How Corneal Thickness Affects IOP Measurements

Jan 6, 2021 | Optometry

How Corneal Thickness Affects IOP Measurements

You and your ophthalmologist rely on an accurate reading of your eyes’ intraocular pressure in order to diagnose and treat glaucoma. But what if those readings are not what they appear to be?

Tonometry, a key aspect of eye care, is the measure of fluid pressure within the eye. Whether by using probes that make physical contact with the eye or by delivering puffs of air, tonometers measure fluid pressure within the eye by determining the resistance of the probe against the cornea. Intraocular pressure, or IOP, that falls outside normal parameters occurs when the eye cannot properly drain its fluid, leading to build-up within the eye and subsequent irreversible damage to the optic nerve. But a reading of such pressure relies on a cornea that also falls within normal parameters—a cornea that is thicker or thinner than average will affect the way the tonometer reports pressure. Here’s a better look at how corneal thickness affects IOP measurements.

Thicker Corneas

A “false positive” on any kind of test can be shocking to a patient, and this certainly includes a test for encroaching blindness. A cornea that is thicker than average will present additional resistance to a tonometer probe, which, in turn, will report higher intraocular pressure than what may be present. Corneal thickness is believed to be a highly heritable trait, and while such a characteristic doesn’t affect a patient’s day-to-day life, family history can be useful information in determining whether a high IOP reading is accurate or due to a false positive.

Thinner Corneas

While thicker corneas may lead ophthalmologists to take additional precautions, patients who have thinner corneas than average may mislead their tonometers in a much more dangerous fashion. Just as thicker corneas can inaccurately report high pressure, the lower resistance of a thinner cornea can misrepresent intraocular pressure in the other direction. This “false negative” reading can be a serious concern, because fluid can build up within the eyes unbeknownst to doctor and patient alike while the tonometer reads intraocular pressure as normal. An unusually low IOP reading in healthy eyes, however, can aid ophthalmologists in correctly deducing corneal thinness.

Diagnosing Corneal Thickness

To account for these variations in how corneal thickness affects IOP measurements, ophthalmologists supplement tonometry with pachymetry, the measure of corneal thickness. In order to correctly diagnose glaucoma and avoid letting at-risk patients slip by, a pachymeter is among the key ophthalmic supplies any office needs. Pachymetry in conjunction with tonometry will give ophthalmologists accurate readings and help them protect their patients’ sight.