What is the role of tonometry in the evaluation 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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Tonometry (see also Other tonometric methods in Other Tests)

IOP varies from hour-to-hour in any individual. The circadian rhythm of IOP usually causes it to rise most in the early hours of the morning; IOP also rises with a supine posture.

When checking IOP in both eyes, the method used (Goldmann applanation is the criterion standard) and the time of the measurement should all be recorded.

Previous tonometry readings, if available, should be reviewed (eg, Is the reading reproducible? What method was used to obtain the reading? What time of the day was it? Where does it fall on the diurnal pressure curve? Do both eyes have similar measurements?).

In obese patients, the possibility of a Valsalva movement causing an increased IOP should be considered when measured in the slit lamp by Goldman applanation. Measurement should be tried via Tono-Pen, Perkins, or pneumotonometer with the patient resting back in the examination chair.

A difference between the 2 eyes of 3 mm Hg or more indicates greater suspicion of glaucoma. An average of 10% difference between individual measurements should be expected. The measurements should be repeated on at least 2-3 occasions before deciding on a treatment plan. The measurement should be completed in the morning and at night to check the diurnal variation, if possible. (A diurnal variation of more than 5-6 mm Hg may be suggestive of increased risk for POAG.) Early POAG is suspected strongly when a steadily increasing IOP is present.

Pachymetry affects applanation tonometry values and, therefore, should be checked on the initial examination (see also Pachymetry and Other tonometric methods in Other Tests).

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