Taking IOP Measure Beyond Goldmann

Growing evidence shows NCT and other emerging approaches to measure IOP compare favorably to Goldmann tonometry.

BY Karen Blum

Ophthalmologists face a wide range of technologies to help in assessing IOP and monitoring glaucoma patients. While the Goldmann applanation tonometer (GAT) remains the “gold standard” for IOP measurement, it has its limitations, and a number of other contact and non-contact tonometers on the market may play a helpful role for children, patients with irregular or scarred corneas, or in situations where certified medical professionals are not available to perform contact tonometry.

GAT has held the gold standard title for 20 years. However, studies over the past five years “have shown that it is not as accurate as we would like it to be,” says Kelechi Ogbuehi, PhD, consultant associate professor of optometry and vision sciences at King Saud University in Riyadh, Saudi Arabia. GAT tends to read pressures much lower than they truly are, he says, and accuracy can be affected if patients’ corneas are thicker or have scarring. “But basically, old habits die hard. Everyone has been using Goldmann forever so they continue to use it,” says Dr. Ogbuehi, who has published several journal articles on tonometry.

NCT has Come Far

On the market now are many noncontact tonometers (NCTs). The original air puff-style tonometers from 30 years ago “had gotten a bad name,” says Peter Netland, MD, PhD, chairman of ophthalmology at the University of Virginia School ofMedicine in Charlottesville. The devices were large, he says, and many people remember “getting a blast of air, knocked over practically.” Plus, they were not very accurate. By contrast, the wide variety of noncontact tonometers available today are smaller, portable or handheld, produce a sensitive puff of air and for the most part are more accurate.

A recent study found the Nidek RKT-7700 to read IOPs on average 2.7 mm Hg higher than GAT.

“Goldmann applanation is still the gold standard, but we’re finding many of the new tonometers are approaching the accuracy of the Goldmann and have many distinct clinical advantages,” Dr. Netland says.

NCTs are particularly helpful when regulations preclude the use of contact tonometers by medical students or anyone other than ophthalmologists, Dr. Netland says, or in community settings where screenings are more often done by other health professionals. Because they do not make contact with the eye, NCTs do not need to be sterilized.

Dr. Netland and colleagues have used a portable NCT called the PT100 (Reichert Technologies) successfully in a glaucoma screening program that medical students perform. In a comparison test of the device’s performance to GAT in 98 eyes, the two tonometers were in agreement within 3 mm Hg or less in nearly 93% of eyes in normal patients.¹

NCT as Time Saver

For Dr. Ogbuehi, NCTs help save time in a busy practice, as he rotates among several clinics.

The icare tonometer uses disposable soft, rubbery tips and is said to read IOP so fast that children hardly notice.

When pressed for time to perform diagnostic tests, “more and more we are finding we have to delegate tasks like taking IOP to my nurses or ancillary staff,” Dr. Ogbuehi says. “The advantage of this machine is that someone who has been trained for 20 minutes can use it. It’s just point and shoot, really.”

Device manufacturers calibrate their instruments to the GAT, but because the GAT reads IOP lower, they try to ensure that the NCTs read higher by about 2 mm Hg in normal eyes and in eyes that have glaucoma or ocular hypertension, so they are measuring closer to the true IOP, according to Dr. Ogbuehi. On the whole, he says, NCTs cannot be interchangeable with GAT but they are “accurate enough to give you a guideline.”

A recent study he conducted compared the performance of GAT with two NCTs — the Nidek RKT-7700 and the Topcon CT-80, in one eye of 49 healthy subjects.² The RKT-7700 read IOPs on average 2.7 mm Hg higher than GAT; the CT-80 read pressures on average 3 mm Hg higher than GAT. The RKT-7700 was discontinued about five years ago, according to Nidek, and has been replaced by the M3, which combines NCT with an auto-refractor and auto-keratometer. In Dr. Ogbuehi’s study, both NCTs demonstrated a tendency to overestimate IOPs in relation to GAT in eyes with thicker corneas.

Dr. Ogbuehi says he has used the CT-80 in other studies, but wants to investigate the RTK-7700 more. It’s a newer tonometer and one of few that combines an automated keratometer and autorefractor in the same unit. “I’m beginning to question whether trying to make it a jack of all trades has compromised the accuracy of measuring IOP,” he says. However, because the sample size he used was small, “I can’t say anything definite yet.”

NCTs are “rudimentary, but for what they do, they are actually quite good,” Dr. Ogbuehi says. He recommends that clinicians make independent judgments on the devices by testing them on a small sample of patients with and without eye disease, and compare those measurements to the Goldmann.

Other Approaches in Tonometry

A range of other tonometers also can be helpful in certain patient populations, Dr. Netland says. They include:

► icare tonometer (icare USA). In children, Dr. Netland and colleagues have embraced the icare tonometer, also called the Rebound, whose disposable soft, rubbery tips are tiny, like the head of a pin, in a device that reads so fast that children hardly notice. This tonometer has been so helpful in Dr. Netland’s practice that it has almost eliminated the need for anesthesia, except in children who are extremely uncooperative or difficult to measure. “It does make brief contact with the eye but it’s so transient and non-disruptive to patients they literally don’t feel it,” he says.

► Perkins tonometer (Haag-Streit USA). Another tonometer Dr. Netland has found helpful is the handheld Perkins tonometer. The Perkins works like GAT but has a counter weight so one can use it to measure IOP in supine patients — either during exams under anesthesia — or in patients who cannot easily use the slit lamp.

► Pascal Dynamic Contour Tonometer (Ziemer Ophthalmology). In patients with scarred or irregular corneas, some eye-care professionals find hand-held pneumotonometers, or the Dynamic Contour Tonometer (DCT), which fits on a slit-lamp, to provide more accurate measurements, according to Dr. Netland.

► Tonopen (Reichert Technologies). This portable tonometer can be used in upright or supine patients. It measures IOP off the center of the cornea, but the device is highly sensitive to factors that distort the corneal responses, and not as accurate outside the normal range, he says.

“Ultimately, all these measurements are surrogates for what the real IOP is in the eye,” Dr. Netland says. “We try to find the ones that are the most accurate.”

Variable Measurements

Tonometers can vary widely in their measurements, according to a review paper by researchers in Scotland.³ Investigators from the University of Aberdeen compiled data from 99 studies comparing the performance of eight types of tonometers, including DCT, NCTs and handheld applanation tonometers (HAT), to GAT. For most tonometers, approximately half of the measurements taken were estimated to be within 2 mm Hg of GAT. The NCTs and HAT studied performed a bit better, with 66% of NCT measurements and 59% of HAT measurements within 2 mm Hg of GAT.

It’s a rapidly changing market, says Jonathan Cook, PhD, a statistician and coauthor of the Aberdeen paper. Not only do tonometermodels change quickly, but also the companies producing them frequently merge, which he says made his research more challenging.

When monitoring patients over time, “it probably makes sense to follow up people with the same tonometer, because of concerns of equivalence of findings between different tonometers,” adds coauthor Augusto Azuara-Blanco, PhD, FRCOOphth, professor of health services research and honorary consultant ophthalmologist.

Newer Approaches to Measuring IOP

Manufacturers continue to produce tonometers with new properties. They include the Diaton tonometer (BiCOM Inc.), a handheld device that measures IOP through the eyelid, and the Ocular Response Analyzer (Reichert), which administers a rapid air puff and records two applanation measurements: one while the cornea moves inward, another as it returns. The average of the two values provides a Goldmann-correlated IOP measurement. Further advances might bring about NCTs that are more accurate, implantable, or even operable by patients themselves.

“If you talk to patients, many of them would like to have a tonometer so they could check the pressure themselves,” Dr. Azuara-Blanco says. This would make NCTs more like in-home devices thatmeasure blood pressure and glucose. “We are seeing the beginning of handheld tonometry that has the potential of being used by patients at home, but it’s still expensive” he says.

There also is great interest among researchers in tonometers that can give clues to other properties of the cornea or eye, Dr. Azuara-Blanco says. For example, he says, DCTs could indirectly reflect some measure of blood volume going into the eye, and that the Ocular Response Analyzer could detect some properties of the cornea, like its elasticity.

Also on the horizon are manometers or pressure sensors that could be implanted directly into the eye to continually measure IOP, Dr. Netland says, although the first candidates likely would be patients with keratoprostheses or intraocular lenses the sensors could attach to. “These sensors and other devices, such as contact lens pressure-sensing devices, should allow continuous intraocular pressure monitoring, which will provide a wealth of new information for clinicians and their patients,” he says. OM

The handheld Perkins tonometer works like GAT but has a counter weight so one can use it on supine patients.

References

1. Salim S,Linn DJ, Echols JR, Netland PA. Comparison of intraocular pressure measurements with the portable PT100 noncontact tonometer and Goldmann applanation tonometry. Clin Ophthalmol. 2009;3:341-344.

2. Ogbuehi KC, Mucke S, Osuagwu UL. Influence of central corneal thickness on measured intraocular pressure differentials: Nidek RKT-7700, Topcon CT-80 NCTs and Goldmann Tonometer. Ophthalmic Physiol Opt. 2012;32:547-555.

3. Cook JA, Botella AP, Elders A, et al; Surveillance of Ocular Hypertension Study Group. Systematic review of the agreement of tonometers with Goldmann applanation tonometry. Ophthalmology. 2012;119:1552-1557.