The course of axons through the retina and optic nerve head

Lamina Cribrosa Microstructure in Nonhuman Primates With Naturally Occurring Peripapillary Retinal Nerve Fiber Layer Thinning

Purpose

The lamina cribrosa (LC) is hypothesized to be the site of initial axonal damage in glaucoma with the circumpapillary retinal nerve fiber layer thickness (RNFL-T) widely used as a standard metric for quantifying the glaucomatous damage. The purpose of this study was to determine in vivo, 3-dimensional (3D) differences in the microstructure of the LC in eyes of nonhuman primates (NHPs) with naturally occurring glaucoma.

Methods

Spectral-domain optical coherence tomography (OCT) scans (Leica, Chicago, IL, USA) of the optic nerve head were acquired from a colony of 50 adult rhesus monkeys suspected of having high prevalence of glaucoma. The RNFL-T was analyzed globally and in quadrants using a semi-automated segmentation software. From a set of 100 eyes, 18 eyes with the thinnest global RNFL-T were selected as the study group and 18 eyes with RNFL-T values around the 50th percentile were used as controls. A previously described automated segmentation algorithm was used for LC microstructure analysis. Parameters included beam thickness, pore diameter and their ratio (beam-to-pore ratio [BPR]), pore area and shape parameters, beam and pore volume, and connective tissue volume fraction (CTVF; beam volume/total volume). The LC microstructure was analyzed globally and in the following volumetric sectors: quadrants, central and peripheral lamina, and three depth slabs (anterior, middle, and posterior).

Results

Although no significant difference was detected between groups for age, weight, or disc size, the study group had significantly thinner RNFL than the control group (P < 0.01). The study group had significantly smaller global and sectoral pore diameter and larger BPR compared with the control group. Across eyes, the global RNFL-T was associated positively with pore diameter globally. BPR and CTVF were significantly and negatively associated with the corresponding RNFL-T in the superior quadrant.

Conclusions

Global and sectoral microstructural differences were detected when comparing thin and normal RNFL-T eyes. Whether these LC differences are the cause of RNFL damage or the result of remodeling of the LC requires further investigation.

Translational Relevance

Our findings indicate structural alterations in the LC of NHP exhibiting natural thinning of the RNFL, a common characteristic of glaucomatous damage.

Visible-Light Optical Coherence Tomography Fibergraphy of the Tree Shrew Retinal Ganglion Cell Axon Bundles

We seek to develop techniques for high-resolution imaging of the tree shrew retina for visualizing and parameterizing retinal ganglion cell (RGC) axon bundles in vivo. We applied visible-light optical coherence tomography fibergraphy (vis-OCTF) and temporal speckle averaging (TSA) to visualize individual RGC axon bundles in the tree shrew retina. For the first time, we quantified individual RGC bundle width, height, and cross-sectional area and applied vis-OCT angiography (vis-OCTA) to visualize the retinal microvasculature in tree shrews. Throughout the retina, as the distance from the optic nerve head (ONH) increased from 0.5 mm to 2.5 mm, bundle width increased by 30%, height decreased by 67%, and cross-sectional area decreased by 36%. We also showed that axon bundles become vertically elongated as they converge toward the ONH. Ex vivo confocal microscopy of retinal flat-mounts immunostained with Tuj1 confirmed our in vivo vis-OCTF findings.

Retrolaminar Demyelination of Structurally Intact Axons in Nonhuman Primate Experimental Glaucoma

Purpose

To determine if structurally intact, retrolaminar optic nerve (RON) axons are demyelinated in nonhuman primate (NHP) experimental glaucoma (EG).

Methods

Unilateral EG NHPs (n = 3) were perfusion fixed, EG and control eyes were enucleated, and foveal Bruch's membrane opening (FoBMO) 30° sectoral axon counts were estimated. Optic nerve heads were trephined; serial vibratome sections (VSs) were imaged and colocalized to a fundus photograph establishing their FoBMO location. The peripheral neural canal region within n = 5 EG versus control eye VS comparisons was targeted for scanning block-face electron microscopic reconstruction (SBEMR) using micro-computed tomographic reconstructions (µCTRs) of each VS. Posterior laminar beams within each µCTR were segmented, allowing a best-fit posterior laminar surface (PLS) to be colocalized into its respective SBEMR. Within each SBEMR, up to 300 axons were randomly traced until they ended (nonintact) or left the block (intact). For each intact axon, myelin onset was identified and myelin onset distance (MOD) was measured relative to the PLS. For each EG versus control SBEMR comparison, survival analyses compared EG and control MOD.

Results

MOD calculations were successful in three EG and five control eye SBEMRs. Within each SBEMR comparison, EG versus control eye axon loss was -32.9%, -8.3%, and -15.2% (respectively), and MOD was increased in the EG versus control SBEMR (P < 0.0001 for each EG versus control SBEMR comparison). When data from all three EG eye SBEMRs were compared to all five control eye SBEMRs, MOD was increased within the EG eyes.

Conclusions

Structurally intact, RON axons are demyelinated in NHP early to moderate EG. Studies to determine their functional status are indicated.

Optical Coherence Tomography and Optical Coherence Tomography Angiography: Essential Tools for Detecting Glaucoma and Disease Progression

Early diagnosis and detection of disease progression are critical to successful therapeutic intervention in glaucoma, the leading cause of irreversible blindness worldwide. Optical coherence tomography (OCT) is a non-invasive imaging technique that allows objective quantification in vivo of key glaucomatous structural changes in the retina and the optic nerve head (ONH). Advances in OCT technology have increased the scan speed and enhanced image quality, contributing to early glaucoma diagnosis and monitoring, as well as the visualization of critically important structures deep within the ONH, such as the lamina cribrosa. OCT angiography (OCTA) is a dye-free technique for noninvasively assessing ocular microvasculature, including capillaries within each plexus serving the macula, peripapillary retina and ONH regions, as well as the deeper vessels of the choroid. This layer-specific assessment of the microvasculature has provided evidence that retinal and choroidal vascular impairments can occur during early stages of glaucoma, suggesting that OCTA-derived measurements could be used as biomarkers for enhancing detection of glaucoma and its progression, as well as to reveal novel insights about pathophysiology. Moreover, these innovations have demonstrated that damage to the macula, a critical region for the vision-related quality of life, can be observed in the early stages of glaucomatous eyes, leading to a paradigm shift in glaucoma monitoring. Other advances in software and hardware, such as artificial intelligence-based algorithms, adaptive optics, and visible-light OCT, may further benefit clinical management of glaucoma in the future. This article reviews the utility of OCT and OCTA for glaucoma diagnosis and disease progression detection, emphasizes the importance of detecting macula damage in glaucoma, and highlights the future perspective of OCT and OCTA. We conclude that the OCT and OCTA are essential glaucoma detection and monitoring tools, leading to clinical and economic benefits for patients and society.

Visible-Light Optical Coherence Tomography Fibergraphy of the Tree Shrew Retinal Ganglion Cell Axon Bundles

We seek to develop techniques for high-resolution imaging of the tree shrew retina for visualizing and parameterizing retinal ganglion cell (RGC) axon bundles in vivo. We applied visible-light optical coherence tomography fibergraphy (vis-OCTF) and temporal speckle averaging (TSA) to visualize individual RGC axon bundles in the tree shrew retina. For the first time, we quantified individual RGC bundle width, height, and cross-sectional area and applied vis-OCT angiography (vis-OCTA) to visualize the retinal microvasculature in tree shrews. Throughout the retina, as the distance from the optic nerve head (ONH) increased from 0.5 mm to 2.5 mm, bundle width increased by 30%, height decreased by 67%, and cross-sectional area decreased by 36%. We also showed that axon bundles become vertically elongated as they converge toward the ONH. Ex vivo confocal microscopy of retinal flat-mounts immunostained with Tuj1 confirmed our in vivo vis-OCTF findings.

Structure-function assessment in glaucoma based on perimetric sensitivity and en face optical coherence tomography images of retinal nerve fiber bundles

Many studies have assessed structure-function relations in glaucoma, but most without topographical comparison across the central 30°. We present a method for assessing structure-function relations with en face images of retinal nerve fiber layer (RNFL) bundles allowing topographical comparison across much of this retinal area. Forty-four patients with glaucoma (median age 61 years) were recruited and tested with Optical Coherence Tomography (OCT) and perimetry. Six rectangular volume scans were gathered, and then montaged to provide en face views of the RNFL bundles. We calculated the proportion of locations showing a perimetric defect that also showed an en face RNFL defect; and the proportion of locations falling on an RNFL defect that also showed a perimetric defect. A perimetric defect for a location was defined as a total deviation (TD) value equal to or deeper than -4 dB. We found that the median (IQR) number of locations with abnormal RNFL bundle reflectance that also had abnormal TD was 78% (60%) and for locations with abnormal TD that also had abnormal RNFL bundle reflectance was 75% (44%). We demonstrated a potential approach for structure-function assessment in glaucoma by presenting a topographic reflectance map, confirming results of previous studies and including larger retinal regions.

The Retinal Nerve Fiber Layer: How William F. Hoyt Opened Our Eyes to It

Retinal nerve fiber layer (RNFL) assessment based on optical coherence tomography has become an essential structural parameter in the evaluation of the visual pathway. Yet, it was the trailblazing efforts of one individual, William F. Hoyt, MD, who in the 1970s published a series of landmark reports, which lay the foundation for evaluating the RNFL. With the aid of a direct ophthalmoscope, red-free photographic techniques, and an inquisitive mind, Hoyt added an entirely new dimension to the importance of careful ophthalmoscopy. This article chronicles the discoveries and publications that allowed Hoyt and his coworkers to establish the importance of analysis of the RNFL.

The Horizontal Raphe of the Human Retina and its Watershed Zones

The horizontal raphe (HR) as a demarcation line dividing the retina and choroid into separate vascular hemispheres is well established, but its development has never been discussed in the context of new findings of the last decades. Although factors for axon guidance are established (e.g., slit-robo pathway, ephrin-protein-receptor pathway) they do not explain HR formation. Early morphological organization, too, fails to establish a HR. The development of the HR is most likely induced by the long posterior ciliary arteries which form a horizontal line prior to retinal organization. The maintenance might then be supported by several biochemical factors. The circulation separate superior and inferior vascular hemispheres communicates across the HR only through their anastomosing capillary beds resulting in watershed zones on either side of the HR. Visual field changes along the HR could clearly be demonstrated in vascular occlusive diseases affecting the optic nerve head, the retina or the choroid. The watershed zone of the HR is ideally protective for central visual acuity in vascular occlusive diseases but can lead to distinct pathological features.

Location of Disc Hemorrhage and Direction of Progression in Glaucomatous Retinal Nerve Fiber Layer Defects

PURPOSE

To investigate the relationship between glaucomatous disc hemorrhage (DH) location with respect to the nasal or temporal margin of the retinal nerve fiber layer defect (RNFLD) and the direction of RNFLD widening.

MATERIALS AND METHODS

This is a retrospective, cross-sectional study. Patients with any documented episode of glaucomatous DH throughout the follow-up period, and definite RNFLD widening on retinal nerve fiber layer photographs were analyzed. The location of DH was recorded as either nasal or temporal, and the direction of RNFLD widening was investigated. The laterality of DH location on the RNFLD border and the direction of RNFLD widening were correlated with each other. We also compared clinical parameters between eyes with nasal versus temporal margin DHs.

RESULTS

We analyzed 123 eyes from 116 patients with definite widening of the RNFLD and glaucomatous DH at the border between the healthy and damaged retinal nerve fiber layer. The most common diagnosis was normal-tension glaucoma (109, 87.9%). The most frequent location of DH was the temporal margin of an inferotemporal RNFLD (75, 61.0%), and the most frequent pattern of change in RNFLD was temporal widening (89, 72.4%). The absolute congruency was 82.9% and the total congruency was 99.2%. No significant differences were identified between eyes with nasal versus temporal margin DHs.

CONCLUSIONS

The lateral location of DH on the RNFLD border was highly congruent with the direction of RNFLD widening. This correspondence of laterality between DH development and progression of RNFLD may suggest an intimate structural relationship during the pathogenesis of DH, at the enlarging RNFLD border.

Optic nerve as a source of activated retinal microglia post-injury

Using mice expressing green fluorescent protein (GFP) from a transgenic CD11c promoter we found that a controlled optic nerve crush (ONC) injury attracted GFP^hi retinal myeloid cells to the dying retinal ganglion cells and their axons. However, the origin of these retinal myeloid cells was uncertain. In this study we use transgenic mice in conjunction with ONC, partial and full optic nerve transection (ONT), and parabiosis to determine the origin of injury induced retinal myeloid cells. Analysis of parabiotic mice and fate mapping showed that responding retinal myeloid cells were not derived from circulating macrophages and that GFP^hi myeloid cells could be derived from GFP^lo microglia. Comparison of optic nerve to retina following an ONC showed a much greater concentration of GFP^hi cells and GFP^lo microglia in the optic nerve. Optic nerve injury also induced Ki67⁺ cells in the optic nerve but not in the retina. Comparison of the retinal myeloid cell response after full versus partial ONT revealed fewer GFP^hi cells and GFP^lo microglia in the retina following a full ONT despite it being a more severe injury, suggesting that full transection of the optic nerve can block the migration of responding myeloid cells to the retina. Our results suggest that the optic nerve can be a reservoir for activated microglia and other retinal myeloid cells in the retina following optic nerve injury.

Attenuation Coefficients From SD-OCT Data: Structural Information Beyond Morphology on RNFL Integrity in Glaucoma

PURPOSE

The purpose of this study is to explore the attenuation coefficient (AC) of the retinal nerve fiber layer (RNFL) in spectral domain optical coherence tomography (OCT) images, in healthy eyes and eyes affected by glaucoma. To assess the relation between RNLF AC, disease severity, RNFL thickness, visual field sensitivity threshold, spatial location and age.

PATIENTS AND METHODS

We analyzed peripapillary circle scans of a clinical OCT device (Spectralis OCT, Heidelberg Engineering, Heidelberg, Germany) in 102 glaucoma patients and 90 healthy controls. The images were fully automatically converted into depth-resolved AC images. Next, the median AC within the RNFL was calculated based on the Spectralis segmentation. We compared the RNFL AC between healthy, mild, moderate and advanced glaucomatous eyes and assessed the correlation with patient characteristics such as age and visual field sensitivity threshold (HFA, Carl Zeiss Meditec, Dublin, CA) in a generalized estimating equations (GEE) model. Finally, we explored the ability to discriminate between glaucomatous and healthy eyes by RNFL AC.

RESULTS

Median RNFL AC decreased with increasing disease severity up to moderate glaucoma (P<0.001) in all 4 sectors around the optic nerve head. The largest relative decrease occurred in the nasal sector. The RNFL AC (AUC, 0.834±0.028) effectively discriminated healthy from glaucomatous eyes, although RNFL thickness (AUC, 0.975±0.013) performed even better (P<0.001). Prediction of visual field sensitivity improved significantly when RNFL thickness was augmented with RNFL AC as covariates (P<0.001).

CONCLUSIONS

This study demonstrated that RNFL AC provides complementary information on the RNFL's health compared with RNFL thickness measurements alone.

Regional Retinal Ganglion Cell Axon Loss in a Murine Glaucoma Model

Purpose

To determine if retinal ganglion cell (RGC) axon loss in experimental mouse glaucoma is uniform in the optic nerve.

Methods

Experimental glaucoma was induced for 6 weeks with a microbead injection model in CD1 (n = 78) and C57BL/6 (B6, n = 68) mice. From epoxy-embedded sections of optic nerve 1 to 2 mm posterior to the globe, total nerve area and regional axon density (axons/1600 μm2) were measured in superior, inferior, nasal, and temporal zones.

Results

Control eyes of CD1 mice have higher axon density and more total RGCs than control B6 mice eyes. There were no significant differences in control regional axon density in all mice or by strain (all P > 0.2, mixed model). Exposure to elevated IOP caused loss of RGC in both strains. In CD1 mice, axon density declined without significant loss of nerve area, while B6 mice had less density loss, but greater decrease in nerve area. Axon density loss in glaucoma eyes was not significantly greater in any region in either mouse strain (both P > 0.2, mixed model). In moderately damaged CD1 glaucoma eyes, and CD1 eyes with the greatest IOP elevation exposure, density loss differed by region (P = 0.05, P = 0.03, mixed model) with the greatest loss in the temporal and superior regions, while in severely injured B6 nerves superior loss was greater than inferior loss (P = 0.01, mixed model, Bonferroni corrected).

Conclusions

There was selectively greater loss of superior and temporal optic nerve axons of RGCs in mouse glaucoma at certain stages of damage. Differences in nerve area change suggest non-RGC responses differ between mouse strains.

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.

Pattern of Visual Field Loss in Primary Angle-Closure Glaucoma Across Different Severity Levels

PURPOSE

To investigate the patterns of visual field (VF) defects in primary angle-closure glaucoma (PACG) across different severity levels and to assess hemifield differences within each severity level.

DESIGN

Cross-sectional study.

PARTICIPANTS

Three hundred four patients diagnosed with PACG were recruited from glaucoma clinics at a Singapore hospital.

METHODS

Point-wise total deviation values were recorded from the static automated perimetry (Swedish interactive threshold algorithm standard program 24-2; Humphrey model 750 [Carl Zeiss Meditec, Dublin, CA]) printouts. Patients were excluded if they had unreliable VFs (fixation losses >33% and false-positive responses >15%), had undergone only 10-2 VF testing, had VF defects not typical of glaucoma, or had undergone cataract extraction. Mild, moderate, and severe VF loss were defined by a mean deviation of -6.00 dB or more, -6.01 to -12.00 dB, and -12.01 dB or less, respectively. Each hemifield was divided into regions according to glaucoma hemifield test sectors. The average mean deviation (MD) of each region was obtained using total deviation values.

MAIN OUTCOME MEASURES

Between- and within-hemifield differences of the regions across the severity levels.

RESULTS

After excluding ineligible cases, 249 patients with PACG were included in the analysis. Mean age of the patients was 65.7±8.6 years, with a 1:1 gender ratio. The number of patients who had mild, moderate, and severe VFs was 72 (28.9%), 78 (31.3%), and 99 (39.8%), respectively. For between-hemifield comparisons, all regions in the superior hemifield had worse MDs compared with their counterparts in the inferior hemifield across the severity spectrum. Likewise, for within-hemifield comparisons, MDs of the regions gradually worsened with increasing distance from the fixation point.

CONCLUSIONS

In this group of clinic-based PACG patients, the superior hemifield was found to be affected more severely than the inferior hemifield, and the differences between them increased with worsening disease severity. The damage was consistently more pronounced in the nasal area.

The non-human primate experimental glaucoma model

The purpose of this report is to summarize the current strengths and weaknesses of the non-human primate (NHP) experimental glaucoma (EG) model through sections devoted to its history, methods, important findings, alternative optic neuropathy models and future directions. NHP EG has become well established for studying human glaucoma in part because the NHP optic nerve head (ONH) shares a close anatomic association with the human ONH and because it provides the only means of systematically studying the very earliest visual system responses to chronic intraocular pressure (IOP) elevation, i.e. the conversion from ocular hypertension to glaucomatous damage. However, NHPs are impractical for studies that require large animal numbers, demonstrate spontaneous glaucoma only rarely, do not currently provide a model of the neuropathy at normal levels of IOP, and cannot easily be genetically manipulated, except through tissue-specific, viral vectors. The goal of this summary is to direct NHP EG and non-NHP EG investigators to the previous, current and future accomplishment of clinically relevant knowledge in this model.

Diagnostic specificities of retinal nerve fiber layer, optic nerve head, and macular ganglion cell-inner plexiform layer measurements in myopic eyes

PURPOSE

To evaluate and compare the diagnostic specificities of peripapillary retinal nerve fiber layer (RNFL) thickness, macular ganglion cell-inner plexiform layer (GC-IPL) thickness, and optic nerve head (ONH) measurements in nonglaucomatous myopic individuals.

METHODS

In a prospective, cross-sectional study, participants underwent a complete ophthalmic examination, a screening automated visual field test, and axial length measurement. The study eye then underwent optic nerve head and macular scanning using spectral-domain optical coherence tomography (OCT) instrumentation to determine RNFL thickness, GC-IPL thickness, and ONH measurements. False-positive rates for each of the OCT-derived parameters, using predefined criteria for an abnormal test, were calculated. Comparative analysis was performed using the McNemar test.

RESULTS

Data from 43 eligible subjects were analyzed. The mean age was 30±6.8 years (range, 22 to 50 y) with an average axial length of 25.26±1.21 mm (range, 23.06 to 29.07 mm) and mean spherical equivalent of -4.50±1.93 D (range, -1.00 to -9.00 D). The false-positive rate was higher when using RNFL parameters compared with both ONH (47% vs. 7%, respectively; P<0.001) and GC-IPL (47% vs. 26%, respectively; P=0.049) parameters. The false-positive rate was higher when using GC-IPL parameters, compared with ONH parameters (26% vs. 7%, respectively; P=0.039).

CONCLUSIONS

Caution should be exercised when relying on OCT-derived RNFL and GC-IPL thickness values to diagnose glaucoma in myopic individuals. OCT-derived ONH parameters perform better than RNFL and GC-IPL parameters and may increase diagnostic specificity in this population.

Structure-Function Relationship between Frequency-Doubling Technology Perimetry and Optical Coherence Tomography in Glaucoma

Purpose: To assess the relationship between the retinal nerve fibre layer (RNFL) thickness and the frequency-doubling technology perimetry (FDT) outcome. Methods: Sixty-two healthy individuals and 72 glaucoma patients were prospectively selected. All participants underwent a reliable FDT and optical coherence tomography (OCT). Pearson correlations were calculated between the unlogged threshold values of FDT and RNFL thicknesses measured by OCT. Results: Mild to moderate correlations were found between a few points from FDT and RNFL thicknesses in the vertical axis. The nasal superior area of FDT and the RNFL thickness at the 7-o'clock position had the strongest correlation (0.434, p < 0.001). Conclusions: The poor agreement between FDT and OCT parameters suggests that both instruments assess different characteristics of glaucomatous optic neuropathy. The map obtained validates previously reported clinical findings and contributes to a better understanding of the structure-function relationship in glaucoma. © 2014 S. Karger AG, Basel.

Alzheimer's disease in the human eye. Clinical tests that identify ocular and visual information processing deficit as biomarkers

Alzheimer's disease (AD) is the most common form of dementia with progressive deterioration of memory and cognition. Complaints related to vision are common among AD patients. Several changes in the retina, lens, and in the vasculature have been noted in the AD eye that may be the cause of visual symptoms experienced by the AD patient. Anatomical changes have been detected within the eye before signs of cognitive impairment and memory loss are apparent. Unlike the brain, the eye is a unique organ that can be visualized noninvasively at the cellular level because of its transparent nature, which allows for inexpensive testing of biomarkers in a clinical setting. In this review, we have searched for candidate biomarkers that could enable diagnosis of AD, covering ocular neurodegeneration associated with functional tests. We explore the evidence that suggests that inexpensive, noninvasive clinical tests could be used to detect AD ocular biomarkers.

High-resolution imaging of retinal nerve fiber bundles in glaucoma using adaptive optics scanning laser ophthalmoscopy

PURPOSE

To detect pathologic changes in retinal nerve fiber bundles in glaucomatous eyes seen on images obtained by adaptive optics (AO) scanning laser ophthalmoscopy (AO SLO).

DESIGN

Prospective cross-sectional study.

METHODS

Twenty-eight eyes of 28 patients with open-angle glaucoma and 21 normal eyes of 21 volunteer subjects underwent a full ophthalmologic examination, visual field testing using a Humphrey Field Analyzer, fundus photography, red-free SLO imaging, spectral-domain optical coherence tomography, and imaging with an original prototype AO SLO system.

RESULTS

The AO SLO images showed many hyperreflective bundles suggesting nerve fiber bundles. In glaucomatous eyes, the nerve fiber bundles were narrower than in normal eyes, and the nerve fiber layer thickness was correlated with the nerve fiber bundle widths on AO SLO (P < .001). In the nerve fiber layer defect area on fundus photography, the nerve fiber bundles on AO SLO were narrower compared with those in normal eyes (P < .001). At 60 degrees on the inferior temporal side of the optic disc, the nerve fiber bundle width was significantly lower, even in areas without nerve fiber layer defect, in eyes with glaucomatous eyes compared with normal eyes (P = .026). The mean deviations of each cluster in visual field testing were correlated with the corresponding nerve fiber bundle widths (P = .017).

CONCLUSIONS

AO SLO images showed reduced nerve fiber bundle widths both in clinically normal and abnormal areas of glaucomatous eyes, and these abnormalities were associated with visual field defects, suggesting that AO SLO may be useful for detecting early nerve fiber bundle abnormalities associated with loss of visual function.

A mathematical model for describing the retinal nerve fiber bundle trajectories in the human eye: average course, variability, and influence of refraction, optic disc size and optic disc position

Previously we developed a mathematical model for describing the retinal nerve fiber bundle trajectories in the superior-temporal and inferior-temporal regions of the human retina, based on traced trajectories extracted from fundus photographs. Aims of the current study were to (i) validate the existing model, (ii) expand the model to the entire retina and (iii) determine the influence of refraction, optic disc size and optic disc position on the trajectories. A new set of fundus photographs was collected comprising 28 eyes of 28 subjects. From these 28 photographs, 625 trajectories were extracted. Trajectories in the temporal region of the retina were compared to the existing model. In this region, 347 of 399 trajectories (87%) were within the 95% central range of the existing model. The model was extended to the nasal region. With this extension, the model can now be applied to the entire retina that corresponds to the visual field as tested with standard automated perimetry (up to approximately 30° eccentricity). There was an asymmetry between the superior and inferior hemifields and a considerable location-specific inter-subject variability. In the nasal region, we found two "singularities", located roughly at the one and five o'clock positions for the right optic disc. Here, trajectories from relatively widespread areas of the retina converge. Associations between individual deviations from the model and refraction, optic disc size and optic disc position were studied with multiple linear regression. Refraction (P = 0.021) and possibly optic disc inclination (P = 0.09) influenced the trajectories in the superior-temporal region.

Patterns of peripapillary retinal nerve fiber layer thinning in vigabatrin-exposed individuals

PURPOSE

To explore the relationship of peripapillary retinal nerve fiber layer (ppRNFL) thinning in individuals exposed to the antiepileptic drug vigabatrin with respect to 2 separate variables: cumulative vigabatrin exposure and severity of vigabatrin-associated visual field loss (VAVFL).

DESIGN

Cross-sectional observational study.

PARTICIPANTS

Subjects were older than 18 years, 129 with vigabatrin-treated epilepsy (vigabatrin-exposed group) and 87 individuals with epilepsy never treated with vigabatrin (nonexposed group).

METHODS

All subjects underwent ppRNFL imaging using spectral-domain optical coherence tomography. Eighty-four vigabatrin-exposed individuals underwent Goldmann kinetic perimetry. The visual field examined from the right eye was categorized as normal (n = 47), mildly abnormal (n = 18), or moderately to severely abnormal (n = 19). In 91 vigabatrin-exposed individuals, the cumulative vigabatrin exposure could be ascertained: 41 subjects received 1000 g or less, 23 subjects received more than 1000 g but equal to or less than 2500 g, 16 subjects received more than 2500 g but equal to or less than 5000 g or less, and 11 subjects received more than 5000 g.

MAIN OUTCOME MEASURES

Differences in ppRNFL thickness across the twelve 30° sectors: (1) among all nonexposed individuals and all vigabatrin-exposed individuals, (2) between each vigabatrin-exposed group, according to cumulative vigabatrin exposure, and the nonexposed group, (3) among different vigabatrin-exposed subjects grouped according to cumulative vigabatrin exposure, and (4) among vigabatrin-exposed subjects grouped according to severity of VAVFL.

RESULTS

The ppRNFL was significantly thinner in vigabatrin-exposed compared with nonexposed individuals in most 30° sectors (P<0.004). The temporal, temporal superior, and temporal inferior 30° sectors, as well as the nasal 30° sector, were not affected. There was a trend for increasing ppRNFL thinning with increasing cumulative vigabatrin exposure. The nasal-superior 30° sector was significantly thinner in group 1 (≤1000 g) compared with nonexposed individuals (P<0.05) and in vigabatrin-exposed individuals with normal visual fields compared with nonexposed individuals (P<0.05).

CONCLUSIONS

After vigabatrin exposure in individuals receiving cumulative doses of 1000 g or less or in the presence of normal visual fields, ppRNFL thinning in the nasal superior 30° sector may occur. With higher cumulative doses of vigabatrin exposure, additional ppRNFL thinning was observed. The temporal aspects of the ppRNFL are spared, even in individuals with large cumulative vigabatrin exposures and moderate or severe VAVFL.

Perspectives on biomechanical growth and remodeling mechanisms in glaucoma()

Glaucoma is a blinding diseases in which damage to the axons results in loss of retinal ganglion cells. Experimental evidence indicates that chronic intraocular pressure elevation initiates axonal insult at the level of the lamina cribrosa. The lamina cribrosa is a porous collagen structure through which the axons pass on their path from the retina to the brain. Recent experimental studies revealed the extensive structural changes of the lamina cribrosa and its surrounding tissues during the development and progression of glaucoma. In this perspective paper we review the experimental evidence for growth and remodeling mechanisms in glaucoma including adaptation of tissue anisotropy, tissue thickening/thinning, tissue elongation/shortening and tissue migration. We discuss the existing predictive computational approaches that try to elucidate the potential biomechanical basis of theses growth and remodeling mechanisms and highlight open questions, challenges, and avenues for further development.

High-resolution imaging of the retinal nerve fiber layer in normal eyes using adaptive optics scanning laser ophthalmoscopy

Purpose

To conduct high-resolution imaging of the retinal nerve fiber layer (RNFL) in normal eyes using adaptive optics scanning laser ophthalmoscopy (AO-SLO).

Methods

AO-SLO images were obtained in 20 normal eyes at multiple locations in the posterior polar area and a circular path with a 3–4-mm diameter around the optic disc. For each eye, images focused on the RNFL were recorded and a montage of AO-SLO images was created.

Results

AO-SLO images for all eyes showed many hyperreflective bundles in the RNFL. Hyperreflective bundles above or below the fovea were seen in an arch from the temporal periphery on either side of a horizontal dividing line to the optic disc. The dark lines among the hyperreflective bundles were narrower around the optic disc compared with those in the temporal raphe. The hyperreflective bundles corresponded with the direction of the striations on SLO red-free images. The resolution and contrast of the bundles were much higher in AO-SLO images than in red-free fundus photography or SLO red-free images. The mean hyperreflective bundle width around the optic disc had a double-humped shape; the bundles at the temporal and nasal sides of the optic disc were narrower than those above and below the optic disc (P<0.001). RNFL thickness obtained by optical coherence tomography correlated with the hyperreflective bundle widths on AO-SLO (P<0.001)

Conclusions

AO-SLO revealed hyperreflective bundles and dark lines in the RNFL, believed to be retinal nerve fiber bundles and Müller cell septa. The widths of the nerve fiber bundles appear to be proportional to the RNFL thickness at equivalent distances from the optic disc.

Localization of papillofoveal bundles in primates

Axons in the fovea are precisely organized to ensure accurate vision. We investigated the morphologic characteristics and localization of nerve bundles in the optic nerve in primates. Macaque eyes were studied for conventional and immunostaining, and also marmoset eyes for carbocyanine dye tracing. Locally confined lesions associated with similar findings to human age-related macular degeneration (ARMD) were also evaluated. Axons of retinal ganglion cells formed fasciculi near their origin, and these fasciculi formed bundles thereafter. In the retinal nerve fiber layer, ascending bundles assembled stratification adding proximal bundle underneath successively. Bundles in the arcuate zone displayed a characteristic fine, parallel arrangement, whereas those in the outside zone intermingled with undefined reticular bundles as they approached the optic nerve head. Macular bundles remained in groups and were distributed in the temporal wedge of the optic nerve head. Orthograde and retrograde tracing revealed that these bundles formed confined groups of various sizes and, ultimately, a specific group of small bundles located in the innermost row, near the central vessels. In addition, these bundles showed evidence of focal degenerative deterioration in eyes with ARMD. Papillomacular bundles have a characteristic alignment and configuration. Foveal bundles that compose the confined group closest to the optic trunk (which we term papillofoveal bundles) appear to have functional significance with respect to the isolated lesions that accompany central vision loss or preservation.

Role of cerebrospinal fluid pressure in the pathogenesis of glaucoma

The pathogenesis of normal (intraocular) pressure glaucoma has remained unclear so far. As hospital-based studies showed an association of normal-pressure glaucoma with low systemic blood pressure, particularly at night, and with vasospastic symptoms, it has been hypothesized that a vascular factor may play a primary role in the pathogenesis of normal-pressure glaucoma. That assumption may, however, be contradicted by the morphology of the optic nerve head. Eyes with normal-pressure glaucoma and glaucomatous eyes with high-intraocular pressure can show a strikingly similar appearance of the optic nerve head, including a loss of neuroretinal rim, a deepening of the optic cup, and an enlargement of parapapillary atrophy. These features, however, are not found in any (other) vascular optic neuropathy, with the exception of an enlargement and deepening of the optic cup in arteritic anterior ischaemic optic neuropathy. One may additionally take into account (i) that it is the trans-lamina cribrosa pressure difference (and not the trans-corneal pressure difference, i.e. the so-called intraocular pressure) which is of importance for the physiology and pathophysiology of the optic nerve head; (ii) that studies have shown that the anatomy of the optic nerve head including the intraocular pressure, the anatomy and biomechanics of the lamina cribrosa and peripapillary sclera, retrobulbar orbital cerebrospinal fluid pressure and the retrobulbar optic nerve tissue pressure may be of importance for the pathogenesis of the highly myopic type of chronic open-angle glaucoma; (iii) that studies have suggested a physiological association between the pressure in all three fluid filled compartments, i.e. the systemic arterial blood pressure, the cerebrospinal fluid pressure and the intraocular pressure; (iv) that an experimental investigation suggested that a low cerebrospinal fluid pressure may play a role in the pathogenesis of normal (intraocular) pressure glaucoma; and (v) that recent clinical studies reported that patients with normal (intraocular) pressure glaucoma had significantly lower cerebrospinal fluid pressure and a higher trans-lamina cribrosa pressure difference when compared to normal subjects. One may, therefore, postulate that a low cerebrospinal fluid pressure may be associated with normal (intraocular) pressure glaucoma. A low systemic blood pressure, particularly at night, could physiologically be associated with a low cerebrospinal fluid pressure, which leads to an abnormally high trans-lamina cribrosa pressure difference and as such to a similar situation as if the cerebrospinal fluid pressure is normal and the intraocular pressure is elevated. This model could explain why patients with normal (intraocular) pressure glaucoma tend to have a low systemic blood pressure, and why eyes with normal (intraocular) pressure glaucoma and eyes with high-pressure glaucoma, in contrast to eyes with a direct vascular optic neuropathy, show profound similarities in the appearance of the optic nerve head.

Loss of the low-frequency component of the global-flash multifocal electroretinogram in primate eyes with experimental glaucoma

PURPOSE

To study relationships between glaucoma-sensitive components identified with frequency-domain analysis of global-flash multifocal electroretinogram (mfERG), regional retinal nerve fiber layer thickness (RNFLT), and local visual field sensitivity (VS).

METHODS

Eleven macaque monkeys, including four controls and seven with unilateral laser-induced trabecular meshwork scarification and ocular hypertension, were observed with optical coherence tomography (OCT), full-field light-adapted flash ERG, 103-hexagon global-flash mfERG (MFOFO), and static perimetry. The effects of experimental glaucoma on mfERG were assessed in the frequency domain. Relations between root mean square (RMS) amplitude of a glaucoma-sensitive frequency range and peripapillary RNFLT (standard 12° OCT circular scan), and between RMS amplitude and VS were studied.

RESULTS

Experimental glaucoma led to a dramatic and consistent power loss in the low-frequency (<25 Hz) band of mfERG. The RMS of this low-frequency component (LFC) correlated significantly with the regional RNFLT. The r(2) of linear fits was 0.39 (P < 0.001) for cross-sectional group data and 0.60 after correction for intersubject variability. The r(2) of linear fits for longitudinal data from individual animals was as high as 0.78 (P < 0.001). Local LFC RMS amplitude also correlated significantly with interpolated VS for hexagons. The r(2) for exponential fits of hexagon LFC RMS amplitudes (inner three rings) versus VS (dB) was 0.29 to 0.52 (P < 0.001) for the group and up to 0.95 in individuals.

CONCLUSIONS

The significant correlations between regional measures of global-flash mfERG, RNFLT, and VS suggest that LFC RMS amplitude provides a useful index for objective quantification of local RGC function and monitoring of early changes in glaucoma.

Modeling the patterns of visual field loss in glaucoma

PURPOSE

A computer model was developed to test the assumption that diffuse neural loss can result in the field loss pattern characteristic of glaucoma.

METHODS

The anterior visual pathways comprised the retinal ganglion cells, and their axons up to the optic nerve head (ONH) were modeled in a computer program. Axon resistance to stress was accounted for depending on the location on the ONH, taking into consideration the presence or absence of vessels in the area. Damage patterns were applied to the axons at the ONH, and the corresponding dendritic fields were removed accordingly. A visual field was extracted and represented on a gray scale after a predetermined stage of damage was reached. Two patterns of damage were considered, a diffuse damage produced by randomly removing fibers and an ordered anteroposterior elimination.

RESULTS

Random damage never rendered a pattern loss. Ordered centrifugal fiber loss may produce a radial pattern more conspicuous when the vessels are endowed with a protective role. In both cases, scotomas tend to be detectable earlier in more peripheral locations, attributable to the increasing size of the receptive fields with eccentricity.

CONCLUSIONS

The model shows that pattern loss typical of glaucoma cannot be solely the result of a random loss of fibers. Anteroposterior damage of the ONH can explain radial progression of scotomas if a protective role is introduced for the central vessels.

The morphology and spatial arrangement of astrocytes in the optic nerve head of the mouse

We evaluated the shapes, numbers, and spatial distribution of astrocytes within the glial lamina, an astrocyte-rich region at the junction of the retina and optic nerve. A primary aim was to determine how the population of astrocytes, collectively, partitions the axonal space in this region. Astrocyte processes labeled with glial fibrillary acidic protein (GFAP) compartmentalize ganglion cell axons into bundles, forming "glial tubes," and giving the glial architecture of the optic nerve head in transverse section a honeycomb appearance. The shapes of individual astrocytes were studied by using transgenic mice that express enhanced green fluorescent protein in isolated astrocytes (hGFAPpr-EGFP). Within the glial lamina the astrocytes were transverse in orientation, with thick, smooth primary processes emanating from a cytoplasmic expansion of the soma. Spaces between the processes of neighboring astrocytes were spatially aligned, to form the apertures through which the bundles of optic axons pass. The processes of individual astrocytes were far-reaching-they could span most of the width of the nerve-and overlapped the anatomical domains of other near and distant astrocytes. Thus, astrocytes in the glial lamina do not tile: each astrocyte participates in ensheathing approximately one-quarter of all of the axon bundles in the nerve, and each glial tube contains the processes of about nine astrocytes. This raises the mechanistic question of how, in glaucoma or other cases of nerve damage, the glial response can be confined to a circumscribed region where damage to axons has occurred.

Optical coherence tomography scan circle location and mean retinal nerve fiber layer measurement variability

PURPOSE

To investigate the effect on optical coherence tomography (OCT) retinal nerve fiber layer (RNFL) thickness measurements of varying the standard 3.4-mm-diameter circle location.

METHODS

The optic nerve head (ONH) region of 17 eyes of 17 healthy subjects was imaged with high-speed, ultrahigh-resolution OCT (hsUHR-OCT; 501 x 180 axial scans covering a 6 x 6-mm area; scan time, 3.84 seconds) for a comprehensive sampling. This method allows for systematic simulation of the variable circle placement effect. RNFL thickness was measured on this three-dimensional dataset by using a custom-designed software program. RNFL thickness was resampled along a 3.4-mm-diameter circle centered on the ONH, then along 3.4-mm circles shifted horizontally (x-shift), vertically (y-shift) and diagonally up to +/-500 microm (at 100-microm intervals). Linear mixed-effects models were used to determine RNFL thickness as a function of the scan circle shift. A model for the distance between the two thickest measurements along the RNFL thickness circular profile (peak distance) was also calculated.

RESULTS

RNFL thickness tended to decrease with both positive and negative x- and y-shifts. The range of shifts that caused a decrease greater than the variability inherent to the commercial device was greater in both nasal and temporal quadrants than in the superior and inferior ones. The model for peak distance demonstrated that as the scan moves nasally, the RNFL peak distance increases, and as the circle moves temporally, the distance decreases. Vertical shifts had a minimal effect on peak distance.

CONCLUSIONS

The location of the OCT scan circle affects RNFL thickness measurements. Accurate registration of OCT scans is essential for measurement reproducibility and longitudinal examination (ClinicalTrials.gov number, NCT00286637).

Peripapillary nerve fiber layer thickness profile determined with high speed, ultrahigh resolution optical coherence tomography high-density scanning

PURPOSE

To determine the retinal nerve fiber layer (RNFL) thickness profile in the peripapillary region of healthy eyes.

METHODS

Three-dimensional, Fourier/spectral domain optical coherence tomography (OCT) data were obtained as raster scan data (512 x 180 axial scans in a 6 x 6-mm region centered on the optic nerve head [ONH]) with high-speed, ultrahigh-resolution OCT (hsUHR-OCT) from 12 healthy subjects. RNFL thickness was measured on this three-dimensional data set with an in-house software program. The disc margin was defined subjectively in each image and RNFL thickness profiles relative to distance from the disc center were computed for quadrants and clock hours. A mixed-effects model was used to characterize the slope of the profiles.

RESULTS

Thickness profiles in the superior, inferior, and temporal quadrants showed an initial increase in RNFL thickness, an area of peak thickness, and a linear decrease as radial distance from the disc center increased. The nasal quadrant showed a constant linear decay without the initial RNFL thickening. A mixed-effects model showed that the slopes of the inferior, superior, and nasal quadrants differed significantly from the temporal slope (P = 0.0012, P = 0.0003, and P = 0.0004, respectively).

CONCLUSIONS

RNFL thickness is generally inversely related to the distance from the ONH center in the peripapillary region of healthy subjects, as determined by hsUHR-OCT. However, several areas showed an initial increase in RNFL, followed by a peak and a gradual decrease.

Evaluating the optic disc and retinal nerve fiber layer in glaucoma. I: Clinical examination and photographic methods

Glaucoma is a leading cause of blindness worldwide and is characterized in part by specific changes in the optic disc and retinal nerve fiber layer. Currently, subjective clinical examination and fundus photography are the most common ways of detecting structural change in glaucoma and monitoring its progression. In the first part of this two-part article, the authors overview structural changes of the optic disc and retinal nerve fiber layer in glaucoma and describe and evaluate photographic methods for observing these changes. In the second part of this article (this issue), recent developments in computer-based optical imaging techniques that allow objective evaluation of the optic disc and retinal nerve fiber layer are described.

The relationship between retinal ganglion cell function and retinal nerve fiber thickness in early glaucoma

PURPOSE

To compare relative reduction of retinal ganglion cell (RGC) function and retinal nerve fiber layer (RNFL) thickness in early glaucoma by means of steady-state pattern electroretinogram (PERG) and optical coherence tomography (OCT), respectively.

METHODS

Eighty-four persons with suspected glaucoma due to disc abnormalities (GS: mean age 56.6 +/- 13.8 years, standard automated perimetry [SAP] mean deviation [MD] -0.58 +/- 1.34 dB) and 34 patients with early manifest glaucoma (EMG, mean age 65.9 +/- 10.7 years, SAP MD -2.7 +/- 4.5 dB) were tested with PERG and OCT. Both GS and EMG patients had small refractive errors, corrected visual acuity > or =20/25, and no systemic or retinal disease other than glaucoma.

RESULTS

MDs from age-predicted normal values were larger for PERG amplitude (GS: -1.113 dB; EMG: -2.352 dB) compared with the PERG-matched RNFL thickness (GS: -0.217 dB; EMG: -0.725 dB). Deviations exceeding the lower 95% tolerance intervals of the normal population were more frequent for PERG amplitude (GS: 26%; EMG: 56%) than PERG-matched RNFL thickness (GS: 6%; EMG: 29%).

CONCLUSIONS

In early glaucoma, reduction in RGC electrical activity exceeds the proportion expected from lost RGC axons, suggesting that a population of viable RGCs in the central retina is dysfunctional. By combining PERG and OCT it is, in principle, possible to obtain unique information on reduced responsiveness of viable RGCs.

Spatiotemporal receptive field properties of epiretinally recorded spikes and local electroretinograms in cats

Background

Receptive fields of retinal neural signals of different origin can be determined from extracellular microelectrode recordings at the inner retinal surface. However, locations and types of neural processes generating the different signal components are difficult to separate and identify. We here report epiretinal receptive fields (RFs) from simultaneously recorded spikes and local electroretinograms (LERGs) using a semi-chronic multi-electrode in vivo recording technique in cats. Broadband recordings were filtered to yield LERG and multi unit as well as single unit spike signals. RFs were calculated from responses to multifocal pseudo-random spatiotemporal visual stimuli registered at the retinal surface by a 7-electrode array.

Results

LERGs exhibit spatially unimodal RFs always centered at the location of the electrode tip. Spike-RFs are either congruent with LERG-RFs (N = 26/61) or shifted distally (N = 35/61) but never proximally with respect to the optic disk. LERG-RFs appear at shorter latencies (11.9 ms ± 0.5 ms, N = 18) than those of spikes (18.6 ms ± 0.4 ms, N = 53). Furthermore, OFF-center spike-RFs precede and have shorter response rise times than ON-center spike-RFs. Our results indicate that displaced spike-RFs result from action potentials of ganglion cell axons passing the recording electrode en route to the optic disk while LERG-RFs are related to superimposed postsynaptic potentials of cells near the electrode tip.

Conclusion

Besides contributing to the understanding of retinal function we demonstrate the caveats that come with recordings from the retinal surface, i.e., the likelihood of recordings from mixed sets of retinal neurons. Implications for the design of an epiretinal visual implant are discussed.

Circulation and axonal transport in the optic nerve

Retinal ganglion cells are the output cells of the retina whose axons are under considerable metabolic stress in both health and disease states. They are highly polarised to ensure that mitochondria and enzymes involved in the generation of ATP are strategically concentrated to meet the local energy demands of the cell. In passing from the eye to the brain, axons are protected and supported by glial tissues and the blood supply of the optic nerve head is regulated to maintain the supply of oxygen and nutrients to the axons. In spite of this, the optic nerve head remains the point at which retinal ganglion cell axons are most vulnerable to the effects of increased intraocular pressure or ischaemia. Considerable work has been undertaken in this area to advance our understanding on the pathophysiology of axon damage and to develop new strategies for the prevention of retinal ganglion cell death.

Histological measurement of retinal nerve fibre layer thickness

PURPOSE

Accurate assessment of the retinal nerve fibre layer (RNFL) is central to the diagnosis and follow-up of glaucoma. The in vivo measurement of RNFL thickness by a variety of digital imaging technologies is becoming an important measure for early detection, as well as for follow-up, of glaucomatous damage. However, when drawing clinical inference concerning the state of the RNFL, it is important to have valid reference data on RNFL thickness in both healthy and diseased eyes. In this review, we summarize the knowledge currently available about RNFL thickness in human and primate eyes.

METHODS

A review of the literature on histological analysis of RNFL thickness in the context of glaucomatous damage.

CONCLUSIONS

Six studies have so far analysed RNFL thickness. Despite the diverse study methodology taken, a consistent feature of all the data is that the superior and inferior quadrants of the peripapillary retina are thicker than the nasal and temporal quadrants; that the RNFL thickness rapidly diminishes with increasing distance from the disc margin; and that apparently at different locations the ratio of axons to supportive tissue varies significantly. We conclude that limited data are available to describe the normal variation in RNFL thickness in the normal human eye. Further studies may help better characterize the RNFL thickness in health and disease and to facilitate the correlation with clinical methods for nerve fibre layer assessment.

Quantification of interpoint topographic correlations of threshold values in glaucomatous visual fields

PURPOSE

The authors sought to quantify neighboring and distant interpoint correlations of threshold values within the visual field in patients with glaucoma.

METHODS

Visual fields of patients with confirmed or suspected glaucoma were analyzed (n = 255). One eye per patient was included. Patients were examined using the 32 program of the Octopus 1-2-3. Linear regression analysis among each of the locations and the rest of the points of the visual field was performed, and the correlation coefficient was calculated. The degree of correlation was categorized as high (r(2) > 0.66), moderate (0.66 > or = r(2) > 0.33), or low (r(2) < or = 0.33). The standard error of threshold estimation was calculated.

RESULTS

Most locations of the visual field had high and moderate correlations with neighboring points and with distant locations corresponding to the same nerve fiber bundle. Locations of the visual field had low correlations with those of the opposite hemifield, with the exception of locations temporal to the blind spot. The standard error of threshold estimation increased from 0.6 to 0.9 dB with an r(2) reduction of 0.1.

CONCLUSION

Locations of the visual field have highest interpoint correlation with neighboring points and with distant points in areas corresponding to the distribution of the retinal nerve fiber layer. The quantification of interpoint correlations may be useful in the design and interpretation of visual field tests in patients with glaucoma.

Peripheral visual field defects after macular hole surgery: a complication with decreasing incidence

AIM

To prospectively evaluate peripheral visual fields after vitrectomy for idiopathic macular holes.

METHODS

Goldmann perimetry was performed in 105 patients before, as well as 6 weeks and 12 months after macular hole surgery.

RESULTS

Only one patient (< 1%) with a stage III macular hole developed an asymptomatic postoperative visual field defect. The scotoma was wedge-shaped, peripherally located in the temporal quadrant, and remained unchanged during the following 12 months.

CONCLUSION

Peripheral visual field defects after macular hole surgery can be a complication of very low incidence. A rather low pressure set during air-fluid exchange as well as special aspects of the surgical technique may be responsible for this low incidence of peripheral visual field defects.

Mapping the visual field to the optic disc in normal tension glaucoma eyes

PURPOSE

To establish the anatomical relationship between visual field test points in the Humphrey 24-2 test pattern and regions of the optic nerve head (ONH) DESIGN: Cross-sectional study.

PARTICIPANTS

Glaucoma patients and suspects from the Normal Tension Glaucoma Clinic at Moorfields Eye Hospital.

METHODS

Sixty-nine retinal nerve fiber layer (RNFL) photographs with well-defined RNFL defects and/or prominent bundles were digitized. An appropriately scaled Humphrey 24-2 visual field grid and an ONH reference circle, divided into 30 degrees sectors, were generated digitally. These were superimposed onto the RNFL images. The relationship of visual field test points to the circumference of the ONH was estimated by noting the proximity of test points to RNFL defects and/or prominent bundles. The position of the ONH in relation to the fovea was also noted.

MAIN OUTCOME MEASURES

The sector at the ONH corresponding to each visual field test point, the position of the ONH in relation to the fovea, and the effect of the latter on the former.

RESULTS

A median 22 (range, 4-58), of a possible 69, ONH positions were assigned to each visual field test point. The standard deviation of estimations was 7.2 degrees. The position of the ONH was 15.5 degrees (standard deviation 0.9 degrees ) nasal and 1.9 degrees (standard deviation 1.0 degrees ) above the fovea. The location of the ONH had a significant effect on the corresponding position at the ONH for 28 of 52 visual field test points.

CONCLUSIONS

A clinically useful map that relates visual field test points to regions of the ONH has been produced. The map will aid clinical evaluation of glaucoma patients and suspects, as well as form the basis for investigations of the relationship between retinal light sensitivity and ONH structure.

Optic nerve head structure in glaucoma: astrocytes as mediators of axonal damage

Increased intraocular pressure (IOP) is recognised as the principal risk factor for the development of glaucomatous cupping of the optic disc. The hypothesis that it disrupts the function of retinal ganglion cell axons by increasing mechanical forces on the lamina cribrosa of the optic nerve head has received considerable experimental support. However, many patients with glaucoma will have progressive cupping even though the IOPs remain within the normal range, suggesting that mechanical compression is unlikely to be the sole cause of optic nerve damage. Clinical studies have emphasised the role of other factors, such as optic nerve head ischaemia, in generating optic disc cupping. One of the outstanding problems in understanding optic nerve head dysfunction in glaucoma has been the elucidation of the pathways that could integrate the effects of IOP and ischaemia to generate the characteristic changes seen. This review considers the role that optic nerve head astrocytes might play in the initiation of axon damage, based on the hypothesis that these cells are sensitive to mechanical or ischaemic factors and are important for the maintenance of retinal ganglion physiology. It discusses their role in the remodelling of the structure of the lamina cribrosa and the effect that this might have on axon function. Recent evidence has shown that the modulation of astrocyte activity, for example by the reduction of the production of nitric oxide, may prevent retinal ganglion cell death in ocular hypertension. The possibility that astrocyte-axon interactions are important in the development of glaucomatous optic neuropathy suggests new avenues of therapeutic intervention, not related to the control of IOP, that would prevent retinal ganglion cell death in glaucoma.

The similarity of protein expression in trabecular meshwork and lamina cribrosa: implications for glaucoma

The purpose of the present investigation was to compare protein expression in various ocular cells and tissues including the human trabecular meshwork (TM) and the lamina cribrosa (LC). To conduct the comparisons, we primarily utilized autofluorography of one-dimensional (1D) and high resolution, two-dimensional (2D) polyacrylamide gels of proteins from radiolabelled tissues and cultured cells. Results from the investigations indicated that patterns of protein expression from TM and LC were the most similar among the ocular cells and tissues compared.Specifically, these autofluorographic ' fingerprints' indicated that proteins in TM and LC cultured cells and tissue were exceptionally similar (a) in band position and intensity (1D gels) and (b) in spot congruence (2D gels) as compared to other ocular cells and tissues. We conclude that the TM and the LC, two ocular tissues intimately linked to the pathogenesis of primary open-angle glaucoma, display remarkable similarity in protein expression. This finding may have implications for the molecular etiology of glaucoma.

Ophthalmoscopic evaluation of the optic nerve head

Optic nerve diseases, such as the glaucomas, lead to changes in the intrapapillary and parapapillary region of the optic nerve head. These changes can be described by the following variables: size and shape of the optic disk; size, shape, and pallor of the neuroretinal rim; size of the optic cup in relation to the area of the disk; configuration and depth of the optic cup; ratios of cup-to-disk diameter and cup-to-disk area; position of the exit of the central retinal vessel trunk on the lamina cribrosa surface; presence and location of splinter-shaped hemorrhages; occurrence, size, configuration, and location of parapapillary chorioretinal atrophy; diffuse and/or focal decrease of the diameter of the retinal arterioles; and visibility of the retinal nerve fiber layer (RNFL). These variables can be assessed semiquantitively by ophthalmoscopy without applying sophisticated techniques. For the early detection of glaucomatous optic nerve damage in ocular hypertensive eyes before the development of visual field loss, the most important variables are neuroretinal rim shape, optic cup size in relation to optic disk size, diffusely or segmentally decreased visibility of the RNFL, occurrence of localized RNFL defects, and presence of disk hemorrhages.

In vivo morphometry of the lamina cribrosa and its relation to visual field loss in glaucoma

PURPOSE

The lamina cribrosa has been proposed as a site of origin of the optic nerve damage in glaucoma. The purpose of this study was to investigate, in vivo, the clinical features of the lamina cribrosa pores of glaucomatous patients and to relate their morphometric characteristics to the extent of their visual field loss.

METHODS

Images of the internal lamina cribrosa surface of 60 glaucomatous patients and 15 normal subjects were acquired, in vivo, using a scanning laser ophthalmoscope (SLO). A purposely developed technique of image processing was employed to objectively evaluate pore morphometry, with particular regard to their geometrical characteristics (circularity and elongation). Visual function was assessed by automated perimetry (Humphrey Field Analyser).

RESULTS

Normal subjects showed approximately round lamina pores. In glaucomatous patients, pores become more elongated and less circular with increasing field loss (p = 0.009 and p < 0.001, respectively).

CONCLUSIONS

Scanning laser ophthalmoscopy and a new technique of image processing were employed, for the first time, to the investigation in vivo of the lamina cribrosa of glaucomatous patients, in relation to the extent of visual field loss. The results indicated differences in the lamina cribrosa pore morphometry associated with increasing severity of the disease. These changes may represent the result of compressing and shearing forces applied to the laminar plates.

The human fetal retinal nerve fiber layer and optic nerve head: a DiI and DiA tracing study

The organization of the primate nerve fiber layer and optic nerve head with respect to the positioning of central and peripheral axons remains controversial. Data were obtained from 32 human fetal retinae aged between 15 and 21 weeks of gestation. Crystals of the carbocyanine dyes, DiI or DiA, and fluorescence microscopy were used to identify axonal populations from peripheral retinal ganglion cells. Peripheral ganglion cell axons were scattered throughout the vitreal-scleral depth of the nerve fiber layer. Such a scattered distribution was maintained as the fibers passed through the optic nerve head and along the optic nerve. There was a rough topographic representation within the optic nerve head according to retinal quadrant such that both peripheral and central fibers were mixed within a wedge extending from the periphery to the center of the nerve. There was no indication that the fibers were reorganized in any way as they passed through the optic disc and into the nerve. The present results suggest that any degree of order present within the fiber layer and optic nerve is not an active process but a passive consequence of combining the fascicles of the retinal nerve fiber layer. Optic axons are not instructed to establish a retinotopic order and the effect of guidance cues in reordering fibers, particularly evident prechiasmatically and postchiasmatically, does not appear to be present within the nerve fiber layer or optic nerve head in humans.

Quantitative analysis of the lamina cribrosa in vivo using a scanning laser opthalmoscope

PURPOSE

Structural changes in the lamina cribrosa have been implicated in the pathogenesis of glaucomatous optic nerve atrophy. The purpose of this study was to develop a method for morphometric analysis of the lamina cribrosa pores in vivo, using a scanning laser ophthalmoscope.

METHODS

A prototype Zeiss confocal laser scanning ophthalmoscope was used to acquire images of the lamina cribrosa. The images were digitised and aligned to compensate for eye movements. Thirty-two consecutive images were averaged to reduce noise. The images were processed to adjust for luminance gradients prior to segmentation and analysis. Details of the image processing are described.

RESULTS

The end result of processing the images was a binary (black and white) image that can be used for automated computer assisted measurements. The pores of the lamina cribrosa are well represented and retain their overall shape in the binary image, as judged by superimposing the binary image on the unprocessed image. We also established the repeatability, reproducibility and intercession variability of this technique. Repeated images of the internal lamina cribrosa of 10 patients were acquired by two observers in two separate visits, and the images were processed before automated computer measurements. The parameters evaluated were number of pores, area covered by the pores and area covered by the visible lamina cribrosa. The coefficient of variation for number of pores, pore area and lamina area was 6.9%, 2.1% and 4.3% for observer A and 5.5%, 2.1% and 5.8% for observer B. Pearson product moment correlation coefficient between the two observers was 0.94, 0.99 and 0.97 for the above parameters respectively. There was no significant difference between the measurements on visit 1 and 2 for both observers.

CONCLUSIONS

The technique described allows, for the first time, in vivo morphometry of the internal lamina cribrosa surface. This method has good reproducibility, suggesting future clinical applications.