Measurement of intraocular pressure during corneal flap preparation.

To the Editor: We read with interest the study by Vetter et al, “Intraocular Pressure During Corneal Flap Preparation: Comparison Among Four Femtosecond Lasers in Porcine Eyes,” which appeared in the June 2011 issue of Journal of Refractive Surgery.1 Vetter et al measured the changes in intraocular pressure (IOP) during lamellar fl ap creation using four different femtosecond lasers. We also analyzed IOP changes that occur during both external eye compression and fl ap creation in an animal model,2-4 and we would like to address the method used by Vetter et al in their study. Vetter et al measured IOP in the vitreous cavity using a needle inserted through the optic nerve. However, as found in previous studies of our own, we believe the vitreous cavity may not be the most appropriate place to measure IOP when a cannulation method is used. In our fi rst study,2 a signifi cant difference was found in IOP measured in the anterior chamber versus the vitreous cavity. Furthermore, we found that IOP measured within the vitreous cavity during the external IOP-increase induction was close to the IOP measured in the anterior chamber during the fi rst seconds of the procedure. Afterwards, IOP in the vitreous cavity reached a “plateau,” while IOP in the anterior chamber continued to rise. As the cannulation method of measuring IOP depends on the liquid fl ow through the cannula and tubes, the high viscous nature of the vitreous gel may occlude or decrease the fl ow within the lumen of the cannula placed in the vitreous cavity. This fi nding may be one reason for these differences in IOP measured between the anterior chamber and vitreous cavity. We believe we have provided evidence to suggest that the anterior chamber is a better place to monitor the changes in IOP due to surgical maneuvers. Intraocular pressure recorded in the vitreous cavity (with a cannula) is lower, and thus less sensitive to changes in IOP. Therefore, we do not believe that using the vitreous cavity for cannulation is the best way to detect differences in intraoperative IOP changes among several surgical devices. José L. Hernández-Verdejo, OD, PhD Miguel A. Teus, MD, PhD Madrid, Spain