How is intraocular pressure (IOP) measured in the evaluation of a glaucoma suspect?

Updated: Jul 27, 2020
  • Author: Robert H Graham, MD; Chief Editor: Hampton Roy, Sr, MD  more...
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Elevated IOP is a definite and important risk factor for developing glaucomatous damage but is not sufficient for a diagnosis. [20, 21, 14, 22] The prevalence of POAG is higher with increasing IOP. One tenth of patients with ocular hypertension develop field loss within 10 years. Each year, about 1% of all individuals with increased IOP progress to glaucomatous damage. As many as 50% of patients with glaucomatous optic neuropathy or visual field changes have IOP of less than 21 mm Hg on initial evaluation. Some eyes undergo damage at IOP of less than 18 mm Hg; others tolerate IOP of more than 30 mm Hg.

A pressure of 10-21 mm Hg is considered normal; a nongaussian distribution occurs with a skew toward higher pressures.

The diurnal variation is as follows:

  • 2-6 mm Hg - Normal

  • Greater than 10 mm Hg - Variation suggestive of glaucoma

Peak usually occurs in the morning hours.

Goldmann-type applanation tonometry is the criterion standard for IOP measurement. In patients who are obese, handheld tonometry may be more accurate by minimizing strain to fit to the slit lamp.

Landers et al proposed an ibopamine challenge test to differentiate patients who are glaucoma suspect from those who have either stable glaucoma or progressive glaucoma. [23]

Common pitfalls in IOP measurement are as follows:

  • Too much/too little fluorescence occurs

  • Meridians are not averaged in patients with high astigmatism

  • The upper lid is not held gently

  • Patients should breathe normally

  • Unless dangerously elevated, IOP is checked numerous times before initiating treatment to assess diurnal variation

  • Abnormally thick corneas may result in artificially high IOP measurements by applanation tonometry, while abnormally thin corneas may result in artificially low IOP measurements

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