How Much Does Eye Pressure Fluctuate?

 For the first two years after I was diagnosed with glaucoma, I had my eye pressure checked once every six months. I was under the impression that eye pressure is fairly steady and that large fluctuations are unusual. I certainly did not expect that I would experience very large fluctuations in eye pressure within very short periods of time (such as minutes). However, fluctuations in my eye pressure are something I have observed frequently over the course of collecting more than 18,000 eye pressure measurements.

 

I have seen my eye pressure almost double within about 10 minutes. I have also seen it nearly halve within about 10 or 15 minutes (particularly as a result of exercise). I would prefer to minimize such fluctuations. In particular, I try to do everything I can to eliminate large upward fluctuations in my eye pressure. So I am seeing these large changes in spite of my efforts to minimize them.

Sometimes I feel these fluctuations must be related to my unique situation. After all, I have pigmentary glaucoma and it seems reasonable that people with primary open angle glaucoma (POAG) might experience something different. However, when I mentioned this concept to an expert in the field, he said there was no reason to suspect differences of this nature between pigmentary glaucoma patients and those with POAG. Therefore, my current assumption is that many glaucoma patients could experience the rapid fluctuations in eye pressure that I do. 

A couple days ago a friend sent me some of her eye pressure measurements. At 7:24 AM her eye pressure was 17.0 in her right eye. She checked it again 12 minutes later and it had jumped to 24.3! (She was rushing to get ready to go to the airport and taking care of a few other things.) Even though her situation is very different from my own (she has POAG, for example), her eye pressure fluctuations reminded me of exactly the kinds of eye pressure fluctuations I see in my own monitoring.

There have been days when I have measured my eye pressure over 200 times because I wanted to really understand the nature of quick changes like this. Some times I have collect 10 or even 18 measurements in rapid-fire succession while trying to monitor the rate of change in my eye pressure. After collecting so many eye pressure measurements, I recognize a very clear pattern. With about 90% certainty, I know which activities and situations will raise my eye pressure and which ones will lower my eye pressure.

It is important to note that I use a Reichert AT555 tonometer for most of my eye pressure measurements. This is a non-contact tonometer. This particular tonometer has been shown to be comparable to Goldman applanation tonometry. It is considered very accurate, with a small caveat or two. In particular, when eye pressure is very low, the AT555 seems to report a lower value than Goldman and when eye pressure is above normal, the AT555 seems to report a higher value than Goldman. I think it is impossible to say which instrument is giving the more accurate value in these cases, but I do believe one sees a larger range of eye pressure values when using a non-contact tonometer for eye pressure measurements.

In regard to eye pressure, the most unexpected situation I can imagine is for my eye pressure to be unchanged at different times of the day or under different circumstances. Everyone knows blood pressure fluctuates with the conditions of the moment, but my eye pressure fluctuates far more than my blood pressure. (I assume that is related to the fact that I have glaucoma.)

I recently learned about the methods used in the Quaranta study to calculate a diagnostic value called diastolic ocular perfusion pressure (DOPP). Diastolic ocular perfusion pressure is simply the difference between diastolic blood pressure (the bottom number) and eye pressure. 

In the eye, critical areas such as the optic nerve head can be deprived of blood flow if the eye pressure becomes too high in relationship to the blood pressure. In other words, even if one has normal eye pressure, low blood pressure could result in critical structures in the eye (such as the optic nerve head) receiving inadequate perfusion. That's why researchers are looking at the relationship between eye pressure and blood pressure. Of course, it is the instantaneous difference in these pressures that determines perfusion. Comparing average values of these two pressurescould be meaningless (if ocular perfusion changes over a range of values).

It has been proposed that vascular risk factors are among the major precipitating factors that lead to the death of optic nerve cells in glaucoma, so the difference in blood pressure and eye pressure is of great importance. Again, since it is the relationship between these two pressures at any moment in time that matters, comparing my blood pressure from yesterday afternoon with my eye pressure from today seems meaningless. Yet that is exactly what was done in the Quaranta study - the diastolic blood pressure and the eye pressure were measured on two different days. These temporally unrelated values were used to calculate a number that is only valid for the relationship between two pressure measurements taken at the same time.

I would like to learn more about why the particular methodology used in the Quaranta study was employed. I assume it may have been used because it has been reported that the eye maintains perfusion over a range of values until a threshold is reached. However, by running some back-of-the-napkin calculations, it seems that my own eye pressure (up to 25) and blood pressure (down to 90/50) come close enough to this threshold that the fluctuations of both those pressures would be an important consideration. Furthermore, I tend to question whether the relationship between blood pressure and eye pressure matters only above a threshold and whether that threshold is a constant.