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Introduction: Femtosecond laser-assisted laser in situ keratomileusis (FS-LASIK) surgery is a surgical procedure performed in the treatment of refractive errors. Given the changes in central corneal thickness, intraocular pressure may be lower and underestimated, which may lead to a late diagnosis of ocular hypertension/glaucoma.

Aims: The aim of this study was to evaluate and compare intraocular pressure, by different methods, before and after FS-LASIK, for correction of myopia or myopic astigmatism.

Methods: Retrospective and observational study, which included eyes undergoing FS-LASIK surgery (November 2020 to November 2022). The intraocular pressure values were measured (preoperatively, on the 1^st, 3^rd and 6^th postoperative month) using contact tonometry (Goldmann applanation tonometry) and non-contact tonometry (by Corvis®ST according to the formulas: pachymetry – Ehlers, Shah, Dresden, Spoerl – and biomechanics – corrected and uncorrected). The intraocular pressure values obtained at 6 months with the different methods were compared with the Goldmann applanation tonometry intraocular pressure values obtained at baseline.

Results: Ninety patients (of a total of 174 eyes) underwent FS-LASIK, with a preoperative spherical equivalent of -3.7±1.7. Preoperatively (mean ± standard deviation) Goldmann applanation tonometry was 15.0±1.9 mmHg; the intraocular pressure values obtained through the Corvis® ST (in mmHg) according pachymetry: Ehlers (13.5±2.6), Shah (14.3±2.3), Dresden (14.5±2.2) and Spoerl (14.5±2.1); and biomechanics: corrected (14.4±1.9) and not corrected (15.3±2.1), respectively. At 6 months postoperatively, all showed statistically significant differences, with the exception of Shah's formula (p=0.074); comparing the preoperative Goldmann applanation tonometry with each of the formulas at the end of the follow-up, the Ehlers formula did not present statistically significant differences (p=0.434), the Shah formula a value of p=0.047 and the others a value of p<0.001.

Conclusion: Femtosecond laser-assisted laser in situ keratomileusis surgery underestimates intraocular pressure measurement by contact tonometry. In non-contact tonometry, at 6-month follow-up, the Shah formula appears to be less influenced by this bias. More studies are needed to evaluate the best method to assess I intraocular pressure measurement after FS-LASIK surgery.