Tseng and colleagues have reported the results of a cross-sectional analysis correlating exercise intensity with the presence of glaucoma in the National Health and Nutrition Examination Survey (NHANES). NHANES is a long-standing, ongoing study in which ~10,000 Americans (adults and children) undergo interviews, examinations, and blood sampling to provide a snapshot of the health status of the US population. The assessments performed in NHANES vary year to year, and in 2005-2006, assessments included both exercise and ocular measures. Exercise was assessed through a questionnaire and seven days of wearing an accelerometer to measure physical activity. Ocular measures included Humphrey Matrix frequency doubling perimetry (FDT) and optic disc photography.
This was a cross-sectional study, and there is no way to infer which came first: exercise or glaucoma
The presence of glaucoma was defined using the Rotterdam criteria (two or more abnormal FDT points and cup-disc ratio - or asymmetry - exceeding 97.5% of the NHANES population) and alternately based on expert review of the optic nerve photographs. By the Rotterdam criteria, the prevalence of glaucoma among US adults over age 40 years was estimated to be 3.1%, while by expert photograph review the prevalence was 0.3%. By both definitions, and using a variety of measures of physical activity, the prevalence of glaucoma was consistently lower in people who exercised more. One possible interpretation of this observation is that exercise forestalls the development of glaucoma. There is biological plausibility to this possibility: exercise is known to lower IOP, which is a key risk factor for the development and progression of glaucoma. However, this was a cross-sectional study, and there is no way to infer which came first: exercise or glaucoma. Perhaps the diagnosis of glaucoma leads some people to reduce or stop their exercise activities, possibly due to the same limitations of visual function that put glaucoma patients at increased risk of falls. As the investigators aptly point out, further study is warranted to more robustly characterize the relationship between exercise and glaucoma development/progression risk.