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ORIGINAL ARTICLE
Year : 2007  |  Volume : 55  |  Issue : 3  |  Page : 191-194

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


Department of Clinical Ophthalmology, Dr. R. P. Centre for Ophthalmic Sciences, AIIMS, New Delhi, India

Date of Submission13-Feb-2006
Date of Acceptance08-Sep-2006

Correspondence Address:
Soman Nair
Unit II, Department of Clinical Ophthalmology, Dr. R. P. Centre for Ophthalmic Sciences, AIIMS, New Delhi
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0301-4738.31938

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  Abstract 

Aim: To evaluate the effect of laser-assisted in situ keratomileusis (LASIK) on the measurement of retinal nerve fiber layer thickness by scanning laser polarimetry using customized corneal compensation in myopes.
Materials and Methods: Scanning laser polarimetry was performed on 54 eyes of 54 healthy patients with myopia using the glaucoma diagnostics variable corneal compensation (GDx VCC) instrument (Laser Diagnostic Technologies, San Diego, California) before and a week after LASIK. The various parameters were compared using the Student's t test.
Results:
No statistically significant change was observed in any of the retinal nerve fiber layer parameters before and after LASIK. Conclusions: While the measurement of retinal nerve fiber layer thickness by scanning laser polarimetry is affected by anterior segment birefringent properties and LASIK would be expected to produce changes in the same, customized corneal compensation using the GDx VCC seems to adequately compensate for these changes.

Keywords: Customized corneal compensation, laser-assisted in situ keratomileusis, scanning laser polarimetry.


How to cite this article:
Dada T, Chaudhary S, Muralidhar R, Nair S, Sihota R, Vajpayee RB. Evaluation of retinal nerve fiber layer thickness measurement following laser in situ keratomileusis using scanning laser polarimetry. Indian J Ophthalmol 2007;55:191-4

How to cite this URL:
Dada T, Chaudhary S, Muralidhar R, Nair S, Sihota R, Vajpayee RB. Evaluation of retinal nerve fiber layer thickness measurement following laser in situ keratomileusis using scanning laser polarimetry. Indian J Ophthalmol [serial online] 2007 [cited 2024 Mar 29];55:191-4. Available from: https://journals.lww.com/ijo/pages/default.aspx/text.asp?2007/55/3/191/31938

Definition of the GDx parameters analyzed

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Definition of the GDx parameters analyzed

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

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

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Scanning laser polarimetry (SLP) is a noninvasive diagnostic technique used to estimate the peripapillary retinal nerve fiber layer (RNFL) thickness.[1],[2],[3] 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.[8],[9]

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.[10],[11] 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 Top


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's t test was applied to determine if the difference was statistically significant. A P value < 0.05 was deemed to be statistically significant.


  Results Top


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.


  Discussion Top


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.[13],[14] 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.[16],[17]

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.[17],[18] 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. [11],[17],[19]

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.

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.[20],[21],[22],[23]

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.[11],[24] 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 Top


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.

 
  References Top

1.
Hollo G, Suveges I, Nagymihaly A, Vargha P. Scanning laser polarimetry of the retinal nerve fibre layer in primary open angle and capsular glaucoma. Br J Ophthalmol 1997;81:857-61.  Back to cited text no. 1
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Weinreb RN, Zangwill L, Berry CC, Bathija R, Sample PA. Detection of glaucoma with scanning laser polarimetry. Arch Ophthalmol 1998;116:1583-9.  Back to cited text no. 2
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Yamada N, Chen PP, Mills RP, Leen MM, Stamper RL, Lieberman MF, et al . Glaucoma screening using the scanning laser polarimeter. J Glaucoma 2000;9:254-61.   Back to cited text no. 3
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Greenfield DS, Knighton RW, Huang XR. Effect of corneal polarization axis on assessment of retinal nerve fiber layer thickness by scanning laser polarimetry. Am J Ophthalmol 2000;129:715-22.  Back to cited text no. 7
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Zhou Q, Weinreb RN. Individualized compensation of anterior segment birefringence during scanning laser polarimetry. Invest Ophthalmol Vis Sci 2002;43:2221-8.  Back to cited text no. 8
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Choplin NT, Scalhorn SC, Sinai M, Tanzer D, Tidwell JL, Zhou Q. Retinal Nerve Fiber Layer Measurements do not change after LASIK for High Myopia as measured by Scanning Laser Polarimetry with Custom Compensation. Ophthalmology 2005;112:92-7.  Back to cited text no. 10
    
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Hollo G, Katsanos A, Kothy P, Kerek A, Suveges I. Influence of LASIK on scanning laser polarimetric measurement of the retinal nerve fibre layer with fixed angle and customised corneal polarisation compensation. Br J Ophthalmol 2003;87:1241-6.  Back to cited text no. 11
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Mitchell P, Hourihan F, Sandbach J, Wang JJ. The relationship between glaucoma and myopia; The Blue Mountains Eye Study. Ophthalmology 1999;106:2010-5.  Back to cited text no. 13
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Emara B, Probst LE, Tingey DP, Kennedy DW, Willms LJ, Machat J. Correlation of intraocular pressure and central corneal thickness in normal myopic eyes and after laser in situ keratomileusis. J Cataract Refract Surg 1998;24:1320-5.  Back to cited text no. 16
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Halkiadakis I, Anglionto L, Ferensowicz M, Triebwasser RW, van Westenbrugge JA, Gimbel HV. Assessment of nerve fiber layer thickness before and after laser in situ keratomileusis using scanning laser polarimetry with variable corneal compensation. J Cataract Refract Surg 2005;31:1035-41.  Back to cited text no. 17
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Greenfield DS, Knighton RW, Feuer WJ, Schiffman JC, Zangwill L, Weinreb RN. Correction for corneal polarization axis improves the discriminating power of scanning laser polarimetry. Am J Ophthalmol 2002;134:27-33.  Back to cited text no. 18
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Lee AG, Kohnen T, Ebner R, Bennett JL, Miller NR, Carlow TJ, et al . Optic neuropathy associated with LASIK. J Cataract Refract Surg 2000;26:1581-4.  Back to cited text no. 20
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24.
Centofanti M, Oddone F, Parravano M, Gualdi L, Bucci MG, Manni G. Corneal birefringence changes after laser assisted in situ keratomileusis and their influence on retinal nerve fiber layer thickness measurement by means of scanning laser polarimetry. Br J Ophthalmol 2005;89:689-93.  Back to cited text no. 24
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25.
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    Tables

  [Table - 1], [Table - 2]


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