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Background: Laser in situ keratomileusis (LASIK) leads to inaccurate measurement of intraocular pressure (IOP). This study aimed to determine the efficacy and reliability of IOP measurement on the peripheral cornea after LASIK.
Methods: The IOP was measured in 52 eyes of 26 subjects before LASIK and one week and one month after LASIK using non-contact tonometry. The measurements were made on the central and peripheral cornea and the results were subjected to statistical analysis.
Results: The mean measured IOP on the central cornea was decreased by 6.425 ± 2.544 mmHg at one week and 5.752 ± 3.863 mmHg at one month post-operatively. The mean IOP measured on the peripheral cornea was decreased by only 0.921 ± 2.054 mmHg at one week post-operatively. Most notably, the mean IOP measured on the peripheral cornea was increased by only 0.158 ± 2.979 mmHg at one month post-operatively with no statistical significance (p > 0.05). Furthermore, a linear regression of ablation depth versus change in IOP measured on the central cornea was demonstrated, but the peripheral IOP did not display such a statistically significant correlation with the depth of ablation at both one week and one month after operation.
Conclusion: The IOP measured on the peripheral cornea is closer to the actual IOP. Therefore, it is more accurate and reliable to measure IOP on the peripheral cornea than on the centre after LASIK.
Laser in situ keratomileusis (LASIK) is one of the most important surgical options for treatment of myopia. The ablation of corneal tissue inherently causes changes in corneal thickness, corneal curvature and corneal rigidity, which leads to inaccurate measurement of the intraocular pressure (IOP).1 Many studies have shown that LASIK causes a lowering of IOP as measured using both Goldmann applanation and non-contact tonometers by an amount dependent on the amount of tissue removed, the change of corneal curvature and corneal rigidity.2 Unfortunately, no reliable correction formula for IOP measurement after LASIK has been established based on previous studies. The proposed equations for correcting the measured IOP by the ablation depth or corrected refractive error do not fit well with clinical phenomena.3
As the peripheral cornea is outside the ablation zone and is less disturbed after LASIK,4 we assumed that measuring IOP on the peripheral cornea (inferior cornea, between the ablation zone and corneoscleral limbus, beyond the edge flap by 1.5 mm but still inside the limbus, Figure 1) using non-contact tonometry would be less affected by LASIK. To test our hypothesis we compared the IOP measured on the periphery versus the central cornea and report our results.
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Many studies have shown that refractive surgery will lead to an inevitable decrease in IOP after surgery. An excimer laser for myopia alters corneal shape, structure, thickness and curvature, especially on the central cornea, inducing inaccuracies in IOP measurement.5 It is important to note that the measurement only records a change of measured values but not a real change in IOP.6 Unfortunately, there is no precise and universal formula for correcting the post-operative IOP in clinics. The aim of the present study was to evaluate the post-operative IOP more accurately by measuring an alternative position on the cornea.
Many factors affect the post-LASIK IOP measurement, including corneal thickness, ablated corneal depth, changes in the corneal curvature, and thickness of the corneal epithelium.7–10 For example, a decrease of 4.8 mmHg in IOP was found for every 100 µm of central cornea removed by LASIK.11 Faucher, Grégoire and Blondeau12 showed a 2.4 mmHg reduction in IOP for every 73.4 µm decrease in corneal thickness. In the present study we found that the central IOP was decreased significantly after surgery, which correlated with the ablation depth. Corresponding to every 100 µm decrease in ablation depth, a 4.5 mmHg decrease in IOP was observed at one week and 7.8 mmHg decrease in IOP at one month after LASIK, consistent with the previous reports.11,12
While Zadok and colleagues13 reported no significant difference between central and peripheral IOP measured using pneumotonometry, Park, Uhm and Hong14 found that the nasal IOP measurement was 1.8 mmHg higher than the central IOP. Schipper and colleagues4 proposed that measurements in IOP over the temporal part of the cornea are more reliable than the central cornea. In the present study, we found that the peripheral IOP was higher than the central IOP measurement and the difference between them was more significant after LASIK. It is not surprising that the peripheral IOP is greater than the central, as the cornea is thicker in the periphery than in the centre.15 In the present study, we found that the IOP measured in the periphery at one month after surgery was not significantly different from that before the operation; however, central IOP measured at one month after surgery was significantly reduced compared with that before the operation. These results indicate that the increased difference is due to the reduction in IOP in the centre but is unrelated to the true IOP change in the periphery.
We also measured peripheral IOP before and after LASIK and used the difference between the two readings to evaluate the real changes in IOP. Although we observed a slight change in the IOP measured in the peripheral cornea, the change was much lower compared with the change in IOP at the central cornea. Most notably, we found that while a significant decline in IOP measured on the peripheral cornea was observed one week post-operatively, no statistically significant difference in IOP was observed at one month. This might be explained as a response to the healing process of the corneal flap and the use of corticosteroids. At the early healing stage, corneal biomechanical properties, such as cornea structure, thickness, curvature, distribution of collagen fibrils and the deposition of newly synthesised collagen material, might be altered and these will reduce corneal rigidity and cause inaccurate IOP measurement.16 The use of a steroid might increase peripheral IOP; however, the change in IOP is a complex process and there might be a self-regulating mechanism to control IOP. Both corneal hysteresis (which represents the visco-elastic response of the cornea) and the corneal resistance factor (an indicator of overall resistance of the cornea) are significantly lower after refractive surgery.17 These changes tend to recover one month post-operatively, which contributes to the insignificant difference in peripheral IOP measured pre-operatively and post-operatively. To evaluate the reliability of IOP measured on the peripheral cornea, we compared the change in IOP and the post-operative IOP with the ablation depth for both the central and peripheral cornea. Although linear correlations were demonstrated for IOP measured on the central cornea, no correlation was found between the ablation depth and the peripheral IOP, which provides further evidence that the peripheral IOP is less affected by refractive surgery and is more reliable than the IOP measured on the central cornea.
The Pascal dynamic contour tonometer has been proposed as a novel tonometer designed to measure IOP independent of central corneal thickness.18 The Pascal tonometer is a reliable device for the measurement of IOP particularly after corneal refractive surgery;19 however, to conduct the measurement, this tonometer requires a round tear film contour. Although the Pascal tonometer is very useful in healthy eyes and in patients with ocular dryness, which frequently occurs after LASIK, it exhibits low reliability, which might be clinically significant.20 In contrast, in the present study, we adopted the non-contact tonometer measurement, thus avoiding the disadvantages of contact tonometry.
In addition, although Pascal tonometry is not affected by central corneal thickness, differences in IOP measurement have been reported between the Pascal and Goldmann tonometers and this was due to corneal stiffness.21 It is known that the changes in corneal stiffness and biomechanical properties are more significant in the centre after LASIK. In contrast to the IOP measured on the central cornea using Pascal tonometry, our method of measuring on the peripheral cornea could minimise the effects due to changes in corneal stiffness and biomechanical properties and thus be more accurate. Although our measurement method has several advantages compared with the methods based on the central cornea, future studies that recruit a large number of subjects and involve long-term follow up are necessary to confirm that the method of measuring IOP on the peripheral cornea is reliable and accurate.