How is intraocular pressure (IOP) used in the categorization and treatment of primary open-angle glaucoma (POAG)?

Updated: Mar 16, 2020
  • Author: Kristin Schmid Biggerstaff, MD; Chief Editor: Inci Irak Dersu, MD, MPH  more...
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However, IOP is the only clinical risk factor that has been able to be successfully manipulated to date. Categorizing and managing patients based on their IOP and determining when IOP should be treated to prevent optic nerve damage became the forefront issue of glaucoma management for most of the last half of the 20th century.

Several studies over the years have shown that as IOP rises above 21 mm Hg, the percentage of patients developing visual field loss increases rapidly, most notably at pressures higher than 26-30 mm Hg. A patient with an IOP of 28 mm Hg is about 15 times more likely to develop field loss than a patient with a pressure of 22 mm Hg. Therefore, a patient population of those with elevated IOP should not be thought of as homogeneous. Furthermore, before initiating treatment of a patient based on a specific IOP measurement, the following factors should be considered regarding that IOP level obtained:

  • Variability of tonometry measurements per examiner (usually found to be about 10%, or 1-2 mm Hg)

  • Effect corneal thickness has on accuracy of IOP measurements (see Other Tests)

  • Diurnal variation of IOP (often highest in the early morning hours, but maximum IOP can be at any time of day in some patients)

  • In addition, remember that while normal eyes have a diurnal variation of approximately 3-4 mm Hg, glaucomatous eyes have even higher variation (>10 mm Hg). Note: Multiple readings should be taken over time and should be considered with correlative evidence of visual field and optic nerve examination before any diagnosis or therapy is rendered.

A study by Costa et al supports the need to more accurately assess the relationship of 24-hour IOP to 24-hour diastolic perfusion pressure in patients with glaucoma. Future methodology that performs noninvasive, real-time IOP measurements throughout the 24 hours of the day may enable a more complete understanding of the roles that IOP and blood pressure have to the etiology of glaucomatous damage and progression of the disease. [3]

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