In both traditional primary open-angle glaucoma and normalpressure glaucoma, the IOP level is still the most important risk factor for glaucoma damage.174 Furthermore, the Early Manifest Glaucoma Trial showed that for each mmHg lowering of IOP from baseline, there is a 10% decrease in the rate of progression.175 From only a few instruments just a few decades ago, there seems now to be a bewildering array of devices to measure IOP. The Goldmann tonometer has stood the test of time but is beginning to show some traces of gray. By now, everyone should recognize that thin corneas will cause the Goldmann to under read the IOP and a thick cornea will likely cause the Goldmann to over read. Because the relationship is not linear, no current formula can accurately convert Goldmann readings to ‘true’ IOP with any given corneal thickness.

Some newer tonometers have shown independence of corneal thickness and may, therefore, be more accurate in eyes on the thick and thin extremes of corneal thickness. These include the pneumotonometer, the dynamic contour tonometer and the hysteresis non-contact tonometer. However, it is still true that most of our current understanding of the treatment of glaucoma is based on Goldmann readings. Therefore, it may take some time for the newer, perhaps more accurate, devices to work their way into the diagnostic and treatment construct that has served so well for the past 60 years or so.

Other tonometers may be particularly useful in certain situations. For example, the pneumotonometer and the Tono-Pen may be more accurate than the Goldmann when the cornea is irregular or scarred.The pneumotonometer and the non-contact tonometer

may be able to obtain reasonably accurate readings through a soft contact lens. While not very accurate for day-to-day use, the Proview tonometer, because it does not need anesthetic and is user friendly, may be helpful for home tonometry, in obtaining diurnal IOP estimations and in patients who cannot be tested with corneal contact instruments.The non-contact tonometers, pneumotonometers, and Tono-Pen may be useful for screening situations.

Distribution of intraocular pressure in the general population

There have been a number of studies on the distribution of IOP in the normal population (Table 4-1). In a classic study, Leydhecker and co-workers176 performed Schiøtz tonometry on more than 10 000 normal individuals.They found the mean ( SD) IOP to be 15.8 2.6 mmHg.At first glance the pressure readings appeared to be distributed in a normal fashion (also referred to as a Gaussian or bell-shaped

distribution). However, closer inspection of the data revealed that the distribution was not Gaussian, but rather skewed to the right.181,184

This distinction is important because it means we cannot define an upper limit for IOP by adding 2 or 3 standard deviations to the mean. This conclusion is supported by a number of studies, all of which have found a much higher prevalence of elevated IOP (e.g., 20 or 21 mmHg) than would be predicted by Gaussian statistics (Table 4-2).