Topical glaucoma agent offers real IOP reduction in study

January 15, 2006

Plymouth, England—Latanoprost (Xalatan, Pfizer Ophthalmics), a potent topical ocular antihypertensive agent, does not seem to cause changes in the central corneal thickness after 2 months of follow-up. This result contradicts previous studies that reported a statistically significant decrease in the central corneal thickness, according to George D. Pappas, MD.

The importance of measuring the central corneal thickness in the context of ocular hypertension and glaucoma is well recognized,1,2 according to Dr. Pappas.

"Clinical measurement of IOP involves estimation of the force required to applanate a circular area of axial cornea of 3.06 mm in diameter using Goldmann tonometry.3 It was previously believed that at this diameter, corneal rigidity and the capillary attraction of the tear film exert equal and opposite forces with regard to applanation and therefore could be disregarded. However, we now recognize that a thick cornea, which results from increased rigidity, requires increased force to flatten it, and this leads to an overestimation of the IOP. Conversely, a thin cornea leads to underestimation of the IOP,4 which has attendant consequences in the context of glaucoma management," Dr. Pappas explained. He is a clinical fellow in Vitreoretinal Surgery, Royal Eye Infirmary, Peninsula School of Medicine, Plymouth, England.

Patients who had just received a diagnosis of glaucoma underwent measurement of the central corneal thickness. All corneas were measured using ultrasound pachymetry (Model 885, Carl Zeiss Meditec) immediately before the patients started treatment with latanoprost 0.005% nocte and then 2 months later.

Statistical analysis was performed using the paired-samples t-test.

Fifty-two eyes were included in the study. The mean central corneal thickness before treatment with latanoprost was started was 542.1 μm (standard deviation [SD], 36.04 μm). The mean central corneal thickness after 2 months of treatment with latanoprost was 538.8 μm (SD, 36.87 μm). The difference between the two did not reach statistical significance.

"Latanoprost was not found to cause a change in the central corneal thickness after 2 months of topical treatment. This is in contrast to previous studies, the results of which have suggested that latanoprost does cause a statistically significant reduction in the central corneal thickness.6-8 The reduction in IOP caused by topical latanoprost is therefore a real reduction in IOP rather than an apparent reduction in IOP secondary to a change in the central corneal thickness," Dr. Pappas explained. The study is continuing with the enrollment of more subjects.

References

1. Herndon LW, Choudhri SA, et al. Central corneal thickness in normal, glaucomatous and ocular hypertensive eyes. Arch Ophthalmol 1997;115:1137-1141.

2. Brandt JD, Beiser JA, Gordon MO, Kass MA. Central corneal thickness and measured IOP response to topical ocular hypotensive medication in the Ocular Hypertension Treatment Study. Am J Ophthalmol 2004:138:717-722.

3. Goldmann H. Applanation tonometry. In: Newell F, ed. Glaucoma. Transactions of the Second Conference. New York: Josiah Macy Jr. Foundation, 1957:167-220.

4. Doughty MJ, Zaman ML. Human corneal thickness and its impact on intraocular pressure measures: a review and meta-analysis approach. Surv Ophthalmol 2000;44:367-408.

5. Pappas GD, Achar A, Smith M, Booth AP. The effect of latanoprost on central corneal thickness. ARVO 2005, World Glaucoma Meeting 2005.