Science: Is Goldmann Applanation Tonometer (GAT) the Gold Standard?
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The following article is based on:
Accurate Tonometry: Is Help on the Way? - Glaucoma Today

by Dan Eisenberg, MD; extensively modified and edited by david

Accurate Tonometry: Is Help on the Way?

It does not appear that the community of clinicians (ophthalmologists, glaucoma specialists) as a whole is en route to change anytime soon.

There is no shortage of technology or of the will to invent, but the forces against change in the professional community have, to date, been insurmountable. This article details the various obstacles that appear to be halting progress.

Self-tonometry among glaucoma patients is growing, and one of the most popular tonometers for home use is a Reichert model with the same advanced technology mentioned below. Patients with one of these tonometers and following the appropriate IOP monitoring protocol will have a far better (more reliable, more accurate, more comprehensive) intraocular pressure database for themselves than their specialist will have for them. That's the surprising reality today, and the obstacles described below are partly responsible for this sad state of affairs.

WHY DO WE NEED MORE ACCURATE TONOMETRY?

The Goldmann applanation tonometer (GAT) is considered accurate, highly reliable, and simple to use. Moreover, this instrument is extremely well established in clinical use and research. Every article that I (Dan Eisenberg, MD) have reviewed on the topic of tonometry includes the cliché that the GAT is the gold standard. Nearly every review and discussion of tonometry includes the statement as well. It is typically declared as a fact beyond question. Why, then, should we abandon the so-called gold standard? Because GAT is not, in fact, a gold standard.

The amount of literature detailing the failings of the GAT is just as extensive as the literature extolling it. I will not revisit all the known instances when the GAT fails to obtain a correct pressure or to obtain any pressure at all, but I will note that to accept this tonometer as a standard of any type requires a total disregard for its proven failings. Really, how can we accept the GAT as a standard when it is accurate only some of the time and only when the right conditions and restrictions are met? As an aside, gold is not really a standard, either, because it is a commodity with a value that fluctuates at market price.

THE HISTORICAL ARGUMENT

In the historical argument, new research must use the GAT to be consistent with prior literature that used this tonometer. This circular thinking precludes any change ever, because old literature cannot be altered. It also assumes that new technology can never produce new information. What is the purpose of research if we cannot learn anything new?

THE COMPARISON ARGUMENT

New tonometers must agree with the GAT to check the accuracy of the readings. In addition to the assumptions of the historical stance, the comparison argument assumes that the GAT is ideal, so any new tonometer must produce similar results. Why would anyone develop a new tonometer if this were true? The need to compare everything to the GAT is completely illogical, yet it appears to be mandatory by consensus. It is certainly a major hindrance of technological innovation.

Maurice Langham, a tonometry researcher and the inventor of the Langham pneumatonometer, among other things, once told me that he had to include a Goldmann setting on his tonometer so that clinicians could compare his tonometer's readings with those from their GAT to convince them that his instrument was accurate. It did not matter that the literature showed pneumatonometry to be much closer to true IOP than Goldmann applanation tonometry, as measured by manometry [1]; they did not trust any instrument that deviated from the Goldmann. No tonometer is an acceptable control for another tonometer, especially not one with the well-documented shortcomings of the GAT.

Without a true reference standard, manometric IOP, it is impossible to determine the accuracy of one instrument versus another, because the errors of both are either contrasting or compensating while the true pressure remains unknown.

Another example of an advanced tonometer needing to provide a less-accurate GAT-compatible reading is the Reichert Ocular Response Analyzer. It provides the corneal compensated IOPcc value, as well as a Goldman-compatible IOPg value.

THE NOSTALGIA ARGUMENT

We are human, so we generally like to do what we have always done. It is familiar, comfortable, and less stressful than change. Like the comparison argument, the nostalgia argument precludes all new instruments. It also assumes that what we have done has done well by our patients. The literature suggests this is not true. Elevated IOP is routinely missed in patients with thin corneas. Eyes with a keratoprothesis are at very high risk of blindness from glaucoma, because the GAT cannot measure their IOP. The GAT also fails in children and in eyes with corneal scarring or nystagmus. Our nostalgia appears to be paired with a fair amount of amnesia.

THE SIMPLICITY ARGUMENT

Many eye care practitioners like the GAT, because it works via gravity and it is easy to understand the mechanism and principles. Instruments like the pneumatonometer (model 30; Reichert),, Pascal Dynamic Contour Tonometer (Ziemer Ophthalmic Systems), Ocular Response Analyzer ("ORA") (Reichert) and the 7CR (Reichert) are arguably more complicated (or at least less familiar), so may is harder to grasp the mechanics and theory behind them. Most of us likely do not understand our cell phones, but we have no trouble accepting and using them. Nevertheless, we are wary of sophisticated tonometers.

Albert Einstein said, “It can scarcely be denied that the supreme goal of all theory is to make the irreducible basic elements as simple and as few as possible without having to surrender the adequate representation of a single datum of experience.” The GAT satisfies the first section but fails the latter.

THE ECONOMIC ARGUMENT

The economic argument is the most strongly touted in many arguments in favor of keeping the GAT. All of the high-tech instruments are more expensive to purchase initially. However, intraocular pressure is currently the #1 treatable risk factor for glaucoma and arguably the only treatable risk factor. Glaucoma specialists therefore have a single metric (IOP) that rises in importance above the others. The instrument used by this profession should not be the cheapest, lowest tech instrument in the entire practice. Compared to perimeters, OCT's and most other equipment in a glaucoma practice, the cost of any tonometer, whether it be an inexpensive Goldmann, or a modern Pascal DCT, is not going to have a material impact on the finances of the practice. But the tonometer does have a material impact on the practice of medicine for glaucoma patients and their physicians.

Another old argument used to justify GAT is that they don't have the disposables costs of the more advanced tonometers. There is a small expense for disposables for most of the modern tonometers (with the exception of the two Reichert models (ORA, 7CR).

The fact that some professionals continue use a Goldmann tonometer with a reusable (non-disposable) prism is not something to cite as an advantage of Goldmann. Sure, your practice can avoid a small disposables cost, but the practice should then be expending proper effort on disinfection (and many practices do not do this).

Disinfection of the Goldman applanation tonometer: a systematic review - PubMed
https://pubmed.ncbi.nlm.nih.gov/29784162/

There is growing a worldwide trend toward the use of disposable prisms on Goldmann tonometers. Therefore, when you compare best practices, there is no cost disadvantage to the use of a modern tonometer. Even disregarding that, the small cost of disposables should not be an argument against using modern tonometer technology in a professional practice.

We can even make a compelling argument that the ammortized cost per measurement (and total cost of ownership) is lower for the ORA (Reichert) than for GAT -- when GAT best practices are followed (as they should always be). GAT requires more frequent calibration than ORA. ORA does not have disposables costs, while, GAT with best practices does. Staff labor costs to use the ORA are less, and having a lesser skilled staff member use the ORA does not compromise its accuracy, as is the case with GAT.

If we are only concerned about simple economics (the initial purchase cost), the GAT is the clear winner -- they are dirt cheap. But then why would we, as a profession, continue purchasing expensive perimeters, OCTs, surgical equipment, etc., if our standard is that our instruments have to be cheap and cannot advance with technology? If we put our priorities where they should be -- greater accuracy -- we can do much better with the newer instruments.

Another new tonometer not mentioned by Dan Eisenberg above is the Corvis ST (Oculus). Like the Ocular Response Analyzer (Reichert), it compensates for corneal biomechanics.

CONCLUSION

The dogma that the GAT is the gold standard remains the single greatest hindrance to the development and dissemination of new, more accurate tonometers. I cannot explain why eye care practitioners as a group continue to prefer 60-year-old technology and to reject decades of literature demonstrating its flaws.

Disclaimer: this article has been heavily edited by FitEyes. You can find the original, unedited version here.

Dan Eisenberg, MD

Dan Eisenberg, MD, is a glaucoma specialist at The Shepherd Eye Center in Las Vegas. He acknowledged no financial interest in any product or company mentioned herein. Dr. Eisenberg may be reached at (702) 731-2088**; glaucoma@cox.net.

References

1. Eisenberg DL, Sherman BG, McKeown CA, Schuman JS. Tonometry in adults and children. A manometric evaluation of pneumatonometry, applanation, and TonoPen in vitro and in vivo. Ophthalmology. 1998;105(7):1173-1181.

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