What is the target pressure?

Take Home Points

What is the goal of glaucoma treatment? It is to help persons retain all the useful vision possible for the remainder of their lives without badly bothering them by treatment. In general, we succeed at this with the vast majority of patients. During the descriptions of open angle glaucoma, we mentioned that it is not a disease of elevated pressure in half of those with the disease. So, if your pressure starts out normal and that’s the dangerous level for you, it wouldn’t help to normalize the pressure, would it? In a very important clinical trial (the Collaborative Normal Tension Glaucoma Study), persons with open angle glaucoma and normal baseline pressure before treatment were randomly given either no pressure lowering, or their pressure was lowered by 30% by drops, laser or surgery. Another key study (the Early Manifest Glaucoma Study) assigned people with untreated glaucoma (many of whom started with normal pressures) to either no therapy or a combination of drops and a laser treatment. The pressures fell in the treated group by about 25% from their untreated level. Both studies showed a real benefit by lowering from a normal pressure level to a lower pressure within the normal range.

It may seem amazing that prior to 1990, doctors thought that making the pressure normal was successful treatment. In fact, many published studies report that they were successful at treating glaucoma when the pressure was reduced below 21 millimeters of mercury (the upper limit in persons who don’t have glaucoma). This was true whether the baseline pressure was 22 or 35! Lowering eye pressure is essential to prevent glaucoma damage. We can't possibly tell how we’re doing if we don’t have a meaningful goal. Remember that the final outcome that we want to have is to keep useful vision—so it is the visual field test results over time that are the final measure of how you’re doing. It takes several years to know whether the field test is stable or not. In the mean time, we need a substitute measurement that tells us how we are doing. That’s where the target pressure comes in.

Given the results of the major clinical research studies, lowering the pressure by at least 20-25% is a pretty good general target zone. But what was the initial level? Say you go to a new eye doctor and on that visit it is determined that you have open angle glaucoma. To set a target range for treatment by drops, laser or surgery, we need to know the range of pressure that caused the glaucoma damage that has been injuring your optic nerve head or visual field. It could be in the so-called normal range, or it could be higher. Yet, I have seen hundreds of patients put on drops at that first visit with only one pressure taken. If you’re going to be on drops for the rest of your life (or undergoing a procedure for pressure), you should have a good idea of what the starting pressure is. No one likes extra visits to the doctor, but we don’t want to rely on only one pressure reading before treatment begins in order to determine the treatment goal for the next 15 years. At our Center, we ask patients to come back at least 2 more times before starting treatment. We start the therapy on the third visit. Three examples of what happens when we do this are shown in Figure 19. The pressure recordings are from 3 patients whose first day pressure was 25. After 3 measurements, the first patient’s average was really 25, the second one averaged lower than that (21), and the third had an average untreated pressure of 30. Without the extra measurements, we would have guessed wrong in 2 out of 3. Eye pressure fluctuates day to day, and fluctuates even more in glaucoma patients. A target set by lowering things 20% in these 3 comes to 3 different pressures when we have a better idea of the baseline.

Baseline Eye Pressure
Figure 19: Baseline Eye Pressure. An illustration of baseline eye pressure determination by making 3 visits in 3 different patients. The average baseline, untreated pressure was the same in each person (25) but after 3 visits, the true baseline was very different among the 3 persons. The target range set for each patient is lower than their baseline by 20%

I have been asked whether there’s a lower limit to the target we set. Can it get too low? For most patients the target is never lower than 12 and for most eyes, 12 is not too low. It’s true that surgery can lower the pressure so low it leads to blurred vision. Interestingly, there are lots of eyes than can have a pressure of 5 or less for years and see just fine, while others at this low level are in trouble.

The initial drops versus initial surgery study (Collaborative Initial Glaucoma Treatment Study) had a more detailed approach to how much the pressure should be lowered. For those with very early damage, the target lowering was about 20% lower than baseline. Those with bad damage had a target 40% below baseline. Many glaucoma specialists use the general idea that your target needs to be lower if you have more advanced disease. The concept is based on the idea that since we can’t get back what is lost, the person with serious damage has less remaining reserve. We don’t want to leave pressure too high in advanced disease, since such patients have higher stakes from any further loss.

One approach to determining if the target is achieved when starting eye drops is to use one eye as a comparison for the other. In general, your two eyes have similar variation in their pressures. While it is not perfect, the correlation is far above random, so if pressure goes from lower to higher in the right eye between visits, it will often go in the same direction in the other eye. If we start a new eye drop in the right eye and leave the left temporarily untreated, we can get a decent idea of what the drop did to pressure by comparing the right before and after drops with the untreated left eye on the two visits. This also gives us a good idea of whether any new symptoms might be side effects of the drop, since they would usually only happen in the right eye. But, eyes don’t exactly move up and down together, so we will often keep the single-eye trial going for another visit or two to see for sure that the drop achieved the target level. Then, we try it in the second eye. There’s no guarantee that the two eyes will behave identically, but in general they do. In some patients, the single-eye trial is not appropriate, and eye drops are started in both eyes.

Most often, working at a referral center, persons coming to us are already receiving treatment prescribed elsewhere. Some of these people have been taking the same drops for 10 years. I often recommend that we find out whether the drops are still “working” by stopping them in one eye for a short time. This is called a unilateral stop trial and is perfectly safe for a short time like a week, since glaucoma doesn’t damage eyes in that short a time. If you’re going to take drops for 10 more years, it makes sense to see if they’re doing anything. I admire and understand patients who are reluctant to do this. They have taken seriously the concept that they must always take their drops. But, the stop trial can help to fix important problems. Imagine that eye pressure fluctuates up and down and that drops are holding it down, but not completely eliminating the variation. You roll along seeing the doctor for 2 or 3 years. Then, on the next visit, the pressure is higher than the target. It might be that you forgot the drops that day. It might be that the eye just had a bad day (big stress or change in health temporarily can raise eye pressure). Many times, we have seen that this leads doctors to prescribe more drops that very day. If we had resisted the temptation to write the new prescription and just measured again a few days later, we could have determined whether the new drop was really needed, instead of being unnecessary. In past studies, we have found a substantial number of patients are prescribed more drops than they need, probably due to this sequence of events.

We must take pressure fluctuation above the target very seriously, especially if it happens more than once. Recent research has shown that it may be as bad to have a pressure that is varying a lot as it is to have a pressure that on average is above the target. Swings in pressure could have bad effects on the stress generated in the eye wall and on blood flow and ganglion cells. One of the areas that some have studied is how the pressure behaves at night. We all have variations in our body functions from day to night. Our hormone levels follow the sun and moon, and our blood pressure and our eye pressure vary at night compared to daytime. At this time, there are no practical recommendations that have been proven to tell us that pressure measured at night is more important than that measured in the daytime. Some doctors feel that measuring pressure through the course on one day at the hospital is a worthwhile indication of how the patient is doing. There is presently not enough strong evidence that this helps to justify doing it routinely.

We sometimes change the target upwards or downwards. If a patient with glaucoma has done very well over a period of years, we may have set the target pressure lower than it really needs to be. We can then try a higher target level with careful monitoring of the visual field tests for a period of 2 years. On the other hand, we may find that a patient is getting worse in their visual field or optic nerve head structure at the set target level. That means that we have to lower the target range—typically by another 20% compared to the original target—as well as making sure that the patient is truly adhering to the treatment between visits.

Among glaucoma specialists, there are some very good doctors who claim that the target pressure idea is not really needed. I find these arguments difficult to understand. We need to know what we’re doing and the idea behind our present target setting is based on very good clinical research. We may not know the exact best target for every glaucoma patient, but every glaucoma patient should have a target pressure to guide the short-term and medium-term treatment, as we look for longer-term stability or worsening in the visual field test.

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