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

The Effect of Corneal Refractive Surgery on Glaucoma

Laser-assisted refractive procedures have become very popular in the last two decades. As a result, a "generation" of patients with altered corneal properties is emerging. These patients will require both cataract extraction and glaucoma follow-up in the future. Since the glaucoma examination largely depends on the corneal properties, the reshaped postrefractive surgery cornea poses a challenge in the diagnosis, follow-up, and management of the glaucomatous patient. In order to overcome this problem, every patient who is planned to undergo corneal refractive surgery must have a thorough glaucoma examination in order for the ophthalmologist to be able to monitor their patients for possible glaucoma development and/or progression. Some examinations such as tonometry are largely affected by the corneal properties, while others such as the evaluation of the structures of the posterior pole remain unaffected. However, the new imaging modalities of the anterior segment in combination with the most recent advances in tonometry can accurately assess the risk for glaucoma and the need for treatment.

Scanning laser polarimetry in glaucoma

Glaucoma is an acquired progressive optic neuropathy which is characterized by changes in the optic nerve head and retinal nerve fiber layer (RNFL). White-on-white perimetry is the gold standard for the diagnosis of glaucoma. However, it can detect defects in the visual field only after the loss of as many as 40% of the ganglion cells. Hence, the measurement of RNFL thickness has come up. Optical coherence tomography and scanning laser polarimetry (SLP) are the techniques that utilize the evaluation of RNFL for the evaluation of glaucoma. SLP provides RNFL thickness measurements based upon the birefringence of the retinal ganglion cell axons. We have reviewed the published literature on the use of SLP in glaucoma. This review elucidates the technological principles, recent developments and the role of SLP in the diagnosis and monitoring of glaucomatous optic neuropathy, in the light of scientific evidence so far.

The glaucoma detection capability of spectral-domain OCT and GDx-VCC deviation maps in early glaucoma patients with localized visual field defects

PURPOSE

To evaluate and compare the glaucoma detection capabilities afforded by retinal nerve fiber layer (RNFL) thickness and deviation maps obtained using Cirrus spectral domain optical coherence tomography (Cirrus OCT), and GDx employing variable corneal compensation (GDx-VCC) in glaucoma patients with early, localized visual field (VF) loss.

METHODS

This prospective controlled, comparative study was performed on 42 eyes with localized VF defects, and 42 age/refractive error-matched healthy eyes. All participants were imaged by both imaging devices at the same visit. The area of the RNFL defect in each deviation map, corresponding to a VF defect, was analyzed by direct counting of color-coded superpixels in each device. Receiver operating characteristic (ROC) curves were constructed and compared between Cirrus OCT and GDx-VCC.

RESULTS

The areas under the ROCs (AUCs) of RNFL quadrant thicknesses in hemifields with visual field (VF) defects did not differ significantly (Cirrus OCT; 0.961, GDx-VCC; 0.919, P = 0.07). However, Cirrus OCT afforded a better diagnostic ability, by deviation map analysis, than did GDx-VCC (0.972 vs 0.887, P = 0.02).

CONCLUSIONS

The RNFL thicknesses assessed by either Cirrus OCT or GDx-VCC were comparable in terms of early glaucoma diagnostic capability. However, when areas containing RNFL defects were analyzed via deviation mapping, Cirrus OCT was better than GDx-VCC.

Event-based progression detection strategies using scanning laser polarimetry images of the human retina

Monitoring glaucoma patients and ensuring optimal treatment requires accurate and precise detection of progression. Many glaucomatous progression detection strategies may be formulated for Scanning Laser Polarimetry (SLP) data of the local nerve fiber thickness. In this paper, several strategies, all based on repeated GDx VCC SLP measurements, are tested to identify the optimal one for clinical use. The parameters of the methods were adapted to yield a set specificity of 97.5% on real image series. For a fixed sensitivity of 90%, the minimally detectable loss was subsequently determined for both localized and diffuse loss. Due to the large size of the required data set, a previously described simulation method was used for assessing the minimally detectable loss. The optimal strategy was identified and was based on two baseline visits and two follow-up visits, requiring two-out-of-four positive tests. Its associated minimally detectable loss was 5-12 μm, depending on the reproducibility of the measurements.

Quantification of change in axonal birefringence following surgical reduction in intraocular pressure

BACKGROUND AND OBJECTIVE

the purpose of this study was to examine the hypothesis that retinal nerve fiber layer (RNFL) birefringence increases following surgical reduction of intraocular pressure (IOP).

PATIENTS AND METHODS

twenty-six glaucomatous eyes requiring trabeculectomy or drainage implant were enrolled. Optical coherence tomography (OCT), scanning laser polarimetry (SLP), and IOP measurements were performed preoperatively and 3 months postoperatively. The OCT and SLP images were aligned using a new algorithm that aligns the vessels in an OCT image to those in the corresponding SLP reflectance image. The SLP retardance values at the location of the OCT scan circle were then extracted using the OCT scan circle position inferred by the algorithm. Sixty-four corresponding RNFL segments were extracted from SLP and OCT to calculate RNFL birefringence. A significant birefringence change was defined as 1.96 times the weighted test-retest standard deviation in four contiguous segments.

RESULTS

preoperative IOP (19.3 ± 6.1 mm Hg) was significantly (P < .001) lower than postoperative IOP (10.4 ± 3.7 mm Hg). Average birefringence magnitude did not change (P = .19) postoperatively. Localized birefringence magnitude increased significantly in 6 (23%) eyes and decreased significantly in 7 (27%) eyes.

CONCLUSION

in this cohort, variable changes in localized birefringence were observed following surgical reduction of IOP.

Relationship between pattern electroretinogram, standard automated perimetry, and optic nerve structural assessments

PURPOSE

To examine the relationship between retinal ganglion cell function measured using pattern electroretinogram optimized for glaucoma screening (PERGLA), retinal nerve fiber layer (RNFL) thickness, and optic nerve head topography.

METHODS

Twenty-nine normal, 28 glaucoma, and 37 glaucoma suspect volunteers were enrolled. All participants were age similar. One randomly selected eye underwent complete eye examination, standard automated perimetry (SAP), scanning laser polarimetry with enhanced corneal compensation (GDxECC), optical coherence tomography, Heidelberg retina tomograph (HRT), and PERGLA measurements. PERGLA amplitude (microV) was converted to dB for comparison with SAP mean deviation (MD) and pattern SD. The correlation between PERGLA amplitude in dB and the average of sensitivity values for 16 central test locations of SAP were calculated. Analysis of variance, Pearson and Spearman rank correlations, coefficient of variation, and intraclass correlation coefficients were calculated.

RESULTS

PERGLA amplitude in glaucomatous eyes was significantly lower than normal eyes (0.47+/-0.20 vs. 0.70+/-0.28 microV, P<0.001) but not glaucoma suspects (0.54+/-0.21 microV, P=0.84). PERGLA amplitude was inversely correlated with age (r=-0.31, P=0.002). PERGLA amplitude (in dB) was associated with the sensitivity values of the SAP central 16 test locations (r=0.40, P<0001) across the entire cohort, GDxECC superior RNFL thickness (r=0.38, P<0.001), and HRT Moorfields regression analysis classification (rho=-0.34, P=0.001). The coefficient of variation and intraclass correlation coefficients were 14.5% and 0.89 for PERGLA amplitude, 2.4% and 0.98 for optical coherence tomography average RNFL, 2.2% and 0.97 for GDxECC temporal superior nasal inferior temporal average, and 6.3% and 0.94 for HRT rim area.

CONCLUSIONS

Retinal ganglion cell function measured using PERGLA is reduced in glaucoma and demonstrates modest correlations with central SAP sensitivity values and structural measures of optic nerve topography and RNFL thickness.

Atypical retardation pattern in measurements of scanning laser polarimetry and its relating factors

PURPOSE

To assess the prevalence of atypical retardation pattern (ARP) in scanning laser polarimetry (SLP) with variable corneal compensation (VCC) and enhanced corneal compensation (ECC), and to evaluate the factors relating to typical scan score (TSS).

METHODS

Measurements of SLP-VCC, SLP-ECC, and the Humphrey field analyzer (HFA) were performed in 105 normal subjects and 82 open-angle glaucoma (OAG) patients. ARP was defined as an SLP image with TSS<80.

RESULTS

Prevalence of ARP with SLP-VCC was 13.3 and 14.6% in normal and OAG eyes, respectively, and that with SLP-ECC (2.9% and 1.2%) was lower than SLP-VCC (P<0.009). TSS with SLP-VCC was significantly correlated with age (partial correlation coefficient (PCC)=-0.22, P=0.003) and refractive error (PCC=0.26, P<0.001) after adjusting for each other. TSS with SLP-ECC was significantly correlated with neither age nor refractive error (PCC=-0.02, P=0.788; PCC=0.10, P=0.177, respectively). In OAG eyes, mean deviation (MD) of HFA was significantly correlated with TSS with SLP-VCC and SLP-ECC (PCC=0.35, P=0.001; PCC=0.23, P=0.039, respectively). In SLP-VCC, MD was significantly correlated with retinal nerve fibre layer (RNFL) thickness only after excluding eyes with ARP (P<0.001).

CONCLUSIONS

ARP in SLP-VCC measurements was found in more than 10% of normal or glaucomatous eyes, and TSS was significantly lower in older subjects, more myopic eyes, or eyes with more advanced glaucomatous damage. The presence of ARP disturbs the accuracy of RNFL thickness measurement by SLP-VCC. In SLP-ECC measurements, prevalence of ARP was considerably lower and TSS was not affected by age or refractive error, suggesting the advantages of ECC in clinical practices.

Relationship between scanning laser polarimetry with enhanced corneal compensation and with variable corneal compensation

Purpose

To evaluate the structure-function relationships between retinal sensitivity measured by Humphrey visual field analyzer (HVFA) and the retinal nerve fiber layer (RNFL) thickness measured by scanning laser polarimetry (SLP) with variable corneal compensation (VCC) and enhanced corneal compensation (ECC) in glaucomatous and healthy eyes.

Methods

Fifty-three eyes with an atypical birefringence pattern (ABP) based on SLP-VCC (28 glaucomatous eyes and 25 normal healthy eyes) were enrolled in this cross-sectional study. RNFL thickness was measured by both VCC and ECC techniques, and the visual field was examined by HVFA with 24-2 full-threshold program. The relationships between RNFL measurements in superior and inferior sectors and corresponding retinal mean sensitivity were sought globally and regionally with linear regression analysis in each group. Coefficients of the determination were calculated and compared between VCC and ECC techniques.

Results

In eyes with ABP, R² values for the association between SLP parameters and retinal sensitivity were 0.06-0.16 with VCC, whereas they were 0.21-0.48 with ECC. The association of RNFL thickness with retinal sensitivity was significantly better with ECC than with VCC in 5 out of 8 regression models between SLP parameters and HVF parameters (P<0.05).

Conclusions

The strength of the structure-function association was higher with ECC than with VCC in eyes with ABP, which suggests that the ECC algorithm is a better approach for evaluating the structure-function relationship in eyes with ABP.

New developments in scanning laser polarimetry for glaucoma

PURPOSE OF REVIEW

Scanning laser polarimetry is a technique that is used to evaluate the thickness of the retinal nerve fiber layer. It has been shown to have a high accuracy for diagnosing glaucoma. In a subset of eyes, atypical retardation patterns may be present that do not match the expected retinal nerve fiber layer appearance. This review summarizes recent advances made to reduce the frequency and severity of these patterns. In addition, recent progress in the development of algorithms for detecting progression is discussed.

RECENT FINDINGS

A new measurement algorithm--enhanced corneal compensation--has been developed to improve the instrument's signal-to-noise ratio. Enhanced corneal compensation has been shown to improve the accuracy of scanning laser polarimetry for diagnosing glaucoma. In addition, enhanced corneal compensation improves the relationship between standard automated perimetry and scanning laser polarimetry measurements. Furthermore, research is being done on detecting progression in glaucoma. Recently, a method for simulating progression has been proposed, thereby diminishing the need for long-term studies to validate numerous measurement algorithms.

SUMMARY

With enhanced corneal compensation, the diagnostic accuracy of scanning laser polarimetry has been further improved for glaucoma. Newly developed algorithms for detecting any progressive retinal nerve fiber layer thinning await clinical validation.

Scanning laser polarimetry with enhanced corneal compensation for detection of axonal loss in band atrophy of the optic nerve

PURPOSE

To compare the abilities of scanning laser polarimetry (SLP) with enhanced corneal compensation (ECC) and variable corneal compensation (VCC) modes for detection of retinal nerve fiber layer (RNFL) loss in eyes with band atrophy (BA) of the optic nerve.

DESIGN

Cross-sectional study.

METHODS

Thirty-seven eyes from 37 patients with BA and temporal visual field defect from chiasmal compression and 40 eyes from 40 healthy subjects were studied. Subjects underwent standard automated perimetry and RNFL measurements using an SLP device equipped with VCC and ECC. Receiver operating characteristic (ROC) curves were calculated for each parameter. Pearson correlation coefficients were obtained to evaluate the relationship between RNFL thickness parameters and severity of visual field loss, as assessed by the temporal mean defect.

RESULTS

All RNFL thickness parameters were significantly lower in eyes with BA compared with normal eyes with both compensation modes. However, no statistically significant differences were observed in the areas under the ROC curves for the different parameters between GDx VCC and ECC (Carl Zeiss Meditec, Inc, Dublin, California, USA). Structure-function relationships also were similar for both compensation modes.

CONCLUSIONS

No significant differences were found between the diagnostic accuracy of GDx ECC and that of VCC for detection of BA of the optic nerve. The use of GDx ECC does not seem to provide a better evaluation of RNFL loss on the temporal and nasal sectors of the peripapillary retina in subjects with BA of the optic nerve.

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.

Influence of myelinated retinal nerve fibers on scanning laser polarimetry using variable and enhanced corneal compensation methods

Five eyes with myelinated retinal nerve fibers were imaged using a scanning laser polarimeter with variable corneal compensation and with enhanced corneal compensation (a new research software). Using variable corneal compensation, areas of myelinated retinal nerve fibers uniformly appeared as areas of increased retardation. Using enhanced corneal compensation software, the same myelinated retinal nerve fiber areas adjacent to the optic nerve head appeared as areas of clinically significant thinning of the retinal nerve fiber layer in the superior and inferior quadrants, and as areas of increased retinal nerve fiber layer thickness nasally and temporally. Wedge-shaped myelinated retinal nerve fibers along a retinal nerve fiber bundle and discontinuous to the optic nerve head caused increased retardation with both compensation methods. The results show that influence of myelinated retinal nerve fibers on scanning laser polarimetric results varies according to the compensation method and the retinal location of the myelinated retinal nerve fibers.

Performance of GDx VCC in eyes with peripapillary atrophy: comparison of three circle sizes

PURPOSE

A scanning laser polarimetry (GDx VCC) equips three different sized measurement circles. In eyes with peripapillary atrophy (PPA), the GDx measurement becomes inaccurate when the circle falls on PPA. The aim of this study was to evaluate performance of the three circles of GDx measurement in eyes with PPA.

METHODS

Three different sized circles were compared regarding frequency of PPA, which fell on each circle in 282 open-angle glaucoma (OAG) eyes, reproducibility of GDx parameters in 24 normal and 22 OAG eyes, and ability to detect glaucoma in 50 normal and 50 OAG eyes.

RESULTS

PPA was observed in 230 (82%) of 282 OAG eyes. PPA fell on the small circle (default setting), medium, and large circles in 119 (43%), 38 (14%), and 12 (4%) of the 280 OAG eyes. Reproducibility of GDx parameters was not significantly different among three circles in normal eyes (P>0.05), whereas coefficients of reproducibility of TSNIT average (P=0.006) and superior average (P=0.035) were smaller in the smaller circles in OAG eyes. GDx parameters significantly correlated (P<0.001), but were significantly different (P<0.05) between the small and medium circles. The area under receiver operating characteristic curves for dividing OAG from normal eyes using GDx parameters was similar between the small and medium circles.

CONCLUSIONS

If the medium circles were used, obstructing influences of PPA on GDx measurement could be avoided more often in Japanese OAG eyes with similar reproducibility and comparable ability to detect glaucoma compared to those with the default small circle.

Evaluation of Screening versus Regular Polarimetry

PURPOSE

The purpose of this study is to evaluate a new screening program for the GDx VCC (variable corneal compensation) retinal nerve fiber layer analyzer. This new method uses a fast screening approach to detect retinal nerve fiber layer loss associated with glaucoma. The study evaluates the sensitivity and specificity of the screening program and compares the results with those from the GDx VCC full examination.

METHODS

One hundred seventeen eyes from 60 subjects with a mean age of 60.65 years (range, 41-79 years old) were analyzed. Subjects were divided into a normal or glaucoma group based on an ophthalmic examination. Each individual underwent a GDx VCC screening examination, GDx full examination, Humphrey HFA II 24-2 SITA Standard visual field, intraocular pressure assessment, and dilated optic nerve examination. The outcome for the GDx screening examination was within normal limits, borderline, or outside normal limits and compared with the GDx full examination results.

RESULTS

The sensitivity and specificity for the GDx full examination were 73% and 79%, respectively. The sensitivity and specificity for the screening method were 67% and 91%, respectively. The full examination was more sensitive; however, the screening examination was more specific.

CONCLUSION

The screening examination is more conservative than the full examination algorithm and may miss some individuals with glaucomatous damage that the full examination may identify. However, the number of false-positives for the screening examination is less than the full examination method.