Winter seems especially hard on IOP, with elevated levels found in a recent study. Image courtesy of Getty Images.

Previous studies on IOP have found a higher IOP in winter compared with summer months. A recent study aimed to investigate the factors that influence changes in IOP, confirming seasonal fluctuations.

Dataset A included patients examined with Goldmann applanation tonometry between 2011 and 2018. Dataset B was performed from 1999 to 2002, and dataset C consisted of all first visits during 2012 to 2017. Regression analyses assessed the relationship between the time of year and IOP.

IOP was highest in the colder months (December and January) and lowest in the warmer months (August and September). For dataset A, mean IOP was highest in December (15.7+3.7mm Hg) and January (15.7+3.8mm Hg) and lowest in September (14.5+3.1mm Hg). This suggests conventional quarterly analysis—January to March, for example—can “conceal time-of-year relationships due to inadequate statistical power and timing of IOP variation,” the study notes.

Multiple linear regression analysis, with a November to October reordering, detected an annual, downward IOP trend, and an analysis of dataset B confirmed this. Fourier analysis on datasets A and B combined supported a 12-month IOP cycle for both eyes, and an analysis of dataset C showed a repeating pattern where IOP trended downward around April and then back upward around October.

“Although not discernible with the smaller datasets A and B, it was interesting that the larger dataset C suggested a ‘plateau’ of IOP that existed from about October to April, followed by an ensuing trough that developed from May to September,” the authors noted in their study. “It is evident that we cannot make firm conclusions about this definitive pattern, but it would be interesting to see how even larger datasets from around the world may, or may not, mirror these observations.”

Patients with IOP-lowering medication and a history of cataract surgery were excluded from datasets A and B, but findings from previous studies showed similar results in both healthy and glaucomatous eyes. The authors noted even small IOP differences “can be influential on glaucoma-related outcomes.”

“Although time-related variation in IOP may be small, it is demonstrable with an adequately sized dataset and appropriate methods and may be worthy of further study relative to glaucoma control,” the authors explained in their study. “It is noteworthy that the Early Manifest Glaucoma Trial showed that IOP lowering of just 1mm Hg yielded a 10% to 13% risk reduction in visual field deterioration. Further, the Ocular Hypertension Treatment Trial demonstrated that this amount of IOP lowering resulted in a 10% reduction in the risk of glaucoma conversion. To maintain a level reduction in risk of glaucoma progression, this suggests that, at least for certain patients, management might need to be altered during certain times of the year.”

Exercise has been shown to have an effect on IOP, and with many forms of exercising more possible in the summer months, that could have played a part in these results.

Additionally, seasonal IOP variation magnitude or trends may differ year-to-year depending on the different daily and monthly temperatures. “For example, if the temperature in November is warmer or cooler in one year vs. another, does this have any effects on relative IOP measurements? We believe this may be an interesting question to explore but the effects may be extremely small and difficult to measure without careful adjustment for many other variables,” the authors explained in their study. “The increasing availability of very large datasets may make this a more feasible question to answer, but a potential problem may be that any effects could also be negated by spending most time indoors.”

Morettin CE, Roberts DK, Newman TL, et al. Time-of-year variation in intraocular pressure. J Glaucoma. August 17, 2021. [Epub ahead of print].