Tonometry

Eye pressure measurement is most often done with a tool called a tonometer (Figure 10). The tonometer used most now was invented by Hans Goldmann, a Swiss eye doctor who invented many other important diagnosis tools for glaucoma. This instrument is part of the big machine that is attached to a chin rest device that slides up in front of the patient with a pair of binoculars for the doctor to see things magnified and with a bright light. After numbing the eye with eye drop anesthesia, the Goldmann tonometer presses against the eye. The force with which the eye pushes back is used to estimate the pressure inside the eye. The tonometer is highly accurate and is the “gold standard” for glaucoma. Patients should not hold their breath during measurement (you can slowly breathe through your nose). For most patients, the eyelids have to be held out of the way with the doctor’s (or technician’s) fingers or they will make the measurement impossible—or worse, they can artificially raise the measured pressure by pushing on the eye.

Goldmann tonometer
Figure 10: Goldmann tonometer. Drawing shows how the Goldmann applanation tonometer rests against and flattens the cornea to measure eye pressure. (Right) Photograph of the tonometer with its tip (top of instrument) that contacts the cornea.

Tonometry is usually easy to go through, since the eye has been numbed by drops, so you feel nothing. During the actual measurement, the instrument can rub some cells from the cornea if it is not done gently. If the patient rubs his or her eye during the 20 minutes afterward, the cornea can also be scratched. This rarely happens, but can cause substantial pain or a feeling like something is in the eye. The doctor should be made aware if this happens.

Pressure is measured in units like an old-style thermometer's column of mercury. The range of normal eye pressure is from about 10 to 20 in units of millimeters of mercury. But, pressure measuring doesn’t tell us who has glaucoma. Half of those who have open angle glaucoma have a pressure that is always within the "normal" range. So, having a higher than normal pressure is a contributing risk factor, and should lead to a full detailed glaucoma evaluation, but it isn’t glaucoma. Those with angle closure glaucoma more often have higher than normal pressure, but not always.

Eye pressure varies a bit during an average day and from day to day. It can change 4 millimeters of mercury up or down in those with untreated glaucoma from morning to night. It is, on average, higher first thing in the morning and lower in the evening, though this is not true of every person. It is higher when we are lying down than when we are standing (mostly this relates to how high our eye is compared to our heart). Hanging upside down or doing headstands, for example, causes eye pressure to go much higher (see section How should I change my life?). There is some evidence that greater variation in eye pressure may be worse for glaucoma patients. Some doctors advocate trying to estimate the degree of variability by measuring patients more often, on different days, at different times of day, or throughout one whole long day of measuring (called diurnal measurement). However, recent research suggests that this is not worth the trouble.

Recently, newer tonometers were invented to solve problems that come up with pressure measuring. First, the Goldmann tonometer is hard to use when the cornea (the clear front of the eye) isn’t normal in shape. Second, we’ve known for a long time that the pressure reads differently depending on how thick the cornea is, called central corneal thickness or CCT. This is measured with a small instrument called a pachymeter. The normal cornea is about half a millimeter thick, and the tonometer depends on the cornea being that thick to be accurate. If your cornea is somewhat thicker or thinner than the average, eye pressure will read differently. Thinner corneas read too low and thicker ones read too high. Persons with thinner corneas have been shown to be more likely to develop open angle glaucoma when they start out as suspects (see section How did you get glaucoma?) While several new instruments have been designed, they all have issues that keep them from being perfect enough to avoid these problems. The new instruments have disadvantages of their own (like being much more expensive to use). Of course, the doctor and patient want to have the best reading on what eye pressure is, so accurate tonometry is important. But, it is even more important to know how much the pressure has been changed from before therapy to afterward (see section What is the target pressure?). That is much less dependent on having the exact true eye pressure and more dependent on having lots of pressure measurements on different days.

One new tonometer, the ICare, has solved some problems well. It can measure pressure without putting in anesthetic drops, so for patients who are allergic to those drops we can now get a good reading. Our Center of Excellence has recently worked to get approval for a "take-home" version of this tonometer, and we have had persons purchase their own professional grade tonometer, but this is very expensive. Equally important, we can get good pressures in the office in infants and children much more often with this tonometer. That means fewer times when a child must be put under anesthesia to find out what their pressure is (see section Children and glaucoma)

Take Home Points on Tonometry

If you would like to support the cost of providing and maintaining this book with a charitable donation of any size, please click here.