What do we measure with various techniques when assessing IOP?

This article reviews briefly three important aspects of tonometry: physics, physiology, and glaucoma. With respect to the physical principles of tonometry, deformation of the globe is a key component: force tonometers measure intraocular pressure (IOP) by deforming the globe whereas pressure tonometers do not. Pressure tonometers can provide measurements that are closer to absolute pressure than force tonometers. Tonometry is a physiological measurement that evaluates IOP over time by assessing pulsation in consideration of the pressure gradient from the cornea to the optic nerve. It seems that the outflow characteristics of the trabecular meshwork also modify the pulse amplitude. When diagnosing glaucoma, tonometry alone does not adequately discriminate between normal and diseased eyes. A long-term measurement may be desirable but this, in turn, evokes new problems. Finally, lowering the IOP improves the blood flow in the optic nerve head.