2020–2021 BCSC Basic and Clinical Science Course™
Chapter 2: Intraocular Pressure and Aqueous Humor Dynamics
IOP Distribution and Relation to Glaucoma
Pooled data from large epidemiologic studies indicate that the mean IOP in the general population of European ancestry is approximately 15.5 mm Hg, with a standard deviation of 2.6 mm Hg. IOP has a non-Gaussian distribution with a skew toward higher pressures, especially in individuals older than 40 years (Fig 4-2). IOP is also genetically influenced. The value 21 mm Hg (2 standard deviations above the mean) was traditionally used both to separate normal from abnormal pressures and to define which patients required ocular hypotensive therapy. However, it is now understood that glaucoma is a multifactorial disease process for which IOP is an important risk factor. Many patients with glaucoma consistently have IOPs ≤ 21 mm Hg, and most individuals with IOP >21 mm Hg do not develop glaucoma. Further, the cutoff value of 21 mm Hg is statistically flawed, given the non-Gaussian distribution of IOP values in the population. Consequently, screening for glaucoma based solely on the criterion of IOP >21 mm Hg misses up to half of the people with glaucoma and optic nerve damage in the screened population.
Figure 2-4 Frequency distribution of IOP: 5220 eyes in the Framingham Eye Study.
(Modified from Colton T, Ederer F. The distribution of intraocular pressures in the general population. Surv Ophthalmol. 1980;25: 123–129.)
General agreement has been reached that, for the population as a whole, there is no clear level below which IOP can be considered “normal” or safe and above which IOP can be considered “elevated” or unsafe. IOP is a continuous risk factor across its entire range: the higher the IOP, the greater the risk for glaucoma. Although other risk factors affect an individual’s susceptibility to glaucoma, all current treatments are designed to reduce IOP.
Sommer A, Tielsch JM, Katz J, et al. Relationship between intraocular pressure and primary open-angle glaucoma among white and black Americans. The Baltimore Eye Survey. Arch Ophthalmol. 1991;109(8):1090–1095.
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.