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94 EW GLAUCOMA March 2013 February 2011 Seasonal focus for glaucoma testing coming to light by Maxine Lipner EyeWorld Senior Contributing Writer Monthly clues on visual field sensitivity T he time of year when glaucoma visual field and IOP testing is done may have an impact on outcomes, according to Stuart K. Gardiner, Ph.D., associate scientist, Devers Eye Institute, Legacy Research Institute, Portland, Ore. During winter months visual fields tend to have their greatest sensitivity, recent study results published in the January issue of Ophthalmology showed. This effect was particularly noteworthy in northern areas where seasonal changes were greatest. This study is an outgrowth of the highly touted Ocular Hypertension Treatment Study (OHTS) that considered whether reducing intraocular pressure helped to prevent ocular hypertensive patients from developing full-blown glaucoma. "We were intrigued by the fluctuations (in data)," Dr. Gardiner said. "We know that perimetry is variable from test to test, and we don't quite know where all of the variability comes from." Dr. Gardiner and fellow investigators wondered whether it might be possible to make the test less variable over time and wanted to delve into the OHTS data further. Monitoring monthly variations Included in the OHTS were 1,636 participants who were followed over a total of 33,873 visits. Depending on where the patient's clinic was located, OHTS data was sorted into six geographic zones from the Southeast to the Pacific Northwest. When Dr. Gardiner and fellow inves- tigators reviewed this data they determined that when it came to IOP, study results showed seasonal variations, ranging from 0.14 to 0.39 mm Hg, in all of the six geographic zones. The IOPs were highest in the winter, peaking in the months of January and February, in all regions. Likewise, changes in visual field sensitivity indicated a significant seasonal effect on visual field sensitivity in five of the six regions. Magnitudes here ranged from 0.04 dB to 0.21 dB, peaking in February. Dr. Gardiner sees the fact that this seasonal effect exists as a key finding. "There has been one other report of some sort of seasonal effect from a group in the Netherlands, but that wasn't as big a data set," Dr. Gardiner said. In this study, published in the October 2012 issue of Investigative Ophthalmology and Visual Science, investigators looked at changes in the season, but they didn't look at this by month, he pointed out. This other trial likewise showed that there might be a seasonal effect at work, but Dr. Gardiner stressed that the studies were independent of each other and not confirmatory. The study led by Dr. Gardiner indicated that the farther north patients resided, the bigger the seasonal changes in visual field sensitivity. "It's where there is a bigger change in the seasons that you get a bigger seasonal effect," Dr. Gardiner said. "It wasn't very big in California because the seasons don't change much in San Diego, whereas around Detroit and the Great Lakes and those areas, there was a much more substantial change between winter and summer." Investigators found that visual fields were most sensitive during the winter months. Possible causes Dr. Gardiner speculated that one possibility why visual field sensitivity decreases in summer months is because it is much lighter during that time. "The sunlight is much brighter than artificial light," he said. "The visual system adapts to that." As a result, he theorized the visual field becomes less sensitive to contrast because it doesn't need to be as sensitive during this time. If true this would be an effect that is happening in everyone with the eye adapting to light levels, Dr. Gardiner explained. "The alternative is that it has something to do with the eye developing glaucoma," he said. "The implication would be that the high light levels in the summer are damaging the cells in the eye." So in an already damaged eye, such as those studied here where the intraocular pressure was already up, the eye may be more susceptible to other forms of damage, Dr. Gardiner theorized. If so, high light levels could be causing damage that a normal eye would be able to cope with. It is also possible, he pointed out, that this is a temperature rather than a light effect. "We can't rule that out, but a light effect seems far more obvious," he said. "Intuitively, it's far more likely that it's something to do with light levels rather than temperature." The next obvious question, in Dr. Gardiner's view, is whether this seasonal effect on visual field sensitivity and IOP takes place in normal eyes or is reserved for those with glaucoma. Or, it might be that this effect is confined to those who are on the cusp of developing glaucoma. "It could be that the light is doing some damage that we don't quite understand yet," Dr. Gardiner said. "This is speculation, but if that's true then the implication is that wearing sunglasses would be helpful." From a clinical perspective Dr. Gardiner sees this information on seasonal implications as potentially important for practitioners to keep in mind when assessing visual field sensitivity changes, depending on where they live. "If they live in the Great Lakes area where they have large seasonal changes, they could bear in mind that a slight decline in sensitivity in the summer might be seasonal change and it might improve again in the winter," he said. "By contrast, if they see that sensitivity has gone down in the winter then it's actually more serious than they think." Peak visual field sensitivity during this time may be masking the true degree of sensitivity loss. Overall, Dr. Gardiner views this as anything but an endpoint. "For the future, let's figure out what the cause is because I don't see this as the end of the story," he said. "It's the start of the story almost." He stressed that there is a need to know what is causing this. "There are some theories around that could be convincing, but we need to know which one, if it is actually happening, and most importantly, if this is a general characteristic or is telling us something about the disease," he said. The ultimate big picture would be if sunlight was shown to be the cause of damage in these people and could be reduced by wearing sunglasses. EW Editors' note: Dr. Gardiner has no financial interests related to this article. Contact information Gardiner: 503-413-1199, sgardiner@deverseye.org