The influence of keratorefractive interventions on tonometric indices

Relevance Keratorefractive surgery (LASIK, FS-LASIK, PRK) is widely used to correct refractive errors, but it changes the biomechanics of the cornea, affecting the accuracy of intraocular pressure measurements. Maklakov tonometry is widespread in the Russian Federation, but its reliability after surgery requires clarification [1, 2]. Objective To study the effect of keratorefractive surgeries on tonometric measurements. Material and methods A total of 61 people (121 eyes) were examined, the average age was 33.45 ± 6.15 years. Women accounted for 60.3% (73 eyes), men – 39.7% (48 eyes). The control group and LASIK, FS-LASIK, PRK groups were included. Keratometry (Km), Maklakov tonometry (10 g weight), elastotonometry (5; 7.5 and 15 g), correlation analysis between Km and the diameter of the prints were performed. Statistical analysis: Spearman correlation coefficient, p < 0.05. Results LASIK group: the prints of 5 and 7.5 g weights moderately correlated with Km in the central and paracentral zones (r = 0.3 ... -0.5). The 10 g weight did not show a significant relationship with Km. The 15 g weight had a pronounced inverse correlation with Km in the paracentral points (r = -0.5 ... -0.7), i.e. with an increase in corneal curvature, the print diameter decreased. FS-LASIK group: significant correlation was found only for the 10 g weight, showing an inverse relationship with Km in the paracentral zone (r = -0.4 ... -0.5). The other weights did not show statistically significant correlations. PRK group: weak (r < 0.4) correlations were found between Km and the imprints of the 7.5 and 10 g weights in the central zone (1-2 mm). The 5 and 15 g weights did not show significant correlations. Control group: there was virtually no correlation between Km and the imprints of the 5 and 7.5 g weights. The 10 g weight correlated uniformly with Km at all points (r = -0.38 ... -0.60), indicating a stable effect of corneal curvature on tonometry results. The 15 g weight correlated mainly with the curvature of the horizontal meridian (r = -0.37 ... -0.49). The greatest dependence on Km was shown by the results of tonometry with a 10 g load, the least - with a 5 g load. LASIK caused the most pronounced changes in tonometric parameters, FS-LASIK - minimal. A steeper cornea (high Km) led to inflated tonometry results, a flatter one (low Km) - to underestimated ones. Conclusion The greatest influence of keratometry on Maklakov tonometry was noted for a 10 g load, the least - for 5 g. LASIK causes the most pronounced changes in tonometric parameters, FS-LASIK - minimal. A steeper cornea (high Km) leads to an overestimation of IOP, a flatter one (low Km) - to a decrease. This must be taken into account when interpreting tonometry data in patients after refractive surgery [3]. Key words: tonometry; glaucoma; intraocular pressure; corneal curvature