Retinal Nerve Fiber Layer Birefringence Research Articles

Observation on the changes of visual field and optic nerve fiber layer thickness in patients with early diabetic retinopathy

BackgroundDiabetic retinopathy (DR) is a common complication of diabetes mellitus (DM) and is a leading cause of vision loss. Early detection of DR-related neurodegenerative changes is crucial for effective management and prevention of vision loss in diabetic patients. MethodsIn this study, we employed spectral-domain polarization-sensitive optical coherence tomography (SD PS-OCT) to assess retinal nerve fiber layer (RNFL) changes in 120 eyes from 60 types 1 DM patients without clinical DR and 60 age-matched healthy controls. Visual field testing was performed to evaluate mean sensitivity (MS) and mean defect (MD) as indicators of visual function. ResultsSD PS-OCT measurements revealed significant reductions in RNFL birefringence, retardation, and thickness in type 1 DM patients compared to healthy controls. Visual field testing showed decreased MS and increased MD in DM patients, indicating functional impairment correlated with RNFL alterations. ConclusionOur findings demonstrate early neurodegenerative changes in the RNFL of type 1 DM patients without clinical DR, highlighting the potential of SD PS-OCT as a sensitive tool for early detection of subclinical DR-related neurodegeneration. These results underscore the importance of regular ophthalmic screenings in diabetic patients to enable timely intervention and prevent vision-threatening complications.

Early Identification of Retinal Neuropathy in Subclinical Diabetic Eyes by Reduced Birefringence of the Peripapillary Retinal Nerve Fiber Layer.

PurposeTo study birefringence of the peripapillary retinal nerve fiber layer (RNFL) of diabetic eyes with no clinical signs of diabetic retinopathy (DR) or mild to moderate DR stages using spectral-domain polarization-sensitive (PS) optical coherence tomography (OCT).MethodsIn this observational pilot study, circular PS-OCT scans centered on the optic nerve head were recorded in prospectively recruited diabetic and age-matched healthy eyes. From averaged circumpapillary intensity and retardation tomograms plots of RNFL birefringence were obtained by a linear fit of retardation versus depth within the RNFL tissue for each A-scan position and mean birefringence values for RNFL calculated. Spectral-domain OCT imaging (Heidelberg Engineering) was performed to assess peripapillary RNFL thickness and macular ganglion cell complex (GCC).ResultsOut of 70 eyes of 43 diabetic patients (mean ± SD age: 50.86 ± 15.71) 36 showed no signs of DR, 17 mild and 17 moderate nonproliferative DR with no diabetic macular edema. Thirty-four eyes of 34 healthy subjects (53.21 ± 13.88 years) served as controls. Compared with healthy controls (0.143° ± 0.014°/µm) mean total birefringence of peripapillary RNFL was significantly reduced in subclinical diabetic eyes (0.131° ± 0.014°/µm; P = 0.0033), as well as in mild to moderate DR stages (0.125° ± 0.018°/µm, P < 0.0001) with borderline statistically significant differences between diabetic patients (P = 0.0049). Mean birefringence values were significantly lower in inferior compared with superior RNFL sectors (P < 0.0001) of diabetic eyes with no such difference detected in the healthy control group.ConclusionsWe identified evidence of early neuroretinal alteration in diabetic eyes through reduced peripapillary RNFL birefringence assessed by PS-OCT occurring before appearance of clinical microvascular lesions or GCC alterations.

Analysis of retinal nerve fiber layer birefringence in patients with glaucoma and diabetic retinopathy by polarization sensitive OCT.

The retinal nerve fiber layer (RNFL) is a fibrous tissue that shows form birefringence. This optical tissue property is related to the microstructure of the nerve fiber axons that carry electrical signals from the retina to the brain. Ocular diseases that are known to cause neurologic changes, like glaucoma or diabetic retinopathy (DR), might alter the birefringence of the RNFL, which could be used for diagnostic purposes. In this pilot study, we used a state-of-the-art polarization sensitive optical coherence tomography (PS-OCT) system with an integrated retinal tracker to analyze the RNFL birefringence in patients with glaucoma, DR, and in age-matched healthy controls. We recorded 3D PS-OCT raster scans of the optic nerve head area and high-quality averaged circumpapillary PS-OCT scans, from which RNFL thickness, retardation and birefringence were derived. The precision of birefringence measurements was 0.005°/µm. As compared to healthy controls, glaucoma patients showed a slightly reduced birefringence (0.129 vs. 0.135°/µm), although not statistically significant. The DR patients, however, showed a stronger reduction of RNFL birefringence (0.103 vs. 0.135°/µm) which was highly significant. This result might open new avenues into early diagnosis of DR and related neurologic changes.

Optic axis uniformity as a metric to improve the contrast of birefringent structures and analyze the retinal nerve fiber layer in polarization-sensitive optical coherence tomography.

A new metric is used to improve the contrast of birefringent structures in biological tissue using polarization-sensitive optical coherence tomography. This metric, optic axis uniformity (OAxU), is based on the optic axis of birefringence and quantifies the uniformity of the optic axis direction. OAxU provides surprisingly strong contrast for fibrous structures such as muscle and the retinal nerve fiber layer (RNFL). We used OAxU for automatic segmentation of the RNFL in human eyes. From the segmentation, en face images of RNFL thickness and RNFL birefringence were created. The measured birefringence values are consistent with earlier reports.

Reflectance Spectrum and Birefringence of the Retinal Nerve Fiber Layer With Hypertensive Damage of Axonal Cytoskeleton.

PurposeGlaucoma damages the retinal nerve fiber layer (RNFL). This study used precise multimodal image registration to investigate the changes of the RNFL reflectance spectrum and birefringence in nerve fiber bundles with different degrees of axonal damage.MethodsThe reflectance spectrum of individual nerve fiber bundles in normal rats and rats with experimental glaucoma was measured from 400 to 830 nm and their birefringence was measured at 500 nm. Optical measurements of the same bundles were made at different distances from the optic nerve head (ONH). After the optical measurements, the axonal cytoskeleton of the RNFL was evaluated by confocal microscopy to assess the severity of cytoskeletal change.ResultsFor normal bundles, the shape of the RNFL reflectance spectrum and the value of RNFL birefringence did not change along bundles. In treated retinas, damage to the cytoskeleton varied within and across retinas. The damage in retinal sectors was subjectively graded from normal-looking to severe. Change of spectral shape occurred near the ONH in all sectors studied. This change became more prominent and occurred farther from the ONH with increased damage severity. In contrast, RNFL birefringence did not show change in normal-looking sectors, but decreased in sectors with mild and moderate damage. The birefringence of severely damaged sectors was either within or below the normal range.ConclusionsVarying degrees of cytoskeletal damage affect the RNFL reflectance spectrum and birefringence differently, supporting differences in the ultrastructural basis for the two optical properties. Both properties, however, may provide a means to detect disease and to estimate ultrastructural damage of the RNFL in glaucoma.

Cytoskeletal Alteration and Change of Retinal Nerve Fiber Layer Birefringence in Hypertensive Retina

ABSTRACTPurpose: Glaucoma damages the retinal nerve fiber layer (RNFL). Both RNFL thickness and retardance can be used to assess the damage, but birefringence, the ratio of retardance to thickness, is a property of the tissue itself. This study investigated the relationship between axonal cytoskeleton and RNFL birefringence in retinas with hypertensive damage.Materials and Methods: High intraocular pressure (IOP) was induced unilaterally in rat eyes. RNFL retardance in isolated retinas was measured. Cytostructural organization and bundle thickness were evaluated by confocal imaging of immunohistochemical staining of the cytoskeletal components: microtubules (MTs), F-actin, and neurofilaments. Bundles with different appearances of MT stain were studied, and their birefringence was calculated at different radii from the optic nerve head (ONH) center.Results: Forty bundles in eight normal retinas and 37 bundles in 10 treated retinas were examined. In normal retinas, the stain of axonal cytoskeleton was approximately uniform within bundles, and RNFL birefringence did not change along bundles. In treated retinas, elevation of IOP caused non-uniform alteration of axonal cytoskeleton across the retina, and distortion of axonal MTs was associated with decreased birefringence. The study further demonstrated that change of RNFL birefringence profiles along bundles can imply altered axonal cytoskeleton, suggesting that ultrastructural change of the RNFL can be inferred from clinical measurements of RNFL birefringence. The study also demonstrated that measuring RNFL birefringence profiles along bundles, instead of at a single location, may provide a more sensitive way to detect axonal ultrastructural change.Conclusions: Measurement of RNFL birefringence along bundles can provide estimation of cytoskeleton alteration and sensitive detection of glaucomatous damage.

Polarization properties of single layers in the posterior eyes of mice and rats investigated using high resolution polarization sensitive optical coherence tomography.

We present a high resolution polarization sensitive optical coherence tomography (PS-OCT) system for ocular imaging in rodents. The system operates at 840 nm and uses a broadband superluminescent diode providing an axial resolution of 5.1 µm in air. PS-OCT data was acquired at 83 kHz A-scan rate by two identical custom-made spectrometers for orthogonal polarization states. Pigmented (Brown Norway, Long Evans) and non-pigmented (Sprague Dawley) rats as well as pigmented mice (C57BL/6) were imaged. Melanin pigment related depolarization was analyzed in the retinal pigment epithelium (RPE) and choroid of these animals using the degree of polarization uniformity (DOPU). For all rat strains, significant differences between RPE and choroidal depolarization were observed. In contrast, DOPU characteristics of RPE and choroid were similar for C57BL/6 mice. Moreover, the depolarization within the same tissue type varied significantly between different rodent strains. Retinal nerve fiber layer thickness, phase retardation, and birefringence were mapped and quantitatively measured in Long Evans rats in vivo for the first time. In a circumpapillary annulus, retinal nerve fiber layer birefringence amounted to 0.16°/µm ± 0.02°/µm and 0.17°/µm ± 0.01°/µm for the left and right eyes, respectively.

Scanning Laser Polarimetry, but Not Optical Coherence Tomography Predicts Permanent Visual Field Loss in Acute Nonarteritic Anterior Ischemic Optic Neuropathy

Scanning laser polarimetry (SLP) reveals abnormal retardance of birefringence in locations of the edematous peripapillary retinal nerve fiber layer (RNFL), which appear thickened by optical coherence tomography (OCT), in nonarteritic anterior ischemic optic neuropathy (NAION). We hypothesize initial sector SLP RNFL abnormalities will correlate with long-term regional visual field loss due to ischemic injury. We prospectively performed automated perimetry, SLP, and high definition OCT (HD-OCT) of the RNFL in 25 eyes with acute NAION. We grouped visual field threshold and RNFL values into Garway-Heath inferior/superior disc sectors and corresponding superior/inferior field regions. We compared sector SLP RNFL thickness with corresponding visual field values at presentation and at >3 months. At presentation, 12 eyes had superior sector SLP reduction, 11 of which had inferior field loss. Six eyes, all with superior field loss, had inferior sector SLP reduction. No eyes had reduced OCT-derived RNFL acutely. Eyes with abnormal field regions had corresponding SLP sectors thinner (P = 0.003) than for sectors with normal field regions. During the acute phase, the SLP-derived sector correlated with presentation (r = 0.59, P = 0.02) and with >3-month after presentation (r = 0.44, P = 0.02) corresponding superior and inferior field thresholds. Abnormal RNFL birefringence occurs in sectors corresponding to regional visual field loss during acute NAION when OCT-derived RNFL shows thickening. Since the visual field deficits show no significant recovery, SLP can be an early marker for axonal injury, which may be used to assess recovery potential at RNFL locations with respect to new treatments for acute NAION.

Measuring Retinal Nerve Fiber Layer Birefringence, Retardation, and Thickness Using Wide-Field, High-Speed Polarization Sensitive Spectral Domain OCT

We presented a novel polarization sensitive optical coherence tomography (PS-OCT) system for measuring retinal nerve fiber layer (RNFL) birefringence, retardation, and thickness, and report on the repeatability of acquiring these quantities. A new PS-OCT system, measuring at 840 nm, was developed that supports scan angles of up to 40° × 40° with an A-scan rate of 70 kHz. To test the performance and reproducibility, we measured 10 eyes of 5 healthy human volunteers five times each. All volunteers were imaged further with scanning laser polarimetry (SLP). The obtained RNFL birefringence, retardation, and thickness maps were averaged, and standard deviation maps were calculated. For quantitative comparison between the new PS-OCT and SLP, a circumpapillary evaluation within 2 annular segments (superior and inferior to the optic disc) was performed. High quality RNFL birefringence, retardation, and thickness maps were obtained. Within the superior and inferior segments, the mean retardation for individual eyes ranged from 20° to 28.9° and 17.2° to 28.2°, respectively. The quadrant precision over the 5 consecutive measurements for each subject, calculated for the average retardation obtained within the superior and inferior quadrants ranged from 0.16° to 0.69°. The mean birefringence ranged from 0.106°/μm to 0.141°/μm superior and 0.101°/μm to 0.135°/μm inferior, with a quadrant precision of 0.001°/μm to 0.007°/μm. The mean RNFL thickness varied from 114 to 150 μm superior, and 111 to 140.9 μm inferior (quadrant precision ranged from 3.6 to 11.9 μm). The new PS-OCT system showed high image quality and reproducibility, and, therefore, might be a valuable tool for glaucoma diagnosis.

Structural-functional dissociation in glaucoma: An attempt to end controversy

Dear Editor, We read with interest the article titled “Evidence-based approach to glaucoma management” by Garudadri et al.[1] Standard automated perimetry (SAP) is the current standard in glaucoma diagnosis, but 20-40% retinal ganglion cells could be lost before a visual field defect could be picked up by SAP. The reported sensitivity and specificity of image technologies and their agreement with each other are far from ideal. Brain and eye have adaptive strategies to compensate scotoma. Safran and Landis[2] in a review article described the effects of plasticity in the adult visual cortex. Plasticity plays a crucial role in recovery from damage to the visual system. Cortical remapping generates a filling-in of visual field defects. Cortical rearrangement following lesions in the visual pathways helps to compensate for gaps in perception. Filling-in is a major cause of failure of patients with simple chronic glaucoma to recognize their visual defects at an early stage. Qing et al.,[3] have documented that more affected the visual field (SAP), the less affected the functional magnetic resonance image response. This phenomenon may imply a replasticity or a compensatory mechanism of the visual pathway. Peripheral scotoma may have a potentiating effect on the central reserved visual field. Decreased retina input may result in replasticity of the cortical neurons and enhance the information processing and integrity ability of the visual cortex. Measurement of blue chromatic thresholds in the macular region might make it possible to detect functional damage due to glaucoma prior to morphological changes of the optic nerve head of glaucoma patients. Probably a combination of decreased peripheral sensitivity with increased central retinal sensitivity may be suggestive of early glaucoma. Alzheimer's disease and Parkinson's disease cause retinal nerve fiber loss (RNFL). Evidence shows that microtubules in nerve fiber bundles are the major contributors for retinal nerve fiber layer birefringence, and microtubules are expected to disappear after the ganglion cells die. It remains to be clarified whether the time lag between cell death and microtubules disappearance or other subcellular changes differ between primary open angle glaucoma (POAG) and primary angle closure glaucoma. Preferred retinal locus (PRL) and oculomotor re-referencing are adaptive strategies of eye to compensate scotoma.[4] A PRL can be defined as a discrete retinal area that contains the center of target image for greater than 20% of a fixation interval. Oculomotor re-referencing is a phenomenon seen in patients who describe themselves as looking straight ahead when they are fixating with PRL (i.e., when the eye is not in the primary position). Fixation instability is a feature documented in early and moderate POAG.[5] This may be considered as an evidence for ocular adaptive strategy to compensate scotoma because eccentric fixation induces instability. Probably a combination of RNFL and recent onset fixation instability may improve sensitivity and specificity of image technologies. In conclusion, we agree with the authors advice that good history taking, proper clinical examination, documentation of details, supplemented with appropriate investigations, counseling, and proper follow-up enhance the care of the patient and family.

Thickness, Phase Retardation, Birefringence, and Reflectance of the Retinal Nerve Fiber Layer in Normal and Glaucomatous Non-Human Primates

We identified candidate optical coherence tomography (OCT) markers for early glaucoma diagnosis. Time variation of retinal nerve fiber layer (RNFL) thickness, phase retardation, birefringence, and reflectance using polarization sensitive optical coherence tomography (PS-OCT) were measured in three non-human primates with induced glaucoma in one eye. We characterized time variation of RNFL thickness, phase retardation, birefringence, and reflectance with elevated intraocular pressure (IOP). One eye of each of three non-human primates was laser treated to increase IOP. Each primate was followed for a 30-week period. PS-OCT measurements were recorded at weekly intervals. Reflectance index (RI) is introduced to characterize RNFL reflectance. Associations between elevated IOP and RNFL thickness, phase retardation, birefringence, and reflectance were characterized in seven regions (entire retina, inner and outer rings, and nasal, temporal, superior and inferior quadrants) by linear and non-linear mixed-effects models. Elevated IOP was achieved in three non-human primate eyes with an average increase of 13 mm Hg over the study period. Elevated IOP was associated with decreased RNFL thickness in the nasal region (P = 0.0002), decreased RNFL phase retardation in the superior (P = 0.046) and inferior (P = 0.021) regions, decreased RNFL birefringence in the nasal (P = 0.002) and inferior (P = 0.029) regions, and loss of RNFL reflectance in the outer rings (P = 0.018). When averaged over the entire retinal area, only RNFL reflectance showed a significant decrease (P = 0.028). Of the measured parameters, decreased RNFL reflectance was the most robust correlate with glaucomatous damage. Candidate cellular mechanisms are considered for decreased RNFL reflectance, including mitochondrial dysfunction and retinal ganglion cell apoptosis.

Quantification of Change in Axonal Birefringence Following Surgical Reduction in Intraocular Pressure

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

The Relationship between Retinal Ganglion Cell Axon Constituents and Retinal Nerve Fiber Layer Birefringence in the Primate

To determine the degree of correlation between spatial characteristics of the retinal nerve fiber layer (RNFL) birefringence (Delta n(RNFL)) surrounding the optic nerve head (ONH) with the corresponding anatomy of retinal ganglion cell (RGC) axons and their respective organelles. RNFL phase retardation per unit depth (PR/UD, proportional to Delta n(RNFL)) was measured in two cynomolgus monkeys by enhanced polarization-sensitive optical coherence tomography (EPS-OCT). The monkeys were perfused with glutaraldehyde and the eyes were enucleated and prepared for transmission electron microscopy (TEM) histologic analysis. Morphologic measurements from TEM images were used to estimate neurotubule density (rho(RNFL)), axoplasmic area (A(x)) mode, axon area (A(a)) mode, slope (u) of the number of neurotubules versus axoplasmic area (neurotubule packing density), fractional area of axoplasm in the nerve fiber bundle (f), mitochondrial fractional area in the nerve fiber bundle (x(m)), mitochondria-containing axon profile fraction (m(p)), and length of axonal membrane profiles per unit of nerve fiber bundle area (L(am)/A(b)). Registered PR/UD and morphologic parameters from corresponding angular sections were then correlated by using Pearson's correlation and multilevel models. In one eye there was a statistically significant correlation between PR/UD and rho(RNFL) (r = 0.67, P = 0.005) and between PR/UD and neurotubule packing density (r = 0.70, P = 0.002). Correlation coefficients of r = 0.81 (P = 0.01) and r = 0.50 (P = 0.05) were observed between the PR/UD and A(x) modes for each respective subject. Neurotubules are the primary source of birefringence in the RNFL of the primate retina.

Retinal Imaging by Laser Polarimetry and Optical Coherence Tomography Evidence of Axonal Degeneration in Multiple Sclerosis

Optical coherence tomography (OCT) and scanning laser polarimetry with variable corneal compensation (GDx) are similar yet provide information on different aspects of retinal nerve fiber layer (RNFL) structure (thickness values similar to histology for OCT vs birefringence of microtubules for GDx). To compare the ability of OCT and GDx to distinguish eyes of patients with multiple sclerosis (MS) from eyes of disease-free controls and thus identify RNFL abnormalities. We also sought to examine the capacity of these techniques to distinguish MS eyes from those without a history of optic neuritis and to correlate with visual function. Cross-sectional study. Academic tertiary care MS center. Eighty patients with MS (155 eyes) and 43 disease-free controls (85 eyes) underwent both OCT and GDx imaging using protocols that measure RNFL thickness. Areas under the curve (AUC), adjusted for within-patient, intereye correlations, were used to compare the abilities of OCT and GDx temporal-superior-nasal-inferior-temporal average RNFL thicknesses to discriminate between MS and control eyes and to distinguish MS eyes with a history of optic neuritis. Visual function was evaluated using low-contrast letter acuity and high-contrast visual acuity. Average peripapillary RNFL thickness (360 degrees around the optic disc) was reduced in patients with MS compared with controls for both methods. Age-adjusted AUC did not differ between OCT (0.80; 95% confidence interval [CI], 0.72-0.88) and GDx (0.78; 95% CI, 0.68-0.86; P = .38). Optical coherence tomography-measured RNFL thickness was somewhat better at distinguishing MS eyes with a history of optic neuritis from those without (OCT: AUC, 0.73; 95% CI, 0.64-0.82; GDx: AUC, 0.66; 95% CI, 0.57-0.66; P = .17). Linear correlations of RNFL thickness for OCT vs GDx were significant yet moderate (r = 0.67, P < .001); RNFL thickness measures correlated moderately and significantly with low-contrast acuity (OCT: r = 0.54, P < .001; GDx: r = 0.55, P < .001) and correlated less with high-contrast visual acuity (OCT: r = 0.44, P < .001; GDx: r = 0.32, P < .001). Scanning laser polarimetry with variable corneal compensation measurements of RNFL thickness corroborates OCT evidence of visual pathway axonal loss in MS and provides new insight into structural aspects of axonal loss that relate to RNFL birefringence (microtubule integrity). These results support validity for RNFL thickness as a marker for axonal degeneration and support use of these techniques in clinical trials that examine neuroprotective and other disease-modifying therapies.

Relative Course of Retinal Nerve Fiber Layer Birefringence and Thickness and Retinal Function Changes after Optic Nerve Transection

To test the hypothesis that alterations of RNFL birefringence precede changes in RNFL thickness in an experimental model of RGC injury and, secondarily, to determine the time course of RGC functional abnormalities relative to RNFL birefringence and thickness changes. RNFL birefringence was measured by scanning laser polarimetry (GDx VCC; Carl Zeiss Meditec, Inc., Dublin, CA). RNFL thickness was measured by spectral domain optical coherence tomography (SD-OCT, Spectralis HRA+OCT; Heidelberg Engineering, GmbH, Heidelberg, Germany). Retinal function was assessed by three forms of electroretinography (ERG): slow-sequence multifocal (mf)ERG (VERIS; EDI, San Mateo, CA); pattern-reversal (P)ERG (Utas-E3000; LKC Technologies, Inc. Gaithersburg, MD); and photopic full-field flash (ff)ERG (Utas-E3000; LKC Technologies). All measurements were obtained in both eyes of four adult rhesus macaque monkeys (Macaca mulatta) during two baseline sessions, and again 1 week and 2 weeks after unilateral optic nerve transection (ONT). ONT was successfully completed in three subjects. RNFL birefringence declined by 15% 1 week after ONT (P = 0.043), whereas there was no significant change in RNFL thickness (+1%, P = 0.42). Two weeks after ONT, RNFL retardance had declined by 39% (P = 0.018), whereas RNFL thickness had declined by only 15% (P = 0.025). RGC functional abnormalities were present 1 week after ONT, including decreased amplitudes relative to baseline of the mfERG high-frequency components (-65%, P = 0.018), the PERG N95 component (-70%, P = 0.007), and the photopic negative response of the ffERG (-44%, P = 0.005). RNFL birefringence declined before and faster than RNFL thickness after ONT. RGC functional abnormalities were present 1 week after ONT, when RNFL thickness had not yet begun to change. RNFL birefringence changes after acute RGC injury are associated with RGC dysfunction. Together, they reflect RGC abnormalities that precede axonal caliber changes and loss.

Retinal nerve fiber layer birefringence evaluated with polarization sensitive spectral domain OCT and scanning laser polarimetry: A comparison

A polarization-sensitive spectral domain optical coherence tomography (PS-SD-OCT) system is used to measure phase retardation and birefringence of the human retinal nerve fiber layer (RNFL) in vivo. The instrument records three parameters simultaneously: intensity, phase retardation and optic-axis orientation. 3D data sets are recorded in the optic nerve-head area of a healthy and a glaucomatous eye, and the results are presented in various ways: En-face phase-retardation maps of the RNFL are generated from the recorded 3D data and results are compared with scanning laser polarimetry (SLP). The depth information provided by OCT is used to segment the RNFL in the intensity image and measure the RNFL thickness. From the retardation and thickness data, 2D birefringence maps of the RNFL are derived. Circumpapillary plots of RNFL retardation and thickness obtained by PS-SD-OCT are quantitatively compared with those obtained by SLP.

Use of retinal nerve fiber layer birefringence as an addition to absorption in retinal scanning for biometric purposes

We built a device sensitive to the birefringence of the retinal nerve fiber layer for biometric purposes. A circle of 20 degrees diameter on the retina was scanned around the optic disk with a spot of light from a 785 nm laser diode. The nonbirefringent blood vessels indenting or displacing the retinal nerve fiber layer were seen as "blips" in the measured birefringence-derived signal. For comparison, the reflection-absorption signature of the blood vessel pattern in the scanned circle was also measured. The birefringence-derived signal proved to add useful information to the reflectance-absorption signature for retinal biometric scanning.

Intravitreal Colchicine Causes Decreased RNFL Birefringence without Altering RNFL Thickness

To test the hypothesis that longitudinal differences between retinal nerve fiber layer (RNFL) birefringence, measured by scanning laser polarimetry (SLP), and RNFL thickness, measured by optical coherence tomography (OCT), are informative about the state of axonal degeneration. Colchicine was injected into the vitreous cavity of one eye in each of six vervet monkeys (Chlorocebus sabaeus; estimated vitreal concentration: 1 mM, n = 3; 2 mM, n = 1; 10 mM, n = 2); an equivalent volume (approximately 0.1 mL) of sterile saline was injected into fellow control eyes. RNFL birefringence was measured by SLP before injection and every 10 minutes after injection for 2 hours. RNFL thickness was measured by OCT before injection and 2 hours later. After isolating each retina, biopsy specimens were obtained from the inferotemporal arcade region, approximately 2 mm from the center of the optic disc, using a 2-mm trephine and were processed for transmission electron microscopy (TEM). Retinas were then flat-mounted and stained with an antibody against polymerized beta-III-tubulin. RNFL birefringence measured by SLP decreased over time in all six colchicine-injected eyes, appearing to reach a plateau of -20% +/- 7% (P < 0.0001) approximately 100 minutes after injection. There were no significant differences between quadrants (P = 0.44) and no apparent dose effect (P = 0.87). The change in vehicle-injected control eyes was -3% +/- 3% (P = 0.06; NS). The change in RNFL thickness measured by OCT was +1% +/- 4% (P = 0.81; NS) in colchicine-injected eyes and +6% +/- 6% (P = 0.13; NS) in control eyes. There was no evidence of macular edema by fundus biomicroscopy, stereo fundus photography, or OCT. TEM revealed disorganization of microtubules, swelling of mitochondria, and blurred axonal membrane borders in colchicine-injected eyes. Flat-mounted retinas stained with an antibody against polymerized beta-III-tubulin showed only a mild reduction of peripapillary stain intensity in the colchicine-injected eyes compared with controls. Intravitreal injection of colchicine caused microtubule disruption within the axons of the RNFL in nonhuman primate eyes. This was manifest as a reduction of RNFL birefringence, without alteration of RNFL thickness, suggesting that such discrepancies can be informative about the status of axonal degeneration.

Microtubules Contribute to the Birefringence of the Retinal Nerve Fiber Layer

The retinal nerve fiber layer (RNFL) exhibits birefringence that is due to the oriented cylindrical structure of the ganglion cell axons. Possible birefringent structures include axonal membranes, microtubules (MTs), and neurofilaments. MTs are generally assumed to be a major contributor, but this has not been demonstrated. In this study, the MT depolymerizing agent colchicine was used to evaluate the contribution of MTs to RNFL birefringence. Retinal nerve fiber bundles of isolated rat retina were observed through an imaging polarimeter set near extinction. Images were taken over an extended period. During baseline, the tissue was perfused with a physiological solution. During a treatment period, the solution was switched either to a control solution identical with the baseline solution or to a similar solution containing colchicine. The contrast of nerve fiber bundles was used to follow change of RNFL birefringence over time. When imaged by the polarimeter, birefringent retinal nerve fiber bundles appeared as either bright or dark stripes. Bundles displayed as bright stripes were used to follow changes in retardance. The contrast of nerve fiber bundles was stable in control experiments. However, in treatment experiments, bundles were bright during the baseline period, but the contrast of bundles decreased rapidly when the colchicine solution was applied; bundles were barely visible after 30 minutes of treatment. After 70 minutes, the bundle contrast was close to zero at all wavelengths studied (440-780 nm). MTs make a significant contribution to RNFL birefringence. The decrease of RNFL birefringence in glaucoma may indicate a loss of MTs.

Birefringence of the primate retinal nerve fiber layer

The purpose of this study was to measure the peripapillary retinal nerve fiber layer (RNFL) thickness, phase retardation (PR), and depth-resolved birefringence (Δ n) of the normal primate eye using Enhanced Polarization-Sensitivity Optical Coherence Tomography (EPS-OCT). Both eyes of two rhesus monkeys were imaged with EPS-OCT. A multiple incident polarization state nonlinear fitting algorithm was used to determine RNFL phase retardation. RNFL thickness (RNFLT) was determined from the corresponding EPS-OCT intensity image and phase retardation per unit depth (PR/UD, proportional to Δ n) was calculated by dividing PR by RNFLT. Peripapillary area maps consisting of pixels uniformly distributed along a radius from 0.8 to 1.8 mm from the center of the optic nervehead were constructed for RNFLT, PR, and PR/UD. Average PR/UD in the superior and inferior quadrants was 18°/100 μm (Δ n=4.2×10 −4) and average PR/UD in the nasal and temporal quadrants was 6.3°/100 μm (Δ n=1.5×10 −4). Relative magnitude of PR radial gradient is similar to that of RNFLT radial gradient and no radial gradient was observed for PR/UD. Polarization-dependent amplitude attenuation per unit depth (PDAA/UD) was 0.02 rad/100 μm in thick RNFL regions. RNFL birefringence was higher in the arcuate bundles compared to nasal and temporal fibers ( P=0.001). Birefringence was nearly equal in nasal and temporal quadrants. No statistically significant ( P=0.01) radial gradient of birefringence was observed in any quadrant. RNFL birefringence is believed to originate from anisotropic structures within the cytoskeleton of the parallel axons. Birefringence differences presented in this study cannot be explained by the known axon diameter distribution around the optic nervehead and suggest other sources of the birefringence signal including neurotubules and neurofilaments.