How The Goldmann Applanation Tonometer (GAT)

IOP is usually measured in an indirect way by measuring the tension on the surface of the eye, which is extrapolated to estimate the internal pressure. Therefore, methods to measure the IOP are influenced by corneal properties, like thickness and biomechanics. The Goldmann Applanation Tonometer (GAT) remains the most common method and the gold standard for measuring IOP. (Clement et al., 2016).

Principle of GAT:
The GAT depends on the Imbert-Fick principle, which states that the pressure inside a sphere is equal to the force needed to flatten its surface divided by the area of flattening, (P=F/A), where P=pressure, F=force, and A=area.
The GAT measures the force needed to flatten an area of the cornea of 3.06 mm in diameter. At this diameter, the resistance of the cornea to flattening is counterbalance by the capillary attraction
The prism is advanced until it is just touching the surface of the cornea. Two semicircular mires are seen, which represent the contact between the tear film and the outer surface of the prism. The dial of the tonometer is adjusted until the inner edges of the mires are aligned together. At this point, the reading of the tonometer, multiplied by 10, is equal to the IOP. (Kanski & Bowling,
It showed that the normal adult CCT 535 µm +/-11.6% i.e. 473-597 µm. A statistically significant correlation between CCT and IOP was found, a 10% difference in CCT results in 3.4 +/- 0.9 mmHg difference in IOP. This observation was less evident in normal eyes (1.1 +/- 0.6 mm Hg for a 10% difference in CCT), but stronger in eyes with chronic diseases (2.5 +/- 1.1 mm Hg for a 10% difference in CCT), and much higher in eyes with acute diseases (10.0 +/- 3.1 mm Hg for a 10% difference in CCT). (Doughty & Zaman,