you determine target pressures?
Dr. Rick Wilson: I evaluate the extent of the glaucoma damage, and add in the additional risk factors, such as family history of damage at low IOPs, decreased circulation, migraines, low blood pressure, and cardiac arrhythmias. I combine that information with life expectancy to arrive at a target IOP.
P: Which of the risk factors do you consider the most important?
Dr. Rick Wilson: The extent of damage. If damage is advanced, the IOP needs to be around 12 mm Hg, unless the damage was occurring in the mid teens. Then it may need to be 10 mm Hg, or even lower. Nerves that have suffered moderate damage may tolerate IOPs in the mid teens. Minimally damaged nerves that were being damaged by IOPs significantly higher may do well with IOPs in the high teens or 20 mm Hg.
P: How do you determine that the target IOP has stopped progression?
Dr. Rick Wilson: The key is to follow the patients closely with disc exams and visual field tests to see if they remain stable at an IOP level that is fairly constant.
P: Last week we learned from Dr. Henderer that thinking has changed about the effect of central corneal thickness on the measurement of IOP. Would you please comment?
Dr. Rick Wilson: The first factor about the cornea that affected the IOP measurement was the thickness of the central cornea. A thinner cornea caused the measured IOP to be lower than the actual IOP; a thicker cornea caused the measured IOP to be higher than the actual IOP. Now we know that other factors, such as the elastic and viscous properties of the cornea, combine to affect the cornea’s resistance to being deformed by the tonometer and also affect the measured IOP.
P: Are fluctuating IOPs a concern?
Dr. Rick Wilson: Fluctuating IOPs seem to be harder on the optic nerve than constant IOPs.
P: Many patients worry if their IOP increases 2 to 4 mm Hg between appointments. Some doctors (including mine) tend to dismiss those variations as normal and nothing to be concerned about. Is there a consensus about those variations?
Dr. Rick Wilson: Remember that IOP normally varies, on average, 4 mm Hg during the day, but a patient with glaucoma has IOP swings that average 11 to 12 mm Hg. Therefore, between visits, a difference of a few millimeters, but still in the target range, is acceptable. The larger the swing, especially if the IOPs are above the target range, the more suspicious I become that the control of IOP is adequate.
P: How much damage to the optic nerve can be stopped by lowering the IOP?
Dr. Rick Wilson: I feel that almost all of it can be stopped if the IOP is lowered enough. For most people, an IOP of 12 mm Hg is adequate. For some people, that may be an IOP as low as 6 to 8 mm Hg. I have only had one patient in whom damage progressed when I lowered the IOP to 8 and 9 mm Hg, with no neurologic cause found on work-up at the University of Pennsylvania.
P: For most patients, which is more of a problem: an excess production of fluid or an insufficient outflow of fluid?
Dr. Rick Wilson: It seems that most patients with glaucoma have insufficient outflow. Steroid responders on oral steroids are most likely to produce an excess of fluid.
P: Is there a level of IOP at which everyone will suffer glaucomatous damage?
Dr. Rick Wilson: The chances of glaucoma damage are directly related to IOP, but there does not seem to be a level at which everybody above that level will get damage in five years. Clearly, the longer the IOP is elevated, the greater the chance of damage.
P: If a person has a healthy-looking optic nerve, but an IOP of 30 mm Hg, should that person be treated for glaucoma?
Dr. Rick Wilson: Most doctors treat patients with IOPs of 30 mm Hg, even with healthy-looking nerves. Perhaps we older doctors remember that many of the patients with high IOPs who did not develop glaucoma damage had very thick corneas. If the adjustment for the corneal thickness were included in the IOP equation, those patients’ IOPs would be normal or close to normal.
In my practice, I have had a number of children who were treated with medicine for glaucoma for years, but when their corneas were measured and found to be very thick (say 840 microns versus an average around 540 microns), I judged them not to have glaucoma at all.
P: I had a shunt implant in September. My IOP is now 5 mm Hg. I am taking Pred Forte six times a day. Do steroids always increase the intraocular pressure? How low an IOP is too low?
Dr. Rick Wilson: Steroids seem to increase the build-up of debris in the trabecular meshwork. That causes the IOP to increase in the segment of the population sensitive to steroids. If a trabeculectomy or shunt is diverting the debris away from the trabecular meshwork, the steroids may not have much effect on IOP. The steroids encourage the eye to be healthy and make a normal amount of fluid, but do not seem to increase aqueous production above normal.
P: Is it possible to get a different IOP reduction with the same type drugs, such as the prostaglandins, Xalatan and Travatan? Can one of them sometimes be more effective than the other?
Dr. Rick Wilson: Yes. In one large, well-conducted study, Lumigan produced roughly 1/2 mm Hg lower IOP than Xalatan and Travatan, which were equal in effect. However, in individual patients, the amount of IOP and eye redness varied greatly. Most of the time, it does not help to switch among the prostaglandins until other medical possibilities have been exhausted.
P: Is the IOP normally about the same in both eyes?
Dr. Rick Wilson: It is frequently similar in both eyes. Secondary glaucomas — like pseudoexfoliation, pigmentary, traumatic or inflammatory glaucoma — may be markedly different.
P: Does intraocular pressure vary with the seasons?
Dr. Rick Wilson: Yes. It seems higher in the winter, as I remember. It also varies with the time of the month. The variation and average IOP increase with age in the U.S., though not in Japan.
P: Is less aqueous humor produced as we age?
Dr. Rick Wilson: Yes, but that is overly counterbalanced by the fall-off in aqueous leaving the eye among the muscle bundles of the ciliary body.
P: Does living in a city that is 7,000 feet above sea level increase the level of IOP?
Dr. Rick Wilson: I don’t think I would be concerned about 7,000 feet unless your optic nerve had lost 95% of its fibers, and even then I would be skeptical.
P: Should glaucoma patients avoid having the air puff test often used at optometrists’ offices? I’ve heard it’s bad for corneas.
Dr. Rick Wilson: I don’t know that it is bad for corneas, but it is not accurate enough for someone with glaucoma.
P: Does underwater diving affect the IOP?
Dr. Rick Wilson: Not much. It does increase the partial pressure of oxygen and carbon dioxide in the blood, both of which would seem to be helpful for the glaucoma patient. My only warning to divers is to practice care with hygiene in defogging their masks, and with mask squeeze and trauma to the eye if they have a trabeculectomy that is still working.