NOTICE: If you are engaged in self-tonometry and you have questions about your daily intraocular pressure fluctuations, you should read this comment about the difference in the way fluctuations are defined by mainstream ophthalmology and the fluctuations we observe in self-tonometry.
I will further preface this article by saying that I do not believe there has been a study done where subjects with normal, healthy eyes performed self-tonometry the way we are doing it.
I urge caution in comparing self-tonometry data against the data being discussed by Dr. Rick Wilson below.
It is good to try to understand as much as we can. However, I believe it is not good to misinterpret your own self-tonometry data -- or to jump to incorrect conclusions based on the apples and oranges fallacy.
On Wednesday, May 18, 2005, Dr. Rick Wilson a glaucoma specialist at Wills, and the glaucoma chat group discussed "Fluctuating IOP's."
Moderator: Tonight's topic concerns fluctuating intraocular pressures (IOPs). In a recent chat, you said studies have shown that fluctuating IOPs are more of a risk factor than a somewhat higher, but steady IOP. Why is that?
Dr. Rick Wilson: We are not sure. Several studies have shown glaucoma patients do not auto-regulate their circulation as well as patients without glaucoma. In other words, if a patient's blood pressure increases suddenly, it pushes more blood into the eye, and the added blood flow and pressure cause more aqueous to be made. A normal eye would sense the change and constrict the blood vessels to reduce the flow of the blood under higher pressure back to normal.
P: Is that called "autoregulation?"
Dr. Rick Wilson: Correct. Autoregulation should take place with decreased blood flow or pressure, increased metabolic needs of the eye, etc. Clearly, a fluctuating IOP requires the eye to continually auto-regulate the blood flow to keep the optic nerve well supplied with oxygen and nutrients. That may be onerous for the glaucomatous eye.
P: Isn't some amount of IOP fluctuation normal even in healthy eyes? For example, aren't pressures expected to be higher in the morning than in the later part of the day? In any case, how much of a spread in IOP is considered normal?
Dr. Rick Wilson: Less than 4 mm Hg of fluctuation is normal. Glaucoma patients who are not treated with medication that dampens the swing of IOP average fluctuation of around 11 mm Hg. That is clearly more difficult for the eye to handle.
P: Are you saying that in untreated glaucomatous eyes, regardless of blood pressure, there is an average variation in IOP of 11 mm Hg over a given amount of time?
Dr. Rick Wilson: Yes, over a 24-hour period.
P: In pigmentary glaucoma (which I have), I had always associated a fluctuation in IOP of around 11 mm Hg. Is it even higher than, or is it now thought to be no higher than, fluctuation in the other glaucomas?
Dr. Rick Wilson: I would guess the fluctuation seen in pigmentary glaucoma is similar to that of open-angle glaucoma, except when there is enough movement of the iris (from impact exercise or dilation of the pupil) to liberate a storm of pigment. The pigment then blocks the trabecular meshwork and can cause rises in IOP of 10 to 40 mm Hg.
P: What is the pattern of eye pressure during the day and night?
Dr. Rick Wilson: More people have their highest IOP in the morning than have it in the afternoon. Many have their highest IOPs near the time they awaken. One patient in Chicago has a drop in IOP of 18 mm Hg during the first half hour of being awake in the morning.
P: Is it true that prostaglandins are especially effective because they dampen fluctuations in IOP?
Dr. Rick Wilson: Since it now seems that fluctuating IOP is a risk factor for glaucoma progression, it makes sense to use the medication that flattens the diurnal curve of IOP as much as possible. Prostaglandins are the best at that.
P: What is a diurnal curve?
Dr. Rick Wilson: We have diurnal (relating to or occurring in a 24-hour period) curves of almost all physiologic functions such as IOP, serum cortisol, temperature, etc. It has been said that different diurnal curves of temperature, activity, and alertness break up more marriages than any other single factor, e.g., the husband is a morning person and the wife is a night owl.
P: Are fluctuating IOPs a greater risk factor in open- or closed-angle glaucoma, or is it the same for both?
Dr. Rick Wilson: The studies have concerned open-angle glaucoma. Since angle-closure glaucoma is a different disease, it is not at all clear that fluctuating IOPSs are as much of a risk factor for angle-closure glaucoma as for open-angle glaucoma.
P: Some glaucoma patients get upset because their pressures are one or two mm Hg higher than the last time the pressures were measured. Some patients have also attributed a small decrease in IOP to some activity or to a supplement they took. On any given day, how much can the pressure fluctuate in a patient using glaucoma medications?
Dr. Rick Wilson: If the patient is on multiple medications to dampen the fluctuation, then I would expect a fluctuation that would be close to normal, that is, about 4 mm Hg.
P: What would those multiple medications be?
Dr. Rick Wilson: The patient would need to be taking 3X/day (three times a day) medicine three times a day, if they are being taken alone or with a prostaglandin. The medicines are Alphagan and Trusopt/Azopt. If the patient uses them together or combines them with a beta-blocker, then twice-a-day use may be acceptable to flatten out the diurnal curve.
P: What if the patient is combining a 3 X/day drop with a prostaglandin?
Dr. Rick Wilson: Then the patient is washing out the 3X/day drop more quickly with the prostaglandin effect on outflow. A response rate greater than 8 mm Hg certainly would not be expected with Alphagan, Trusopt or Azopt.
P: What is the IOP difference between fluctuating IOPs and a spike?
Dr. Rick Wilson: A spike is just a rapid rise in IOP, usually for just a few hours, and means there is considerable fluctuation. I have never seen spikes defined with numbers.
P: It seems counter-intuitive that prostaglandin analogues should flatten the diurnal curve when the uveoscleral outflow route is said to be pressure insensitive (as opposed to trabecular outflow, which supposedly responds positively in response to an increase in IOP). So how do prostaglandins flatten the diurnal curve?
Dr. Rick Wilson: It relates mostly to how long-acting the agent is. Prostaglandins are the longest-acting. After you have been on prostaglandins for some time, it takes six weeks to get away from the last noticeable effect. For beta blockers, that is three weeks. Therefore, prostaglandins produce the most even response, followed by the beta blockers, followed by Alphagan and Azopt/Trusopt, which are about the same, with the latter two slightly more effective after eight hours.
P: Does fluctuating blood pressure cause IOP to fluctuate considerably?
Dr. Rick Wilson: No. Fluctuating eye pressure is much more related to how much fluid the eye is making at any particular time. Blood pressure does play a role in this, but I do not think it is a major one unless it is quite high or low.
P: Glaucoma patients typically have their IOPs checked every three months, six months, or every year. How can doctors really monitor their IOPs with so few checks and no diurnal curve tests?
Dr. Rick Wilson: Usually, patients' IOPs are not checked at intervals longer than three months unless the patient has proved to be fairly stable over the last year or two. The optic nerve and visual fields are being monitored as thoroughly as the IOP.
P: What kind of fluctuation occurs in eyes with normal-tension glaucoma (NTG)?
Dr. Rick Wilson: Increased fluctuation in NTG is seen, but because the IOP by definition has to stay within normal limits, the fluctuation is less, say 3 to 6 mm Hg.
P: Has anyone observed large fluctuations in pressures in healthy eyes that stayed healthy, or in healthy eyes that only later developed nerve damage?
Dr. Rick Wilson: The answer to the first question is probably, but I don't remember a study to that effect. The answer to the second question is yes.
P: Is there any relationship between fluctuating IOPs and fluctuating acuity?
Dr. Rick Wilson: Yes, if the fluctuation is excessive and the optic nerve damage is advanced or the circulation is compromised.
P: When someone does headstands or other forms of activity that might cause a person to strain hard, such as lifting weights or biking on hills, how long would an IOP spike last, how large would it be, and is it likely to cause damage?
Dr. Rick Wilson: As I remember, standing upside down on your head normally causes a rise in IOP from 16 to 36 mm Hg. It does not take long for the IOP to return to normal after the straining or being upside down. Biking on hills with the head high over the heart should cause the least rise in IOP and has many beneficial effects. The rise in CO2 (carbon dioxide) increases blood flow and lowers the IOP.