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Mitsugu Shimmyo, Anna Djougarian, Joshua R. Ehrlich, Nathan M. Radcliffe; Seasonal IOP Variation: True Effect or Ecological Fallacy?. Invest. Ophthalmol. Vis. Sci. 2012;53(14):4170.
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Previous studies have found that there is seasonal variation in intraocular pressure (IOP), with lower IOP recorded during the summer months as compared to the winter months. We sought to confirm these findings and to assess whether this effect can be explained by an ecological inference fallacy.
We interrogated a large database of Goldmann-correlated IOP measurements obtained with the Ocular Response Analyzer (ORA; Reichert, Corp., Buffalo, NY) on all consecutive patients seen in a clinical ophthalmology practice in New York City over a five-year period from 2006 to 2010. We first performed a cross-sectional (ecological) analysis to determine IOP differences between Winter (December, January and February) Spring (March, April and May), Summer (June, July and August) and Fall (September, October and November) in all patients (glaucoma and normal). In the longitudinal assessment, we considered only normal patients (without corneal pathology or glaucoma diagnoses) with greater than 3 IOP measurements in separate seasons over one calendar year. Paired t-tests were used to compare seasonal variation among individuals (Bonferroni corrected p-value=0.01)
For the ecological analysis, we considered 87,352 IOP measurements, and average IOP over a 5-year period from 2006 to 2010 are shown in Figure 1. IOP measurements were lower during the summer (M=13.6±5.0) as compared to the fall (M=14.0±5; p<0.001), winter (M=14.3±5.0; p<0.001) and spring (M=14.3±5.0; p<0.001) months respectively. For the longitudinal analysis, we included 955 normal patients with at least 3 ORAs in a single calendar year and no more than one per season (2,428 total observations). Pair-wise comparisons demonstrated lower IOP during the summer (M= 13.6±4.8) months compared to fall (M=14.1±5.4; p=1.0), winter (M=14.1±5.4, p=0.8) and spring (M=14.3±5.3) months, however the difference was only statistically significant between summer and spring (p=0.01).
While in groups of individuals IOP may be lower during the summer months, this result was not as pronounced considering normal patients with multiple IOP assessments. These differences may be due to a lack of statistical power in determining small differences between groups or they may be explained by ecological patterns.
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