D5 Tonometry: Measurement of Intraocular Pressure

Topic

The Goldmann applanation tonometer measures the force required to flatten a small area of ​​the cornea to a fixed, predetermined size. If the cornea is treated as a thin membrane that resists flattening solely due to the pressure inside the eye—rather than any inherent corneal rigidity—then that flattening force, divided by the fixed area used by the Goldmann tonometer, corresponds exactly to the intraocular pressure; thus, the Goldmann tonometer can calculate intraocular pressure simply based on the force needed to flatten that specific corneal area.

The air-puff tonometer measures something similar without making direct contact with the cornea: it directs a brief jet of air at the cornea and measures the air pressure or time required to flatten the cornea to that same reference point, rather than measuring the force exerted by an instrument physically touching it. In contrast, the rebound tonometer—marketed, for example, under the name iCare—allows a very lightweight probe to briefly strike the cornea and bounce back; it measures how the probe decelerates and rebounds upon contact, because a cornea subject to higher intraocular pressure offers greater resistance to this contact, slowing the probe differently than a cornea subject to lower intraocular pressure.

The assumption underlying the Goldmann tonometer's calculation of intraocular pressure based on flattening force—namely, that the cornea itself offers no resistance to flattening—is not entirely accurate: the cornea possesses inherent rigidity that depends, among other factors, on the central corneal thickness of the specific eye in question; this corneal rigidity also resists flattening, adding to the resistance caused by the actual intraocular pressure. When a person's central corneal thickness exceeds the thickness assumed during Goldmann applanation tonometer calibration, the additional corneal rigidity resulting from that greater thickness requires more applanation force than the actual intraocular pressure alone would dictate; consequently, the Goldmann tonometer overestimates the person's actual intraocular pressure. Conversely, when a person's central corneal thickness is less than the thickness assumed during calibration, the cornea offers less resistance than expected, requiring less applanation force, and the Goldmann tonometer underestimates the person's actual intraocular pressure.

Therefore, the intraocular pressure measured by the Goldmann tonometer is corrected by factoring in the central corneal thickness—determined via methods such as corneal topography—to arrive at a value closer to the actual intraocular pressure: the measured pressure is adjusted downward for individuals with greater-than-average central corneal thickness and upward for those with less-than-average thickness. This correction is significant because intraocular pressure—once measured and adjusted for central corneal thickness—serves as an approximation of the risk of optic nerve damage (as described by glaucoma models) rather than a perfect measure of that risk; for instance, a person with a thin central cornea could have an actual intraocular pressure higher than the uncorrected Goldmann reading suggests, and without adjusting for corneal thickness, this margin of error could cause their true risk of optic nerve damage to go undetected.

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