IOP is one of the most reliable indicators for assessing glaucoma risk and the need for intervention. However, there is no uniform standard for accurately determining IOP after corneal refractive surgery due to corneal ablation. In this study, we used NCT and two common techniques to estimate pre- and postoperative IOP in patients undergoing SMILE and FS-LASIK surgery and compared the measurements using different correction formulas. With the preoperative IOP as the standard reference, we found that IOPs determined no significant change after surgery at 1 month in patients receiving SMILE surgery, while IOPd, IOPo, and IOPk showed no change in patients undergoing FS-LASIK surgery.
Devices commonly used to estimate IOP, such as the NCT, Goldmann applanation tonometer (GAT), and ocular response analyzer (ORA), tend to underestimate post-operative IOP values after corneal refractive surgery. For instance, it has been found that the IOP value measured by NCT after LASIK surgery decreased by (0.029 ± 0.003) mmHg for every 1 μm ablation of the central cornea. Thus, an average ablation of 15 μm per diopter of correction resulted in an IOP change of approximately 0.5 mmHg. A study of 47 patients undergoing LASIK surgery found that the NCT-measured IOP decreased by (5.65±1.71) mmHg after surgery compared to preoperative values. In another study, the IOP measured by NCT in 93 patients undergoing LASIK surgery decreased by (5.41 ± 1.89) mmHg one month after surgery and (5.73 ± 2.03) mmHg three months after surgery. A substantial decrease in GAT-derived IOP has also been described for patients undergoing FS-LASIK surgery. The same trend has been reported after SMILE surgery as well. A study measuring IOP in 60 patients undergoing SMILE surgery showed that the preoperative NCT measurement decreased by (3.91±1.97) mmHg and the GAT-measured IOP value decreased by (5.51 ± 2.42) mmHg three months after surgery. These differences between pre- and postoperative IOP measurements can be attributed to intraoperative changes in corneal thickness, curvature, and biomechanics parameters, all of which significantly affect GAT and NCT measurements.
ORA has emerged as a new type of NCT that may be less sensitive than GAT or NCT to changes in corneal biomechanical properties after LASIK surgery. During ORA examination, a controlled air pulse is puffed onto the eye, and two IOP values are recorded. The mean value of the two IOP measurements is then used to simulate Goldmann IOP, while the calculated difference between the two IOP values is used to determine corneal hysteresis, corneal resistance factor, and corneal-compensated IOP. Although ORA provides more complete biomechanical data than GAT, one study found that ORA-derived IOP after FS-LASIK surgery was 0.67 ± 2.07 mmHg lower than the preoperative value. In another study, ORA indicated a corneal-compensated IOP at three months after SMILE that was 2.51 ± 2.35 mmHg lower than the preoperative value. These two studies suggest that biomechanical-corrected IOP by ORA may still be inaccurate for measuring IOP after corneal refractive surgery.
Clinical studies and experiments in animal models have shown that corneal refractive surgery does not lead to intraocular hypotension, suggesting that the reduced IOP measurements recorded after surgery need to be corrected. Pentacam is a common method to correct IOP, which takes corneal thickness into account. However, many correction formulas have been proposed and the appropriate choice of formulas is controversial. One study found the Shah correction formula is an appropriate way to correct postoperative IOP after LASIK surgery. Nevertheless, two studies in which the Ehlers correction formula was used to correct IOP in 105 patients undergoing LASIK or 62 patients undergoing epithelial LASIK surgery showed that the difference between pre- and postoperative IOPe was not significant, indicating that the Ehlers correction formula can be used to correct IOP after either type of LASIK. Similar results were obtained in another study comparing Pentacam-corrected IOP and IOP measured by dynamic contour tonometry before and after LASIK.
In this study, we found that different correction formulas may be chosen in FS-LASIK and SMILE surgery groups. The difference between pre- and postoperative IOPs was insignificant, suggesting that Pentacam’s Shah formula is the most appropriate method for accurately calculating IOP after SMILE surgery. Besides, we found that the Dresden, Orssengo-Pye, and Kohlhaas formulas all generated similar IOP measurements compared to preoperative values in patients undergoing FS-LASIK. However, we did not identify the Ehlers formula as an appropriate method for correcting IOP values after FS-LASIK. The difference in the correction formula may be associated with the different corneal ablation methods, in which FS-LASIK creates a corneal flap with a femtosecond laser while SMILE extracts an intrastromal lenticule by femtosecond laser.
The various Pentacam correction formulas gave considerably different IOP values even when the formulas used a common estimated corneal thickness. This reflects the different ways that the correction algorithms account for the effects of corneal thickness on IOP. Except for the Kohlhaas formula, which takes into account both the thickness and the curvature of the anterior surface of the cornea, the other four correction formulas of Pentacam include only the corneal thickness and show the same trend in IOP evaluation: as the thickness increases, the corrected IOP value decreases. A previous study defined the standard corneal thickness as 550 μm for the Dresden formula and 545 μm for the Ehlers formula, yet when the corrected IOP value changed by 1 mmHg, the corneal thickness corrected by the Dresden formula changed by 25 μm, whereas the Ehlers formula changed by 15 μm. Thus, correction algorithms may require substantially different adjustments in corneal thickness in order to correct the same IOP fluctuation.
Corvis ST has recently emerged as a more convenient and faster method than Pentacam for examining corneal biomechanics and measuring IOP. However, only a few studies have compared IOP values before and after corneal refractive surgery, reporting that IOP obtained using Corvis ST is more reliable than that measured by conventional NCT. In a study of myopic patients, Corvis-derived IOP was found to be in good agreement with the GAT value. However, in another study, Corvis ST was used to detect IOP of ex vivo human globes, which is manually changed by adjusting the internal inflation rig. IOP corrected by Corvis ST was found to be correlated with corneal thickness and had a large mean difference of (7.5±3.2) mmHg compared to true IOP measured by the internal sensor.
In our study, we found that preoperative Corvis-derived IOP differed significantly from the corresponding postoperative values after both SMILE and FS-LASIK surgery, suggesting that Corvis ST is unsuitable for measuring IOP after corneal refractive surgery. In addition, the repeatability of Corvis ST measurements was poor in our study. Nevertheless, the average differences between pre- and postoperative Corvis-derived IOP were 1.09 ± 1.95 for SMILE and 1.52 ± 2.01 for FS-LASIK, which were significantly lower than the corresponding differences with NCT.
In this study, we compared the IOP 1 month after surgery with the preoperative measurements for the evaluation of different IOP-measuring techniques and correction formulas. As the IOP after corneal refractive surgery tends to fluctuate during the early postoperative period due to surgical procedures and frequent corticosteroid use, which becomes stable after 1 month, we chose 1 month as the time point. With no gold standard of IOP measurements after corneal refractive surgery available, we chose the preoperative IOP values as the reference and assumed them to be unchanged 1 month after surgery. However, during this recovery period, patients were routinely treated with tobramycin/dexamethasone eye drops for three days and loteprednol eye drops for 30 days to prevent postoperative inflammation and promote wound healing. Since these formulations can increase IOP, brimonidine tartrate eye drops were also prescribed to keep the pressure under control, which may all contribute to IOP change after surgery. So future studies should measure it at three and six months after surgery, perhaps even longer, when patients are free of topical drugs. Another limitation of our study is that IOP was not always measured at the same time of the day, which may have affected our results since IOP can change slightly within 24 h.
In summary, our study shows that the Shah correction formula of Pentacam is the most appropriate method for correcting postoperative IOP in patients undergoing SMILE surgery. In contrast, the Dresden, Orssengo-Pye, and Kohlhaas formulas are identified as the most suitable methods for obtaining reliable IOP values after FS-LASIK surgery. Our study may help guide the selection of appropriate IOP correction formulas after corneal refractive surgery.