What is the role of Goldmann applanation tonometry in the workup of penetrating keratoplasty and glaucoma (PKPG)?

Updated: Dec 30, 2020
  • Author: Shibandri Das, MD; Chief Editor: Inci Irak Dersu, MD, MPH  more...
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When the graft surface is smooth, the epithelium is intact, and the mires are regular, Goldmann applanation tonometry can be used to measure the IOP. Marked corneal astigmatism causes an elliptical fluorescein pattern. To obtain an accurate reading with the Goldmann applanation tonometer, the clinician should rotate the prism so that the red mark on the prism holder is set at the least curved meridian of the cornea (along the negative axis). Alternatively, 2 pressure readings, taken 90 degrees apart, can be averaged. The accuracy of applanation tonometry in general is reduced in certain situations, such as the presence of corneal edema, scars, or bloodstaining or any condition that thickens or alters the cornea. Corneal epithelial edema and stromal edema predispose to inaccurately low readings, and pressure measurements taken over a corneal scar will be falsely high. Thin corneas result in underestimation, and thick corneas result in overestimation.

A meta-analysis comparing the accuracy of various tonometers against the diagnostic standard Goldmann applanation concluded that noncontact tonometers and handheld applanation tonometers appeared to best correlate with Goldmann applanation measurements (66% and 59%, respectively). This is important to keep in mind since measurement of IOP with Goldmann applanation in the early postoperative period is not recommended because the corneal surface is irregular. [33]

The pneumatic tonometer is a better choice for measurement of IOP in a patient with corneal surface irregularities. The pneumatic tonometer has a pressure-sensing device that consists of a gas-filled chamber covered by a Silastic diaphragm. The gas in the chamber escapes through an exhaust vent. As the diaphragm touches the cornea, the gas vent is reduced in size and the pressure in the chamber rises. Because this instrument applanates only a small area of the cornea, it has the advantage of measuring the IOP when corneal surface irregularities such as corneal scars or edema are present or when only a small portion of the cornea is visible (large tarsorrhaphy). In addition, in patients with neurotrophic corneas, this device can be used to measure the IOP with minimal disturbance of the epithelium.

In patients with complete tarsorrhaphy, an attempt must be made to measure the IOP by digital palpation. Measuring the IOP in the normal eye using one of the standard techniques (eg, Goldmann applanation tonometer) is helpful; then digital palpation can be performed on both eyes.

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