The influence of corneal thickness on the diagnosis and management of glaucoma.

Despite the recognition that glaucoma is far more complex than simply “hypertension of the eye,” tonometry remains the method by which glaucoma commonly is detected. Similarly, as pressure-lowering therapies remain our primary treatment, clinicians generally assess therapeutic response with tonometry. Amazingly, despite the numerous and accelerating advances in our ability to monitor the structural and functional status of the optic nerve, we continue to use tonometry technology dating from the 1950s. Over the last decade a gradual reappraisal of tonometry has occurred, with the grudging recognition that the “gold standard” of applanation tonometry is far less accurate and subject to far more artifact than has generally been appreciated. The inaccuracy and artifact caused by altered corneal thickness can lead to misdiagnosis, incorrect treatment, or neglect of glaucoma. In first describing their applanation tonometer, Goldmann and Schmidt discussed the effect of central corneal thickness (CCT) on intraocular pressure (IOP) as measured by the new device. Believing that variations in corneal thickness occurred rarely in the absence of corneal disease, they assumed a CCT of 500 M but acknowledged that at least theoretically, CCT might influence applanation readings. It has since become apparent that CCT is more variable among clinically normal individuals than Goldmann and Schmidt recognized. It is clear that CCT indeed has a positive correlation with IOP as measured by Goldmann applanation tonometry, with the effect on measured IOP clinically significant in some cases. The past decade has seen numerous reports of increased CCT in patients with ocular hypertension (OHT). Argus found that CCT was greater in the patients with OHT compared with both the control and glaucoma patients. In a larger study, Herndon et al found that the CCT of the eyes of patients with OHT was significantly greater than that of eyes of patients with primary openangle glaucoma, or of eyes of normal controls. Bron et al found similar results among patients with OHT, primary open angle glaucoma (POAG), and POAG with concurrent diabetes. Herman et al found that the CCT of ocular hypertensive subjects was significantly greater compared with age-matched normotensive controls. The Ocular Hypertension Treatment Study (OHTS) is a prospective, randomized multicenter clinical trial comparing medical treatment to close observation among patients deemed at moderate risk of developing glaucoma due to ocular hypertension. We recently reported that the average CCT of 1,220 OHTS subjects was some 30 to 40 M thicker than normal values in the literature. Taken together, these and other studies suggest that a considerable subset of patients currently classified as having OHT may simply have thicker than average but otherwise healthy corneas that result in an overestimation of what is likely a normal, true IOP. In the OHTS, depending upon how one chooses to correct IOP for CCT (see below), as many as 52% may have had true IOPs 21 mm Hg at their baseline visit. The opposite situation may occur among some patients classified as having normal tension glaucoma (NTG), with thin corneas leading to an under-estimation of IOP. Morad et al found that patients diagnosed with NTG had thinner corneas than patients with POAG and normal controls. Copt et al measured the CCT among patients classified as having POAG, NTG, and ocular hypertension. In addition to confirming that patients with OHT had thicker corneas than their control and POAG counterparts, they likewise found that patients classified as having NTG had thinner corneas. Might this tonometry artifact extend even to our ability to measure the clinical response to therapy? In some Address correspondence and reprint requests to Dr. James D. Brandt, Department of Ophthalmology, University of California, Davis 4860 Y Street, Suite 2400, Sacramento, CA 95817-2307. Journal of Glaucoma 10(Suppl 1):S65–S67 © 2001 Lippincott Williams & Wilkins, Inc.