Tonometry and intraocular pressure measurement

What it is

Tonometry is the measurement of IOP, the intraocular pressure.

IOP is one of the five vital signs of the eye, and the five are asked as a set before anything else: visual acuity, pupillary reaction, intraocular pressure, EOM movement, and color vision.

The normal range, and why the time of day is written down

The normal range is 10-21 mmHg, with an average of 15 mmHg.

IOP has a diurnal variation, so always record the time of day at which the measurement was taken. A single number with no time against it cannot be compared with the next one.

The methods, best to worst

IOP is commonly measured by:

• Goldmann Applanation Tonometry (GAT): the gold standard, performed using the slit-lamp with a special tip, a prism.
• Tono-Pen: its benefit is portability and the use of disposable probe tips. Use it instead of applanation when the cornea is scarred or asymmetric, because GAT is inaccurate on such a cornea.
• Air puff: non-contact, and the least reliable of them.
• Digital IOP.

A topical anesthetic is used for GAT and for the Tono-Pen, and that is one more reason neither is used on an eye that may be open.