Laser in situ keratomileusis (LASIK) is currently the most popular method of surgical correction for myopia, hyperopia, and astigmatism. The outcomes have resulted in high patient satisfaction. However, some cases of retinal damage have been reported postoperatively. Several studies show that high intraocular pressure (IOP) elevation during suction ring application can lead to retinal breaks and detachments in predisposing patients.12 Some studies report other forms of retinal damage including Bruch's membrane disruption, choroidal neovascularization, macular hemorrhage, and macular hole formation after LASIK.134 Sachs et al5 reported that IOP elevation during suction ring application can rise up to 77 to 229 mmHg. Short fluctuations of IOP do not lead to visual field defects in most patients undergoing LASIK.6 However, histological evaluation of retinal nerve fiber layer estimated that up to 50% may be lost prior to the appearance of initial visual field damage in glaucoma patients.7
Several imaging devices are currently available to provide objective quantitative measurements of retinal nerve fiber layer thickness: scanning laser slit-lamp microscopy, scanning laser polarimetry, and optical coherence tomography (OCT). The resolution of the OCT (Stratus OCT, Carl Zeiss Meditec, Jena, Germany) is 8 to 10 nm, which is better than any other available glaucoma imaging device (Fig 1).
The purpose of this study was to determine whether the increase in IOP during LASIK suction can induce decreased retinal nerve fiber layer thickness as assessed by OCT.
PATIENTS AND METHODS
Nineteen patients (38 eyes) were enrolled in the study. Mean patient age was 27.8 years (range: 18 to 33 years). Mean preoperative spherical equivalent refractive error was -4.9 diopters (D) (range: -2.0 to -8.5 D). All patients were examined preoperatively as well as postoperatively at days 1 and 3, and 1, 2, and 12 weeks. The examination included visual acuity measurement, slit-lamp microscopy of the anterior and posterior segments, visual field testing, and tonometry. Additional examinations performed prior to and 1 week and 3 months after LASIK included peripapillary retinal nerve fiber layer thickness measurements using OCT-3 Stratus.
Figure 1. OCT-3 Stratus (Carl Zeiss Meditec, Jena, Germany).
Figure 2. Measurements were performed using circumpapillary scan pattern (arrow).
Optical coherence tomography is a diagnostic technique that provides cross-sectional tomography of retinal structures in vivo. Optical int erf erome try is used to resolve the distances of reflective structures within the eye. It is analogous to ultrasound B-scan but has better resolution, approximately 10 nm in the retina. The reproducibility of the method in normal subjects is approximately 7%.8 Low coherence light from a superluminescent diode source, operating at 840 nm (infrared light), is divided in two beams: one incident on the retina and the other incident on a translating mirror. The two reflected beams are recombined, and optical interference is detected by photodiode. A computer algorithm was used to profile the inner and outer retinal boundaries for each tomogram.9 A circumpapillary scan pattern, which is typically a 3.4-mm diameter, is used because this effectively intercepts all of the nerve fibers that emanate from the optic disc while avoiding inaccurate measurements resulting from peripapillary atrophy (Fig 2).
Measured nerve fiber layer thicknesses were compared to normative database, and mean nerve fiber layer thicknesses were automatically computed using integrated protocols, "retinal nerve fiber layer thickness serial analysis" (Fig 3) and " retinal nerve fiber layer thickness average" (Fig 4). Optical coherence tomography mean retinal nerve fiber layer thickness values before and after LASIK were compared using the Student paired t test. The LASIK procedure was performed using the Hansatome microkeratome (Bausch & Lomb, Rochester, NY) and the NIDEK EC-5000 193-nm argonfluoride excimer laser (NIDEK, Gamagori, Japan).
The results are summarized in the Table. Mean time of microkeratome suction ring application was 30 seconds (range: 20 to 50 seconds). Intraocular pressure was normal at all pre- and postoperative examinations. Preoperatively, the mean retinal nerve fiber layer thickness obtained by OCT was 104.2 ?9.0 µm at 1 week postoperative the mean thickness was 101.9?6.9 µm and 106.7±6.0 µm at 3 months. Mean retinal nerve fiber layer thicknesses obtained by OCT were unchanged 1 week and 3 months after LASIK (P≥. 05, for all comparisons, Student paired t test).
The corneal wound healing process and flap interface can change the accuracy of retinal nerve fiber layer measurements made by optic devices. A tendency towards decreased postoperative retinal nerve fiber layer thickness was demonstrated by Roberts et al10 when assessing the effect of LASIK with scanning laser polarimetry (GDx). Subsequently, it was proven that LASIK induces a decrease of the polarimetrie retinal nerve fiber layer thickness, but usually does not lead to concomitant decrease in data obtained with OCT or scanning laser tomography.11
In the present study, the postoperative mean retinal nerve fiber layer thickness was not statistically significantly lower than the preoperative value. From this outcome, it seems that transient IOP elevation during LASIK does not lead to any harmful retinal nerve fiber layer damage in young myopic patients. This study confirms earlier studies by Whitson et al12 who found that LASIK did not adversely affect optic nerve head morphology or retinal nerve fiber layer thickness as measured by Heidelberg retina thomograph. Previous investigations13 have reported retinal nerve fiber layer thickness measurements after LASIK obtained with scanning laser polarimetry (SLP), scanning laser thomography (SLT), and OCT. Mean retinal nerve fiber layer thicknesses obtained by SLP were thinner 1 week and 4 weeks after LASIK, whereas mean OCT and SLT retinal nerve fiber layer thickness measurements were unchanged 1 week and 4 weeks after LASIK compared to preoperative values.
Figure 3. Retinal nerve fiber layer serial analysis.
Figure 4. Retinal nerve fiber layer thickness average analysis.
Retinal Nerve Fiber Layer Measurements (µm) Obtained With OCT
It is generally accepted that LASIK does not typically lead to IOP-related intraocular damage or visual field defects. However, case reports have documented bilateral optic neuropathy after LASIK.14 Lee et al14 emphasize that the etiology of acute anterior or retrobulbar optic neuropathy is unknown but may be related to the marked increase in IOP that occurs during a portion of the procedure. A report by Piette et al15 used ophthalmodynamometry to increase the IOP of normal, healthy eyes and found that the rim area, rim volume, cup area, cup volume, cup-to-disc ratio, mean cup depth, maximum cup depth, and mean retinal nerve fiber layer thickness showed significant changes during IOP elevation. All measured parameters returned to their original values after pressure resolution except mean retinal nerve fiber layer thickness. The authors concluded that transient elevation of IOP results in measurable alterations in optic nerve head topography.
Laser in situ keratomileusis performed on young myopic patients does not have a significant effect on retinal nerve fiber layer thickness as measured by OCT. Care should be taken, however, to avoid possible retinal nerve fiber layer damage during LASIK in patients suspected to have glaucoma. Additional studies are needed to further evaluate the risk of possible optic nerve or retinal nerve fiber layer damage by elevated IOP during LASIK.
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Retinal Nerve Fiber Layer Measurements (µm) Obtained With OCT