Tonometry

Having replaced Schiotz indentation tonometry many years ago, Goldmann applanation tonometry is considered the ‘gold standard’ in measurement of intraocular pressure. Goldmann readings are, however, influenced by factors including corneal thickness and curvature. These factors are discussed in detail below.

The Pascal Dynamic Contour Tonometer (DCT) is a newer IOP measurement device that incorporates a flexible measurement tip to evenly distribute applanation across the corneal surface. This negates the effect of differences in corneal thickness. The DCT offers the potential for obtaining true IOPs for eyes in which there has been significant corneal disruption, e.g., eyes that have had LASIK.

The Proview™ tonometer (Bausch & Lomb) is a spring-based, non-electrical instrument that allows patients to monitor their own intraocular pressure at home. It uses a phosphene-based method of feedback in which a patient presses the instrument tip against upper nasal eyelid until a glowing pressure phosphene is perceived in the lower temporal field of view. The pressure against the lid at that point corresponds to IOP. IOPs obtained with the Proview have recently been found to be significantly lower than those measured with Goldmann and thus may overestimate the effects of pressure-lowering drugs.

Ocular pressures between 10 and 21mmHg are considered ‘statistically normal’ with the mean value being around 16 mmHg. However at least one in six people with POAG never have pressures above 21 mmHg. Thus, there is no ‘normal’ ocular pressure; rather IOP is merely a risk factor for glaucoma. The higher the IOP, the greater the risk of glaucoma.

Ocular pressures tend to follow a diurnal curve, being higher in the early morning and lower in the evening. Patients with POAG may have wider diurnal curve than those without glaucoma. Therefore, serial tonometry, in which tonometry measurements are taken at several different times on the same day, may be useful in determining the range of ocular pressures.

When measuring the IOP, care should be taken not to apply pressure to the eye (e.g., when lifting the upper eyelid) in order to not artificially raise the pressure reading.

In general, IOP tends to increase with age by an average of about 1.2 mmHg per decade of life.

Pachymetry

Pachymetry typically involves use of either an ultrasound or an optical probe to assess corneal thickness. It has recently become a very popular and useful procedure because of the need to adjust Goldmann IOP measures to compensate for differences in corneal thickness between patients.

Use of Pachymetry to Adjust Goldmann IOP Readings

The original calculations used to determine IOP using Goldmann applanation tonometry were based on an assumed corneal thickness of 520 microns. Central corneal thicknesses of greater than 520 microns would cause the true IOP to be overestimated (i.e., the Goldmann readings would be too high) and thicknesses of less than 520 microns would cause the IOP to be underestimated (i.e., the Goldmann readings would be too low). Although these relationships were well known, few doctors considered them when using Goldmann readings to diagnose or follow glaucoma.

Several years ago, a randomized, multi-center clinical trial called the Ocular Hypertensive Treatment Study (OHTS) was conducted to evaluate progression to glaucoma in a group of ocular hypertensives with IOPs greater than 24 mmHg and no signs of glaucoma (optic nerve cupping, visual field loss, etc.). Half the subjects were given hypotensive medication, and the other half received none. After five years, 9.5% of non-treated subjects progressed to POAG, but only 4.4% of treated subjects progressed. The study demonstrated that prophylactic treatment of ocular hypertensive patients can decrease the risk of glaucoma development.

An additional goal of the OHTS was to assess a number of risk factors beyond ocular hypertension for developing glaucoma. Central corneal thickness (CCT), as measured by pachymetry, was one of the factors considered. CCT was found to be a powerful predictor for progression to POAG in the study patients; the thinner the cornea, the greater the risk of developing glaucoma.

As group, Blacks had a significantly thinner corneas than did the Caucasians in the study, and this was associated with an increased risk of converting to glaucoma.(9) Similar to the thinner corneas found in the Black ocular hypertensives, Blacks in general have significantly thinner central corneas than do Caucasians. They also have higher rates of glaucoma and glaucoma-related blindness.

It has been suggested that the relationship between corneal thickness and glaucoma is associated with the fact that IOP measurements of patients with thin corneas underestimate true IOP because of the applanation procedure used by the Goldmann. This might mean that thin cornea patients would not be diagnosed as readily or treated as vigorously if Goldmann IOP was the only criterion used.

(As a group, the ocular hypertensive OHTS subjects were found to have thicker corneas than normals and POAG patients, which suggests that perhaps they were grouped in this category because of an IOP overestimation.)

Subsequent to the OHTS, several well-designed studies have suggested that corneal thickness is also a predictor of field loss in patients with glaucoma.

Not using a CCT adjustment may mean that patients (e.g., Blacks) with thin corneas might have their true IOPs underestimated, which could lead to a delay in diagnosis, inadequate treatment, and a higher risk of blindness. Patients who have undergone LASIK or other corneal-thinning surgeries may also present with artifactually low IOPs. Conversely, patients with corneal edema, Fuch’s dystrophy, or other cornea-thickening conditions may have artifactually high IOPs. It should be noted that corneal thickness does not affect the true IOP itself, but only affects the Goldmann measured IOP.

A number of adjustment tables, such as the one shown in Table 1, which is based on the average CCT of about 550 microns for Caucasians, have been proposed. Different correction tables for use with races (e.g,. Blacks) having different mean CCTs might be needed.

Table 1. Goldmann IOP correction values (mmHg) for central corneal thickness (CCT). Add or subtract values shown from measured IOP. This table assumes a mean CCT of 545 microns. (13-15)