Eyeworld

EW GLAUCOMA 38 December 2015 by Tony Realini, MD, MPH associate professor of ophthalmol- ogy, and director of the glaucoma unit, University of Brescia, Italy. What drives these regular and irregular variations? "Anything that you can do while you're alive can affect your IOP," said Arthur Sit, MD, Roches- ter, Minn. In particular, changes in body position can cause significant IOP changes. "In some yoga head- stand positions, IOP can double," he added. Unfortunately, of all the body positions, IOP is lowest in the seated position with the neck neither flexed nor extended; this is the po- sition in which Goldmann tonom- etry is typically performed. Thus, office-based IOP measurements rep- resent a best-case scenario, or if you are worried about progression, per- haps a worst-case scenario because the low IOP can falsely reassure you that the patient is well controlled. We have also learned in recent years that IOP is highest at night or early morning, when we are in the supine position during sleep. "Nocturnal IOP elevation may be fundamental to understanding and managing glaucoma progression in some patients," Dr. Quaranta said. Interestingly, these position- al IOP changes are not related to changes in aqueous inflow or trabec- ular outflow but rather to episcleral venous pressure (ESVP) changes. "ESVP changes by an amount that explains the changes in IOP," Dr. Sit said. Most IOP peaks missed Peak IOP is important both on its own and because higher peaks are associated with higher variability. "Diurnal IOP parameters are in- terrelated," said Marcelo Hatanaka, MD, Brazil. "Peak IOP is the most important parameter with regard to glaucoma progression." The occurrence of peak IOP at night has clinical consequences. "Single in-office IOP measurements are the easiest but are least likely to detect peaks," Dr. Hatanaka said. "We miss the peaks of IOP in two-thirds of our patients by relying solely on office hours IOP measure- ment," said Kaweh Mansouri, MD, Switzerland. New tools for comprehensive IOP assessment In an ideal world, we would mea- sure IOP continuously, either for a short time—such as a single 24-hour period—or on a more long-term or even permanent basis. Accomplish- ing this with current technology requires patients to spend the night in a properly equipped facility and be awakened every hour or two all night long. This is impractical, ex- pensive, non-physiologic, and gener- ally unacceptable to both patients and providers. The current middle ground is a diurnal (daytime) curve obtained in a single day during office hours, or a multiday curve in which diurnal variability is inferred by reconstruct- ing the curve from a series of visits at different times of the day, Dr. Hatanaka said. New tools on the near horizon will facilitate the assessment of both after-hours peak IOP and 24-hour circadian IOP variability. Several tonometers are being designed or adapted for home use, either by the patient or a family member, to facilitate acquisition of IOP measurements outside tradition- al office hours. The Triggerfish (Sensimed, Lausanne, Switzerland) is a contact lens-based system that infers changes in IOP based on changes in corneal-scleral junction curvature. IOP is measured with high frequency over a 24-hour peri- od, and readings are wirelessly trans- mitted to a receiver worn around the orbital rim. This device holds great promise for many patients, but there are im- portant limitations to consider. Chief among these is the very nature of the data Triggerfish provides. "Everything we know about IOP and glaucoma is based on Goldmann applanation tonometry," said Gustavo de Moraes, MD, MPH, New York. Triggerfish output is not even measured in mm Hg, he added. This concern was echoed by Felipe Medeiros, MD, San Diego, who pointed out, "You do not get absolute IOP values so you have to rely on the patterns." Another device in development is the EYEMATE (Implandata Oph- thalmic Products GmbH, Hannover, Addressing IOP variability in clinical practice T he goal of glaucoma therapy is to preserve vision. The only therapy that has been shown to accomplish this is to lower the intraocular pressure (IOP). But which IOP is the most important? Is it the peak IOP, the mean IOP, or is the IOP variability also worth targeting on its own? These issues are addressed by Tony Realini, MD, in this month's "Glaucoma editor's corner of the world." IOP variability has been shown in some studies to be an important independent risk factor for glaucoma progression. But we rarely have good information about a patient's IOP variability. This plays into our legitimate fear that we may be missing some important data in treating our glaucoma patients. It doesn't surprise us that the IOP may vary greatly with position or activity, and this opens up an enormous area of uncertainty about what target pressure we should be aiming to reach. Under the best of clinical circumstances, we only measure a tiny fraction of the pressures experienced by the eye of a glaucoma patient. Most of the time this is enough. But in cases where visual field loss continues despite pressures that appear well controlled, we must look for other factors. Unfortunately, confounding issues are in abundance—poor compliance, in- creased susceptibility to pressure (normal tension glaucoma), and peak pressures that we are missing. But that's not all. Dr. Realini and others suggest that increased IOP variability should be added to this list. At the moment, we often have no choice but to treat glaucoma patients with incomplete clinical information. We know they are losing visual field, but we can't be sure about the role of compliance, increased optic nerve vulnerability, or IOP variability. Having more accurate pressure information—the mean, the peaks, and ranges—would give us greater insight into what therapy would be best. But without this information, we sometimes just have to increase medications or perform surgery, since lowering IOP is our only therapy. There are many technologies on the horizon that promise to achieve continuous IOP monitoring. So in the near future we will have a complete set of IOP data. This will be very helpful because it will give us a much more accurate target IOP and will allow us to better gauge the efficacy of our treatments. Meanwhile, surgical therapies and longer duration med- ical therapies are being developed that can achieve more consistent long-term IOP control— regardless of whether the IOP problem is too high of a peak or too much variability. The work that Dr. Realini reviews in this article is more evidence of the bright future of glaucoma care. Reay H. Brown, MD, glaucoma editor I ntraocular pressure (IOP) remains the sole therapeutic tar- get for glaucoma management. The most common clinical approach to IOP reduction is to lower peak IOP. This is accomplished by setting a target range IOP below which the patient is considered ad- equately controlled. In recent years, the role of IOP variability—fluctua- tion throughout the day and from day to day—has been implicated as a potentially important factor to consider when assessing the adequa- cy of IOP control. Some, but not all, of the major glaucoma clinical trials support a role for high IOP vari- ability as a risk factor for glaucoma progression. Measuring IOP variability remains a clinical challenge for two related reasons: Many measurements are needed over time, and there is no widely available, inexpensive way to measure IOP outside of office hours. The latter is about to change, and when it does, IOP variability will come to the forefront of timely glaucoma topics. Natural IOP variability "IOP is a dynamic physiologic parameter with regular circadian variations and unpredictable short- term and long-term fluctuations," said Luciano Quaranta, MD, PhD, Glaucoma editor's corner of the world

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