**1. Introduction**

Glaucoma is an acquired optic neuropathy in which destruction of ganglion cells and fibers leads to irreversible visual field loss. The prevalence of glaucoma, a leading cause of visual impairment and blindness worldwide [1,2], in the general population is about 2%. Increased intraocular pressure (IOP) is a primary risk factor for glaucoma development. IOP evalua‐ tion is used to assess disease control and treatment response, and lowering IOP has resulted in reducing the rates of disease progression over 5 years [3-7]. These data confirmed that ele‐ vated IOP is a pathophysiologic basis for glaucoma; therefore, accurate IOP measurement is critical in glaucoma.

Goldmann applanation tonometry (GAT), the gold standard for measuring IOP, estimates the IOP based on the force needed to flatten the corneal apex to a diameter of 3.06 mm. This area was chosen empirically to offset the surface tension of the tear film, which tends to draw the tonometry tip toward the eye, and the corneal and ocular rigidity, which affect the applanation force needed independent of the IOP level. When applanating this area, a gravi‐ tational force of 0.1 g corresponds to an IOP of 1 mmHg. Goldmann and Schmidt [8] found that when large variations in the central corneal thickness (CCT) occur, the accuracy of the GAT values can be affected.

The corneal rigidity affects the IOP measurements. The corneal biomechanics are more com‐ plex than central pachymetry alone and include viscosity, bioelasticity, hydration, regional pachymetry, and likely other as yet undetermined factors [9,10].

© 2013 Bolivar et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
