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Original Article

Evaluation of retinal nerve fiber layer thickness measurement following laser in situ keratomileusis using scanning laser polarimetry

Dada, Tanuj MD; Chaudhary, Sunil MBBS; Muralidhar, Rajamani MD, FRCS; Nair, Soman MD; Sihota, Ramanjit MD, FRCS; Vajpayee, Rasik B MS, FRCS

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Indian Journal of Ophthalmology: May–Jun 2007 - Volume 55 - Issue 3 - p 191-194
doi: 10.4103/0301-4738.31938
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Abstract

Scanning laser polarimetry (SLP) is a noninvasive diagnostic technique used to estimate the peripapillary retinal nerve fiber layer (RNFL) thickness.123 It uses a plane polarized laser beam that is phase retarded when it double passes the birefringent RNFL. The extent of this retardation is an estimate of the RNFL thickness. The RNFL thickness is obtained assuming a conversion factor of 0.67 nm/µm.4 However, the collagen fibers of the cornea and to a lesser extent the lens also contribute to the retardation of the light and may therefore influence the measurements.5 The older version of the commercially available instrument for SLP, the glaucoma diagnostics (GDx) nerve fiber analyzer (Laser Diagnostic Technologies, Inc, San Diego, CA, USA) used a fixed corneal compensation with a magnitude of 60 nm and an axis 15o nasally downward.6 However 15% of individuals have a corneal polarization axis that is significantly different from that assumed by the fixed compensator.7 Therefore the currently used version of the instrument (version 5.2.3) incorporates variable corneal compensation (VCC) which measures the anterior segment birefringence for the particular eye and automatically corrects the resultant RNFL thickness.89

Refractive corneal procedures such as photorefractive keratectomy and laser-assisted in situ keratomileusis (LASIK) are known to alter the corneal polarization.10 A decrease in the RNFL parameters after LASIK has been observed with fixed corneal compensation.11 With VCC, the results have been variable. Some studies have reported changes in certain parameters, while others have failed to demonstrate a change following the procedure.1011 However, studies using optical coherence tomography found no change in the RNFL thickness after LASIK.12 This suggests that variations in the measurement of the nerve fiber index (NFI) thickness following LASIK using SLP are probably artifacts caused by alterations in corneal polarization.

To the best of our knowledge there is no study evaluating the changes in RNFL thickness following LASIK using SLP in Indian eyes (Medline search). The aim of the study was to examine whether the changes in cornea following LASIK alters the RNFL thickness measurement performed by GDx, VCC on Indian eyes.

Materials and Methods

Fifty-four eyes of 54 consecutive patients with myopia were selected for the study. Informed consent was obtained from all the patients. One eye was randomly chosen if both eyes were eligible and posted for surgery. The inclusion criteria were a spherical refractive error ranging from -2.0 diopter (D) to -12.0D, a central corneal thickness of >500 mm, absence of glaucoma or other ocular disease except ammetropia.

Patients with best corrected visual acuity < 20/40, an intraocular pressure >22 mmHg, peripapillary atrophy, previous ocular or laser surgery were excluded from the study.

Complete medical and ocular history was obtained from all the patients. All patients selected for the study underwent a complete ophthalmological examination including refraction, slit-lamp examination, Goldmann applanation tonometry, noncontact fundus examination using a +90D lens and indirect ophthalmoscopy. Corneal topography (Orbscan II, Bausch and Lomb) and ultrasonic pachymetry were also performed for all the patients. Patients on contact lenses were advised to discontinue them for a month prior to surgery.

The LASIK procedure was carried out using the VISX Star S2 (VISX Inc, Santa Clara, CA) excimer ablation system. The hansatome microkeratome (Bausch and Lomb, Rochester NY) with the 9.5 mm plate and intended depth of 160 mm was used to create a superior hinge flap. Ammetropia was then corrected by excimer laser ablation of the stromal bed. An optical zone diameter of 6.5 mm was chosen but was reduced if the calculated minimum residual stromal bed was determined to be < 270 mm. The transition zone was fixed at 8.0 mm for all patients. The average suction time was 40 seconds.

Following LASIK the patient was reviewed at day 1, day 7 and one month. At each visit the patient underwent measurement of uncorrected visual acuity, best corrected visual acuity, corneal topography and slit-lamp examination together with examination of the posterior segment using the +90D lens.

The RNFL analysis was done using the GDx VCC instrument, software version 5.2.3, (Laser Diagnostic Technologies, Inc., San Diego, CA, USA) by an experienced examiner a day prior to the procedure with undilated pupils. A macular scan was obtained to determine the anterior segment birefringence followed by a scan for the peripapillary RNFL. The centration of the ellipse around the disc was checked and if needed was modified by the examiner. This was followed by measurement of the RNFL retardation and thickness.

The follow-up examination was done seven days following the procedure, again using VCC. Five parameters were analyzed for any changes - the temporal superior nasal inferior temporal (TSNIT) average, the superior average, the inferior average, the nerve fiber index (NFI) and TSNIT standard deviation. The difference between the pre and postoperative RNFL parameters measured using the GDx VCC instrument were statistically analyzed using the Minitab software (Minitab version 11.12 32 bit). Student'sttest was applied to determine if the difference was statistically significant. APvalue < 0.05 was deemed to be statistically significant.

Results

Among the 54 patients, all completed the procedure without undue complications. The mean age (±S.D.) was 23.4 ± 5.6 years. The mean preoperative spherical equivalent (±S.D.) was -4.71 ± 2.39 D. The mean value of the various parameters is indicated in Table 1. No significant change was noted in any of the parameters following LASIK. We actually noted a small albeit statistically insignificant increase in TSNIT, inferior and superior average.

Table 1
Table 1:
Scanning laser polarimetry parameters measured with the GDx VCC before and after LASIK (7 days postop)

Discussion

LASIK is now the most commonly performed refractive procedure and is being done in a large number of a relatively young population. A fair percentage of this population has one or more risk factors apart from myopia for the development of glaucoma.1314 As LASIK alters corneal curvature and thickness, the normative parameters of diagnosing glaucoma, such as tonometry and perimetry may not be reliable.15 While intraocular pressure measurement may be affected by a, change in corneal thickness, there is little experience with automated perimetry after LASIK.1617

SLP has proven to be sensitive and specific for the diagnosis of glaucoma and may be of value in the evaluation of glaucoma suspects who have had LASIK.1718 For this it is mandatory that these measurements remain stable following LASIK. However, the RNFL parameters have been found to decrease following LASIK when these measurements were made with the fixed corneal compensator.10 The reports with customized compensation vary however, with some showing no change at all and some reporting changes in select parameters.111719

In our study RNFL thickness was measured in healthy nonglaucomatous adult myopes using SLP before and after LASIK. VCC was used to account for the anterior segment birefringence in each case. The peripapillary RNFL was assessed using five standard RNFL parameters as described in Table 2. Our study failed to reveal a significant decrease in any of the parameters as noted in Table 1. We, on the other hand, found a small increase in all the parameters and a drop in the nerve fiber index. These changes however did not reach statistical significance. This implies that the RNFL thickness undergoes no change following LASIK if measured with the SLP using VCC.

Table 2
Table 2:
Definition of the GDx parameters analyzed

One limitation of our study was that the short follow-up period precluded the assessment of LASIK-induced RNFL injury. This is especially important as several reports in the literature have described the risk of occurrence of ischemic events and progression of glaucoma following LASIK.20212223

The studies with VCC have shown a change in corneal slow polarization axis. This change is related directly to the ablation depth. Choplin et al. noted that when the measurements were repeated without compensating for this change, the parameters were significantly different.10 However, when these changes were neutralized using corneal compensation the differences disappeared. Halkiadakis et al. also did not find any changes in the RNFL thickness measurements when the readings were taken a month after LASIK.17

Hollo et al. noted that while the superior maximum thickness and the inferior maximum thickness remained unchanged after LASIK when measured with the customized corneal compensation, the superior and inferior ratios decreased when measured with the GDx VCC.11 Since the superior/nasal ratio and the inferior/nasal ratio remained stable after LASIK, he suggested that this could be because of an inadequate compensation for the temporal sector, which in turn influences the global parameters also.

Centofanti et al. found that the superior average and the standard deviation showed a strong correlation to the corneal polarization axis shift and the corneal ablation depth. These differences were however neutralized following compensation for corneal polarimetric changes. Interestingly, the NFI was altered only when customized corneal compensation was applied.24

In the studies mentioned above, the follow-up periods were six days and eight days respectively.1124 The follow-up in our study was therefore maintained at one week in order to make the results obtained comparable with the conclusions of the western studies. Also, in a study by Avunduk et al., thickness of the keratocyte activation zone was obtained using confocal scanning microscopy in human subjects following LASIK.25 The peak measurements indicating active healing were observed at one week. In our study, the RNFL parameters measured at one week were found to be comparable to the baseline values. Thus the GDx VCC adequately compensates for the changes in birefringence that may occur during this period of active healing.

The findings of these studies need to be interpreted with caution. There are always limitations in the percentage of change that can be detected with SLP-VCC. The machine cannot detect changes of < 5 mm magnitude. Further, the ability of the instrument to indicate localized RNFL defects is still in question.26 Discrepancies in the results of the various studies might be due to the small number of patients evaluated or may probably reflect racial differences between the various patient groups studied. Such findings would need to be confirmed by studying a larger number of patients in a multi-centric study.

Conclusion

Our study analyzed the changes occurring in the RNFL measurement using GDx VCC following LASIK in a series of healthy myopic Indian eyes. It also confirmed the hypothesis that since VCC neutralizes corneal birefringence, changes occurring in the latter parameter should not affect the RNFL measurements obtained from GDx VCC following LASIK. Thus postLASIK measurements can be compared with the preLASIK readings without the need for additional baseline imaging.

Source of Support:

Nil

Conflict of Interest:

None declared.

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Keywords:

Customized corneal compensation; laser-assisted in situ keratomileusis; scanning laser polarimetry

© 2007 Indian Journal of Ophthalmology | Published by Wolters Kluwer – Medknow