EyeWorld India September 2017 Issue

Evolution of tonometry: Are we overdue for a new clinical standard? by Tony Realini, MD, MPH The Goldmann tonometer’s shortcomings are well documented; might newer tonometers provide a better clinical standard? I t is well established that IOP is a powerful predictor of glau- coma progression. For decades, the Goldmann tonometer has remained the clinical standard instru- ment for measuring IOP. This instru- ment’s shortcomings have been well documented, the most important of which is its susceptibility to measure- ment error on the basis of structural and biomechanical properties of the cornea, such as central corneal thick- ness (CCT) and corneal hysteresis (CH). In recent years, innovative new tonometers have been developed, some expressly designed to minimize these CCT- and CH-related artifacts. Some have the added advantage of being operable by patients for self- tonometry, providing the opportu- nity for expanded collection of IOP data beyond the standard limits of sporadic office hours measurements. Might one of these new tonom- eters supplant Goldmann tonometry as a better clinical standard? What bar would a new tonometer have to clear in order to drive a paradigm shift in IOP measurement technique? At the 2017 American Glaucoma Society annual meeting in Coronado, California, several glaucoma special- ists presented data suggesting that new tonometers may provide better IOP information than Goldmann tonometry—specifically that IOP measurements with some of the new tonometers may correlate better with glaucoma progression risk than Gold- mann tonometry. Ocular Response Analyzer and IOPcc The Ocular Response Analyzer (ORA, Reichert Technologies, Depew, New York) operates as a non-contact tonometer, applying a metered jet of air to the corneal surface and measur- ing the pressure at two applanation points: one as the jet is increasing in pressure, and another as the jet of air is decreasing in pressure and the cornea returns to its resting configu- ration. The instrument measures a Goldmann equivalent IOP (IOPg), corneal hysteresis (CH)—a measure of the viscoelastic properties of the cornea—and an estimate of IOP com- pensated for corneal biomechanical effects (IOPcc). Bianca Susanna, MB , Uni- versity of California, San Diego, conducted a prospective study evaluating the relationship between IOP measured with a variety of tonometers and the observed rate of visual field progression by standard automated perimetry. In addition to Goldmann applanation tonometry (GAT), IOP was collected using the iCare rebound tonometer (RBT, Icare, Raleigh, North Carolina) and the ORA (IOPcc). Over an average of 2.5 years of follow-up, the mean IOP measured by GAT, RBT, and ORA was 14.2 mmHg, 13.3 mmHg, and 15.8 mmHg, respectively. “Mean IOPcc from the ORA had the strongest relationship with visual field loss over time,” Dr. Susanna said, “and was significantly stronger than the models using IOP from GAT or RBT.” In fact, while CH remained significant in models using IOP measurements from RBT (and nearly significant in a GAT model, with a p- value of 0.067), CH was not significant in the ORA IOPcc model. This suggests that IOP and CH each contain independently relevant in- formation about visual field progres- sion risk, and that the ORA IOPcc measurement (but not IOP from GAT or RBT) might be considered an aggregate measure that incorpo- rates both the relevant IOP and CH information. In clinical terms, “IOPcc may be more valuable than Goldmann IOP when assessing the risk of visual field progression in glaucoma patients,” Dr. Susanna said. Triggerfish contact lens sensor The Triggerfish contact lens sensor (CLS, Sensimed, Lausanne, Switzer- land) recently gained FDA approval as a tool for continuous IOP meas- urement over a 24-hour period. The device does not strictly measure IOP, but rather detects changes in ocular volume and from these changes infers associated changes in IOP. “The CLS captures continuous structural changes at the corneoscle- ral junction that correlate to volume changes that are significantly associ- ated with rates of visual field progres- sion,” said Gustavo de Moraes, MD , Columbia University, New York. The structural component is crucial, Dr. de Moraes said, because “pressure alone does not explain why people progress. How the eye responds to that pressure, and to changes in pressure, is also important in determining progression risk.” He presented the results of a study in which 24-hour CLS record- ing was conducted in 445 eyes of 445 glaucoma patients. These patients had previously completed an average of 10 visual field tests over an average of 5 years preceding their CLS ses- sion. The baseline visual field mean deviation of the group was approxi- mately –7 dB, and over the period of study, the mean rate of progression was –0.48 dB/year. The data were analyzed to deter- mine which correlated better with visual field progression: the mean of all the Goldmann IOP readings collected during the 5 years of study, or the CLS data collected during the single 24-hour recording session at the end of the 5-year period. “The CLS signal recorded in one session did better than Goldmann tonometry over 5 years,” he said. “A single 24-hour recording with the CLS provided a signature that correlated with the rates of visual field progression 5 years prior to its recording better than the mean Gold- mann IOP collected over years. “Twenty four-hour recording with the CLS may be useful to iden- tify patients at high risk for progres- sion and may aid in clinical decision- making,” Dr. de Moraes said. He cautioned that this was a study of past progression and does not indicate that the CLS data is predictive of future progression, but added, “The results of studies testing its predictive value are pending.” Practical applications Virtually every clinical health param- eter measured today is done so using equipment and techniques that were unimagined 50 years ago. In contrast, IOP is measured using virtually the same equipment and methods that were in routine use a half-century ago. Given the many known limi- tations of Goldmann tonometry, there is room for improvement. The notion that the IOP measurements obtained by different tonometers might provide different levels of information on glaucoma progres- sion risk is intriguing. Future studies may clarify the relative strengths and limitations of these new tonometers, and perhaps—if the data is compel- ling—onemay emerge as the new clinical standard. EWAP Editors’ note: The physicians have no financial interests related to their com- ments. Contact information de Moraes: demoraesmd@gmail.com Susanna: binsusanna@gmail.com 53 September 2017 EWAP GLAUCOMA