Scanning laser polarimetry of the retinal nerve fibre layer in primary open angle and capsular glaucoma

Imaging of the optic nerve and retinal nerve fiber layer: an essential part of glaucoma diagnosis and monitoring

Because glaucomatous damage is irreversible early detection of structural changes in the optic nerve head and retinal nerve fiber layer is imperative for timely diagnosis of glaucoma and monitoring of its progression. Significant improvements in ocular imaging have been made in recent years. Imaging techniques such as optical coherence tomography, scanning laser polarimetry and confocal scanning laser ophthalmoscopy rely on different properties of light to provide objective structural assessment of the optic nerve head, retinal nerve fiber layer and macula. In this review, we discuss the capabilities of these imaging modalities pertinent for diagnosis of glaucoma and detection of progressive glaucomatous damage and provide a review of the current knowledge on the clinical performance of these technologies.

Measurement of retinal nerve fiber layer thickness in normal and glaucomatous Cocker Spaniels by scanning laser polarimetry

OBJECTIVE

To measure changes in the thickness of the retinal nerve fiber layer in normal and early glaucomatous dogs with scanning laser polarimetry.

ANIMALS STUDIED

A total of 45 eyes, 32 normal and 13 glaucomatous eyes, of American Cocker Spaniels with primary glaucoma were used. All eyes were evaluated through a complete neuro-ophthalmic examination, tonometry, gonioscopy, slit-lamp biomicroscopy, and indirect ophthalmoscopy prior to enucleation.

METHODS

The retinal nerve fiber layer thickness was measured in anesthetized animals with scanning laser polarimetry (Nerve fiber analyzer, GDx; Laser Diagnostic Technologies, LTD, San Diego, CA, USA). Glaucomatous eyes retained some vision at the time of this study.

RESULTS

The mean +/- SD of the retinal nerve fiber layer thickness was 141.69 +/- 18 microm for normal dogs and 105.08 +/- 23.86 microm for visual glaucomatous dogs. The average retinal nerve fiber layer thickness in the superior and inferior retinal quadrants was 148.03 +/- 8.5 and 141.06 +/- 8.73 microm, respectively, for normal dogs, and 106.61 +/- 25.77 and 107.08 +/- 24.99 microm in the superior and inferior retinal quadrants, respectively, for glaucomatous dogs. The superior to nasal retinal nerve fiber layer thickness ratio was 1.45 for normal dogs and 1.26 for visual glaucomatous dogs.

CONCLUSIONS

Using scanning laser polarimetry it was possible to detect changes in retinal nerve fiber layer thickness in glaucomatous dogs at early stages of the disease. Therefore, this instrument has the potential to improve the clinical management of canine glaucoma by detecting progressive changes to the retinal nerve fiber layer.

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.

Study of patients with ocular hypertension with scanning laser polarimetry and short-wavelength automatic perimetry

AIMS

To compare and correlate retinal nerve fiber layer (RNFL) measurements obtained by scanning laser polarimetry (SLP) with defects detected by short-wavelength automatic perimetry (SWAP) in eyes with ocular hypertension (OHT).

METHODS

SLP and SWAP were performed in 96 eyes of 48 consecutive patients with OHT.

RESULTS

Twenty-five eyes (26%) had SWAP visual field defects. Twenty-seven eyes (28.1%) had abnormal RNFL evaluation defined by the GDx neural network ('number' > 29). Fourteen eyes of 10 patients (14.5%) had abnormal RNFL evaluation and SWAP visual field defects. RNFL thickness measurements were significantly reduced in eyes with abnormal SWAP. A weak but statistically significant correlation between the 'number' and pattern standard deviation (r = 0.3, p = 0.006) and the corrected pattern standard deviation (r = 0.3, p = 0.007) in SWAP was found. Areas of abnormal RNFL thickness corresponded to the localization of the SWAP visual field defects in corrected pattern deviation plots in 10 of the 14 eyes with defects in both tests.

CONCLUSIONS

SWAP visual field defects frequently coexist and correspond with abnormalities of RNFL detected by SLP in eyes with OHT. In certain eyes, however, the two methods detect different glaucoma properties.

Nerve fiber layer measurement using scanning laser polarimetry with fixed corneal compensator in normal cynomolgus monkey eyes

PURPOSE

The purpose of this study was to examine retinal nerve fiber layer thickness in normal cynomolgus monkeys using a scanning laser polarimeter with a fixed corneal compensator (GDx FCC), and to clarify the reproducibility and symmetries (right-left differences) between both eyes for the GDx parameters.

METHODS

GDx parameters were measured in 36 normal eyes of 18 cynomolgus monkeys aged 4.0-5.5 years. The retardation values (thickness parameters) at peripapillary and macular areas obtained from the GDx FCC were measured and calculated thickness, ratio, and modulation parameters in the superior and inferior quadrants. Mean and standard deviation (SD), coefficient of variation (CV), and binocular differences were obtained for each parameter from three independent measurements made during a 1-week period. Correlation between both eyes in macular retardation and baseline values, which indicated the combined minimum retardation values for the nasal and temporal quadrants, and between macular retardation and baseline values were analyzed.

RESULTS

The intraocular pressure values (mean +/- SD, n = 18) obtained for the right and left eyes were 20.7 +/- 3.8 and 20.0 +/- 3.2 mm Hg, respectively (no significant differences in both eyes). No significant differences between right and left eyes were detected for any GDx parameters. All parameters showed small right-left differences. The CVs (SD/mean x 100) for all parameters were less than 10%. Highly significant correlations were seen between bilateral eyes for macular retardation (r = 0.936, p < 0.0001) or baseline values (r = 0.946, p < 0.0001). A significant correlation (r = 0.883, p < 0.0001) was also seen between macular retardation and baseline values.

CONCLUSIONS

Considering individual differences in corneal birefringence, GDx parameters obtained from a GDx FCC may be useful for the objective evaluation of time-related changes in individual eyes or for binocular comparisons in cynomolgus monkeys.

Diurnal variation of intraocular pressure and its correlation with retinal nerve fiber analysis in Turkish patients with exfoliation syndrome

PURPOSE

The purpose was to evaluate the diurnal variation (DV) of intraocular pressure (IOP) in patients with exfoliation syndrome (XS), to measure retinal nerve fiber layer (RNFL) thickness by using scanning laser polarimetry, and to compare these measurements with those of normal subjects.

METHODS

Forty-five subjects with XS and 40 healthy, age/sex matched subjects were recruited into the study. A detailed ophthalmologic examination was performed. IOP measurements were obtained at 08:00 am, 12:00 pm, 03:00 pm, and 06:00 pm. The XS group was further divided into DV > or = 5 mmHg and DV < 5 mmHg groups and also according to the existence of IOP fluctuation. The IOP measurements and RNFL thickness measurements were compared between the groups.

RESULTS

The mean IOP value was found to be highest in the morning both in the XS and control groups. IOP showed a gradual decrease from 8.00 am to 6.00 pm in the control group, whereas a second peak at 03:00 pm was observed in the XS group. There was a fluctuation in 53.3% of the XS group, while none of the healthy subjects showed fluctuation. Superior and inferior ratios were statistically lower in XS patients than those in control subjects (p<0.05). Moreover, in patients with XS showing a DV > or = 5 mmHg and/or a fluctuation, the superior ratio, inferior ratio, the number, superior average and superior integral were significantly different (all p values <0.05) from those of control subjects.

CONCLUSIONS

As the XS patients with high diurnal IOP variation and fluctuating pattern of IOP had lower RNFL thickness measurements, it is crucial to follow up these patients by performing scanning laser polarimetry in order to discover any possible glaucomatous damage at an earlier stage than with the use of conventional visual field analysis.

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.

Influence of pupillary diameter, ciliary muscle tone, and ambient light on nerve fiber layer measurements with scanning laser polarimetry

PURPOSE

To evaluate the influence of the pupillary diameter, ciliary muscle tone, and room light on nerve fiber layer measurements with the scanning laser polarimeter (GDx).

METHODS

One randomly selected eye of ten normal volunteers was examined with the GDx in a two-day protocol under eight testing conditions (pilocarpine 1%, phenylephrine 10%, tropicamide 1%, or no drops with room lights on or off). The twelve parameters, obtained by the GDx, were compared under the eight testing conditions, using two way ANOVA for repeated measurements and Tukey HSD post hoc test.

RESULTS

Ten of the twelve parameters were statistically significantly different (P < 0.05) when measured under the three medication or no medication conditions, controlling for the ambient light status. There were no significant differences when measured with the light on or off, controlling for use of drops.

CONCLUSIONS

Nerve fiber layer measurements with the GDx were influenced by drugs affecting pupillary diameter, but not by the status of room light or ciliary muscle tone.

Image analysis of optic nerve disease

Existing methodologies for imaging the optic nerve head surface topography and measuring the retinal nerve fibre layer thickness include confocal scanning laser ophthalmoscopy (Heidelberg retinal tomograph), optical coherence tomography, and scanning laser polarimetry. For cross-sectional screening of patient populations, all three approaches have achieved sensitivities and specificities within the 60-80th percentile in various studies, with occasional specificities greater than 90% in select populations. Nevertheless, these methods are not likely to provide useful assistance for the experienced examiner at their present level of performance. For longitudinal change detection in individual patients, strategies for clinically specific change detection have been rigorously evaluated for confocal scanning laser tomography only. While these initial studies are encouraging, applying these algorithms in larger numbers of patients is now necessary. Future directions for these technologies are likely to include ultra-high resolution optical coherence tomography, the use of neural network/machine learning classifiers to improve clinical decision-making, and the ability to evaluate the susceptibility of individual optic nerve heads to potential damage from a given level of intraocular pressure or systemic blood pressure.

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.

Imaging in glaucoma

Structural assessment using the imaging technologies discussed herein provides reproducible quantitative measurements of posterior segment ocular structures. These measurements have been found to provide useful data for glaucoma detection in various regions of the posterior segment. Further studies are needed to evaluate the utility of these technologies for pre-perimetric glaucoma detection and for monitoring glaucoma progression over an extended period.

Influence of subfoveal choroidal neovascularisation on macular imaging with scanning laser polarimetry of the retinal nerve fibre layer

PURPOSE

To investigate the influence of subfoveal choroidal neovascularisation (CNV) on macular imaging performed using scanning laser polarimetry (SLP) of the retinal nerve fibre layer.

METHODS

SLP was performed on 22 consecutive patients with angiographically verified CNV, and on 23 healthy control subjects. One eye per subject was evaluated using the GDx Nerve Fibre Analyser. Regularity of the corneal retardation on the macular SLP images was assessed according to three criteria: (1) magnitude of the 'macular ratio', defined as the ratio of mean retardation values along two axes (the axis with the maximum retardation and the perpendicular one, corresponding in healthy eyes to minimum retardation); (2) the values of GDx parameters which are independent of quadrant position (ellipse modulation and ellipse average); and (3) the frequency of the regular 'bow-tie' polarisation pattern.

RESULTS

'Macular ratio' was significantly higher in the CNV group than in the control group (P<<0.001). Ellipse modulation did not differ between the groups, but ellipse average was higher in the CNV group (P=0.016). The variance for each of these two parameters was significantly higher for the CNV group (P<<0.001 for both comparisons). A 'bow-tie' pattern polarisation was seen in 23 of the 23 control eyes, but only in 7 of the 22 CNV eyes (P<<0.001).

CONCLUSION

The results show that CNV influences the macular image obtained with SLP. This suggests that measurements with SLP may be disturbed for eyes with CNV when the customised corneal compensation method, which makes use of the macular retardation image, is employed.

Comparison between fixed-angle and customised corneal-polarisation compensation methods in scanning laser polarimetric measurement of the retinal nerve fibre layer in glaucoma

PURPOSE

To investigate the differences between the results of scanning laser polarimetric (SLP) measurements of the retinal nerve fibre layer thickness (RNFLT) made using two different corneal-polarisation techniques; customised (SLP-C), and fixed-angle (SLP-F) compensations.

METHODS

Both SLP-C and SLP-F were performed on 37 consecutive phakic patients with chronic open-angle glaucoma, and on 14 healthy control subjects. One randomly selected eye per subject was evaluated.

RESULTS

Both SLP-C and SLP-F parameters were able to discriminate between the glaucoma group and the control group, except in the case of the ellipse modulation, which differed significantly between the two groups with SLP-C (P=0.017), but not with SLP-F (P=0.056). When SLP-C and SLP-F values were compared, inferior maximum thickness and ellipse standard deviation were significantly lower with SLP-C in both groups (P<0.05 for each parameter). Superior maximum thickness was significantly lower in glaucoma with SLP-C than with SLP-F (P=0.006) and tended to be lower with SLP-C than with SLP-F in the control group (P=0.053). In the glaucoma group, it was only with SLP-C that a significant (positive) correlation between the superior maximum thickness and the inferior hemifield mean sensitivity (MS) (r=0.653, P<0.001), and between the inferior maximum thickness and the superior hemifield MS (r=0.420, P=0.023) was found. The other global and sectoral SLP parameters showed significant correlation with the corresponding visual field parameters with both techniques.

CONCLUSION

Our findings suggest that SLP measurements with customised compensation provide more realistic results for RNFLT than those made with the conventional fixed-angle compensation.

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.

Sensitivity and specificity of the GDx: clinical judgment of standard printouts versus the number

PURPOSE

The Number is a standard parameter of the GDx that reportedly distinguishes normal and glaucomatous eyes. The authors evaluated the sensitivity and specificity of the Number and examined whether expert clinical judgment of GDx printouts leads to a better separation.

MATERIALS AND METHODS

Two experienced observers judged 800 GDx scans on 400 randomly presented printouts from 200 glaucoma patients and 200 age-matched normal subjects. The diagnosis was based on the symmetry analysis printout and was per patient rather than per eye. The observers assessed sensitivity for all glaucoma patients together, and separately for mild, moderate, and severe glaucoma. Their specificity was determined in the group of normal subjects. The same procedure was performed for the Number, at various critical values.

RESULTS

Both observers discriminated better than the Number. At a critical value of 23, the specificity of the Number was 81.5%, which matched the lowest specificity of the 2 observers: 82.5% and 92.0% for observers 1 and 2, respectively. At these specificities, the sensitivity of the 2 observers and of the Number were 92.0%, 89.5%, and 85.5%, respectively. The sensitivity increased with the severity of glaucoma. The Kappa values for intraobserver agreement were 0.80 and 1.0.

CONCLUSIONS

The Number yielded acceptable sensitivity and specificity values at a critical value of 23 in this test population. However, the clinical judgments of the printouts by both expert observers resulted in a better separation between normal and glaucomatous eyes, particularly in the group with mild glaucoma.

GDx in glaucoma

The purpose of this paper is to review the clinical applications of GDx in the diagnosis and follow-up of glaucoma. The limits and potential of GDx technology are discussed.

Effect of peripapillary chorioretinal atrophy on GDx parametersin patients with degenerative myopia

PURPOSE

To determine the effect of peripapillary chorioretinal atrophy in degenerative myopia on the retinal nerve fibre layer (RNFL) thickness measurements performed by scanning laser polarimetry.

METHODS

Polarimetric RNFL analysis was done in 41 degenerative myopic eyes of 25 patients with spherical refractive errors between -7.50 and -22.00 D (mean -12.50 D;SD 3.5). Analysis was also performed on 46 eyes of 24 age-matched healthy subjects with spherical refractive errors between -1.00 and +1.00 D (mean -0.25 D; SD 0.50) with GDx Nerve Fiber Analyzer (Version 1.0.12). All of the myopic patients had clinically evident peripapillary chorioretinal atrophy in their fundus examination.

RESULTS

In myopic patients most of the GDx parameters were statistically different from the control group (P < 0.05). Although average thickness, ellipse average, number, inferior maximum and inferior average were found to be higher than the healthy subjects,the modulation parameters and the ratio-based parameters were significantly decreased in degenerative myopic patients (P < 0.05). The TSNIT (temporal, superior, nasal, inferior, temporal) graph showed irregular high spikes with loss of the regular double hump pattern, especially in quadrants with large chorioretinal atrophy and visible sclera.

CONCLUSION

Peripapillary chorioretinal atrophy increased retardation values, which might be explained by the high reflectivity of the bare sclera in these areas.

Differences by quadrant of retinal nerve fiber layer thickness in healthy eyes

PURPOSE

The purpose of this study was to determine the distribution of retinal nerve fiber layer thickness by quadrant in healthy eyes.

PATIENTS AND METHODS

Forty subjects with healthy eyes were included in the study. A complete ophthalmic examination was performed, including determination of visual fields by automated perimetry. The subjects had no family history of retinal disease or glaucoma. Forty right eyes and 40 left eyes were measured. Retinal nerve fiber layer thickness was measured using simultaneous stereophotographs with stereophotogrammetry at the disc margin. For the total disc and four quadrants, absolute retinal nerve fiber layer thickness and optic disc area was determined using a magnification correction formula.

RESULTS

Comparison of the quadrants showed that the retinal nerve fiber layer thicknesses of the superior, inferior, and nasal quadrants were significantly thicker than the temporal quadrant (P = 0.00006 for right and left eyes). No significant differences were observed between the superior, inferior, and nasal quadrants in the right eye. In the left eye, the superior retinal nerve fiber layer thickness was significantly greater than the inferior (P = 0.028) and nasal retinal nerve fiber layer thicknesses (P = 0.00006), while the inferior was not significantly different from the nasal. No significant difference in quadrant retinal nerve fiber layer thickness was noted between eyes.

CONCLUSIONS

Measurements of retinal nerve fiber layer thickness in healthy eyes show the thinnest quadrant to be the temporal, with the other three quadrants having similar thicknesses. This pattern corresponds to the histologic measurements of retinal nerve fiber layer thickness.

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

AIM

To investigate the influence of laser assisted in situ keratomileusis (LASIK) on the values for retinal nerve fibre layer thickness (RNFLT) as measured with scanning laser polarimetry (SLP) during the healing process of the cornea after LASIK.

METHODS

SLP with the GDx instrument was performed on 20 consecutive healthy subjects without any eye disease undergoing LASIK for ametropia correction. The SLP measurements were performed before the surgery, and at 1 and 3 days, as well as at 3 months, after LASIK. Thickness data from images of one randomly selected eye per subject were analysed using the ANOVA and Duncan multiple comparison tests. Correlation coefficients between RNFLT data and the treatment parameters were also calculated.

RESULTS

Somewhat similar results were found for the different retinal areas. The measured values for superior average RNFLT decreased significantly at all time points compared to the preoperative baseline (p<0.003, Duncan test), but increased significantly between postoperative day 1 and the final visit at 3 months (p=0.025, Duncan test). Inferior average RNFLT in the early postoperative days was significantly smaller than at 3 months after LASIK (p<0.05, Duncan test), and tended to be smaller than at baseline. The thickness values before surgery and at the final visit, however, showed no significant difference (p=0.698, Duncan test) in this region. Ellipse average RNFLT was significantly smaller in the early postoperative days than the baseline value before LASIK. However, the measured value had significantly increased again by the time of the final visit (p<0.02, Duncan test). This value at the final visit showed no difference from the baseline value (p=0.46, Duncan test). The changes in the nasal average and temporal average RNFLT were not statistically significant. No correlation was found between the change in the SLP measured thickness values and central corneal thickness at baseline and its change after surgery, nor with the change in cylindrical correction due to LASIK, or the length of the suction time during surgery (p>0.05 for all correlations).

CONCLUSION

The SLP technique is sensitive to the corneal optical properties, and RNFLT as measured with SLP shows changes after LASIK. Most of these changes, however, diminish with time after surgery, and the values tend to return to the preoperative results during the first 3 months of corneal healing following uncomplicated LASIK. It appears that in uncomplicated cases the transient RNFLT changes are artefacts and do not imply pathological thickness alterations due to LASIK.

Scanning laser polarimetry in a selected group of patients with glaucoma and normal controls

PURPOSE

To evaluate the ability of scanning laser polarimetry to discriminate between subjects with glaucoma with specific patterns of visual field defect and normal controls.

METHODS

This cross-sectional, prospective study in a glaucoma practice, focused on subjects with glaucoma with predefined types of visual field defect, including advanced (group A, n = 14), localized (group L, n = 46), or mixed (diffuse and localized) defects (group M, n = 22) and normal controls (n = 32). Scanning laser polarimetry was performed in one study eye per subject. Two methods of analysis were used: a subjective analysis, in which examination printouts with the image of the optic disk manually blocked were classified by two observers masked to the diagnosis, and a logistic regression analysis of the retardation parameters included in the printouts.

RESULTS

The observers correctly identified 97% of the controls and 68% of subjects with glaucoma (overall correct classification of 77%), with 93%, 70%, and 47% of patients from groups A, L, and M, respectively, being correctly identified. The best discrimination obtained with the logistic regression correctly identified 69% of controls and 94% of glaucoma subjects (overall correct classification of 87%). The performance was only slightly better for cases from group A compared with L and M.

CONCLUSIONS

Subjective assessment of the scanning laser polarimetry standard printout of single eyes might not be sensitive enough to detect cases of glaucoma with localized or milder mixed types of visual field defect. The discriminating ability of scanning laser polarimetry improves slightly when logistic regression analysis is employed.

Effects of cataract extraction with intraocular lens placement on scanning laser polarimetry of the peripapillary nerve fiber layer

PURPOSE

To determine the effects of cataract extraction with intraocular lens placement on scanning laser polarimetry of the peripapillary nerve fiber layer.

DESIGN

Cohort study.

METHODS

Thirty-four eyes of 33 patients undergoing phacoemulsification cataract surgery with intraocular lens were imaged using the GDx Nerve Fiber Analyzer within 4 weeks before and at least 4 weeks after cataract surgery. Preoperative and postoperative mean images were compared. The effects of cataract severity and type, and intraocular lens material, were also examined.

RESULTS

Comparison of preoperative and postoperative mean scanning laser polarimetry measurements revealed that eyes in which acrylic intraocular lenses (n = 11) were placed had a significant increase in scanning laser polarimetry for the total band circumference, and superior, inferior, and nasal quadrants (P < or =.041). Placement of silicone (n = 15) and polymethylmethacrylate (n = 8) intraocular lenses did not result in statistically significant changes in scanning laser polarimetry measurements. However, eight of 34 eyes (23.5%) had changes of 15% or more postoperatively in the total circumference measurement, including eyes with acrylic, silicone, and polymethylmethacrylate intraocular lenses.

CONCLUSIONS

Cataract extraction with intraocular lens was associated with a statistically significant change in mean scanning laser polarimetry values in eyes that received an acrylic intraocular lens. In individual patients, clinically important changes (15% or greater) in scanning laser polarimetry measurements were not uncommon irrespective of intraocular lens type. New baseline scanning laser polarimetry measurements may be warranted in eyes that undergo cataract extraction with intraocular lens.

Scanning laser polarimetry versus frequency-doubling perimetry and conventional threshold perimetry: changes during a 12-month follow-up in preperimetric glaucoma. A pilot study

PURPOSE

To evaluate the possibility for detecting the progression in preperimetric primary open angle glaucoma (POAG) using scanning laser polarimetry (SLP), frequency-doubling technology (FDT), and conventional automated perimetry (AP).

PATIENTS AND METHODS

22 eyes of 11 preperimetric POAG patients were evaluated using SLP (Nerve Fiber Analyzer, GDx), AP (Humphrey 24-2 threshold test) and FDT (30 degrees threshold test). All eyes had intraocular pressure (IOP) higher than 21 mmHg before treatment, but were consistently lower than 22 mm Hg with unchanged topical medication before and during the study. At the initial evaluation session optic nerve heads showed mild glaucomatous changes but the visual fields were normal (MD better than 2.0 dB, Glaucoma Hemifield Test: within normal limits or borderline). AP and FDT measurements were repeated 6 months later, and all three tests were repeated 12 months after the first investigation.

RESULTS

IOP, AP and FDT measurements showed no statistically significant changes during the 12-month follow up period. In contrast to this, a tendency for a glaucomatous type decrease was seen with SLP in the retinal nerve fibre layer (RNFL) thickness parameters (mean superior and inferior sector thickness values, ellipse average thickness and maximal modulation). The mean decrease of RNFL thickness in the superior and inferior sectors was 2.77 microm and 2.48 microm, respectively. Using the two-way nested ANOVA, which considers the relation between the right and left eyes of the subjects, the decrease was statistically significant (p=0.021) for the inferior sector RNFL thickness.

CONCLUSION

The results suggest that scanning laser polarimetry is a useful technique to detect and measure glaucomatous progression in early glaucoma. Scanning laser polarimetry of the RNFL may help to detect and quantify early progression even if worsening is not seen with perimetry and FDT tests.

The value of polarimetry in the evaluation of the optic nerve in glaucoma

Scanning laser polarimetry (SLP) is a technology used to measure the thickness of the human retinal nerve fiber layer (NFL) in vivo. SLP has been demonstrated to well differentiate between glaucomatous, ocular hypertensive, and normal eyes, despite overlapping data. Recently increased interest is seen in the polarizing properties of the cornea and crystalline lens that may lead to spurious measurements. Although the instruments that use SLP compensate for these anterior segment polarizing properties, recent interest has focused on the extent and effects of incomplete compensation. If well compensated in all, SLP may better separate diseased from normal eyes. Its promising role in the follow-up of glaucoma is still under investigation.

An evaluation of the retinal nerve fiber layer thickness by scanning laser polarimetry in individuals with dementia of the Alzheimer type

PURPOSE

To determine, using scanning laser polarimetry, whether or not the retinal nerve fiber layer (RNFL) is altered in dementia of the Alzheimer type (DAT).

METHODS

Thirty individuals with mild to moderate DAT and 30 healthy age-matched controls participated in the study. Fundus images were acquired with a Nerve Fiber Analyzer. RNFL thickness measurements were obtained under an ellipse located 1.75 disc diameter from the optic nerve head (ONH) center.

RESULTS

No differences in RNFL thickness were observed between DAT and healthy subjects. The regional distribution of RNFL thickness was similar between the two test groups, with the RNFL being thickest in the superior and inferior retinal segments relative to the nasal and temporal regions.

CONCLUSIONS

Our data indicate that the RNFL is not altered in DAT, at least in the earlier stages of the disease.

Effects of artefacts on scanning laser polarimetry of retinal nerve fibre layer thickness measurement

AIMS

To investigate the effects of artefacts on scanning laser polarimetry of the retinal nerve fibre layer.

METHODS

Six eyes of six normal volunteers and an artificial nerve fibre layer were examined using the nerve fibre analyser II. The retinal nerve fibre layer thickness (RNFLT) was measured in each of four 90 degree quadrants, superior (S), temporal (T), inferior (I), and nasal (N), at 1.5 disc diameters from the disc margin. Study 1: Measurement in normal eyes. The amount of maximum error in RNFLT measurements was investigated as follows: (1) the intensity setting of the laser beam was changed to be as weak as possible or to be as strong as possible; (2) the intentional offsets of the laser beam axis in relation to the pupil were made in four directions; (3) the eye was rotated by shifting the head 45 or 90 degrees; (4) the right eye was measured by moving it to the left eye position on the head rest. Study 2: Measurements on an artificial nerve fibre layer. The birefringence measurements were confirmed with a plastic disc, which has a radial arrangement of birefringence. The plastic disc with black paper was fixed at the right eye position or the left eye position on the head rest. The retardation of the laser beam by the plastic disc on the black paper was measured. The retardation of the plastic disc was checked by an automatic birefringence evaluation system (ABR-10A, Uniopt Co, Ltd, Shizuoka).

RESULTS

Study 1: The effects of the rotated eye and the measurement of the opposite eye position were significant. The eyes rotated 90 degrees showed quite a different pattern in which the thicker and thinner locations of the RNFLT are switched. The nasal RNFLT of the baseline and the 90 degree rotated eye are 41.9 (SD 6.0) microm and 122.5 (11.2) microm, respectively (p<0.0001, Scheffe multiple comparison test). Study 2: The uniform retardation of the plastic disc was observed with the ABR-10A. The NFA detects the retardation of the plastic disc which the retardation map showed as a double humped pattern.

CONCLUSIONS

Study 2 indicated that the amount of corneal compensation was not small. The cause of significant influences by the rotated eyes and right eyes measurement in left eye position were thought to be incorrect corneal compensation. To increase the diagnostic ability of SLP, an improved compensation of the cornea is thought to be important.

Nerve fibre layer measurement of the Hong Kong Chinese population by scanning laser polarimetry

PURPOSE

To obtain normal retinal nerve fibre layer (RNFL) measurements by scanning laser polarimetry in a local Hong Kong Chinese population and to identify the correlation of the measurements with age.

METHODS

One hundred and fifty-nine normal Hong Kong Chinese volunteers of different ages were recruited for this study. RNFL values were measured using a Nerve Fibre Analyzer GDx (Laser Diagnostic Technologies, CA).

RESULTS

Mean peripapillary RNFL measurements at the superior, inferior, temporal and nasal regions were 95.0 +/- 15.6, 97.0 +/- 16.3, 47.0 +/- 9.0 and 43.9 +/- 15.1 microns respectively (mean +/- SD). There was a highly significant negative correlation in average RNFL values with increasing age (approximately 1.9 microns per decade, p = 0.001). A significant negative correlation with increasing age was also identified in the RNFL values in all four regions and they decreased by similar amounts (p < 0.05). No correlation with age was observed for relative ratio parameters such as superior/nasal ratio, inferior/nasal ratio and max. modulation (p > 0.05).

CONCLUSION

RNFL values of the Hong Kong Chinese population appeared to decrease over time cross-sectionally. Since they decreased by similar amounts at four regions, use of relative ratio parameters for analysis has an advantage over absolute values, as they were not affected by age.

Use of the GDx to detect differences in retinal nerve fibre layer thickness between normal, ocular hypertensive and early glaucomatous eyes

PURPOSE

The GDx is a scanning laser polarimeter that has been developed to allow the quantitative analysis of retinal nerve fibre layer (RNFL) thickness. The purpose of this study was to determine whether differences in the RNFL thickness between normal, ocular hypertensive and glaucomatous eyes could be detected using the GDx.

METHODS

RNFL analysis was carried out using the GDx on 33 normal, 145 ocular hypertensive (OHT) and 44 glaucomatous (POAG) eyes. The inclusion criteria for entry into the study did not include the clinical appearance of the RNFL or optic disc, thus eliminating an important source of selection bias. The Kruskal-Wallis one-way analysis of variance was used to determine whether any significant differences existed among the groups in mean total and quadrantic RNFL thickness. When significant differences were found, specific inter-group analysis was carried out using the Mann-Whitney U-test.

RESULTS

Significant differences in RNFL thickness were found for the mean total, superior and inferior quadrant values between normals and OHT eyes as compared with POAG eyes. No significant differences were found for these values between the normal and OHT eyes.

CONCLUSION

Analysis of RNFL thickness using the GDx was able to detect differences between POAG eyes compared with normal and OHT eyes, although there was considerable overlap between groups. Further assessment of the technique is required to determine whether it may be useful in screening for the presence of early glaucomatous damage.

Asymmetries of the retinal nerve fibre layer thickness in normal eyes

AIMS

To investigate the variation in the retinal nerve fibre layer thickness in detail in normal eyes with a scanning laser polarimeter.

METHODS

The retinal nerve fibre layer thickness (RNFLT) was measured in 94 normal volunteers with a scanning laser polarimeter. The mean RNFLT around a 10 pixel-wide ellipse located concentrically with the disc of 1.5 disc diameters was calculated for 16 sectors each of 22.5 degrees. The symmetry of the RNFLT distribution with respect to the horizontal midline for individual eyes and to the vertical meridian for the two eyes was examined.

RESULTS

The RNFLT was thicker on the inferior side than on the superior side for the temporal four pairs of 22.5 degrees sectors, and the differences were significant in two of the four temporal pairs (p<0.007). The RNFLT was thicker in the superior than in the inferior side for the nasal four pairs of the sectors, and the differences were significant in three of the four nasal pairs (p<0.04). The mean RNFLT was significantly thicker in the right eyes than in the left eyes in the four temporal sectors (p<0.02), and significantly thicker in the left eyes than in the right eyes in the inferior two nasal sectors (p<0.01).

CONCLUSIONS

Asymmetries of the RNFLT in normal eyes with respect to the horizontal midline and to the vertical meridian for the two eyes were found. These asymmetries should be considered when retinal nerve fibre layer loss is evaluated during the course of a disease process.

Retinal nerve fiber layer measurements using scanning laser polarimetry after photorefractive keratectomy

PURPOSE

To assess the effects of corneal depth changes on retinal nerve fiber layer (RNFL) postoperative measurements in myopic patients who had undergone photorefractive keratectomy (PRK).

METHODS

A total of 120 myopic patients underwent PRK for myopia (range -2 to -10 diopters) and were divided into three groups according to their myopic correction: lower than 3 diopters (low myopia group), between 3 and 6 diopters (medium myopia group), over 6 diopters (high myopia group). RNFL parameters were evaluated preoperatively and 5 days, 3 months and 6 months after surgery, using a GDx NFA II scanning laser ophthalmoscope.

RESULTS

Significant changes were seen in the symmetry, superior maximum, and average thickness, comparing baseline with six-month measurements (p=0.008, 0.027, 0.015 respectively). Dividing the sample according to attempted myopic correction, it was found that mean postoperative RNFL thickness was significantly lower after PRK only in the high myopia group. Mean RNFL thickness did not change with time (p=0.884). Ablation depth was correlated with a change in RNFL thickness by the sixth postoperative month for each group. These variables were significantly related only in the high myopia group (p=0.003).

CONCLUSIONS

As polarised light penetrates the ablation area, morphological and functional changes might affect Gdx NFA II measurements. It must always be borne in mind that RNFL thickness can decrease either in cases of glaucoma or after PRK for high myopia, so close attention must be paid to interpreting these measurements in patients who have undergone PRK.

Effects of contact lenses on scanning laser polarimetry of the peripapillary retinal nerve fiber layer

PURPOSE

To determine the effects of contact lenses on scanning laser polarimetry of the peripapillary nerve fiber layer.

METHODS

In a prospective study using the Nerve Fiber Analyzer (Laser Diagnostic Technologies, San Diego, California), retinal nerve fiber layer thickness in 22 subjects (51 eyes) was imaged with and without contact lenses (disposable and nondisposable daily wear soft and rigid gas permeable). Measurements of the circumference and of each quadrant were compared using paired Student t test.

RESULTS

Nerve Fiber Analyzer measurements with and without contact lenses were not significantly different for any of the contact lens types tested (P > or = .11), using either hyperopic (to +4 diopters) or myopic (to -8.5 diopters) lenses.

CONCLUSION

Contact lens wear and refractive power of the eye within the range tested do not significantly affect scanning laser polarimetry of the peripapillary nerve fiber layer.