Influence of post-LASIK corneal healing on scanning laser polarimetric measurement of the retinal nerve fibre layer thickness

Scanning Laser Polarimetry and Retinal Thickness Analysis before and after Laser in Situ Keratomileusis

Purpose

To evaluate changes in retinal nerve fiber layer (RNFL) thickness after laserin situ keratomileusis (LASIK) using a scanning laser Polarimeter with fixed corneal compensation (GDx) and the retinal thickness analyzer (RIA).

Methods

Thirty-eight eyes of 19 healthy subjects (10 female and 9 male; mean age 37.0±8.8 years) underwent GDx and RTA measurements before and after LASIK. All subjects revealed mild to high myopia (mean spherical refraction: −4.0±2.75 D). Measurements using GDx were followed by RTA measurements after pupil dilation. All measurements were performed the day before LASIK and 1 week postoperatively.

Results

GDX revealed a decrease in nerve fiber layer thickness measurements after LASIK, but did not reach statistical significance (p>0.05). Using RTA, mean RNFL thickness (MRNFL) and RNFL cross sectional area decreased significantly after LASIK (p=0.03 and p=0.02, respectively).

Conclusions

Scanning laser polarimetry revealed a slight decrease in RNFL thickness measurements after LASIK. MRNFL and RNFL cross section were significantly lower after LASIK using RTA. The changes might be artifacts in a small group of myopic subjects.

Refractive surgery and the glaucoma patient

PURPOSE OF REVIEW

Patients considering corneal refractive surgery undergo extensive preoperative testing, but current protocols may not address the management of glaucoma appropriately. This review outlines the current body of literature on the diagnostic and management challenges that exist in the treatment of glaucoma patients undergoing laser ablative surgery, and makes recommendations to improve current perioperative protocols.

RECENT FINDINGS

As permanent structural alterations to the cornea after laser-assisted in-situ keratomileusis surgery make Goldmann applanation tonometry inaccurate, the advent of new diagnostic modalities and recommendations to accurately measure postoperative intraocular pressure (IOP) and subtle damage to the optic nerve have been further analyzed and tested.

SUMMARY

As IOP is the only modifiable risk factor in the treatment of glaucoma to date, traditional diagnostic and treatment algorithms may not be appropriate for refractive surgery patients. Glaucoma remains a relative contraindication to refractive procedures, but as new diagnostic modalities emerge, our ability to diagnose and manage these patients may improve. More uniform recommendations need to be implemented to improve our long-term management of these patients.

[Assessment of the nerve fiber layer thickness after LASIK using scanning laser polarimetry with variable corneal compensation]

AIM

To evaluate the reliability of scanning laser polarimetry with variable corneal compensation after laser-assisted in-situ keratomileusis (LASIK).

METHODS

Thirty-six eyes of 18 consecutive patients who had LASIK where included in the study. For each eye, one scanning laser polarimetry with variable corneal compensation (GDxVCC) was performed before LASIK (GDx no 1). One month after surgery, two other scanning laser polarimetry procedures were performed: one used the corneal compensation measured preoperatively (GDx no 2) and one used a new corneal compensation, measured postoperatively (GDx no 3). The values measured preoperatively were compared first to the values obtained postoperatively with GDx no 2, then to the values obtained postoperatively with GDx no 3 (Wilcoxon test).

RESULTS

When corneal compensation was re-measured postoperatively (GDx no 3), the measurements were reliable for 35 eyes (97%). We found no significant differences between the preoperative and postoperative values except for the NFI (p=0.032). When the preoperative corneal compensation value was used in the postoperative measurements (GDx no 2), the exam was not reliable for 13 eyes, and it could not be performed at all for two eyes. For the remaining eyes, the values of four parameters were significantly modified: superior thickness (p=0.03), superior ratio (p=0.0005), inferior ratio (p=0.009), and ellipse modulation (p=0.39). The values of the other parameters (average thickness and inferior average) remained unchanged.

CONCLUSION

The use of a preoperative customized corneal compensation for the realization of post-LASIK measurements does not provide reliable results. The GDxTMVCC can be considered a reliable tool after LASIK only when the corneal compensation is re-evaluated postoperatively.

Prospective evaluation of factors associated with post-LASIK corneal birefringence with scanning laser polarimetry

PURPOSE

To identify factors that affect corneal birefringence, such as the corneal polarization axis (CPA) and corneal polarization magnitude (CPM), after laser in situ keratomileusis (LASIK) using scanning laser polarimetry with a variable corneal polarization compensator.

METHODS

The CPA and CPM from 42 patients (42 eyes) who underwent LASIK and 24 normal untreated subjects (24 eyes) were measured 1 week before and 1 week after LASIK. Changes in the CPA and CPM after LASIK were studied using Student t test and Pearson correlation coefficient.

RESULTS

In the LASIK group, the post-LASIK CPA and CPM significantly changed, whereas both values did not change in normal subjects. Among 5 candidate factors [preoperative CPA (pre-CPA), preoperative CPM (pre-CPM), age, change in corneal curvature, and ablation depth], the change in CPA was significantly correlated with the pre-CPA (r=-0.793) and pre-CPM (r=0.339). The change in CPM was correlated with the pre-CPA (r=-0.455) and pre-CPM (r=-0.411). Over 83% case of postoperative corneal birefringence can be explained by only 2 parameters with regression analysis.

CONCLUSIONS

The pre-CPA and pre-CPM are strongly associated with LASIK-induced changes in CPA and CPM. Variations in age, changes in corneal curvature, and ablation depth do not contribute to changes in either parameter.

Effect of laser in situ keratomileusis and laser-assisted subepithelial keratectomy on retinal nerve fiber layer thickness

PURPOSE

To evaluate and compare the influence of laser in situ keratomileusis (LASIK) and laser-assisted subepithelial keratectomy (LASEK) on the peripapillary retinal nerve fiber layer (RNFL) thickness.

SETTING

Observational case series.

METHODS

Twenty nonglaucomatous patients with mild to moderate myopia were included. One eye in each patient was randomly selected to have LASIK (Group 1, n = 20), and LASEK was performed in the fellow eye (Group 2, n = 20). The RNFL thickness measurements were performed using optical coherence tomography (OCT) preoperatively and 1 day and 1 month postoperatively. The RNFL thickness parameters were compared using repeated-measures 1-way analysis of variance (ANOVA) followed by multiple comparison tests in both groups. The main outcome parameter was RNFL thickness as determined by OCT before and after LASIK and LASEK.

RESULTS

The mean age of the patients was 21 years +/- 3.2 (SD). The mean preoperative spherical equivalent was -4.23 +/- 1.71 diopters (D) in Group 1 and -4.16 +/- 1.68 D in Group 2. The mean ablation depth was 73.35 +/- 27.85 microm and 66.35 +/- 23.64 microm in groups 1 and 2, respectively. In Group 1, the mean average RNFL thickness, inferior average, superior average, temporal average, and nasal average were 98.2 +/- 5.6 microm, 128.6 +/- 11.9 microm, 120.38 +/- 10.2 microm, 65. 8+/- 7.1 microm, and 80.5 +/- 18.8 microm, respectively. None of the RNFL thickness parameters or RNFL ratios were found to have a significant change after surgery. In Group 2, the average RNFL thickness, inferior average, superior average, temporal average, and nasal average were 98.47 +/- 5.9 microm, 128.38 +/- 14.3 microm, 125.1 +/- 8.8 microm, 66.4 +/- 8.2 microm, 73.6 +/- 15.8 microm, respectively. No significant change was observed in any of the parameters following surgery. No significant alterations were noted in the RNFL parameters following LASIK or LASEK.

CONCLUSIONS

Laser in situ keratomileusis and LASEK did not significantly affect the RNFL thickness parameters postoperatively.

Optic Disc and Retinal Nerve Fiber Layer Analysis With Scanning Laser Tomography After LASIK

PURPOSE

To examine whether LASIK induces changes in the optic disc and retinal nerve fiber layer using scanning laser tomography.

METHODS

Prospective, consecutive study of 53 myopic eyes in 38 patients (mean age: 35.7 +/- 10.4 years; range: 22 to 58 years). Preoperative average refractive error was -6.0 +/- 2.4 diopters (D) (spherical equivalent) (range: -2.0 to -10.0 D). Optic disc morphology and retinal nerve fiber layer thickness were evaluated with scanning laser tomography preoperatively and at 7 and 13 months postoperatively.

RESULTS

No statistically significant differences were noted between the pre- and postoperative optic disc and retinal nerve fiber layer measurements.

CONCLUSIONS

Our results suggest that transient extreme elevation of intraocular pressure during LASIK does not affect the optic disc morphology or retinal nerve fiber layer thickness in normal myopic eyes for at least 1 year after surgery.

Evaluation of enhanced corneal compensation in scanning laser polarimetry: comparison with variable corneal compensation on human eyes undergoing LASIK

PURPOSE

To investigate the potential advantages of an enhanced corneal compensation algorithm (ECC) compared with variable corneal compensation (VCC), in scanning laser polarimetry.

METHODS

One eye of 15 healthy ametropic subjects was imaged immediately before and 7 days after uncomplicated LASIK surgery. Anterior segment birefringence was assessed at each measurement session. For post-LASIK calculations, either actual post-LASIK corneal retardation (VCC and ECC groups), or the pre-LASIK corneal retardation (VCC* and ECC* groups) was used.

RESULTS

The typical scan score-value was higher for ECC both before and after LASIK (P < 0.01), and it was not influenced by LASIK in either compensation method. Both the axis and the magnitude of corneal birefringence altered after LASIK (P < 0.01). After LASIK, with VCC* all eyes showed uncompensated birefringence, but with ECC* none did. LASIK had no effect on retinal nerve fiber layer measurements in VCC and ECC methods. In VCC* the LASIK-induced retinal nerve fiber layer thickness change was significant (P < 0.01) in the temporal, superior, and nasal quadrants. After LASIK, the number of significantly altered sectors along the TSNIT plot showed no difference when comparing VCC and ECC, VCC and ECC*, or ECC and ECC*. In contrast, significantly more sectors were altered in VCC* than in either VCC or ECC* (P < 0.001).

CONCLUSIONS

The new ECC software is more effective than VCC in neutralization of atypical polarization pattern and the uncompensated corneal retardation.

Corneal birefringence changes after laser assisted in situ keratomileusis and their influence on retinal nerve fibre layer thickness measurement by means of scanning laser polarimetry

AIM

To evaluate changes in corneal polarisation properties and their influence on peripapillary retinal nerve fibre layer (RNFL) thickness measurements after laser assisted in situ keratomileusis (LASIK) by means of scanning laser polarimetry (SLP) with variable corneal polarisation compensator (VCC) in normal white subjects.

METHODS

SLP was performed by means of GDx VCC on 32 eyes of 32 normal subjects who underwent LASIK for ametropia correction. Corneal polarisation axis and magnitude and RNFL thickness were measured before and 8 days after LASIK. RNFL thickness data and corneal polarimetric data of one randomly selected eye per subject were analysed by the Wilcoxon signed ranks test. Correlations between corneal ablation depth, corneal polarimetric changes, and RNFL thickness changes were investigated using Spearman's rho test.

RESULTS

The corneal polarisation axis significantly shifted from 15.1 degrees (17.0 degrees ) to 6.9 degrees (12.9 degrees ) (p = 0.00006) after LASIK and this change showed a strong correlation with corneal ablation depth (rho = -0.7, p = 0.00002). Among GDx parameters, TSNIT, SUP, and SD showed significant changes after LASIK and for SUP and SD these changes were well correlated with the shift in corneal polarisation axis (rho = 0.54, p = 0.03 and rho = 0.45, p = 0.01, respectively). SUP and SD changes were neutralised after compensating for corneal polarimetric changes but not TSNIT changes. NFI, a discriminating parameter, was found to be affected after LASIK only after compensating for corneal polarimetric changes.

CONCLUSIONS

LASIK induces a shift in corneal polarisation axis which is responsible for inaccuracies in RNFL thickness measurements. A customised compensation for corneal polarimetric changes after LASIK allows normalisation of some of the thickness parameters except for TSNIT and NFI.

Considerations of Glaucoma in Patients Undergoing Corneal Refractive Surgery

Glaucoma patients present a unique set of challenges to physicians performing corneal refractive surgery. Corneal thickness, which is modified during corneal refractive surgery, plays an important role in monitoring glaucoma patients because of its effect on the measured intraocular pressure. Patients undergo a transient but significant rise in intraocular pressure during the laser-assisted in situ keratomileusis (LASIK) procedure with risk of further optic nerve damage or retinal vein occlusion. Glaucoma patients with filtering blebs are also at risk of damage to the bleb by the suction ring. Steroids, typically used after refractive surgery, can increase intraocular pressure in steroid responders, which is more prevalent among glaucoma patients. Flap interface fluid after LASIK, causing an artificially low pressure reading and masking an elevated pressure has been reported. The refractive surgeon's awareness of these potential complications and challenges will better prepare them for proper management of glaucoma patients who request corneal refractive surgery.

[Laser in situ keratomileusis (LASIK) and scanning laser ophthalmoscopy]

BACKGROUND

To determine the influence of Laser in situ keratomileusis (LASIK) on the measurements of retinal thickness and optic nerve head topography using the Retinal Thickness Analyzer (RTA).

METHODS

RTA measurements were performed before and after LASIK. Forty-eight eyes of 25 healthy subjects were included. Mean age was 40.0+/-10.6 years. Mean preoperative refractive error (spherical equivalent) was -3.3+/-3.6 dpt, and 0.2+/-0.9dpt postoperatively. Correlation between ablation depth and duration and change of retinal thickness postoperatively was performed. All patients received a pachymetry preoperatively.

RESULTS

Postoperatively, mean retinal nerve fiber layer thickness (MRNFL) and cross sectional area (RNFL cross section area) significantly decreased after LASIK (MRNFL preop: 0.18 mm, postop: 0.11 mm, P =0.026, RNFL cross section preop: 1.17 mm(2), postop: 0.71 mm(2), P =0.015). Ablation depth revealed a significant correlation with changes in retinal thickness measurements postoperatively (Delta MRNFL: Ablation depth, P =0.001, r=-0.5). Duration of the ablation was not significantly correlated to the MRNFL measurements postoperatively (P =0.27, r=-0.08). No correlation was found between the central corneal thickness and the change in retinal thickness after LASIK (P =0.51).

CONCLUSION

Due to changes in corneal architecture after LASIK, measurements using RTA reveal a decrease in MRNFL and RNFL cross section area. These changes are likely artifacts. Further studies with a longer follow-up are desirable.

Scanning laser polarimetry of retinal nerve fibre layer thickness after laser assisted in situ keratomileusis (LASIK): stability of the values after the third post-LASIK month

BACKGROUND

Monitoring the retinal nerve fibre layer thickness (RNFLT) is essential in the diagnosis and treatment of glaucoma. In a previous study we found that a decrease of the polarimetric RNFLT observed in the early period after laser-assisted in situ keratomileusis (LASIK) disappears or tends to disappear by the third post-LASIK month.

PURPOSE

To study the stability of the "recovered" polarimetric retardation values between the third and twelfth month after LASIK.

METHODS

Scanning laser polarimetry (SLP) with the classic GDx Nerve Fiber Analyzer was performed on 13 consecutive healthy subjects with no eye disease who underwent LASIK for ametropia correction. Measurements were performed preoperatively, then at 3 and 12 months postoperatively.

RESULTS

Inferior, temporal and nasal average thickness as well as ellipse average thickness and average thickness showed no difference among the three time points (ANOVA, p > 0.05). Superior average thickness was significantly smaller both at three months (Sheffe test, p =0.008) and 12 months (p =0.006) than before LASIK. However, no difference was seen between the values measured at three months and at 12 months after LASIK (p =0.997). A statistically significant interaction between treatment type (myopic or hyperopic correction) and the change of retardation was found for the superior average thickness (two-way ANOVA, p =0.016). In this quadrant the RNFLT values of the myopic eyes decreased between the baseline and the month 3 measurements but became stable after that; the retardation of the hyperopic eyes remained unchanged throughout.

CONCLUSION

RNFLT measured with the classic GDx device after LASIK shows transient changes probably due to the LASIK-induced alteration of the polarization and the healing process. The polarimetric RNFLT values, however, become stable by the third post-LASIK month, and show no further change until the end of the first year after LASIK. Baseline SLP measurements for long-term glaucoma follow-up can be obtained from the third post-LASIK month onwards.

Corneal changes after laser in situ keratomileusis: measurement of corneal polarization magnitude and axis

PURPOSE

Laser in situ keratomileusis (LASIK) involves ablation of the corneal stroma, which may induce a change in birefringence. The purpose of this study was to determine the effect of LASIK on corneal birefringence by measuring corneal polarization magnitude (CPM) and axis (CPA).

STUDY DESIGN

Cohort study.

METHODS

In this prospective study, we measured the change in CPM and CPA before and after LASIK with a scanning laser polarimeter ([SLP] GDx-VCC; Laser Diagnostic Technologies, San Diego, California). Scans were completed on 23 subjects before and 3 months after LASIK. 14 normal controls were tested twice during the same time interval. Change in CPM, CPA, corneal thickness, and corneal curvature measurements were compared between LASIK and normal subjects.

RESULTS

At baseline, the mean (95% confidence interval) values of CPM, CPA, corneal thickness, and corneal curvature measurements of the total population (n = 37) were 41.6 nm (36.6, 46.5); 31.5 degrees (25.7, 37.3); 548.4 microm (540.0, 556.7); and 7.6 mm (7.5, 7.7), respectively. There were no significant differences in baseline values between normal and LASIK subjects. The reproducibility, measured as the average standard deviation of CPM and CPA measurements in 30 normal control eyes, was 1.95 nm (1.43, 2.48) and 1.69 degrees (0.92, 2.46), respectively. Mean CPA, corneal thickness, and corneal curvature measurements were significantly different in patients after LASIK (all P <.0001). Mean absolute values of the change in both CPM and CPA were significantly greater in LASIK patients (4.8 nm [3.3, 6.4], and 10.4 degrees [6.8, 14.1], respectively) than in normal subjects (2.43 nm [1.53, 3.33], and 1.64 degrees [1.15, 2.14], respectively; both P < or =.05). The absolute value of change in CPA was linearly associated with the absolute value of change in both corneal thickness (R(2) = 0.46) and corneal curvature (R(2) = 0.44).

CONCLUSIONS

LASIK causes a measurable change in corneal birefringence as measured by the CPM and CPA that may be related to loss of corneal tissue. Comparison of SLP measurements before and after LASIK requires eye-specific compensation to adjust for the change in corneal birefringence.

Refractive surgery in glaucoma

PURPOSE OF REVIEW

The purpose of this review is to discuss the topics relevant to the topic of LASIK in glaucoma patients or suspects. The safety of LASIK and the implications of LASIK on glaucoma diagnostic testing are reviewed.

RECENT FINDINGS

Most studies have found no change in the nerve fiber layer thickness or optic disc after LASIK. Several reports of steroid-induced glaucoma after LASIK have been published. Important new syndromes such as the interlamellar stromal cyst have been described. Most investigators, but not all, have found intraocular pressure to be lower after LASIK, most likely an artifact of measurement as a result of reduced central corneal thickness.

SUMMARY

The safety of LASIK in patients with glaucoma has not been proved. However, based on the published literature, LASIK may be a viable option for some glaucoma patients. Although not an absolute contraindication, glaucoma is a relative contraindication, and careful patient education and life-long follow-up is mandatory. The clinician must be ever mindful of the important effects that LASIK may have on diagnostic testing.

Influence of LASIK on scanning laser polarimetric measurement of the retinal nerve fibre layer with fixed angle and customised corneal polarisation compensation

BACKGROUND/AIM

Retinal nerve fibre layer thickness (RNFLT), as measured with scanning laser polarimetry using the fixed angle corneal polarisation compensator (SLP-F), has been found to be reduced after uncomplicated laser assisted in situ keratomileusis (LASIK) compared to the pre-LASIK measurement. Since this virtual RNFLT thinning is attributed to the corneal changes induced by the LASIK, the authors investigated whether customised corneal polarisation compensation (SLP-C), which compensates for the actual corneal polarisation during each measurement, can avoid the LASIK induced, virtual changes of the polarimetric RNFLT values.

METHODS

Scanning laser polarimetry using both the SLP-F and SLP-C methods (GDx-Access, software version 5.0) was performed on 15 consecutive healthy subjects with no eye disease who underwent LASIK for ametropia correction. The SLP measurements were performed before the surgery, then on day 1 and day 6 after LASIK. Thickness data from images of one randomly selected eye per subject were analysed using the ANOVA and Scheffe multiple comparison tests.

RESULTS

Superior maximum, inferior maximum, normalised superior area, and normalised inferior area (SLP parameters representing the RNFLT at the superior and inferior poles of the optic nerve head) remained unchanged with SLP-C (ANOVA, p>0.05) but decreased (superior maximum, normalised superior area, Scheffe test, p<0.05) or tended to decrease (inferior maximum) after LASIK, when measured using SLP-F. In contrast, certain other parameters-namely, superior ratio and inferior ratio (representing the ratios between the superior or the inferior sector and the temporal sector), maximal modulation, and ellipse modulation decreased with SLP-C (Scheffe test, p<0.05), but remained stable with SLP-F (ANOVA, p>0.05) after LASIK. Superior to nasal ratio, symmetry of the superior and inferior RNFLT as well as the parameter showing the probability of having glaucoma (called "the number") remained unchanged with both types of corneal compensation (ANOVA, p>0.05). With SLP-C the parameter ellipse average thickness increased after LASIK (Scheffe test, p = 0.021). No parameter value altered between day 1 and day 6 after LASIK, for either method.

CONCLUSION

The results suggest that the LASIK induced decrease of the polarimetric RNFLT, which is consistently detected with polarimeters when using the fixed angle corneal polarisation compensator, is due to alterations of the corneal polarisation. The use of customised corneal polarisation compensation avoids this virtual decrease of the polarimetric RNFLTHowever, our results suggest an increase of the measured retardation in the temporal quadrant of the SLP-C image after LASIK. Since ratios of parameters using the temporal RNFLT in the denominator are important in the polarimetric glaucoma diagnosis algorithm, their decrease as a consequence of using SLP-C needs further investigation.