Measuring Intraocular Pressure
July 18, 2007
Norma Devine, Editor
On Wednesday, July 18, 2007, Dr. Michael James Pro, a glaucoma specialist at Wills, and the glaucoma chat group discussed “Measuring Intraocular Pressure”.
Moderator: The topic tonight is “Measuring Intraocular Pressure”.
Dr. Pro: First, a little history. One hundred years ago, the first device to check eye pressure, the Schiotz tonometer, was developed. Finally, doctors had something relatively accurate and the results were reproducible. About the same time, the discovery of cocaine as an anesthetic made it possible to use the Schiotz tonometer to check IOP (intraocular pressure). The patient had to lie supine (flat on the back), and the metal device with weights was placed on the eye. The Schiotz tonometer had a few drawbacks. Using it took time and it was inaccurate.
Then, 50 years ago, the Goldmann tonometer was developed. The Goldmann was more accurate and attached to the slit lamp. The IOP was measured with the patient sitting in a chair. Most ophthalmologists today use the Goldmann, which has proven accuracy and reproducibility, and is easy to use.
Other devices, such as the Tono-Pen are handheld and portable, which allows them to be used anywhere. Some eye-care providers use the air-puff tonometer. Many other devices were used in the past, and new devices are being developed.
P: What do you think of the Pascal Dynamic Contour tonometer?
Dr. Pro: That was developed a few years ago. It is no more accurate than the Goldmann for normal or thick corneas and getting a reading takes longer. Like the Goldmann, the Pascal tonometer is attached to a slit lamp. The Pascal tonometer seems to be more accurate in patients with thin corneas or those who had LASIK or corneal refractive surgery. It is more expensive to use because the disposable cover must be discarded after each use.
P: What do you think of the Reichert Ocular Response Analyzer?
Dr. Pro: That device measures “corneal hysteresis”, which is the “flexibility” of the cornea. This concept may relate to glaucoma much as thin corneas were found to be risk factors five years ago. The concept is relatively new in ophthalmology, so we don’t really know much about it yet. Since I have never used this device, I don’t know exactly how it takes a measurement.
P: What do you think of the Proview?
Dr. Pro: The Proview could be used at home by the patient. It is not as accurate as the Goldmann, which is the gold standard.
P: Approximately, what is the error of measurement for a skilled user of the Goldmann tonometer?
Dr. Pro: Plus or minus 2 mm of mercury.
P: My impression, gathered from many sources, is that the Proview is not really useful over a clinically important range of IOP.
Dr. Pro: That’s right. Certainly, many patients are interested in knowing their pressures at different times of the day. An accurate pressure graph for 24 or more hours could really help the doctor treat a patient’s glaucoma. Unfortunately, these home devices have not yet proven to be accurate. The Tono-Pen isn’t cheap, is fairly complex, and is reliable. So far, the cheaper devices have not demonstrated comparable accuracy or reliability.
P: A new Tono-Pen costs $3,500. Used Tono-Pens usually cost several thousand dollars.
P: Can anyone purchase a Tono-Pen?
Dr. Pro: The Tono-Pen was not designed for home use. Using the Tono-Pen requires topical anesthesia, which is dispensed only by prescription. Chronic anesthetic use can lead to a corneal ulcer.
P: Do you make any IOP corrections for central corneal thickness (CCT)?
Dr. Pro: I have no fixed chart or conversion table because no algorithm has proven its accuracy. The Goldmann is accurate for an average corneal thickness of about 540 to 560 microns (the corneal thickness for the device to be accurate). Corneal thickness greater or less than that average introduces a slight error. The amount that the measurement deviates is not necessarily linear. The correction factors in the earliest algorithms were too aggressive. For example, at a thickness of 500 microns, the “true IOP” was 6 mm Hg higher. That, however, has not been proven in an experiment. Many, but not all, glaucoma specialists now understand that in patients whose corneas are thin, the true pressure is higher and the RISK for developing glaucoma is greater. It’s really about understanding a patient’s risk for glaucoma and, if necessary, modifying that risk. Placing too much emphasis on a “true” IOP can be a distraction from the larger picture.
P: In a patient who has very thin corneas and is using glaucoma medications, is an IOP of 20 mm Hg high?
Dr. Pro: Certainly the pressure is at least several points higher. Depending on the overall picture (visual fields, optic nerve examination) that IOP may be too high.
P: What can change IOP from normal to abnormal?
Dr. Pro: We think that drinking six or more cups of coffee can affect the pressure. I sometimes wonder about social stress, but no study has proved that can increase IOP. Playing horn instruments and Yoga headstand positions can elevate the pressure. One study suggested that wearing tight neckties raises pressure, but that was refuted by a later study.
P: What is the best time of day to measure the IOP of a patient using glaucoma medication?
Dr. Pro: It is true that some medications “wear off” and lead to a pressure elevation before the next dose. Most medications have a “peak effect” some time after their instillation. I guess I would answer that question by saying there are a few patients in whom the pressure seems stable when I have examined them, yet the glaucoma is worsening. For those patients, measuring the IOP all day (diurnal curve) helps to assess if they are having a fall-off in drop performance, with a pressure elevation at a certain time. That can lead me to add an eyedrop at a different time in their eyedrop cycle.
P: I don’t understand why eye doctors aren’t more interested in pursuing a safe and accurate home tonometer for their patients. Imagine a diabetic who could only check his blood sugar levels a few times a year in a clinical setting. That’s what it’s like for glaucoma patients. For someone like me, with pigmentary glaucoma and expected large swings in IOP, these few IOP snapshots leave much to be desired. Is the technology for home tonometry really that daunting?
Dr. Pro: As a profession, glaucoma doctors are interested in such devices for their patients. The small size of the market probably causes larger companies with more funds to lose interest. I have had individuals “demo” devices for me, with patients like you in mind. No device yet shows tremendous promise, but we keep looking.
P: Some research questions whether the concept behind the Proview (that a phosphene relates to IOP) is even correct. Do you have an opinion on that?
Dr. Pro: It seems to me that the phosphene concept is really an inaccurate endpoint. I do not know all the data, but I wonder if all individuals experience phosphenes at the same external pressure.
P: How do you measure IOP in infants, toddlers, and children?
Dr. Pro: Infants aren’t too hard. We use the Tono-Pen in the office. Toddlers and young children are harder. They are strong and may fight too much. We sometimes need to take them to the operating room for an exam under anesthesia. There the pressure is checked with a Tono-Pen or a Perkins (sort of a hand-held Goldmann). The anesthesia, however, can lower the intraocular pressure, so the pressure must be taken immediately after they are sedated.
P: Can a patient determine, by pressing on the eyeball with alternating fingertips, if the pressure is high because the eyeball feels hard? Could that damage the eye?
Dr. Pro: Pressing on the eye can elevate the intraocular pressure to over 100 mm Hg! But that elevation is transient. Yes, if the eyeball feels hard, then the pressure may be high.
Moderator: Thank you, Dr. Pro.
Dr. Pro: Thanks for the tough questions! Goodnight.