Issues with Current Tonometers

Issues with Current Tonometer Technology, Review of Published Peer Reviewed Literature

The Goldmann Applanation Tonometer is based upon a proven physical principle, the Imbert-Fick Law. Use of this law to measure intraocular pressure has major drawbacks, which were identified by Professor Goldmann in his original publication: “On Applanation Tonometery by H. Goldmann and T. Schmidt, published in Ophthalmologica 134, 221-242, 1957”.

The original publication was translated from German into English with the assistance of Pamila Seiving, Former Head Librarian, University of Michigan Kellogg Eye Center, and Holger Hansen, M.D., Dr. P.H., Professor of Community Medicine, University of Connecticut School of Medicine. What follows are excerpts taken from the original text, which identify the issues and limitations associated with Applanation Tonometry.

It is important to recognize that non-contact tonometers are not based on the Imbert-Fick Law and therefore are not applanation tonometers. The question to ask is what physical law do they use to estimate eye pressure? No known proven physical law validating the methodogy of non-contact tonometers can be found in the published literature. They also do not have the ability to compensate for any of the measurement errors that Dr. Goldmann so aptly described.

1. Corneal Wetness, Thickness and Elasticity
“Imbert-Fick’s Law States that the pressure of a fluid sphere, that is surrounded by a thin membrane can be measured by a counter pressure, which flattens the membrane. His necessary condition is that the membrane is extremely thin and without stiffness and that no other factors are involved”, page 221.

“The 0.5 mm thick cornea represents by no means an extremely thin membrane without stiffness… The surface of the cornea is moistened by fluid. Consequently, capillary forces will operate when the cornea is flattened, depending on the moistening fluid and the adhesive properties of the flattening device and the cornea”, page 222.

“Thus, a number of new problems arise which the abstraction of Imbert-Fick’s law does not take into account”, page 222.

“Elasticity has the (dimension) characteristics of pressure and, therefore acts as additional pressure”, page 223.

“We have assumed thus far that K1 (rigidity factor) is an expression of completely elastic forces. But because the cornea represents a rheological system, it should be presumed that it “floats” under the influence of deforming forces, i.e., after prolonged impact of those forces the cornea gains a new balance … One can see that in this experiment (Table VII and Figure 9), the tonometrically measured pressure declines in spite of manometrically constant pressure… This is an expression of floating of the cornea”, page 232.

“Under circumstances that vary much from our measurement conditions (abnormally thin or thick cornea, e.g. keratoconus, animal eyes, epithelial edema) errors of several millimeters are to be expected”, page 241.

As stated by Dr. Goldmann, The Goldmann tonometer cannot measure and therefore cannot compensate for the forces due to corneal wetness, thickness and elasticity. These forces affect the accuracy of all tonometers including non-contact tonometers.

Issue: Current tonometer technology cannot measure the forces due to corneal wetness, thickness and elasticity. Therefore, the current technology cannot compensate for these forces, which limits the accuracy of the measurement.

Solution: The FMAT1 optical and force generation system measures the forces due to corneal wetness, thickness and elasticity. The device compensates for these forces, eliminating the measurement errors associated with use of the Imbert-Fick Law.

2. Intraocular Pressure Variation due to the change in Diastolic and Systolic Blood Pressure
“One recognizes the coincidence of the two half rings (Fig 13) but some general technical expertise is necessary because eye pressure varies with the pulsating blood pressure. With correct measuring position, one can see the two half rings moving across from each other, with the inner margins of the half rings moving equally far to the left and the right” , page 240.

“Thus even more since the quickly established eye pressure does not reflect the natural average eye pressure at the time of measurement. The natural average eye pressure at a given point could be recognized only if one could control constant changes due to not only the pulse wave and breathing but also the impact of the outer eye muscles, the changing tension of the lids and emotional influences” , page 241.

Issue: The Goldmann tonometer does not measure the variation in intraocular pressure (IOP) that occurs with the cardiac cycle. IOP is higher in systole than diastole. The pulsating mires of the Goldmann Tonometer are evidence of this. When the inner edges of the mires are aligned, that is the diastolic phase of the cardiac cycle. This can be confirmed by checking the radial pulse when the inner edge of the mires are aligned, which is the end point according to the instruction manual. The mires separate during systole and cannot be aligned. The Goldmann tonometer cannot and does not measure IOP during systole.

The average individual spends one-third of the time in systole and two-thirds in diastole. Because the Goldmann tonometer cannot measure the systolic IOP, the measurement is not a true average IOP. One third of the IOP data is not being measured.

Other tonometers are not measuring the true average IOP as well. They lack the ability to determine at what point in the cardiac cycle data is being captured. The data capture is random. So a true average IOP is not being measured.

The true average IOP is the combination of the IOP during systole and diastole.

When blood pressure is measured both the systolic (SBP) and diastolic (DBP) is recorded. From those two values, the mean arterial pressure (MAP) is calculated, ((2DBP + SBP) / 3) = MAP.

Solution: The FMAT1 optical system is able to determine what phase of the cardiac cycle the measurement is captured. The device measures the IOP during systole and diastole for several cardiac cycles and calculates a true average IOP. The Ocular Pulse Amplitude (OPA) is the variation in IOP with the cardiac cycle. The FMAT1 device measures and displays the true average IOP and the OPA.

3. Repeatability, Precision and Accuracy
Repeatability in a measurement system refers to the ability of an operator to consistently repeat a measurement with minimal variation. Precision is how close the measurements are to each other. The closer repeat measurements are to each other, the less the variation. Accuracy is how close a measured value is to the actual true value.

So the only way you can obtain an accurate measurement is to take multiple repeat measurements, calculate the average or mean value and calculate the variation. An accurate measurement should have minimal variation.

The Goldmann tonometer only takes one measurement, and it is subjective because the user has to line up the scale. So the measurement is affected by the experience of the user and the subjective alignment of the measurement scale, which causes measurement bias. With only one measurement, it is not possible to determine the accuracy of the measurement.

Other tonometers also have the same issue. Multiple measurements must be taken over time, averaged and the variation examined to determine if the measurement is accurate.

Because decisions are made based upon the measured value, it is critically important that the measurement be repeatable, precise and accurate.

Issue: It is not possible to determine the accuracy of a measurement with current tonometer technology. With only one measurement value, the repeatability, precision and accuracy of the measured value cannot be determined.

Solution: The FMAT1 device samples every millisecond for multiple cardiac cycles. The sample data is analyzed for repeatability, precision and accuracy. The mean and percent variation is displayed to the user for his/her assessment.

4. Transmission of Infectious Disease
Laboratory studies have confirmed infectious prions in the tears of individuals with Creutzfeldt – Jakob Disease (Mad Cow), HBV virus in the tears of Hepatitis – B infected individuals, HCV virus in the tears of Hepatitis – C infected individuals and HTLV virus in the tears of individuals infected with the AIDS virus, (Br J Med Res 2014 Apr 30;4(12):2322-2333, J Infect Dis 2012 Aug 15;206(4):478-485, J Clin Microbiol 1995 Aug 33(8): 2202-2203, Ophthalmology 1986 Dec 93;12:1479-1481). For these potentially lethal diseases, the infectious agent is present in the tears before the infected individual manifests the disease.

At this time, Hepatitis-C is the leading cause of death among all infectious diseases in the US, as reported by Kathleen N. Ly, MPH, CDC Division of Viral Hepatitis, (Clinical Advisor, June 2016, Page 14).

Laboratory studies examining the disinfection efficacy of different regimens for tonometers have found that none are 100 % effective at removal of the Hepatitis-B Virus, Hepatitis-C Virus and the AIDS Virus. Studies also confirm that the infectious prions of Creutzfeldt-Jacob Disease (Mad Cow) are resistant to all conventional forms of sterilization, (Am J Ophthalmology 2001 Feb;131(2):184-187, Br J Med Res 2014 Apr 30;4(12):2322-2333, Arch Ophthalmol 1994 Nov;112(11):1406-1407, Arch Ophthalmol 1989 Jul;107(7):983-985).

Studies done in the United States on individuals with Creutzfeld-Jakob Disease (Mad Cow) have confirmed that tonometry is a risk factor for infection: Ocular Tonometry and Sporadic Creutzfeldt-Jakob Disease: A Confirmatory Case-Control Study, Br J Med Res 2014 Apr 30;4(12):2322-2333. The study conclusion was that due to lack of effective disinfection regimens and confirmation that tonometry is a risk factor for infection, disposables are required.

Issue: Lethal infectious agents of Hepatitis B, Hepatitis C, AIDS and Creutzfeld-Jakob (Mad Cow) Disease are present in tears. Disinfection regimens, even if they are followed, do not eliminate these pathogens.

Additionally, contact tonometers can be used without confirmation of disinfection. They can also be used with or without disposable covers. There is no absolute requirement that they must be disinfected before use, or that a disposable cover is used, or that the disposable cover has been changed after it has been used. There is also no means of preventing reuse of a disposable cover or disposable probe.

Air-puff tonometry once thought to be the solution, is not. It is also a risk factor for transmission of infectious disease. Laboratory studies have demonstrated that the puff of air disperses the tear layer causing micro-aerosol formation. “The ease with which droplets, potentially contaminated with human immunodeficiency virus and other viruses, are dispersed is disturbing. Air-puff tonometry may not be aseptic as previously presumed”, (Microaerosol Formation in Non-Contact Air Puff Tonometry. Britt JM, et al. Arch Ophthalmol 1991 Feb;109(2):225-228)

The micro-aerosols are dispersed in a cloud which are inhaled, deposited on external surfaces and deposited on the external surface of the eye. This creates the potential transmission of infection to not only other patients from contaminated surfaces but directly to staff from inhalation of the infectious micro-aerosols or deposition on the ocular surface.

Solution: The FMAT1 device uses a disposable plastic prism. The prism must be changed before a measurement can be performed on a different patient. The device has a detection system that is able to determine if the prism has been used. It is an absolute barrier to the transmission of infectious disease.