Glaucoma Case Studies
Corneal Thickness: Does It Really Help Us Manage Patients?
BY E. RANDY CRAVEN, M.D.

Case history/presentation: Here is a classic case of how new information might confuse the discussion between providers. A patient was seen in consultation by me, and it was determined that the early manifestations of glaucoma were present.

Examination: The patient had GAT readings of 19 and 20 mmHg. There were optic nerve changes consistent with glaucoma, and the visual field showed an early arcuate defect. Pachymetry was 480 microns by ultrasound.

Treatment: I felt the "true" pressures probably were higher than the GAT readings. The patient's pressure goal was set at 14 mmHg and treatment was started. A month later I received a call from the eye provider informing me that the pressure was still at 19. We decided to switch to another monotherapy agent and for the patient to return in a month. When the patient returned, again I received a phone call. This time the IOP had gone up to 20. After speaking with the referring doctor, the provider was adjusting the IOP based on the nomogram from one of the journals. The nomogram suggested adjusting the IOP up by 5 mmHg for every 50 microns of thinning. The patient was 70 microns thinner than that nomogram suggested. I was given the adjusted readings of 19 and 20, while the actual GAT readings were 12 and 13! So the drugs had worked, but the situation was confused by the use of adjusted IOP. Should we be adjusting the IOP?

Discussion: Goldmann had assumed the corneal thickness to be 500 microns when he worked on the applanation tonometer. Later, Ehlers found that the average cornea was about 520 microns and that the tonometer readings varied by about 7 mmHg per 100 microns off of the 520. (Ehlers N, Hansen FK. Central corneal thickness in low-tension glaucoma. ACTA Ophthalmol (Copenh). 1974; 52: 740-746.) Whitacre and co-workers found a 1.8 to 4.9 adjustment for the 100 microns. (Whitacre M, Stein R, Hassanein K. The effect of corneal thickness on applanation tonometry. Am J Ophthalmol. 1993; 115: 592-596.)

Since these investigations, numerous studies have demonstrated pachymetry variability based on race and the type of pachymeter used. Doughty and Zaman reviewed the world literature and found with slit-lamp pachymetry the average is 530 microns versus 544 microns with ultrasound. (Doughty M, Zaman M. Human corneal thickness and its impact on intraocular pressure measures: a review and meta-analysis approach. Surv Ophthalmol. 2000; 44:367-408.) Racial differences show that African Americans tend to have a thinner cornea than whites and that Asians tend to have the thickest corneas (African Americans 539, Whites 562, and Asians 572 microns). (Shih C, Graff Zivin J, Trokel S, Tsai J. Clinical significance of central corneal thickness in the management of glaucoma. Arch Ophthalmol. 2004; 1222: 1270-1275.) So how to figure out how thick or thin a patient's cornea might be is not clear. The reference thickness runs between 500 and 600 microns depending on which article you choose for the reference.

On top of the normal corneal thickness variation, we now have a number of patients who have altered corneal thickness from refractive surgery. A few authors have tried to figure out the effects of refractive surgery on tonometry. The trend is that patients who've had refractive surgery do seem to run a lower GAT pressure after the procedure, but the numbers vary with regard to the amount of change from the thinning or flattening. The numbers show that anywhere from 10 to 70 microns of thinning decreases the IOP by 1 mmHg. Because of the increasing number of refractive surgery patients and the concerns about the corneal thickness influence on IOP even for the nonrefractive patient, new tonometers are being tested and released. The PASCAL dynamic contour tonometer and the Ocular Response Tonometer look at different aspects of the eye and use different methods of tonometry to give a better idea as to what the manometric IOP might be for a given patient.

Unfortunately, as we look at what significance there is to the patient when the IOP deviates from manometry, it is not at all clear. Nearly all of our published glaucoma studies use the unadjusted GAT readings. GAT readings do vary between observers. Phelps and Phelps found that GAT variation was > 3 mmHg for 30% of patients in their study. (Phelps C, Phelps G. Measurement of intraocular pressure: a study of its reproducibility. Graefes Arch Klin Exp Ophthalmol 1976; 198: 39-43.) The clinical significance of a given tonometry reading is that the IOP change should be outside of expected variability and that the reading would impact on the diagnosis or treatment of the disease. Thus, we have variation between observers on GAT readings, different average pachymetry readings between races, variation in pachymetry reading with the type of pachymeter, and different nomograms for adjustment of the IOP toward the manometric readings. In the study by Shih and co-workers, the clinical significance of IOP adjustment was said to be >3.0 mmHg. They found 20.2% of the patients had pachymetry readings with enough deviation from the reference corneal thickness used in the study to cause a significant (>3) IOP adjustment. However, only 8.5% or fewer of the patients in their study actually required a change in therapy as a result of adjusting the IOP.

Most of the large studies evaluating the efficacy of pressure-lowering for patients with existing glaucoma, such as the Early Manifest Glaucoma Trial (EMGT), Advanced Glaucoma Intervention Study, and the Collaborative Initial Glaucoma Treatment Study, look at treatment goals and outcomes with unadjusted IOP. That is, the current practice is to use the real numbers from the tonometer and not adjust the IOP. The EMGT did not show a correlation between thin corneas and the rate or likelihood of deterioration. OHTS demonstrated the risk of developing glaucoma with a cornea of less than 555 microns in the presence of IOP over 25.75 mmHg. Drs. Medeiros, Sample and Weinreb have shown in several ocular hypertensive studies that there is an increased likelihood of finding FDT and SWAP defects for patients with thin corneas.

So what does all this mean? Pachymetry definitely has its place in assessing the glaucoma patient or suspect. The suspect is probably the one most served by pachymetry. If the reading is <555 microns, OHTS suggests less likelihood of developing a visual field defect in the presence of increased IOP. It helps both the suspect and glaucoma patient to see if the GAT readings might be high or low as a result of thick or thin corneas. To show clinically significant deviation for a GAT reading, we would need >3 mmHg, or at least a 40 to 50 microns (average of several studies) difference in the cornea from whatever average we are going to use as a reference.

So for the given patient, consider the race and then start to consider the increased impact of the pachymetry with variance of 50 microns thick or thin from the racial average. (See "When to Consider the Impact of the Ultrasonic Pachymetry Readings on GAT by Race," at left.) For the glaucoma patient, there are no indications in the literature that the IOP should be adjusted. For the suspect, the corneal thickness does impact the way we look at the patient: Thick corneas might mean the patient has falsely increased IOP and really have normal pressure, and thin corneas mean the IOP is probably higher and these patients are likely at a greater risk for developing glaucoma.

Dr. Craven practices with Glaucoma Consultants of Colorado in Littleton, Colo.