Intraocular Pressure

IOP is dependent upon the balance between aqueous humor production and outflow, and is important because the refractive properties of the eye depend upon IOP to maintain the curvature of the cornea.16 The distribution of IOP in the population is 10 to 21 mm Hg and is slightly skewed toward higher values; however, caution should be used in assigning this range as being "normal" for IOP because optic neuropathy can be present in the normal range and can be absent at higher IOPs. Elevated IOP is generally considered greater than 21 mm Hg. Optic nerve damage generally is slow and takes several years for noticeable progression between 20 and 30 mm Hg, while IOP of 40 to 50 mm Hg may lead to rapid optic nerve damage. IOP varies in a cyclic fashion over the 24-hour day. IOP was thought to be lowest at night and at its maximum in the morning; however, more recent evidence suggests that not all individuals follow this pattern. Patients with and without glaucoma may exhibit a rhythm that consists of a peak in IOP right after falling asleep. Nighttime peaks in IOP are detrimental to patients with glaucoma, because systemic blood pressure decreases during the night leading to a low ocular perfusion pressure. Decreased ocular perfusion pressure can lead to further optic nerve damage, therefore highlighting the importance of IOP control throughout a 24-hour period.17-19

IOP is clinically measured by tonometry and can be performed via applanation, indentation, and indirect tonometry.20 CCT affects the accuracy of IOP measurements. Thin corneas (less than 540 microns) can produce falsely low IOP readings, whereas thick corneas (greater than 555 microns) may produce falsely high readings. The potential consequences of this error in measurement may lead to overtreatment of patients with falsely high IOP and under-treatment of patients with falsely low IOP. The OHTS demonstrated that CCT is a strong predictive factor for the development of POAG. Patients with corneas less than 555 microns and IOP greater than 25.75 mm Hg had a 36% risk of progressing from ocular hypertension to glaucoma. The CCT of patient should be taken into account when evaluating a patient's IOP.6 IOP is no longer used as a diagnostic criterion for glaucoma, because the presence of glauco-matous changes can be absent at high IOPs and can be present at lower IOPs.

FIGURE 61-3. Normal fundus of the eye and optic disk and cup. (Reprinted with permission from Lesar TS, Fiscella RG, Edward D. Glaucoma. In: DiPiro JT, Talbert RL, Yee GC, et al., eds. Pharmacotheraphy: A Pathophysiologic Approach, 7th ed. New York: McGraw-Hill, 2008:1553.)

Retinal blood vessds

FIGURE 61-3. Normal fundus of the eye and optic disk and cup. (Reprinted with permission from Lesar TS, Fiscella RG, Edward D. Glaucoma. In: DiPiro JT, Talbert RL, Yee GC, et al., eds. Pharmacotheraphy: A Pathophysiologic Approach, 7th ed. New York: McGraw-Hill, 2008:1553.)

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