Retinal nerve fiber layer progression in glaucoma: a comparison between retinal nerve fiber layer thickness and retardance

Assessment of Peripapillary Retinal Nerve Fiber Layer Optical Density Ratios in Healthy Eyes Using Optical Coherence Tomography with Different Analytical Radii

PURPOSE

To explore the characteristics and determinants of peripapillary retinal nerve fiber layer (RNFL) optical density (OD) by optical coherence tomography (OCT) in healthy eyes with varied analytical radii.

METHODS

Peripapillary OCT scans centered at the optic disc of 150 eyes from 150 healthy subjects (64 males and 86 females) were included. Under 5 analytical circles with different radii (1.45 mm, 1.7 mm, 1.95 mm, 2.2 mm and 2.45 mm), the circumpapillary circular cross-sectional images were exported for further analysis using Image J. Peripapillary RNFL and retinal pigment epithelium (RPE) OD in different quadrants and clock-hours were obtained. RNFL optical density ratio (ODR) was then calculated as RNFL OD divided by RPE OD. A linear mixed-effects model analysis was performed to assess the relationship between RNFL ODR and analytical radius, accounting for axial length, age, spherical equivalent, thickness and image score.

RESULTS

The RNFL ODRs had a double-hump pattern with peaks in the superior and inferior quadrants and troughs in the temporal and nasal areas. In the linear mixed-effects model analysis, a trend of decreasing mean RNFL ODR with increasing analytical radius was found (0.9227 ± 0.0689, 0.9063 ± 0.0620, 0.8916 ± 0.0552, 0.8729 ± 0.0553 and 0.8575 ± 0.0564 respectively, p = 0.034). RNFL ODR values was negatively correlated with age (p < 0.001), positively correlated with corresponding RNFL thickness (p < 0.001). No significant correlation was found between RFNL ODR and image score, axial length and spherical equivalent.

CONCLUSIONS

RNFL ODR profile showed a comparable double-hump configuration with RNFL thickness. RNFL ODR values tended to decrease with larger analytical circles and older age, and increase with corresponding RNFL thickness. These factors should be considered when interpreting RNFL ODR in glaucoma assessment.

Current Status and Future Perspectives of Optic Nerve Imaging in Glaucoma

Being the primary site of degeneration, the optic nerve has always been the focus of structural glaucoma assessment. The technical advancements, mainly of optical coherence tomography (OCT), now allow for a very precise quantification of the optic nerve head and peripapillary retina morphology. By far the most commonly used structural optic nerve parameter is the thickness of the parapapillary retinal nerve fiber, which has great clinical utility but also suffers from significant limitations, mainly in advanced glaucoma. Emerging novel imaging technologies, such as OCT angiography, polarization-sensitive or visible-light OCT and adaptive optics, offer new biomarkers that have the potential to significantly improve structural glaucoma diagnostics. Another great potential lies in the processing of the data already available. Artificial intelligence does not only help increase the reliability of current biomarkers but can also integrate data from various imaging modalities and other clinical measures to increase diagnostic accuracy. And it can, in a more efficient way, draw information from available datasets, such as an OCT scan, compared to the current concept of biomarkers, which only use a fraction of the whole dataset.

A Model of Progression to Help Identify Macular Damage Due to Glaucoma

The central macula contains a thick donut shaped region of the ganglion cell layer (GCL) that surrounds the fovea. This region, which is about 12 degrees (3.5 mm) in diameter, is essential for everyday functions such as driving, reading, and face recognition. Here, we describe a model of progression of glaucomatous damage to this GCL donut. This model is based upon assumptions supported by the literature, and it predicts the patterns of glaucomatous damage to the GCL donut, as seen with optical coherence tomography (OCT). After describing the assumptions and predictions of this model, we test the model against data from our laboratory, as well as from the literature. Finally, three uses of the model are illustrated. One, it provides an aid to help clinicians focus on the essential central macula and to alert them to look for other, non-glaucomatous causes, when the GCL damage does not fit the pattern predicted by the model. Second, the patterns of progression predicted by the model suggest alternative end points for clinical trials. Finally, the model provides a heuristic for future research concerning the anatomic basis of glaucomatous damage.

Characteristics of Optic Disc and Visual Field Changes in Patients with Thyroid-Associated Orbitopathy and Open-Angle Glaucoma

This study aimed to characterize the changes in the visual field (VF) patterns and disc morphology of patients with thyroid-associated orbitopathy (TAO) and open-angle glaucoma (OAG). A retrospective review of the medical records at the Tri-Service General Hospital in Taiwan identified 396 eyes of 198 patients with thyroid-associated glaucoma. A final follow-up of VF examination in 140 eyes revealed 114 eyes with VF defects, indicating disease progression. The characteristics of and changes in disc morphology, optical coherence tomography findings, and VF defects were statistically analyzed. The most common VF defects at the initial diagnosis and the end of the follow-up period were inferior partial arcuate (17%) and paracentral (15%) defects, respectively. The most common VF defect in patients with unspecific disc signs was an unspecific scotoma (13%). The most common optic disc feature was disc cupping (51%), followed by parapapillary atrophy (48%). The most frequent location of nerve fiber layer thinning was the inferotemporal region (48%). VF defects showed a significantly more pronounced progression in the non-nerve fiber bundle group than in the nerve fiber bundle group (p < 0.001). This study details the characteristics and progression of disc morphology and VF defects in patients with TAO and OAG.

A Simple Subjective Evaluation of Enface OCT Reflectance Images Distinguishes Glaucoma From Healthy Eyes

Purpose

We present a subjective approach to detecting glaucomatous defects in enface images and assess its diagnostic performance. We also test the hypothesis that if reflectivity changes precede thickness changes in glaucoma there should be reduced correlation between the modalities in glaucoma compared to controls.

Methods

Twenty glaucoma participants and 20 age-matched controls underwent high-resolution OCT scans of one eye. 4 µm-thick enface slabs were constructed through the retina. Enface indices were depths of first gap in visible retinal nerve fiber bundles (RNFBs) and last visible bundle, subjectively evaluated in six sectors of a 3.5 mm circle around the optic disc. Retinal nerve fiber layer thickness (RNFLT) along the same circle was extracted at angles corresponding to enface indices. Between-group differences were tested by linear mixed models. Diagnostic performance was measured by partial receiver operating characteristic area (pAUC).

Results

First gap and last visible bundle were closer to the inner limiting membrane in glaucoma eyes (both P < 0.0001). Enface indices showed excellent diagnostic performance (pAUCs 0.63-1.00), similar to RNFLT (pAUCs 0.63-0.95). Correlation between enface and RNFLT parameters was strong in healthy (r = 0.81-0.92) and glaucoma eyes (r = 0.73-0.80).

Conclusions

This simple subjective method reliably identifies glaucomatous defects in enface images with diagnostic performance at least as good as existing thickness indices. Thickness and reflectivity were similarly related in healthy and glaucoma eyes, providing no strong evidence of reflectivity loss preceding thinning. Objective analyses may realize further potential of enface OCT images in glaucoma.

Translational Relevance

Novel enface OCT indices may aid glaucoma diagnosis.

Local Glaucomatous Defects of the Circumpapillary Retinal Nerve Fiber Layer Show a Variety of Patterns of Progression

PRECIS

The region of glaucomatous progression, seen on optical coherence tomography (OCT) images of the circumpapillary retinal nerve fiber layer (cRNFL), increases in width and depth in all eyes, but shows a variety of different patterns of loss across eyes.

PURPOSE

The purpose of this study was to examine the patterns of cRNFL loss secondary to glaucomatous progression in a region associated with the superior hemifield of the 24-2/30-2 visual field (VF).

METHODS

Twenty-four eyes (20 patients) with a diagnosis of glaucoma and evidence of progression on OCT had OCT disc cube scans on at least 3 separate visits (mean follow-up 7.4 y; range: 3.9 to 11.4). Circumpapillary b-scans were derived after enface images were aligned to assure that the study region (ie, 0 to -135 degrees, where 0 degree is 9 o'clock, on a right eye) coincided. Within this region, a region of progression (ROP) was defined based on the loss in cRNFL thickness between the first and subsequent visits. The width of the ROP was determined, along with the locations of its leading (close to fixation) and trailing edges. In addition, for each ROP, the location and depth at the point of maximal loss, total loss, and average remaining retinal nerve fiber layer were measured.

RESULTS

The ROP proceeded both toward and away from fixation. Across eyes, the ROP varied widely in width (32 to 131 degrees, mean 82.7 degrees), location, and loss at point of deepest loss (22 to 99 μm, mean 52.9 μm), as well as total cRNFL loss.

CONCLUSIONS

All eyes showed a widening and deepening of the ROP, but a variety of different patterns of progressive cRNFL loss. Thus, one should expect considerable variation in patterns of VF loss. Furthermore, conventional metrics (global or quadrant cRNFL thickness) do not fully depict the progressive changes that can be appreciated by inspecting OCT images.

Vulnerability Zone of Glaucoma Progression in Combined Wide-field Optical Coherence Tomography Event-based Progression Analysis

Purpose

To investigate the spatial characteristics and patterns of structural progression using the combined retinal nerve fiber layer (RNFL) and ganglion cell–inner plexiform layer event-based progression analysis feature provided by the Guided Progression Analysis (GPA) software of spectral-domain optical coherence tomography.

Methods

In this retrospective observational study, we evaluated 89 patients with open-angle glaucoma showing clinically confirmed structural progression within a minimum follow-up period of 3 years. For each eye, the RNFL and ganglion cell–inner plexiform layer GPA data were extracted from serial spectral-domain optical coherence tomography (HD-OCT 4000, Carl Zeiss Meditec, Inc., Dublin, CA) data from 2012 to 2017 (available in commercial report). A combined wide-field GPA map was merged using vascular landmark-guided superimposition of RNFL and ganglion cell–inner plexiform layer GPA event-based progression maps onto the RNFL image (resulting in the GPA PanoMaps: proposed in this study). The pattern of progressive structural changes was evaluated by comparing the baseline combined wide-field OCT deviation maps (PanoMap deviation maps: available in commercial report) and GPA PanoMaps at the time the first progression was detected and the GPA PanoMaps at the last follow-up. Spatial characteristics and patterns of glaucoma structural progression on GPA PanoMaps were evaluated.

Results

Progressive structural progression was detected most frequently at the macular vulnerability zone (MVZ), with the peripapillary and macular progression being well-correlated spatially. Compared with the baseline structural change on PanoMap, the progressive structural changes extended toward the fovea at both the peripapillary and macular areas. A spatial difference was observed between the areas where structural damage was frequently found on PanoMap (peripapillary inferoinferior sector and macular MVZ) and areas where progression was frequently found on GPA PanoMap (peripapillary and macular MVZ).

Conclusions

The patterns of progressive glaucomatous structural changes in both the peripapillary and macular areas were confirmed on the combined wide-field GPA map (GPA PanoMap). An analysis of the progression pattern using the GPA PanoMap facilitates the understanding of the spatial relation between the peripapillary and macular areas in glaucoma.

Retinal Thickness Analysis in High Myopia based on Medial Axis Transforms

This paper presents a retinal thickness analysis method from 3D images acquired by optical coherence tomography (OCT). Given OCT images with segmented boundaries of retinal layers, medial axes of the layers are computed by medial axis transforms (MAT), and thickness is evaluated based on Euclidean distance fields. Since the MAT computes the closest points on the boundary of the layer, it can compute more correct thickness than conventional methods that evaluate Y-axis-aligned thickness. Experimental results show that our method can detect thin-parts around distorted regions, or a clue of high myopia. This is useful for early diagnosis of high myopia and other eye diseases.

Early Glaucoma Discrimination Index

PURPOSE

To develop a new structural algorithm derived from optical coherence tomography (OCT) retinal nerve fiber layer (RNFL) thickness and asymmetry and validate it as a discriminate among normal, suspect, and early primary open-angle glaucoma (POAG).

STUDY DESIGN

A case-controlled observational clinical study.

MATERIALS AND METHODS

In total, 150 subjects (299 eyes) were selected, 61 normal, 46 suspect, and 43 early glaucoma, from Al-Azhar University Hospitals. They were in fifth decade and free from any ocular or systemic diseases affecting the retinal nerve fiber layer. They were investigated by two consecutive perimetry (1 month apart), and three scans of circumpapillary retinal nerve fiber layer (cpRNFL) by using Nidek spectral domain (SD)-OCT 3000 Lite. The cpRNFL thickness (cpRNFLT) and inter-eye asymmetry parameters were analyzed among the three groups. Then some selected parameters were selected and analyzed using a binary logistic regression analysis for developing the new algorithm. The new algorithm was tested for the best fitting, accuracy, and diagnostic ability among the three groups and was validated in the suspect group.

RESULTS

The new algorithm model [early glaucoma discrimination index (EGDI)] works well with only four variables; whole cpRNFLT, inferior quadrant cpRNFLT, inferotemporal clock hour (CH) cpRNFLT, and absolute inter-eye inferior quadrants asymmetry. The highest area under the curve (AUC) obtained from the EGDI among the three groups was 0.854. The validation analysis in the suspect group revealed a higher diagnostic ability in discrimination of early glaucoma with AUC of 0.989 (0.976-1.003).

CONCLUSION

The EGDI showed better diagnostic ability for diagnosis of glaucoma in the pre-perimetric stage. The new OCT algorithm is simple and can be run in any SD-OCT device without dependence on normative data.

HOW TO CITE THIS ARTICLE

Safwat H, Nassar E, Rashwan A. Early Glaucoma Discrimination Index. J Curr Glaucoma Pract 2020;14(1):16-24.

Effect of Cataract Grade according to Wide-Field Fundus Images on Measurement of Macular Thickness in Cataract Patients

Purpose

To investigate the effects of cataract grade based on wide-field fundus imaging on macular thickness measured by spectral domain optical coherence tomography (SD-OCT) and its signal-to-noise ratio (SNR).

Methods

Two hundred cataract patients (200 eyes) with preoperative measurements by wide-field fundus imaging and macular SD-OCT were enrolled. Cataract severity was graded from 1 to 4 according to the degree of macular obscuring by cataract artifact in fundus photo images. Cataract grade based on wide-field fundus image, the Lens Opacity Classification System III, macular thickness, and SD-OCT SNR were compared. All SD-OCT B-scan images were evaluated to detect errors in retinal layer segmentation.

Results

Cataract grade based on wide-field fundus imaging was positively correlated with grade of posterior subcapsular cataracts (rho = 0.486, p < 0.001), but not with nuclear opalescence or cortical cataract using the Lens Opacity Classification System III. Cataract grade was negatively correlated with total macular thickness (rho = −0.509, p < 0.001) and SD-OCT SNR (rho = −0.568, p < 0.001). SD-OCT SNR was positively correlated with total macular thickness (rho = 0.571, p < 0.001). Of 200 eyes, 97 (48.5%) had segmentation errors on SD-OCT. As cataract grade increased and SD-OCT SNR decreased, the percentage of eyes with segmentation errors on SD-OCT increased. All measurements of macular thickness in eyes without segmentation errors were significantly greater than those of eyes with segmentation errors.

Conclusions

Posterior subcapsular cataracts had profound effects on cataract grade based on wide-field fundus imaging. As cataract grade based on wide-field fundus image increased, macular thickness tended to be underestimated due to segmentation errors in SD-OCT images. Segmentation errors in SD-OCT should be considered when evaluating macular thickness in eyes with cataracts.

Parameters of ocular fundus on spectral-domain optical coherence tomography for glaucoma diagnosis

In this review, we summarize the progression of several parameters assessed by spectral-domain optical coherence tomography (SD-OCT) in recent years for the detection of glaucoma. Monitoring the progression of defects in the retinal nerve fiber layer (RNFL) thickness is essential. Imaging and analysis of retinal ganglion cells (RGCs) and inner plexiform layer (IPL), respectively, have been of great importance. Optic nerve head (ONH) topography obtained from 3D SD-OCT images is another crucial step. Other important assessments involve locating the Bruch's membrane opening (BMO), estimating the optic disc size and rim area, and measuring the lamina cribrosa displacement. Still other parameters found in the past three years for glaucoma diagnosis comprise central retinal artery resistive index, optic disc perfusion in optical coherence tomography angiography (OCTA) study, peripapillary choroidal thickness, and choroidal area in SD-OCT. Recently, several more ocular fundus parameters have been found, and compared with the earlier parameters to judge the accuracy of diagnosis. While a few of these parameters have been widely used in clinical practice, a fair number are still in the experimental stage.

Posterior rat eye during acute intraocular pressure elevation studied using polarization sensitive optical coherence tomography

Polarization sensitive optical coherence tomography (PS-OCT) operating at 840 nm with axial resolution of 3.8 µm in tissue was used for investigating the posterior rat eye during an acute intraocular pressure (IOP) increase experiment. IOP was elevated in the eyes of anesthetized Sprague Dawley rats by cannulation of the anterior chamber. Three dimensional PS-OCT data sets were acquired at IOP levels between 14 mmHg and 105 mmHg. Maps of scleral birefringence, retinal nerve fiber layer (RNFL) retardation and relative RNFL/retina reflectivity were generated in the peripapillary area and quantitatively analyzed. All investigated parameters showed a substantial correlation with IOP. In the low IOP range of 14-45 mmHg only scleral birefringence showed statistically significant correlation. The polarization changes observed in the PS-OCT imaging study presented in this work suggest that birefringence of the sclera may be a promising IOP-related parameter to investigate.

The macula in pediatric glaucoma: quantifying the inner and outer layers via optical coherence tomography automatic segmentation

BACKGROUND

Recent Spectralis (Heidelberg, Germany) spectral domain optical coherence tomography (SD-OCT) research software can automatically quantify the thickness of each individual retinal layer. The macular ganglion cell layer (GCL) and ganglion cell complex may be more sensitive for detecting glaucoma than the peripapillary retinal nerve fiber layer (pRNFL). The aim of this study was to characterize and compare the volume of each macular layer in the eyes of children with glaucoma versus those of normal controls.

METHODS

The medical records of children with primary glaucoma and physiologic cupping who had undergone Spectralis SD-OCT imaging of the macula and pRNFL were reviewed retrospectively. Controls were recruited from a separate prospective study. Children with refractive error of <±5 or retinal or neurologic abnormalities were excluded. The average volume of each of the 8 retinal layers in the macula (central 6 mm) and pRNFL were compared among diagnostic groups.

RESULTS

A total of 80 eyes of 80 children were included: 37 glaucoma eyes (25 with primary congenital and 12 with juvenile open-angle glaucoma) and 43 nonglaucoma eyes (28 with physiologic cupping). Eyes with glaucoma had significantly thinner mean macular nerve fiber layers, ganglion cell layers, inner plexiform layers, and pRNFLs than nonglaucomatous eyes: 0.82 ± 0.24 μm versus 1.00 ± 0.12 μm; 0.93 ± 0.22 μm versus 1.13 ± 0.10 μm; 0.80 ± 0.14 μm versus 0.91 ± 0.07 μm; 81.6 ± 26.5 μm versus 102.7 ± 10.0 μm, respectively (P < 0.00556 for all). Eyes without cupping and those with physiologic cupping were equivalent for all variables tested.

CONCLUSIONS

Children with glaucoma have thinning of the three innermost retinal macular layers.

Understanding Glaucomatous Optic Neuropathy: The Synergy Between Clinical Observation and Investigation

Glaucoma is a complex disorder of aging defined by the death of retinal ganglion cells and remodeling of connective tissues at the optic nerve head. Intraocular pressure-induced axonal injury at the optic nerve head leads to apoptosis. Loss of retinal ganglion cells follows a slowly progressive sequence. Clinical features of the disease have suggested and corroborated pathological events. The death of retinal ganglion cells causes secondary loss of neurons in the brain, but only as a by-product of injury to the retinal ganglion cells. Although therapy to lower intraocular pressure is moderately effective, new treatments are being developed to alter the remodeling of ocular connective tissue, to interrupt the injury signal from axon to soma, and to upregulate a variety of survival mechanisms.

Longitudinal Change of Circumpapillary Retinal Nerve Fiber Layer Thickness in Children With Optic Pathway Gliomas

PURPOSE

To evaluate longitudinal changes in circumpapillary retinal nerve fiber layer (RNFL) thickness, as measured by spectral-domain optical coherence tomography (SD OCT), in children with optic pathway gliomas.

DESIGN

Longitudinal cohort study.

METHODS

Global and quadrant-specific circumpapillary RNFL thickness measures were acquired using either a hand-held SD OCT during sedation or a table-top SD OCT in children old enough to cooperate. Vision loss was defined as either a 0.2 logMAR decline in visual acuity or progression of visual field. Percent change in circumpapillary RNFL thickness in eyes experiencing vision loss was compared to eyes with stable vision.

RESULTS

Fifty-five eyes completed 250 study visits. Ten eyes (18%) from 7 patients experienced a new episode of vision loss during the study and 45 eyes (82%) from 39 patients demonstrated stable vision across study visits. Percent decline of RNFL thickness between the baseline visit and first event of vision loss event was greatest in the superior (-14%) and inferior (-10%) quadrants as well as global average (-13%). Using a threshold of ≥10% decline in RNFL, the positive and negative predictive value for vision loss when 2 or more anatomic sectors were affected was 100% and 94%, respectively.

CONCLUSIONS

Children experiencing vision loss from their optic pathway gliomas frequently demonstrate a ≥10% decline of RNFL thickness in 1 or more anatomic sectors. Global average and the inferior quadrant demonstrated the best positive and negative predictive values. Circumpapillary RNFL is a surrogate marker of vision and could be helpful in making treatment decisions for children with optic pathway gliomas.

Relating Retinal Ganglion Cell Function and Retinal Nerve Fiber Layer (RNFL) Retardance to Progressive Loss of RNFL Thickness and Optic Nerve Axons in Experimental Glaucoma

PURPOSE

To relate changes in retinal function and retinal nerve fiber layer (RNFL) retardance to loss of RNFL thickness and optic nerve axon counts in a nonhuman primate (NHP) model of experimental glaucoma (EG).

METHODS

Bilateral longitudinal measurements of peripapillary RNFL thickness (spectral-domain optical coherence tomography, SDOCT; Spectralis), retardance (GDxVCC), and multifocal electroretinography (mfERG; VERIS) were performed in 39 NHP at baseline (BL; median, 5 recordings; range, 3-10) and weekly after induction of unilateral EG by laser photocoagulation of the trabecular meshwork. Multifocal ERG responses were high-pass filtered (>75 Hz) to measure high- and low-frequency component (HFC and LFC) amplitudes, including LFC features N1, P1, and N2. High-frequency component amplitudes are known to specifically reflect retinal ganglion cell (RGC) function. Complete (100%) axon counts of orbital optic nerves were obtained in 31/39 NHP.

RESULTS

Postlaser follow-up was 10.4 ± 7.9 months; mean and peak IOP were 18 ± 5 and 41 ± 11 mm Hg in EG eyes, 11 ± 2 and 18 ± 6 mm Hg in control (CTL) eyes. At the final available time point, RNFL thickness had decreased from BL by 14 ± 14%, retardance by 20 ± 11%, and the mfERG HFC by 30 ± 17% (P < 0.0001 each). Longitudinal changes in retardance and HFC were linearly related to RNFL thickness change (R2 = 0.51, P < 0.0001 and R2 = 0.22, P = 0.002, respectively); LFC N2 was weakly related but N1 or P2 (N1: R2 = 0.07, P = 0.11; P1: R2 = 0.04, P = 0.24; N2: R2 = 0.13, P = 0.02). At zero change from BL for RNFL thickness (Y-intercept), retardance was reduced by 11% (95% confidence interval [CI]: -15.3% to -6.8%) and HFC by 21.5% (95% CI: -28.7% to -14.3%). Relative loss of RNFL thickness, retardance, and HFC (EG:CTL) were each related to axon loss (R2 = 0.66, P < 0.0001; R2 = 0.42, P < 0.0001; R2 = 0.42, P < 0.0001, respectively), but only retardance and HFC were significantly reduced at zero relative axon loss (Y-intercept; retardance: -9.4%, 95% CI: -15.5% to -3.4%; HFC: -10.9%, 95% CI: -18.6% to -3.2%; RNFL thickness: +1.8%, 95% CI: -4.9% to +5.4%).

CONCLUSIONS

Retinal nerve fiber layer retardance and RGC function exhibit progressive loss from baseline before any loss of RNFL thickness or orbital optic nerve axons occurs in NHP EG. These in vivo measures might serve as potential biomarkers of early-stage glaucomatous damage preceding axon loss and RGC death.

Detecting optic nerve head deformation and retinal nerve fiber layer thinning in glaucoma progression

The application of digital imaging technologies including confocal scanning laser ophthalmoscopy (CSLO), optical coherence tomography (OCT), and scanning laser polarimetry (SLP) has significantly improved the detection of optic nerve head (ONH) deformation and progressive retinal nerve fiber layer (RNFL) thinning for assessment of glaucoma progression. Algorithms for change analysis such as topographic change analysis and guided progression analysis perform event analysis of serial ONH surface height topology maps and RNFL thickness/RNFL retardance maps, respectively, providing a topographical display of the location of significant change. With spectral-domain OCT, it is feasible to delineate and measure the lamina cribrosa surface depth in addition to ONH surface depth and RNFL thickness. Growing evidence from experimental and clinical studies indicates that ONH and lamina cribrosa deformation can be observed prior to detectable RNFL thinning and functional loss in glaucoma. These findings lend support to the notion that upon detection of ONH/lamina cribrosa deformation, a time window for therapeutic intervention for better outcomes may exist. The ONH and the lamina cribrosa are therefore important targets for monitoring glaucoma progression. This review summarizes the latest findings comparing the performance of OCT, CSLO, and SLP for detection of progressive ONH and RNFL damages in glaucoma patients and the clinical implication and limitations of studying the morphological alteration of the ONH, lamina cribrosa, and RNFL in the assessment of glaucoma progression.

Ophthalmic imaging

INTRODUCTION OR BACKGROUND

The last two decades have seen a revolution in ophthalmic imaging. In this review we present an overview of the breadth of ophthalmic imaging modalities in use today and describe how the role of ophthalmic imaging has changed from documenting abnormalities visible on clinical examination to the detection of clinically silent abnormalities which can lead to an earlier and more precise diagnosis.

SOURCES OF DATA

This review is based on published literature in the fields of ophthalmic imaging and with focus on most commonly used imaging modalities.

AREAS OF AGREEMENT

New imaging techniques enable non-invasive evaluation of ocular structures at a resolution of a few micrometres. This has led to a re-evaluation of diagnostic criteria for ocular disease, which were previously defined by clinical findings without significant reference to imaging.

AREAS OF CONTROVERSY

Lack of formal training and clinical guidelines regarding use of new imaging techniques in diagnosing and monitoring various ocular conditions. Lack of large normative databases and interchangeability issues between different commercial machines can hinder the detection of disease progression.

GROWING POINTS

Imaging devices are being constantly refined with improved image capture and image analysis tools.

AREAS TIMELY FOR DEVELOPING RESEARCH

Clinical applications of new techniques and devices have yet to be determined using systematic scientific research methods.

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.

Onset and progression of peripapillary retinal nerve fiber layer (RNFL) retardance changes occur earlier than RNFL thickness changes in experimental glaucoma

PURPOSE

Longitudinal measurements of peripapillary RNFL thickness and retardance were compared in terms of time to reach onset of damage and time to reach a specific progression endpoint.

METHODS

A total of 41 rhesus macaques with unilateral experimental glaucoma (EG) each had three or more weekly baseline measurements in both eyes of peripapillary RNFL thickness (RNFLT) and retardance. Laser photocoagulation was then applied to the trabecular meshwork of one eye to induce chronic elevation of intraocular pressure and weekly imaging continued. Pairwise differences between baseline observations were sampled by bootstrapping to determine the 95% confidence limits of each measurement's repeatability. The first two sequential measurements below the lower confidence limit defined the endpoint for each parameter. Segmented linear and exponential decay functions were fit to each RNFL-versus-time series to determine the time to damage onset.

RESULTS

In all, 29 (71%) of the EG eyes reached endpoint by RNFL retardance and 25 (61%) reached endpoint by RNFLT. In total, 33 (80%) reached endpoint by at least one of the RNFL parameters and 21 (51%) reached endpoint by both RNFL parameters. Of the 33 EG eyes reaching any endpoint, a larger proportion reached endpoint first by retardance (n = 26, 79%) than did by RNFLT (n = 7, 21%; P = 0.002). Survival analysis indicated a shorter time to reach endpoint by retardance than by RNFLT (P < 0.001). Of the 21 EG eyes that reached endpoint by both measures, the median duration to endpoint was 120 days for retardance and 223 days for RNFLT (P = 0.003, Wilcoxon test). The time to onset was faster for retardance than that for RNFLT based on either segmented fits (by 31 days; P = 0.008, average R(2) = 0.89) or exponential fits (by 102 days; P = 0.01, average R(2) = 0.89).

CONCLUSIONS

The onset of progressive loss of RNFL retardance occurs earlier than the onset of RNFL thinning. Endpoints of progressive loss from baseline also occurred more frequently and earlier for RNFL retardance as compared with RNFLT.