IOP: The Corneal Thickness Factor
It can have a profound effect on pressure measurement -- and treatment decisions.
BY SILVIA ORENGO-NANIA, M.D.

Intraocular pressure (IOP) is a fundamental parameter of ocular health and disease. Among other things, it's an important tool for diagnosing and managing different types of glaucoma, as well as for assessing the postoperative course of corneal, lenticular and vitreoretinal surgery.

Given that measuring IOP is so important, using an accurate tool to take the measurement is essential. A variety of IOP measuring devices are available, including the Schiotz tonometer, noncontact tonometer, tonopen, pneumotonometer and applanation tonometer, but the Goldmann applanation tonometer is generally accepted as the gold standard. However, even the Goldmann tonometer's accuracy can be undermined by certain factors.

Compensating for an Imperfect Premise

The problem arises because tonometry is based on the Imbert-Fick Law, which fails to take many real-world factors into account. The Imbert-Fick law states that the pressure inside a liquid-filled sphere can be determined by measuring the force required to flatten the surface of the sphere.

For this to work perfectly in clinical use, however, several things would have to be true:

• The surface of the sphere (i.e., the cornea) would have to be perfectly elastic and infinitely thin.
• No other external forces could be exerted on the device measuring the pressure (in this case, the tonometer tip).
• The force applied to flatten the surface wouldn't displace any volume inside the sphere.

Clearly, none of these assumptions is true when measuring pressure in the human eye. In the first place, the cornea offers resistance to indentation that varies with its curvature and thickness, as well as the presence or absence of epithelial or stromal edema. Second, the tonometer tip contacts the precorneal tear film, which produces capillary attraction between it and the tip. Finally, the volume displaced by applanation causes a rise in IOP, varying in degree with the ocular rigidity of the eye.

Because of these problems with the application of the Imbert-Fick Law, Goldmann made several modifications to the tonometer. He revised the formula used to calculate the pressure: IOP (plus a factor for the modulus of elasticity of corneal deformation) equals the force acting on the tonometer tip divided by the area of contact between the tip and the cornea, minus the attractive force produced by surface tension. As a result of this revision, the applanation tip was designed to have a diameter of 3.06 mm, which also helped to minimize the impact of many of these factors.

However, even with these modifications, several sources of error still affect the accuracy of the results. One of the most important is corneal thickness.

The Thickness Factor

Goldmann assumed that all corneas had a thickness of about 0.55 mm, which has turned out not to be the case. Of course, differences in thickness affect the elasticity of the cornea, which in turn affects tonometry readings.

Several studies have uncovered data that show just how much of an impact this factor can have:

• In 1975, Ehlers et al. took applanation IOP measurements of eyes with various intraocular hydrostatic pressures and evaluated how measured IOP was affected by variations in central corneal thickness. He found that applanation IOP measured higher than the hydrostatic pressure when the cornea was thick, and lower than the hydrostatic pressure when the cornea was thin. A central corneal thickness 70 microns less than (or greater than) 550 microns resulted in a difference of about 5 mm Hg in the IOP measurement.
• Herndon and coworkers measured corneal thickness in 109 patients and found that patients diagnosed with ocular hypertension tended to have thicker corneas than normal subjects; patients diagnosed with low-tension glaucoma tended to have thinner corneas.
• La Rossa et al. were the first to compare the corneal thicknesses of African American patients and Caucasian patients. African Americans' corneas were approximately 30 to 50 microns thinner than Caucasian patients' corneas.
• The Ocular Hypertension Treatment Study revealed similar findings: Ocular hypertensive patients tended to have thicker corneas than normal patients, and African Americans tended to have thinner corneas than non-African Americans.

Revising Diagnosis and Treatment

These findings have significant consequences for patient care. For example, I examined a 40-year-old African American male with a strong family history of glaucoma. He was 20/20 in both eyes with a normal visual field, IOPs close to 20 mm Hg and a cup to disc ratio of 0.8 in both eyes. How-ever, pachymetry revealed a corneal thickness of 460 microns. His IOPs were actually 5 to 7 mm Hg higher than measured, so I initiated treatment immediately.

Similarly, I examined a 60-year-old white female with no family history of glaucoma who was taking a beta-blocker for IOP control. Without the medication her IOP had been measured at 24 mm Hg. But pachymetry revealed a corneal thickness of 600 microns, so her actual IOP was 3 to 5 mm Hg lower than measured. That placed her in the normal range, and I was able to take her off the medication.

The change in IOP measurement that accompanies change in corneal thickness is now also being taken into account following refractive surgery, which can alter central corneal thickness. Chatterjee published a formula that corrects the IOP based on the change in corneal thickness following PRK: Reduction in IOP = 1.6 - (0.4 x treatment mean spherical equivalent in diopters).

If you treat patients for glaucoma or perform refractive surgery, it's important to take corneal thickness into account:

• Measure the corneal thickness of all of your glaucoma patients and suspects; factor this in when determining the target IOP range for each patient.
• If a patient doesn't follow the normal expected course, corneal thickness may provide the reason. For example, if a patient's visual field continues to deteriorate despite a low IOP, pachymetry might reveal that the pressure reading is falsely low, and more aggressive treatment may be necessary.
• When a patient undergoes a refractive procedure, document baseline and postoperative pachymetry and adjust the measured IOP accordingly. (Serial corneal thickness measurements aren't necessary unless a patient has undergone a surgical intervention. To date, there's no evidence that thickness changes significantly with age.)

A Step in the Right Direction

Treating glaucoma patients is an art; there's no perfect IOP that will always prevent damage and no perfect medication that will work for everyone. Nevertheless, to maximize successful treatment, accurate IOP measurement is crucial.

It's now clear that our current methods of IOP measurement in clinical practice need revision. Basing treatment on inaccurate IOP measurement can lead to undesirable outcomes -- and the only way to determine the "true" IOP measurement is to take corneal thickness into account.

Dr. Orengo-Nania is associate professor of ophthalmology at Baylor College of Medicine and the Cullen Eye Institute in Houston, Texas. She also serves as chief of ophthalmology at the Veterans Affairs Medical Center.