Intraocular pressure (IOP) is known to fluctuate with a range of systemic, ocular and environmental factors. Now, a longitudinal study from Japan suggests that ambient temperature may make an independent contribution to those changes in patients with stable primary open-angle glaucoma (POAG) receiving medical treatment.
Toru Ikushima and colleagues in the Department of Ophthalmology at Kyoto Prefectural University of Medicine, Kyoto, retrospectively analysed patients treated at the hospital between 2012 and 2023. Eligible participants were aged over 40 years, had attended at least three visits annually for more than three years, had no history of ophthalmic surgery and remained on an unchanged glaucoma medication regimen throughout follow-up.
The final dataset comprised 2,655 Goldmann applanation tonometry measurements from 154 eyes of 77 patients, with a mean age of 65.8 years and an average observation period of 5.2 years. Mean IOP was 12.5 mmHg. The investigators combined the clinical data with hourly meteorological information from Kyoto, including ambient temperature, atmospheric pressure, relative humidity, wind speed, precipitation and sunshine duration.
Using a mixed-effects model designed to account for potential ophthalmic, systemic and environmental confounders, the team found a significant negative association between ambient temperature and IOP. For each 1°C increase in temperature, IOP decreased by an estimated 0.029 mmHg. Mean blood pressure, meanwhile, was independently and positively associated with IOP. Glaucoma type, number of eye drops used, and time of day also significantly influenced measurements.
The temperature effect remained when daily rather than hourly meteorological values were used, with daily ambient temperature showing an estimated association of −0.030 mmHg per 1°C increase.
Subgroup analysis suggested that the relationship may be more pronounced in certain patient types. A significant negative association was seen in eyes with severe-stage glaucoma and in patients receiving three- or four-drug regimens. The association was also significant in the normal-tension glaucoma group, although not in the smaller high-tension glaucoma subgroup. However, the study authors cautioned that the latter finding could reflect insufficient statistical power.
One possible explanation, the researchers noted, is the physiological response to cold exposure. Lower temperatures can increase sympathetic nervous system activity, which may influence aqueous humor formation and consequently IOP. Importantly, however, the observed relationship between temperature and IOP remained significant after blood pressure was included in the multivariable model, suggesting that the two factors may act independently.
The authors acknowledged several limitations with their study, including the retrospective design and an inability to account for factors such as exercise, obesity, and blood biochemistry. The study was also deliberately restricted to patients with stable medication regimens, meaning its findings may not extend to individuals with poorly controlled IOP.
Nevertheless, the results suggest that environmental temperature may be worth considering when interpreting longitudinal IOP measurements – particularly in patients with more advanced stages of the disease. As the authors conclude, ambient temperature should be considered as an independent factor in IOP measurements when developing a more comprehensive approach to POAG management.
