Variations in IOP
Intraocular pressure varies with a number of factors, including the time of day (see the section “Circadian variation”), heartbeat, respiration, exercise, fluid intake, systemic medications, and topical medications (Table 2-1).
Body position has a significant effect on IOP, and the lowest IOP measurements are obtained when a person is seated with the neck in neutral position. IOP is higher when an individual is recumbent rather than upright, predominantly because of an increase in the EVP. Some people have an exaggerated rise in IOP when recumbent; this tendency may be important in the pathogenesis of certain forms of glaucoma. Alcohol consumption causes a transient decrease in IOP. Cannabis also decreases IOP but has not proved clinically useful because of its short duration of action and its side effects. In most studies, caffeine has not been shown to have an appreciable effect on IOP. There is little variation in IOP with age in healthy individuals.
Table 2-1 Factors That Affect Intraocular Pressure
In individuals without glaucoma, IOP varies by 2–6 mm Hg over a 24-hour period, as aqueous humor production, outflow facility, and uveoscleral outflow rate change. Higher mean IOP is associated with wider fluctuation in pressure. The time at which peak IOP occurs varies among individuals. However, 24-hour IOP measurement performed with individuals in their habitual body positions (standing or sitting during the daytime and supine at night) indicates that most people (with or without glaucoma) have peak pressures during sleep, in the early-morning hours, corresponding with a decrease in aqueous humor production, outflow facility, and uveoscleral outflow. During the waking hours, peak pressure often occurs soon after awakening. In selected patients, measurement of IOP outside office hours may be useful in determining why optic nerve damage continues to occur despite apparently adequately controlled pressure.
Liu JH, Zhang X, Kripke DF, Weinreb RN. Twenty-four-hour intraocular pressure pattern associated with early glaucomatous changes. Invest Ophthalmol Vis Sci. 2003;44(4):1586–1590.
Nau CB, Malihi M, McLaren JW, Hodge DO, Sit AJ. Circadian variation of aqueous humor dynamics in older healthy adults. Invest Ophthalmol Vis Sci. 2013;54(12):7623–7629.
IOP fluctuation and glaucoma
A significant body of literature demonstrates an association between IOP fluctuation and the risk of glaucoma progression. However, the data are largely retrospective, based on post hoc analyses of IOP variations between visits in large clinical trials. IOP fluctuation is also closely correlated with the level of mean IOP and appears to be a more important risk factor in certain groups of patients, including those with low IOP. Surgery results in less IOP fluctuation than medical therapy, but the clinical significance of this finding has not been demonstrated. Moreover, the specific measures of variability that may best predict glaucoma progression are not known, and the technology to effectively measure IOP fluctuation in a short time frame is currently limited.
Caprioli J, Coleman AL. Intraocular pressure fluctuation a risk factor for visual field progression at low intraocular pressures in the Advanced Glaucoma Intervention Study. Ophthalmology. 2008;115(7):1123–1129.
Musch DC, Gillespie BW, Niziol LM, Lichter PR, Varma R; CIGTS Study Group. Intraocular pressure control and long-term visual field loss in the Collaborative Initial Glaucoma Treatment Study. Ophthalmology. 2011;118(9):1766–1773.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.