Large, population-based epidemiologic studies have revealed a mean IOP of 15.5 mm Hg, with a standard deviation of 2.6 mm Hg. This led to the definition of “normal” IOP as 2 standard deviations above and below the mean IOP, or approximately 10–21 mm Hg.
Although IOP greater than 21 mm Hg has in the past been defined as “abnormal,” this definition has a number of shortcomings. It is known that IOP in the general population is not represented by a Gaussian distribution but is skewed toward higher pressures (see Fig 2-3). IOPs of 22 mm Hg and above would thus not necessarily represent abnormality from a statistical standpoint. More importantly, IOP distribution curves in glaucomatous and nonglaucomatous eyes show a great deal of overlap. An IOP screening value of 21 or 22 mm Hg, by itself, has little real clinical significance. Several studies have indicated that as many as 30%–50% of individuals in the general population who have glaucomatous optic neuropathy and/or visual field loss have initial screening IOPs below 22 mm Hg. Furthermore, because of diurnal fluctuation, elevations of IOP may occur only intermittently in some glaucomatous eyes, with as many as one-third of the measurements being normal.
The IOP in glaucoma patients may vary widely, by 10 mm Hg or more, over a 24-hour period. Most patients without glaucoma show a diurnal range of 2–6 mm Hg. Patterns of diurnal fluctuation have been broken into several types depending on time of peak pressure: morning, day, night, or flat (meaning little diurnal variation). Most individuals manifest similar patterns from day to day; however, 10%–20% of patients are “erratic,” manifesting different patterns of diurnal IOP fluctuation over time. The shift from daytime upright posture to supine posture at night may be associated with increased nocturnal IOP measurements. Spontaneous asymmetric fluctuations of IOP between fellow eyes occur commonly in individuals without glaucoma and in glaucoma patients.
Thus, single office measurements of IOP do not adequately depict the degree of fluctuation of IOP. Diurnal IOP fluctuations have been associated with progression of glaucoma in some studies. Whether or not fluctuation of IOP is an independent risk factor, elevation of IOP is a strong risk factor for glaucoma progression.
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Corneal thickness affects the measurement of IOP. Thicker corneas resist the indentation inherent in nearly all methods of IOP measurement, including applanation tonometry, airpuff method, and pneumotonometry. Some measurement techniques, such as dynamic contour tonometry (DCT), may be less affected by corneal thickness.
Corneal thickness may be measured (pachymetry) by optical and ultrasonic methods. Average corneal thickness, determined by optical and ultrasonic pachymetry, is approximately 530–545 µm in eyes without glaucoma. Central corneal thickness (CCT) has been found to be increased in groups of patients with the condition termed ocular hypertension (OHT), which is elevated IOP in the absence of identifiable optic nerve damage or visual field loss, and decreased in patients with normal-tension glaucoma. Above-average thickness tends to cause overestimation of IOP, but the relationship between corneal thickness and applanation tonometry measurements is probably not linear. Corneal curvature may also play a role in the measurement of IOP, since more sharply curved (steeper) corneas resist indentation more.
, BrowningAC, ShahS, HamiltonR, DaveD, DuaHS. Effect of corneal thickness on intraocular pressure measurements with the pneumotonometer, Goldmann applanation tonometer, and Tono-Pen.. 2002;43:1389–1392.
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, ZamanML. Human corneal thickness and its impact on intraocular pressure measures: a review and meta-analysis approach.. 2000;44:367–408.