The Role of the Cornea in Managing Glaucoma

In routine clinical practice, intraocular pressure (IOP) represents one of several important parameters used not only in the diagnosis of glaucoma but also for following the progression of this disease and its response to treatment. Certainly, its value as a diagnostic tool hinges upon the reliability of measurements taken. The technique used most commonly for this purpose is Goldmann (Haag-Streit) applanation tonometry (GAT; Figure 71-1). In first describing their applanation tonometer, Goldmann and Schmidt discussed the effect of central corneal thickness (CCT) on IOP as measured by the new device. ¹ They felt that variations in corneal thickness occurred rarely in the absence of corneal disease and assumed a CCT of 520 µm, but acknowledged that, at least theoretically, CCT might influence applanation readings. They started from the hypothesis that the cornea might be considered as a sheath covered by 2 membranes between which almost nonshifting water is located. It has since become apparent that CCT is more variable among clinically normal patients than Goldmann and Schmidt ¹ ever realized. Studies by Von Bahr ^{2, 3} showed that there were large variations in CCT within a normal population, and studies by Ehlers and colleagues ^4-6 demonstrated that this variation in CCT had an effect on applanation-measured IOP. Many studies have since looked at the influence of CCT on IOP measurement with most agreeing that there is an increase in measured IOP with increasing CCT. However, CCT alone accounts for only a small proportion of the interindividual variation in measured IOP. In a manometric study, ⁶ Ehlers and colleagues cannulated 29 otherwise normal eyes undergoing cataract surgery and correlated corneal thickness with errors in GAT. They found that GAT most accurately reflected true intracameral IOP when CCT was 520 µm and that deviations from this value resulted in an over- or underestimation of IOP by as much as 7 mm Hg per 100 µm. Johnson and colleagues ⁷ reported a patient with a CCT of 900 µm with a manometric IOP of 11 mm Hg, but when measured by applanation, the IOP had ranged from 30 to 40 mm Hg while the patient was receiving maximum medical therapy! In a manometric study with the Perkins tonometer (Haag-Streit), Whitacre and colleagues ⁸ demonstrated an underestimation of IOP by as much as 4.9 mm Hg in thin corneas, with thick corneas producing an overestimation by as much as 6.8 mm Hg. This corresponded to a calculated range of 0.18 to 0.49 mm Hg of change in IOP for a 10-µm change in CCT from the mean CCT.

Duke Scholars

Author Leon Walker Herndon Jr. Ophthalmology, Glaucoma