EyeWorld Asia-Pacific March 2016 Issue

March 2016 Glaucoma treatment 36 EWAP SECONDARY FEATURE Views from Asia-Pacific Rajesh S. KUMAR, MD Consultant, Glaucoma Services and Deputy Director of Research Narayana Nethralaya Eye Hospital 121/C Chord Road, 1 st R Block, Rajajinagar, Bangalore 560010, Karnataka, India Tel. no. +919731554850 raj_skumar@yahoo.com I ntraocular pressure (IOP) remains the most important clinical biomarker in the management of glaucoma. Measuring this parameter has to be both accurate and precise; values have to be close enough to the true pressure intraocularly and show good repeatability. While the current gold standard, Goldmann applanation tonometry (GAT), has been ubiquitous in our practice for over 50 years, its inherent deficiencies in relation to central corneal thickness and biomechanics will challenge its continued use. As outlined by Drs. Kotecha and Vajaranant, given that a fair number of patients in the future who present with glaucoma could possibly have had cornea-altering procedures for vision correction and that IOP-correction nomograms are far from reliable, newer devices and technologies have been assessed as potential alternatives. Dynamic contour tonometry (DCT) offers the most reliable and precise modality to measure IOP, independent of corneal parameters; it has been shown to be closer to manometric readings than GAT. However, it is time consuming and cumbersome to use, needs skilled technicians and, sometimes, multiple readings to get acceptable measurements. The Ocular Response Analyzer (ORA) and CorvisST provide IOP measures along with mathematically derived corneal biomechanical values; these devices determine the corneal deformation and hysteresis; ocular pulse amplitude and IOP ranges are potential determinants of the accuracy of these devices. The Mackay–Marg tonometer (Tono-Pen) and rebound tonometer (iCare) are portable devices that are useful in diseased corneas and in children. All these devices correlate well with GAT, but cannot be used interchangeably. IOP has a circadian rhythm and fluctuations have been shown to be an important risk factor for incidence and progression of glaucoma; current devices only allow a single snapshot of IOP and this is a major inadequacy in the glaucoma management armamentarium. Recently, 24-hour IOP monitoring has been attempted using temporary contact lenses or permanently implantable intraocular sensors. The contact lens-based Triggerfish does not measure IOP directly; it records corneal curvature changes that could potentially translate as IOP fluctuations. The circadian IOP-values are based on cosinor rhythmometry and the amplitude, mean, and acrophase (time of peak value) are determined. While results appear to be reproducible and the lens well tolerated, there are limitations in the form of the paraphernalia that are attached to it. One of the biggest challenges will be the problem of “big data” that will be thrown up by these devices. Clinicians should understand that every technique for IOP measurement has its advantages and limitations and that most provide only a single reading of a dynamic event. Understanding circadian IOP patterns with 24-hour monitoring may allow better prognostication and help individualize treatment approaches for glaucoma. Editors’ note: Dr. Kumar declared no relevant financial interests. preparation and could be time consuming in a busy practice, Dr. Kotecha said. Studies show that DCT measurements are more precise when trained technicians perform the measurements, so it may be best for physicians to dedicate trained staff to performing DCT in their practice. The Ocular Response Analyzer (ORA, Reichert Technologies, Depew, NY), by contrast, is a non- contact tonometry method that measures the corneal deformation from a puff of air. This method uses the same principle as traditional non-contact tonometry, but the ORA is able to measure corneal hysteresis, providing a corneal- compensated IOP reading (IOPcc). ORA readings are quick—they take only 2–3 seconds—but their repeatability coefficient may appear relatively large if there are significant variations in IOP caused by the ocular pulse. No study has yet compared the ORA to manometry to determine its accuracy. Evidence suggests that the ORA is less affected by corneal thickness than GAT, but some evidence suggests that the ORA IOPcc is not completely immune to corneal changes post-refractive surgery, Dr. Kotecha said. She thinks this method is more useful for characterizing changes in corneal biomechanics post-refractive surgery. A third option is rebound tonometry, which may be the most useful of all the available methods for diseased or transplanted corneas. In rebound tonometry, a probe is bounced off the cornea or the sclera and the rate of the probe’s deceleration is used to calculate IOP. Rebound tonometry is quick, but readings are affected by corneal thickness, and it requires some skill to hit the same spot on the cornea each time. Two rebound tonometry options are currently available: the ICare transcorneal tonometer (ICare, Vanda, Finland) and the Diaton transpalpebral tonometer (Tonom GmbH, Münster, Germany). Both tonometers are hand-held and require no anesthetic. ICare tonometer readings are affected by corneal thickness but less affected by corneal edema, so they may be more useful than other methods in pathologic or transplanted corneas, Dr. Vajaranant said. The Diaton tonometer measures pressure through the eyelid and rebounds against the sclera, making no corneal contact, so this method could be an alternative for keratoprosthesis patients, she added. Will GAT remain the gold standard? Considering that patients now have more options for laser refractive surgery, corneal inlays, and transplants, Goldmann tonometry is no longer enough, Dr. Kotecha said. All of these tonometry alternatives have their own relative advantages and disadvantages, but they are not interchangeable. It is unclear whether one will win out over the others, but knowing these options, physicians can now choose the method that they feel might be best for each patient’s condition. EWAP Editors’ note: Drs. Kotecha and Vajaranant have no financial interests related to this article. Contact information Kotecha: aachal.kotecha@ucl.ac.uk Vajaranant: thasarat@uic.edu Reexamining - from page 35