Scleral Curvature and LASIK

Letters to the Editor Scleral Curvature and LASIK Dear Editor: We read Mirshahi and Kohnen’s article1 with great interest. The authors suggested there may be anterior traction on the posterior pole secondary to the vacuum activation through the application of the suction ring during LASIK, but do not comment on the basic physiopathology and the technique to prevent the anterior movement of the posterior segment during the creation of the corneal flap. We recently postulated on the importance of the scleral curvature as a determinant in the production of vitreoretinal complications after LASIK (Lavaque AJ, Di Marco S, Liggett PE. Retinal detachment after LASIK. The importance of the scleral curvature. Poster presented at: XXVth PanAmerican Meeting of Ophthalmology, March 20, 2005; Santiago, Chile). Refractive surgeons select the suction ring based on the corneal curvature, but commonly, in myopic eyes the corneal curvature does not correlate with the anterior scleral curvature.2 If the radius of the posterior face of the suction ring does not match the curvature of the sclera, the vacuum activation generates a substantial deformation of the globe wall that induces a secondary tractional phenomenon in the vitreous cavity, which could lead to vitreoretinal complications. For that reason, we recommend the measurement of the scleral curvature as an important evaluative tool before surgery in LASIK candidates. Furthermore, we expect more predictable values in the thickness3 and diameter of the corneal flap when avoiding eyewall deformation during the suction ring and microkeratome interaction. Both variables will influence the refraction outcomes positively, thus avoiding residual aberrations and flap complications.4,5 In summary, the scleral curvature measurement will aid the refractive surgeon greatly during the process of selecting the correct suction ring, avoiding unnecessary traction in the vitreous base and anterior displacement of the posterior pole. We also expect an increased predictability in the postoperative refractive outcome. Future trials concerning this matter are needed. ALEJANDRO J. LAVAQUE, MD SALVADOR DI MARCO, MD TAYGAN YILMAZ, MD PETER E. LIGGETT, MD Hamden, Connecticut References 1. Mirshahi A, Kohnen T. Effects of microkeratome suction during LASIK on ocular structures. Ophthalmology 2005;112: 645–9. 2. Barraquer JI. Lamellar keratoplasty. (Special techniques). Ann Ophthalmol 1972;4:437– 69. 3. Prandi B, Baviera J, Morcillo M. Influence of flap thickness on results of laser in situ keratomileusis for myopia. J Refract Surg 2004;20:790 – 6. 4. Zadok D, Carrillo C, Missiroli F, et al. The effect of corneal flap on optical aberrations. Am J Ophthalmol 2004;138: 190 –3.

5. Nakano K, Nakano E, Oliveira M, et al. Intraoperative microkeratome complications in 47,094 laser in situ keratomileusis surgeries. J Refract Surg 2004;20(suppl):723– 6.

Author reply Dear Editor: We appreciate Dr Lavaque et al’s comments on our work. We agree that the microkeratome suction ring design probably plays a role in creation and conduction of traction at the sclera and at the posterior segment structures secondarily. Introduction of the scleral curvature as a parameter to which the suction ring should relate is an interesting concept. However, the validity of this hypothesis must be proven in a study before it can be recommended. Such a comparative study (regular vs. scleral curvature–matched suction ring) should be performed ideally on the same eye in vivo, which seems impossible for ethical reasons. Also, we should consider that we are currently unable to measure the supposed traction to the vitreous body directly. Considering this matter and because of the rareness of vitreoretinal pathology after LASIK, the clinical advantage of the proposed suction ring will be even harder to prove. Unfortunately, in vitro tests may be misleading, as shown by our study. Nevertheless, in summary we encourage a comparative study of the 2 suction ring designs under in vitro conditions. ALIREZA MIRSHAHI, MD THOMAS KOHNEN, MD Frankfurt, Germany

Vision Loss after PDT Dear Editor: In the last few years, several ocular factors have been suggested to explain the variable outcome of photodynamic therapy with verteporfin for treatment of subfoveal choroidal neovascularization secondary to age-related macular degeneration (AMD) (e.g., baseline visual acuity, choroidal neovascularization size and/or composition), but the possible role of thrombophilia in photodynamic therapy effects and side effects has not been considered.1 Acute severe vision decrease (ASVD) has been observed after standardized photodynamic therapy in about 1.0% of predominantly classic choroidal neovascularizations and in 3.0% to 4.5% of occult or minimally classic lesions. Although the possibility of a nonselective chorioretinal nonperfusion has been suggested in a few patients, the chance that ASVD was related to an abnormal occlusive response of the macular vasculature to photodynamic therapy cannot be definitely ruled out.2 In the course of our routine clinical practice, we have observed only one case of ASVD after photodynamic therapy, with poor visual outcome. This adverse event occurred in a 68-year-old white woman suffering from AMD and subfoveal occult choroidal neovascularization with a minimal classic component (Fig 1 [available at http://aaojournal. org]). Fourteen days after photodynamic therapy, fundus examination, fluorescein angiography, and indocyanine

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