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P. Fillmore, P. Gerding, S. Sayegh; Effect of an Intra–corneal Fluid Interface Following Keratotomy on Intraocular Pressure Measurement by Applanation. . Invest. Ophthalmol. Vis. Sci. 2004;45(13):197.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Laser in situ keratomileusis (LASIK) can alter the measurement of intraocular pressure (IOP) by applanation by thinning of the corneal stroma and by the generation of an intra–corneal fluid interface. The generation of a fluid interface is problematic in patients with an elevated IOP, who are at risk for developing glaucoma, since IOP measurements may be artificially low. As the population of post–LASIK patients ages, accurate measurement of IOP will become increasingly important in order to monitor glaucomatous changes. We present an experimental model that establishes the effect of a fluid interface on IOP measurement. Methods: IOP by applanation tonometry using a Tonopen (Medtronic) was measured in enucleated porcine eyes before keratotomy, following the generation of a corneal cap and following lamellar keratotomy. IOP was maintained by a 22 gauge angiocatheter placed directly into the anterior chamber. A 400 µm corneal cap was cut using the automated corneal shaper (ACS) microkeratome. Following the creation of the corneal cap, a 160 µm lamella of corneal stroma was removed. The 400 µm corneal cap was then repositioned and 0.1 cc BSS was injected under the cap, creating a fluid pocket. In this manner, IOP was measured on both intact and post lamellar keratotomy eyes. IOP was measured by Tonopen, noting both intermediate values of each tap and the final calculated value generated by the internal Tonopen algorithm. Results: The initial IOP measured by Tonopen prior to creation of the corneal caps were 27 and 25 mmHg, dropping to 23 and 21 respectively following the removal of a 160µm corneal cap. The initial measurement of IOP following the creation of an intra–corneal fluid interface was significantly lower than the final IOP result generated by the internal tonopen algorithm (p = 0.0055). Furthermore, the initial IOP measurement also differed significantly from those generated by the sequential taps of the tonopen (p=0.0051 and p=0.0017 respectively for second and third taps vs. initial measurement). Intact eyes yielded consistent values for both the intermediate and the final calculated IOP measurements. Conclusions: Our results show that the presence of a fluid interface beneath the corneal flap following keratotomy resulted in significantly decreased IOP measurements. These results confirm that measurement of IOP through applanation is dependent on both the thickness of the corneal stroma and the physical integrity of the remaining cornea.
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