The epithelial laser in situ keratomileusis (epi-LASIK) refractive procedure is performed to correct low to moderate myopia.1 During the procedure, an epithelial flap is created by separating the basal epithelium and the basement membrane with a mechanical keratome; this creates a cleavage plane between the lamina lucida and the lamina densa.2,3
In most corneal refractive surgical techniques in which a flap is created, a suction ring is used to stabilize the eye and allow the keratome pass. When vacuum is created within the suction ring, the IOP increases to more than 90 mm Hg.4,5 The ocular shape is modified by the shape of the suction ring; as a result, the sclera is indented, which causes the IOP to increase. Several animal6 and human7 studies have used direct manometry to measure IOP increases during flap creation in corneal refractive surgery.
Although refractive procedures are generally safe and effective, the sudden intraoperative increase in IOP is associated with several potential ocular complications. These include reduced retinochoroidal circulation, macular hemorrhage, macular holes, and optic ischemic neuropathy.8–13
Some authors14 suggest that the increase in IOP when suction is applied to the eye and during the excimer laser “shock” in ablation causes vitreoretinal changes after laser in situ keratomileusis (LASIK). These changes are relevant in myopic eyes, in which the majority of refractive procedures are performed.
The variability in the IOP increase in studies in which the flap was created with a microkeratome seems to depend on factors such as the design of the suction ring,5,7 the vacuum pressure used for suctioning,4 or the part of the eye used for cannulation and registration of IOP.6
To our knowledge, no studies of real-time IOP changes during epi-LASIK have been published. The aim of the current study was to measure these IOP changes using an external manometer connected to the anterior chamber and to compare the changes with those induced during conventional LASIK in an animal model.
MATERIALS AND METHODS
This prospective study comprised freshly enucleated porcine eyes. The eyes were divided into 2 groups. In the epi-LASIK group, the flap was created with an Epi-K epikeratome and in the LASIK group, with an M2 mechanical microkeratome (both Moria). All eyes were free of corneal damage on slitlamp microscopy.
The eyes were pressurized with a One Use-Plus power unit (Moria), which has 2 vacuum settings, high (600 mm Hg) and low (250 mm Hg). In both groups, surgery was performed using high vacuum. However, given the long duration of epi-LASIK, the manufacturer advises using the low-vacuum setting when the cutting phase is finished and the epikeratome head is moving backward. Therefore, in the epi-LASIK group, the low-vacuum setting was used during backward movement of the epikeratome head.
The eyes were inflated with a 5% glycosylated solution through the optic nerve using a 27-gauge needle, as described by Kasetsuwan et al.,4 to obtain an IOP of 10 to 15 mm Hg. The IOP was verified with a Perkins applanation tonometer (Clement Clarke). The eyes were placed on a stand for support during surgery. To prevent modifying the IOP, care was taken to avoid exerting pressure on the globe as the eye was manipulated. A 21-gauge, winged infusion set (Valu-Set, BD Biosciences) was inserted through the limbus to position the suction ring over the sclera for subsequent IOP measurement in the anterior chamber (Figure 1); care was taken not to touch the needle. The insertion site was sealed with a drop of epoxy glue to prevent leakage.
Intraocular pressure was measured with a reusable blood-pressure transducer (Sensor Presmeter, Cibertec, S.A.) connected to the anterior chamber. This device is an external sensor that measures pressure via a liquid-filled catheter. A saline-filled silicone tube attached to the catheter was connected to the transducer. The transducer was prepared according to the manufacturer's instructions to ensure a tight seal and that all air was flushed from the system. The recorder was set to 0 to initialize the transducer. Before the procedure began, the transducer was checked for the correct pressure measurement. For calibration, the transducer was connected to a mercury-calibrated column, after which it was confirmed that the pressure in the mercury column and the display connected to the transducer were the same.
The suction ring was applied and a flap created in the cornea in both groups. The same experienced surgeon (L.D.B.) performed all procedures on the same day under direct microscopy visualization. During the procedure, the IOP was recorded continuously from the time of the application of the suction ring until the end of the microkeratome pass with an amplifier connected to the barometric transducer. The IOP was also measured with a handheld Perkins tonometer before and after the suction ring was placed. The IOP level after the procedure had to be at least 10 mm Hg to rule out substantial fluid leakage from the eye.
Statistical analysis was performed using the Student t test and the nonparametric Wilcoxon signed rank test. A P value less than 0.05 was considered statistically significant.
Thirty-four freshly enucleated porcine eyes were evaluated. The epi-LASIK group and the LASIK group each comprised 17 eyes. Table 1 shows the results in the epi-LASIK group, and Table 2 the results in the LASIK group.
The mean IOP during suctioning was 92.57 mm Hg ± 20.86 (SD) in the epi-LASIK group and 113.65 ± 10.78 mm Hg in the LASIK group and during flap creation, 82.09 ± 20 mm Hg and 112.35 ± 11.51 mm Hg, respectively; the differences between groups were statistically significant (P = .0008 and P = .0001, respectively). In the epi-LASIK group, the mean IOP during the low-vacuum phase was 67.28 ± 13.49 mm Hg. The mean IOP during the entire procedure was 80.30 ± 17.13 in the epi-LASIK group and 113.00 ± 10.19 in the LASIK group
The actual IOP immediately before suctioning was 12.47 ± 2.09 mm Hg in the epi-LASIK group and 12.53 ± 1.77 mm Hg in the LASIK group (P = .8). The IOP recorded by the transducer immediately after the procedure was 11.76 ± 1.60 mm Hg and 11.12 ± 1.45 mm Hg, respectively (P = .2).
The mean duration of the procedure was 89.41 ± 3.48 seconds in the epi-LASIK group and 15.06 ± 1.95 seconds in the LASIK group. The difference between groups was statistically significant (P = .0001). The mean suction time was 25.88 ± 1.96 seconds in the epi-LASIK group and 9.00 ± 1.46 seconds in the LASIK group and the mean cutting time, 33.82 ± 2.81 seconds and 6.06 ± 1.14 seconds, respectively. In the epi-LASIK group, the mean low-vacuum time was 29.71 ± 3.29 seconds.
Studies report measuring real IOP during LASIK using a mercury manometer placed in the vitreous cavity5 or an external manometer connected to a cannula placed in the anterior chamber6 or vitreous cavity.7 With external manometers, IOP values recorded during LASIK are different when the intraocular cannula is placed in the vitreous cavity than when it is placed in the anterior chamber; this may be because IOP is transmitted in a different way in a fluid-filled tube depending on the viscosity of the intraocular fluid. To avoid this possible confounding factor in our study, we placed the intraocular cannula in the anterior chamber in all eyes.
Understanding IOP changes that occur when the suction ring is placed on the eye and when the keratome is cutting the cornea is important given that most intraoperative complications in epi-LASIK and LASIK are keratome related. During surgery, the IOP must increase to at least 60 to 65 mm Hg to produce a firm cornea and thus a good-quality flap. This IOP level is usually greater than the ocular systolic blood pressure, which is why some patients experience a “black out” phenomenon during the procedure.
Although increases in IOP are usually well tolerated, they can theoretically cause unwanted retinal and choroidal changes. Dollery et al.11 studied the effect of acute IOP increases on retinal and choroidal circulation after application of a limbal suction cup to porcine eyes for a few seconds. They found a positive relationship between the degree of suction pressure and the increase in IOP. Using fluorescein angiography, they also found that the increases in IOP reduced the blood flow through the retinal and choroidal vessels. Experimental studies of the intraocular vascular changes during acute IOP increases in owl monkeys showed a significant reduction in retinal, choroidal, and optic nerve blood flow.15
Agarwal9 found irreversible retinal vascular damage after IOP was increased up to 80 mm Hg for 3 minutes in rat eyes. Cunha-Vaz16 reports similar findings in a study of retinal ischemia rat eyes caused by increasing the IOP from 110 mm Hg to 160 mm Hg.
Recent studies evaluated the association between LASIK and posterior segment pathology.17 Flaxel et al.18 analyzed ocular changes that might explain the development of post-LASIK retinal pathology. Using an A-scan ultrasound device, they found that after placement of the suction ring, the axial length in the eye significantly increased without a change in anterior chamber depth. Their results indicate that the ocular elongation caused by the surgical procedure is primarily due to changes in vitreous cavity dimensions. However, because Flaxel et al. used enucleated eyes in their study, their findings may not apply to in vivo procedures. Arevalo19 proposed a similar mechanism.
In the current study, the IOP increased in both the epi-LASIK group and the LASIK group, although it followed a different pattern. In the epi-LASIK group, the mean IOP was 92.57 ± 20.86 mm Hg during the suction phase and 82.09 ± 20 mm Hg during the epikeratome head pass. After flap creation and during the low-vacuum pressure phase (backward movement of the epikeratome head), there was a decrease in the mean IOP, especially when half the cornea was not indented by the epikeratome head.
In the LASIK group, the mean IOP was 113.65 ± 10.78 mm Hg during suctioning and 112.35 ± 11.51 mm Hg during the microkeratome pass. These IOP values are lower than those in a previous study6 that evaluated the IOP rise induced by the M2 microkeratome. In the study, the mean IOPs during the suction and cutting phases were higher (122.52 ± 30.40 mm Hg and 160.52 ± 22.73 mm Hg, respectively) than in the present study. This apparent discrepancy is because IOP increases are produced by the vacuum in the suction ring and the external pressure the surgeon applies to the suction ring. In the current study, the surgeon was instructed not to apply external pressure on the suction ring; this was to ensure that the only IOP increases measured were those induced by the suction rings and keratomes. On the other hand, the standard deviations of all measurements in the present study are clearly lower than in the previous paper, which reduces the variability in the IOP values. Thus, in the present study, the statistical power to detect small differences in IOP increases between the 2 devices is higher.
The major drawback of the approach we used is that the IOP increases we report cannot be extrapolated to clinical practice or to the period when the surgeon applies external pressure to the suction ring. Nevertheless, we believe our methodology is better for head-to-head comparisons between techniques that use the same suction system we used.
Although the IOP increase in the LASIK group was higher than in the epi-LASIK group, the duration of the procedure in the LASIK group was only 15.06 ± 1.95 seconds versus 89.41 ± 3.48 seconds in the epi-LASIK group. Thus, the duration was significantly longer and the induced IOP increase significantly lower in the epi-LASIK group. It would be interesting to determine which of these 2 factors (ie, time the eye is under high IOP or the IOP level alone) is more important in terms of ocular safety. The duration of the surgical procedure in the epi-LASIK group may seem surprisingly high, and our results may not apply to other available epikeratomes.
Few studies have examined the relationship between vacuum pressure settings and real-time IOP increases in the eye using different microkeratomes. Kasetsuwan et al.4 used 2 vacuum pressures and found that when using a suction ring vacuum pressure of 488 mm Hg, the IOP increased to a mean of 93.3 mm Hg after application of the suction ring alone and to 82.0 mm Hg during microkeratome passage. However, when the vacuum pressure setting was increased to 600 mm Hg, the IOP rose to 108.0 mm Hg during suction ring application and to 92.5 mm Hg during microkeratome passage; the IOP values did not differ significantly. Bradley et al.7 studied the IOP in human eyes that had lamellar microkeratectomy using 3 microkeratomes; all microkeratomes caused IOP increases greater than the normal ocular systolic blood pressure, although the differences were not significant. Bissen-Miyajima et al.5 measured the IOP changes during LASIK by inserting an intravenous pressure sensor into the vitreous cavity. They found no significant differences in IOP increase between use of single-port suction ring and use of a dual-port suction ring. The total duration of the procedures was also similar.
Findings in previous studies and our results indicate that microkeratome design may be at least as important as the vacuum pressure setting in terms of changes in IOP. We found a higher mean IOP increase in the LASIK group than in the epi-LASIK group during suctioning and cutting. We used the same vacuum settings and the same power unit in both groups; therefore, the IOP differences were solely the result of the differences in the suction rings and microkeratome heads.
In the epi-LASIK group, we decreased the vacuum pressure setting in the power unit from 600 (high vacuum) to 250 mm Hg (low vacuum) during backward movement of the epikeratome head. The mean IOP decreased from 82.09 ± 20 mm Hg to 67.28 ± 13.49 mm Hg; the change in IOP was not statistically significant. The IOP course in the epi-LASIK group is interesting (Figure 2). There was no significant IOP change until the epikeratome head was moved backward and the cornea was no longer indented by the device. In other words, the vacuum reduction in the suction ring alone was not enough to significantly decrease the IOP.
As in every animal study, the results we obtained using our model may not directly apply to the human eye. Differences between porcine eyes and human eyes, such as in corneal thickness and scleral rigidity, may influence the magnitude of the IOP increase during LASIK and epi-LASIK.
In conclusion, manometry can be used to measure real-time IOP changes when an epikeratome is used to create epithelial flaps or a microkeratome to create lamellar flaps. Although the vacuum pressure settings in the power unit were identical and the power unit was the same, the use of different keratomes appears to induce different intraoperative IOP increases, at least in porcine eyes. Further research is needed to increase our knowledge about the IOP changes during corneal refractive surgery procedures.
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