EyeWorld is the official news magazine of the American Society of Cataract & Refractive Surgery.
Issue link: https://digital.eyeworld.org/i/194331
October 2013 February 2011 EW GLAUCOMA 71 Controlling IOP variability key in glaucoma management T he therapeutic value of intraocular pressure (IOP) reduction in glaucoma has been well established in many clinical trials. Less clear is the specific therapeutic goal of IOP reduction. What is the IOP profile of a well-controlled glaucoma patient? We tend to approach IOP reduction from a peak IOP perspective: We set a target IOP and consider the patient well controlled if IOP does not exceed our peak target IOP. Unfortunately, this approach is undermined by the reality of IOP behavior. "At least 50% of patients with glaucoma—whether high-tension or normal-tension glaucoma— have their peak IOP at night, outside of office hours, when it cannot be easily measured," said Michael S. Kook, MD, Seoul, Korea. Thus, our efforts to control peak IOP are limited by our inability to measure therapeutic efficacy at the time when peak IOP is most likely to occur. In recent years, the role of IOP variability in glaucoma progression has been debated. "No prospective studies have been conducted to evaluate the importance of 24-hour IOP variability on glaucoma progression," said Dr. Kook. There have been a few retrospective analyses of clinical trial data, and they have revealed mixed results. One problem with studying IOP variability is a lack of standardization. "There is no standard definition of IOP variability," said Arthur Sit, MD, Rochester, Minn. IOP changes from moment to moment, throughout the 24-hour circadian cycle, and from day to day and month-to-month, he said. "What is the correct time frame? We just don't know." To make matters even more complex, there is also an ultra-high frequency IOP variation that occurs when we blink. "Each blink causes an IOP spike in the range of 8 to 15 by Tony Realini, MD The Triggerfish contact lens-based 24-hour IOP monitor Source: René Goedkoop, MD mm Hg," said Crawford Downs, PhD, Birmingham, Ala. In addition, there is a constant stream of minispikes in excess of 1 mm Hg occurring throughout waking hours. "These dynamic spikes hit the optic nerve head 10,000 to 12,000 times per hour during the day," he said. "Preliminary data suggest that these IOP spikes account for more than 10% of the total IOP-related energy that the eye must absorb during waking hours. IOP spikes should be larger in the elderly and at higher steady-state IOPs, but their importance in the pathophysiology of glaucoma is unknown." The key reason that IOP variability remains such a mystery is because we lack the ability to measure IOP outside of the clinical setting. Unlike the tools developed for patients with diabetes and high blood pressure to self-assess their blood glucose and blood pressure at home, there is no validated method for home tonometry. Even this might not be enough—given how quickly IOP can change, even intermittent monitoring may be inadequate to characterize the IOP fluctuations relevant to the risk of progression. Ob- serving these micro-fluctuations may require continuous IOP monitoring, much like Holter monitoring of cardiac rhythms. Such a device—a continuous IOP monitor—has been a holy grail of IOP researchers. The Triggerfish contact lens-based 24-hour IOP monitor (Sensimed, Lausanne, Switzerland) may be an important step toward that goal. "The Triggerfish device is worn like a normal vision-correcting contact lens," said Kaweh Mansouri, MD, Geneva, Switzerland. The device remotely transmits data to a receiver, the antenna for which is affixed to the periocular skin during the 24-hour recording session. "In numerous clinical trials, the device is safe and well tolerated by both glaucoma patients and healthy subjects, and has been shown to have good reproducibility in both groups," he said. The device detects the changes in corneal curvature that occur with changes in IOP; the output is not in mm Hg but in arbitrary units that cannot be directly mapped back to IOP. Manometry studies in enucleated eyes have shown that the de- foliation, uveitis, or trauma," said Andre Mermoud, MD, Lausanne, Switzerland. "In some cases, there will simply not be enough residual capsule to support an IOL. In these cases, an iris-fixated IOL may be necessary." EW Editors' note: Dr. Ahmed consults for industry. Dr. Harasymowycz has financial interests with Alcon (Fort Worth, Texas), Abbott Medical Optics (Santa Ana, Calif.), and Bausch + Lomb (Rochester, N.Y.). Dr. Mermoud has no financial interests related to the article. vice's signal correlates well with true IOP, but similar experiments in vivo have not been completed, Dr. Mansouri said. The device is approved for clinical use in Europe; studies are under way in the United States to garner FDA approval for its use here. Once approved, there will remain the job of determining how to interpret the sheer volume of data the device produces in a 24-hour period. Until then, our best clinical approach to IOP variability is the office-hours diurnal IOP curve coupled with inter-visit IOP variability assessment. "Our primary goal is to strive to achieve consistently low IOP throughout the 24-hour period," said Dr. Kook. "Consistent reduction of IOP throughout the 24-hour period is the most desired goal in the clinical management of glaucoma." Robert N. Weinreb, MD, La Jolla, Calif., agrees. "Despite safe and effective medications, many patients with glaucoma continue to lose vision. With the information we will glean from 24-hour IOP monitoring, we will learn the role of IOP in causing glaucoma progression. We will better understand the relationship between IOP and the risk of glaucoma. And in the future, treatment will be individualized based on 24hour IOP profiles," he said. EW Editors' note: Drs. Downs and Kook have no financial interests related to the article. Dr. Mansouri has financial interests with Sensimed. Dr. Sit has financial interests with AcuMEMS (Menlo Park, Calif.) and Sensimed. Contact information Downs: cdowns@uab.edu Kook: mskook@amc.seoul.kr Mansouri: kawehm@yahoo.com Sit: sit.arthur@mayo.edu Weinreb: Weinreb@eyecenter.ucsd.edu Advanced continued from page 70 essentially cutting out a bigger circle around the original capsulorhexis." These techniques will successfully guide surgeons through most of these tough cases. Occasionally, however, disaster strikes. "Typically, these are eyes with either pseudoex- Contact information Ahmed: ike.ahmed@utoronto.ca Harasymowycz: pavloh@igmtl.com Mermoud: amermoud@montchoisi.ch