Fran Lowry

May 21, 2012

May 21, 2012 (Fort Lauderdale, Florida) — Intraocular pressure (IOP) and visual field sensitivity peak in the winter months, especially in people with early glaucoma, according to a new study presented here at the Association for Research in Vision and Opthalmology (ARVO) 2012 Annual Meeting.

The seasonal effect was much larger in regions with more extreme seasonal variations in weather, lead investigator Stuart K. Gardiner, PhD, from the Devers Eye Institute in Portland, Oregon, reported.

"For example, the effect size in the North of the US — for example, the states of Michigan and Minnesota — was more than 5 times as large as in the South in states like California, Florida, and Texas," Dr. Gardiner told Medscape Medical News.

"This hasn't been reported before, and we're not sure why," he added.

Managing patients with glaucoma requires longitudinal monitoring of IOP and perimetric visual sensitivity, but detecting disease progression and IOP changes is hampered by factors not related to glaucoma per se that affect the measurement of these parameters, Dr. Gardiner said.

"Many researchers have been aiming to reduce the variability of functional testing in glaucoma, which adds uncertainty and so delays treatment that could prevent blindness," he said. "Our approach has been to start by better understanding why the testing is so variable in the first place, since that should help derive tactics to reduce the variability."

Dr. Gardiner and his team analyzed IOP measurements and visual fields for seasonal variation from 33,873 visits over a median of 12.5 years made by 1636 participants of the Ocular Hypertension Treatment Study (OHTS).

The 22 clinics that participated in OHTS were divided into 6 climatically similar geographic regions of the United States, as follows: Atlantic, Central, North, Pacific Northwest, Southeast, and West. Regions were determined on the basis of the size and timing of seasonal changes in precipitation, temperature, and sunlight hours.

The analysis found that seasonal variations in IOP occurred in all 6 regions. Variations peaked in January and February; the size of the variations ranged from 0.14 mmHg to 0.39 mmHg.

In addition, in 5 of the 6 regions, statistically significant seasonal variations in visual field sensitivity occurred; variations ranged from 0.04 dB to 0.13 dB.

"We need to determine whether this is the first evidence of a yet-unknown disease mechanism or whether it is an artifact of the testing method, which could be removed to reduce variability," Dr. Gardiner said.

The findings may help shed light on the disease process and also may help to reduce clinical test-retest variability, he added.

Commenting on this study for Medscape Medical News, Paul Kaufman, MD, from the University of Wisconsin-Madison, in Madison, Wisconsin, and one of the moderators of the session, said that the data "from a well-done study" were very interesting.

The information revealed in the study "may lead to better understanding of the biological mechanisms by which glaucoma damages vision, and a better understanding of the mechanisms can only help in the development of better treatments," Dr. Kaufman said.

Stephen Foster, MD, clinical professor of ophthalmology at Harvard Medical School and president and CEO of the Massachusetts Eye Research and Surgery Institution in Boston, agreed.

"The data are believable, given the large data set from 1636 patients from multiple areas and time zones throughout the United States studied over 12 years and involving 33,873 observational visits, with the same patterns seen in all areas. Although very small, the measured differences seen in winter vs summer were statistically significant," he told Medscape Medical News.

"What is not at all clear is the clinical significance of such a finding, or whether there is a causal relationship with climatic change," he said.

Dr. Gardiner, Dr. Kaufman, and Dr. Foster have disclosed no relevant financial relationships.

Association for Research in Vision and Ophthalmology (ARVO) 2012 Annual Meeting. Abstract #1751. Presented May 7, 2012.