The a-wave of the dark adapted electroretinogram in glaucomas: are photoreceptors affected?

A parafoveal retinal cones analysis using adaptive-optics retinal camera in patients with primary open angle glaucoma

OBJECTIVES

To study the density, spacing, and regularity of retinal cone photoreceptors using an Adaptive Optics (AO) retinal camera (Rtx1TM, Imagine Eyes, Orsay, France) in patients with Primary Open Angle Glaucoma (POAG) and to compare the outcomes with those of healthy age-matched control subjects.

METHODS

The study included 43 eyes with POAG and 31 eyes of normal subjects. POAG patients were divided into three groups according to the severity of the visual field defect. The AO Rtx1TM was used to obtain images of the parafoveal cone mosaic to calculate cone values. Analysis was performed at two and four degrees of eccentricity from the fovea along the four meridians (nasal, temporal, superior, inferior).

RESULTS

In POAG eyes, the mean ± standard deviation (SD) cone density at 2° considering all meridians was significantly lower than in normal controls (23,058.6 ± 3532.0 cones/mm2, and 25,511.7 ± 3157.5 cones/mm2, respectively; p = 0.003). Cone spacing was 7.3 ± 0.5 µm in POAG and 7.0 ± 0.4 µm in normal controls (p = 0.005), and cone regularity was 90.5 ± 4.9% and 93.5 ± 1.9% in POAG and normal controls, respectively (p < 0.001). At 4° similar trends were observed. However, no significant differences were found among patients with different severity of POAG (p > 0.05).

CONCLUSIONS

Using AO Rtx1TM, significant differences in retinal photoreceptors mosaic pattern were found between POAG eyes and age-matched controls, indicating a reduction in photoreceptors in POAG. No significant differences in retinal photoreceptor values were found among the three POAG groups.

Thicker Inner Nuclear Layer as a Predictor of Glaucoma Progression and the Impact of Intraocular Pressure Fluctuation

Background: Thickening of the inner nuclear layer (INL) or microcystic macular changes has been reported to be implicated in glaucoma patients, but their potential impact on disease progression remains unclear. We investigated the relationship between baseline microcystic macular edema in the INL or INL thickness and subsequent visual field (VF) progression in glaucoma patients. Methods: This retrospective observational study included primary open-angle glaucoma with follow-up exceeding 3 years. We identified macular cystic changes through Spectralis optical coherence tomography and measured the INL thickness using automated segmentation. Glaucoma progression was determined using the Guided Progression Analysis program of the Humphrey filed analyzer, calculating the mean deviation (MD) changes (dB/year). Results: Microcystic macular changes were observed in 12 (7.5%) of 162 patients. Patients with microcystic macular change had thicker INL thickness than those without it (p = 0.010). Progressors had a higher probability of having microcystic macular changes and a thicker average INL thickness than nonprogressors (p = 0.003, p = 0.019). Thicker INL thickness was associated with faster VF progression based on MD slope (dB/year) in the multivariate regression analysis (p = 0.045). Additionally, greater intraocular pressure (IOP) fluctuation was found to be associated with both a thicker INL and the presence of microcystic changes in the multivariate regression analysis (p = 0.003, 0.028). Conclusions: Increased macular INL thickness indicative of INL changes was linked to subsequent VF progression in glaucoma patients. These findings suggest that retinal inner nuclear change could serve as an indicator of progressive glaucoma.

Protective Effects of Pituitary Adenylate-Cyclase-Activating Polypeptide on Retinal Vasculature and Molecular Responses in a Rat Model of Moderate Glaucoma

Despite the high probability of glaucoma-related blindness, its cause is not fully understood and there is no efficient therapeutic strategy for neuroprotection. Vascular factors have been suggested to play an important role in glaucoma development and progression. Previously, we have proven the neuroprotective effects of pituitary adenylate-cyclase-activating polypeptide (PACAP) eye drops in an inducible, microbeads model in rats that is able to reproduce many clinically relevant features of human glaucoma. In the present study, we examined the potential protective effects of PACAP1-38 on the retinal vasculature and the molecular changes in hypoxia. Ocular hypertension was induced by injection of microbeads into the anterior chamber, while control rats received PBS. PACAP dissolved in vehicle (1 µg/drop) or vehicle treatment was started one day after the injections for four weeks three times a day. Retinal degeneration was assessed with optical coherence tomography (OCT), and vascular and molecular changes were assessed by immunofluorescence labeling. HIF1-α and VEGF-A protein levels were measured by Western blot. OCT images proved severe retinal degeneration in the glaucomatous group, while PACAP1-38 eye drops had a retinoprotective effect. Vascular parameters were deteriorated and molecular analysis suggested hypoxic conditions in glaucoma. PACAP treatment exerted a positive effect against these alterations. In summary, PACAP could prevent the severe damage to the retina and its vasculature induced by ocular hypertension in a microbeads model.

Intraocular Adeno-Associated Virus-Mediated Transgene Endothelin-1 Delivery to the Rat Eye Induces Functional Changes Indicative of Retinal Ischemia-A Potential Chronic Glaucoma Model

Endothelin-1 (ET-1) overactivity has been implicated as a factor contributing to glaucomatous neuropathy, and it has been utilized in animal models of retinal ischemia. The functional effects of long-term ET-1 exposure and possible compensatory mechanisms have, however, not been investigated. This was therefore the purpose of our study. ET-1 was delivered into rat eyes via a single intravitreal injection of 500 µM or via transgene delivery using an adeno-associated viral (AAV) vector. Retinal function was assessed using electroretinography (ERG) and the retinal expression of potentially compensatory genes was evaluated by means of qRT-PCR. Acute ET-1 delivery led to vasoconstriction and a significant reduction in the ERG response. AAV-ET-1 resulted in substantial transgene expression and ERG results similar to the acute ET-1 injections and comparable to other models of retinal ischemia. Compensatory changes were observed, including an increase in calcitonin gene-related peptide (CGRP) gene expression, which may both counterbalance the vasoconstrictive effects of ET-1 and provide neuroprotection. This chronic ET-1 ischemia model might be especially relevant to glaucoma research, mimicking the mild and repeated ischemic events in patients with long-term vascular dysfunction. The compensatory mechanisms, and particularly the role of vasodilatory CGRP in mitigating the retinal damage, warrant further investigation with the aim of evaluating new therapeutic strategies.

Assessment of outer retinal thickness and function in mice after experimental optic nerve trauma

BACKGROUND

Optic nerve trauma caused by crush injury is frequently used for investigating experimental treatments that protect retinal ganglion cells (RGCs) and induce axonal regrowth. Retaining outer retinal light responses is essential for therapeutic rescue of RGCs after injury. However, whether optic nerve crush also damages the structure or function of photoreceptors has not been systematically investigated. In this study, we investigated whether outer retinal thickness and visual function are altered by optic nerve crush in the mouse.

METHODS

Wildtype mice underwent optic nerve crush and intravitreal injection of a control solution in one eye with the fellow eye remaining uninjured. Two weeks after injury, the thickness of the ganglion cell region (GCL to IPL) and photoreceptor layer (bottom of the OPL to top of the RPE) were measured using OCT. Retinal function was assessed using flash ERGs. Immunodetection of RGCs was performed on retinal cryosections and RGCs and ONL nuclei rows were counted. Multiple comparison analyses were conducted using Analysis of Variance (ANOVA) with Tukey's post hoc test and P values less than 0.05 were considered statistically significant.

RESULTS

Optic nerve crush injury induced RGC death as expected, demonstrated by thinning of the ganglion cell region and RGC loss. In contrast, outer retinal thickness, photopic and scotopic a-wave and b-wave amplitudes and photoreceptor nuclei counts, were equivalent between injured and uninjured eyes.

CONCLUSIONS

Secondary degeneration of the outer retina was not detected after optic nerve injury in the presence of significant RGC death, suggesting that the retina has the capacity to compartmentalize damage. These findings also indicate that experimental treatments to preserve the GCL and rescue vision using this optic nerve injury model would not require additional strategies to preserve the ONL.

Relationship Between Deep Retinal Macular Vessel Density and Bipolar Cell Function in Glaucomatous Eyes

Purpose

To evaluate the correlation between macular retinal function and the changes in the macular retinal vascular structure in glaucomatous eyes.

Methods

The study included patients with glaucoma who visited Saitama Medical University and underwent optical coherence tomography angiography, and multifocal electroretinographic examinations at the same time between February 2020 and April 2021. Correlations among the ocular parameters, macular vessel density, and multifocal electroretinographic parameters were evaluated using a mixed model.

Results

Forty-one eyes (mean deviation, −12.4 ± 7.8 dB) of 24 subjects (mean age, 75.2 ± 8.3 years) were included in the analysis. There were no significant correlations for macular vessel density in the superficial retinal layer. However, macular vessel density in the deep retinal layer showed a significant positive correlation with P1–N1 amplitude (coefficient = 0.724; P = 0.001). There were no significant correlations between the optical coherence tomography parameters and any of the multifocal electroretinographic parameters.

Conclusions

A decrease in N1–P1 amplitude was observed in glaucomatous eyes in relation to a reduction in macular vessel density in the deep retinal layer, which suggests that ischemia-induced bipolar cell dysfunction may be involved in the intermediate retinal dysfunction associated with glaucoma.

Translational Relevance

Intermediate retinal dysfunction in glaucoma is related to the changes in deep retinal microvasculature.

Mechanism for altered dark-adapted electroretinogram responses in DBA/2J mice includes pupil dilation deficits

PURPOSE

The DBA/2J (D2) mouse is an established model of pigmentary glaucoma, a type of primary open angle glaucoma. Prior studies have documented defects in flash electroretinogram (ERG) responses in D2 mice, but the origin of those defects is not clear. The purpose of this study was to understand the origin of these A-wave and B-wave changes in D2 ERGs.Materials & Methods: To accomplish this, we analyzed the differences between 9-month-old DBA/2J-Gpnmb+ (D2-control) and D2 mouse eyes in relation to ERG responses, intraocular pressure (IOP), outer nuclear layer thickness, and pupil area.

RESULTS

D2 scotopic ERGs showed lower A-wave amplitude and longer implicit time as well as a significant rightward shift in the intensity-response curve. D2 IOP increased at approximately seven months of age and had a weak correlation with the ERG A-wave sensitivity. Outer nuclear layer thickness was not significantly different in D2s compared to D2-control retinas. D2 mouse pupils also showed abnormal pupillary shape and no dilation following treatment with tropicamide eye drops. The pupil size moderately correlated with the A-wave sensitivity and this was pharmacologically replicated in C57Bl/6J mice following administration of pilocarpine to constrict the pupils. However, pilocarpine treatment did not affect ERG amplitudes.

CONCLUSIONS

These data suggest that the smaller pupil sizes prevented light from reaching the photoreceptors and thus contributed to reduced ERG sensitivity in D2 mice. The reduced ERG A-wave amplitude in D2 mice likely results from dysfunctional photoreceptor responses.

Magnesium acetyltaurate prevents retinal damage and visual impairment in rats through suppression of NMDA-induced upregulation of NF-κB, p53 and AP-1 (c-Jun/c-Fos)

Magnesium acetyltaurate (MgAT) has been shown to have a protective effect against N-methyl-D-aspartate (NMDA)-induced retinal cell apoptosis. The current study investigated the involvement of nuclear factor kappa-B (NF-κB), p53 and AP-1 family members (c-Jun/c-Fos) in neuroprotection by MgAT against NMDA-induced retinal damage. In this study, Sprague-Dawley rats were randomized to undergo intravitreal injection of vehicle, NMDA or MgAT as pre-treatment to NMDA. Seven days after injections, retinal ganglion cells survival was detected using retrograde labelling with fluorogold and BRN3A immunostaining. Functional outcome of retinal damage was assessed using electroretinography, and the mechanisms underlying antiapoptotic effect of MgAT were investigated through assessment of retinal gene expression of NF-κB, p53 and AP-1 family members (c-Jun/c-Fos) using reverse transcription-polymerase chain reaction. Retinal phospho-NF-κB, phospho-p53 and AP-1 levels were evaluated using western blot assay. Rat visual functions were evaluated using visual object recognition tests. Both retrograde labelling and BRN3A immunostaining revealed a significant increase in the number of retinal ganglion cells in rats receiving intravitreal injection of MgAT compared with the rats receiving intravitreal injection of NMDA. Electroretinography indicated that pre-treatment with MgAT partially preserved the functional activity of NMDA-exposed retinas. MgAT abolished NMDA-induced increase of retinal phospho-NF-κB, phospho-p53 and AP-1 expression and suppressed NMDA-induced transcriptional activity of NF-κB, p53 and AP-1 family members (c-Jun/c-Fos). Visual object recognition tests showed that MgAT reduced difficulties in recognizing the visual cues (i.e. objects with different shapes) after NMDA exposure, suggesting that visual functions of rats were relatively preserved by pre-treatment with MgAT. In conclusion, pre-treatment with MgAT prevents NMDA induced retinal injury by inhibiting NMDA-induced neuronal apoptosis via downregulation of transcriptional activity of NF-κB, p53 and AP-1-mediated c-Jun/c-Fos. The experiments were approved by the Animal Ethics Committee of Universiti Teknologi MARA (UiTM), Malaysia, UiTM CARE No 118/2015 on December 4, 2015 and UiTM CARE No 220/7/2017 on December 8, 2017 and Ethics Committee of Belgorod State National Research University, Russia, No 02/20 on January 10, 2020.

The Paediatric Glaucoma Diagnostic Ability of Optical Coherence Tomography: A Comparison of Macular Segmentation and Peripapillary Retinal Nerve Fibre Layer Thickness

Simple Summary

Optical coherence tomography (OCT) is an established ophthalmologic diagnostic tool to visualise vital retinal structures. In glaucoma, it is used to quantify the thickness decrease in the peripapillary retinal nerve fibre layer (pRNFL) and in the macula. While glaucoma management in adults incorporates traditional clinical parameters as well as instrumental methods such as OCT, guidelines for paediatric glaucoma focus on conventional methods. Even though some reports encouraging a broader use of OCT in children are present, its diagnostic potential in this particular population has still not been sufficiently analysed. To address this, the present study compares the glaucoma discriminative ability of OCT measurements of the pRNFL and macular layers in a paediatric population. The results indicate a reduction of the pRNFL and of inner macular layer thickness in glaucoma eyes, as well as a high correlation with the presence of glaucoma. The glaucoma discriminative ability can be maximised combining either all pRNFL sectors or the thickness results of the three innermost macular layers, even though sensitivity remains moderate. In conclusion, the OCT measurements of the pRNFL and macular thickness have a strong ability to diagnose paediatric glaucoma. However, OCT should be used in addition to conventional diagnostic tools rather than as a standalone method.

Abstract

Paediatric glaucoma leads to a decreased thickness of the peripapillary retinal nerve fibre layer (pRNFL) and of the macula. These changes can be precisely quantified using spectral domain-optical coherence tomography (SD-OCT). Despite abundant reports in adults, studies on the diagnostic capacity of macular SD-OCT in paediatric glaucoma are rare. The aim of this study was to compare the glaucoma discriminative ability of pRNFL and macular segment thickness in paediatric glaucoma patients and healthy children. Data of 72 children aged 5–17 years (glaucoma: 19 (26.4%), healthy: 53 (73.6%)) examined with SD-OCT (SPECTRALIS^®, Heidelberg Engineering) were analysed retrospectively. The thickness of pRNFL sectors and of macular segment subfields were compared between diseased and healthy participants. Areas under the receiver-operating characteristic curves (AUC), sensitivity, and specificity from logistic regression were used to evaluate the glaucoma discriminative capacity of single and combined pRNFL and macular segments’ thickness. The results revealed a reduced thickness of the pRNFL and of the three inner macular layers in glaucoma patients, which correlates highly with the presence of glaucoma. The highest glaucoma discriminative ability was observed for the combination of pRNFL sectors or inner macular segments (AUC: 0.83 and 0.85, respectively), although sensitivity remained moderate (both 63% at 95% specificity). In conclusion, while confirmation from investigations in larger cohorts is required, SD-OCT-derived pRNFL and macular thickness measurements seem highly valuable for the diagnosis of paediatric glaucoma.

Automated Quantification of Macular Ellipsoid Zone Intensity in Glaucoma Patients: the Method and its Comparison with Manual Quantification

The macular ellipsoid zone intensity (mEZi) is a known marker of disease severity in a number of diverse ocular diseases. The purpose of this study was to establish an automated method (AM) for mEZi quantification and to compare the method's performance with that of a manual method (MM) for glaucoma patients and healthy controls. Seventy-one (71) mild-to-moderate glaucoma patients, 71 severe-glaucoma patients, and 51 controls were enrolled. Both calibration (n = 160) and validation (n = 33) image sets were compiled. The correlation of AM to MM quantification was assessed by Deming regression for the calibration set, and a compensation formula was generated. Then, for each image in the validation set, the compensated AM quantification was compared with the mean of five repetitive MM quantifications. The AM quantification of the calibration set was found to be linearly correlated with MM in the normal-to-severe-stage glaucoma patients (R² = 0.914). The validation set's compensated AM quantification produced R² = 0.991, and the relationship between the 2 quantifications was AM = 1.004(MM) + 0.139. In the validation set, the compensated AM quantification fell within MM quantification's 95% confidence interval in 96.9% of the images. An AM for mEZi quantification was calibrated and validated relative to MM quantification for both glaucoma patients and healthy controls.

Intravitreal Therapy Against the Complement Factor C5 Prevents Retinal Degeneration in an Experimental Autoimmune Glaucoma Model

In glaucoma, studies revealed an involvement of the complement system. In an experimental autoimmune glaucoma model, immunization with an optic nerve homogenate antigen (ONA) led to retinal ganglion cell (RGC) loss, while intraocular pressure (IOP) remained unchanged. Here, we investigated the therapeutic effect of a complement system inhibition in this model. Hence, rats were immunized with ONA and compared to controls. In one eye of the ONA animals, an antibody against complement factor C5 was intravitreally injected (15 μmol: ONA+C5-I or 25 μmol: ONA+C5-II) before immunization and then every two weeks. IOP was measured weekly. After 6 weeks, spectral-domain optical coherence tomographies (SD-OCT), electroretinograms (ERG), immunohistochemistry, and quantitative real-time PCR analyses were performed. IOP and retinal thickness remained unchanged within all groups. The a-wave amplitudes were not altered in the ONA and ONA+C5-I groups, whereas a decrease was noted in ONA+C5-II animals (p < 0.05). ONA immunization provoked a significant decrease of the b-wave amplitude (p < 0.05), which could be preserved in ONA+C5-I, but not in ONA+C5-II animals. ONA animals showed a loss of RGCs (p = 0.001), while ONA+C5-I and ONA+C5-II retinae had similar cell counts as controls. A significant downregulation of apoptotic Bax/Bcl2 mRNA was noted in ONA+C5-I retinae (p = 0.02). Significantly more C3⁺ and MAC⁺ cells were observed in ONA animals (p < 0.001). The amount of C3⁺ cells in both treatment groups was significantly increased (p < 0.01), while the number of MAC⁺ cells in the treated retinas did not differ from controls. The number of activated microglia cells remained unchanged in ONA animals, but was increased in the treatment groups (p < 0.05). Recoverin⁺ cells were diminished in ONA animals (p = 0.049), but not in treated ones. Rho mRNA was downregulated in ONA and in ONA+C5-II retinas (both p = 0.014). Less opsin⁺ cones were observed in ONA animals (p = 0.009), but not in the treated groups. Our results indicate that the C5 antibody inhibits activation of the complement system, preventing the loss of retinal function as well as RGC, cone bipolar, and photoreceptor loss. Therefore, this approach might be a suitable new treatment for glaucoma patients, in which immune dysregulation plays an important factor for the development and progression of glaucoma.

Loss of retinal ganglion cells in a new genetic mouse model for primary open-angle glaucoma

Primary open-angle glaucoma (POAG) is one of the most common causes for blindness worldwide. Although an elevated intraocular pressure (IOP) is the main risk factor, the exact pathology remained indistinguishable. Therefore, it is necessary to have appropriate models to investigate these mechanisms. Here, we analysed a transgenic glaucoma mouse model (βB1-CTGF) to elucidate new possible mechanisms of the disease. Therefore, IOP was measured in βB1-CTGF and wildtype mice at 5, 10 and 15 weeks of age. At 5 and 10 weeks, the IOP in both groups were comparable (P > 0.05). After 15 weeks, a significant elevated IOP was measured in βB1-CTGF mice (P < 0.001). At 15 weeks, electroretinogram measurements were performed and both the a- and b-wave amplitudes were significantly decreased in βB1-CTGF retinae (both P < 0.01). Significantly fewer Brn-3a+ retinal ganglion cells (RGCs) were observed in the βB1-CTGF group on flatmounts (P = 0.02), cross-sections (P < 0.001) and also via quantitative real-time PCR (P = 0.02). Additionally, significantly more cleaved caspase 3+ RGCs were seen in the βB1-CTGF group (P = 0.002). Furthermore, a decrease in recoverin+ cells was observable in the βB1-CTGF animals (P = 0.004). Accordingly, a significant down-regulation of Recoverin mRNA levels were noted (P < 0.001). Gfap expression, on the other hand, was higher in βB1-CTGF retinae (P = 0.023). Additionally, more glutamine synthetase signal was noted (P = 0.04). Although no alterations were observed regarding photoreceptors via immunohistology, a significant decrease of Rhodopsin (P = 0.003) and Opsin mRNA (P = 0.03) was noted. We therefore assume that the βB1-CTGF mouse could serve as an excellent model for better understanding the pathomechanisms in POAG.

Heterozygous Meg2 Ablation Causes Intraocular Pressure Elevation and Progressive Glaucomatous Neurodegeneration

Glaucomatous neurodegeneration represents one of the major causes of irreversible blindness worldwide. Yet, the detailed molecular mechanisms that initiate optic nerve damage and retinal ganglion cell (RGC) loss are not fully understood. Members of the protein tyrosine phosphatase (PTP) superfamily are key players in numerous neurodegenerative diseases. In order to investigate the potential functional relevance of the PTP megakaryocyte 2 (Meg2) in retinal neurodegeneration, we analyzed Meg2 knockout (KO) and heterozygous (HET)-synonym protein-tyrosine phosphatase non-receptor type 9 (Ptpn9)-mice. Interestingly, via global microarray and quantitative real-time PCR (RT-qPCR) analyses of Meg2 KO and HET retinae, we observed a dysregulation of several candidate genes that are highly associated with retinal degeneration and intraocular pressure (IOP) elevation, the main risk factor for glaucoma. Subsequent IOP measurements in Meg2 HET mice verified progressive age-dependent IOP elevation. Ultrastructural analyses and immunohistochemistry showed severe optic nerve degeneration accompanied by a dramatic loss of RGCs. Additionally, HET mice displayed reactive micro-/macrogliosis and early activation of the classical complement cascade with pronounced deposition of the membrane attack complex (MAC) in the retina and optic nerve. When treated with latanoprost, significant IOP lowering prevented RGC loss and microglial invasion in HET mice. Finally, electroretinogram (ERG) recordings revealed reduced a- and b-wave amplitudes, indicating impaired retinal functionality in Meg2 HET mice. Collectively, our findings indicate that the heterozygous loss of Meg2 in mice is sufficient to cause IOP elevation and glaucomatous neurodegeneration. Thus, Meg2 HET mice may serve as a novel animal model to study the pathomechanism involved in the onset and progression of glaucoma.

Analyzing the impact of glaucoma on the macular architecture using spectral-domain optical coherence tomography

Purpose

Using spectral domain optical coherence tomography (SD-OCT) the retina can be segmented automatically to visualize all retinal layers. In glaucoma chronically elevated intraocular pressure leads to a decline of retinal ganglion cells (RGC) which changes retinal architecture. The goal of these analyses was to gain insight into the changes induced by glaucoma within all macular layers using SD-OCT within a closely circumscribed glaucoma cohort.

Materials and methods

SD-OCT measurements with automated retinal layer segmentation were performed in both eyes of primary open-angle glaucoma patients with a defined monocular absolute visual field scotoma in the central 10° of the visual field and in an age-matched healthy control group. Thickness of single retinal layers and entire retina were compared with special attention to the localization of the visual field scotoma in the glaucomatous eyes.

Results

30 eyes of 15 glaucoma patients and 15 eyes of 15 healthy controls were included in this study. Statistical significant thickness differences were detected in the control group between superior and inferior retina for the retinal nerve fiber layer (RNFL), the outer plexiform layer (OPL) and the outer nuclear layer (ONL). In the glaucoma group thickness differences between worse and less affected eyes in the RNFL, the ganglion cell layer (GCL) and the inner plexiform layers (INL) were found. Comparison between healthy and diseased eyes revealed significant thickness differences in the RNFL, GCL, IPL and total retinal thickness but not the outer retinal layers.

Conclusion

Comparison between SD-OCT measurements of the macula between healthy and glaucomatous eyes in a closely circumscribed disease stage showed a pronounced disease impact on the inner but not the outer retina. These results provide evidence that GCL and IPL thickness seem to be good measures to discriminate between affected and unaffected eyes in testing for glaucoma.

Outer retinal layer thickness in patients with glaucoma with horizontal hemifield visual field defects

BACKGROUND/AIMS

To determine the effect of glaucoma on outer retinal layer thickness in eyes with horizontal hemifield visual field (VF) defects.

METHODS

We conducted a cross-sectional study in glaucomatous eyes with repeatable (in three or examinations) horizontal hemifield VF (programme 24-2) defect defined as: all five nasal VF locations immediately either above or below the horizontal midline abnormal in the pattern deviation plot with p<0.5%; no mirror-image adjacent 5 VF locations abnormal in the pattern deviation plot and no non-edge VF locations in the non-affected hemifield abnormal in the pattern deviation plot with p<1%. We used optical coherence tomography to measure thickness of each retinal layer in the temporal macula (12° horizontally and 24° vertically) and computed the absolute (µm) and relative (%) intraindividual asymmetry between the perimetrically normal and abnormal hemimacula.

RESULTS

We included 10 eyes of 8 patients with median age of 67 years and median VF mean deviation of -8.85 dB. The nerve fibre, ganglion cell and inner plexiform layers were significantly thinner in the perimetrically abnormal hemimacula (median asymmetry of -6.4, -11.5 and -3.8 µm, (corresponding to -27.7, -40.5 and -15.7 %), respectively, all p≤0.01). The inner nuclear layer was slightly thicker in the perimetrically abnormal hemimacula (median asymmetry of 1.3 µm (5.0 %), p=0.01). The outer plexiform, outer nuclear and photoreceptor layers asymmetry values were negligible.

CONCLUSION

Our study showed no evidence that glaucoma has an effect on the outer retinal layer thickness. In contrast, a large impact was observed in inner layer thickness.

Ellipsoid Zone Change According to Glaucoma Stage Advancement

PURPOSE

To compare retinal photoreceptor ellipsoid zone (EZ) intensity between normal eyes and those with different stages of glaucoma.

DESIGN

Retrospective cross-sectional study.

METHODS

The study included 37 normal, 38 preperimetric glaucoma, 39 mild-to-moderate glaucoma (visual field [VF] mean deviation [MD]: -7.7 ± 2.0 dB), and 36 severe glaucoma eyes (VF MD: -17.8 ± 3.2 dB). The subjects underwent high-resolution horizontal and vertical line scans through the fovea by spectral-domain optical coherence tomography (SD-OCT). Image processing software was employed to quantify the intensity of the first and second hyperreflective bands corresponding with the external limiting membrane (ELM) and EZ. In order to account for the brightness variation among scans, the relative EZ intensity as the ratio of the second to first reflective band (EZ/ELM) was determined.

RESULTS

The relative EZ intensity in severe glaucoma eyes was significantly lower than in mild-to-moderate glaucoma eyes (2.46 ± 0.38 vs 3.15 ± 0.43, P < .001); also, it was lower in mild-to-moderate than in preperimetric glaucoma eyes (3.15 ± 0.43 vs 3.86 ± 0.44, P < .001). However, the comparison between preperimetric glaucoma and normal eyes showed no significant difference (3.86 ± 0.44 vs 4.06 ± 0.40, P = .751). In 75 glaucomatous eyes with VF defect, there was a significant correlation between relative EZ intensity and VF MD (r = 0.83 and P < .001).

CONCLUSIONS

According to SD-OCT, relative EZ intensity reduction occurs in the mild-to-moderate and severe glaucoma stages. These findings suggest, at least provisionally, that in the course of glaucoma progression, mitochondrial changes in the inner segments of photoreceptors occur. Further investigation is warranted to evaluate the potential clinical significance of EZ intensity reduction in glaucoma.

Low-dose minocycline mediated neuroprotection on retinal ischemia-reperfusion injury of mice

PURPOSE

The aim of this study was to investigate the effect of minocycline (MC) on the survival of retinal ganglion cells (RGCs) in an ischemic-reperfusion (I/R) injury model of retinal degeneration.

METHODS

Retinal I/R injury was induced in the left eye of mice for 60 min by maintaining intraocular pressure at 90 mmHg. Low- or high-dose MC (20 or 100 mg/kg, respectively) was administered by intravenous injection at 5 min after the retinal ischemic insult and then administered once daily until the mice were euthanized. RGCs and microglial cells were counted using immunofluorescence staining. Functional changes in the RGCs were evaluated using electroretinography. The visual function was assessed using an optokinetic test.

RESULTS

The data demonstrated that the effect of MC was dose dependent. Low-dose MC showed protective effects, with reduced RGC loss and microglial activation, while the high-dose MC showed damage effects, with more RGC loss and microglial activation when compared with the vehicle group. The electroretinography and optokinetic test results were consistent with the morphologic observations.

CONCLUSIONS

These data suggested that appropriate concentrations of MC can protect the retina against retinal ischemic-reperfusion injury, while excessive MC has detrimental effects.

Functional Analysis and Immunochemical Analyses of Ca2+ Homeostasis-Related Proteins Expression of Glaucoma-Induced Retinal Degeneration in Rats

The retinal degeneration resulting from elevated intraocular pressure was evaluated through functional and morphological analyses, for better understanding of the pathophysiology of glaucoma. Ocular hypertension was induced via unilateral episcleral venous cauterization in rats. Experimental time was set at 1 and 3 days, and 1, 2, 4, and 8 weeks post-operation. Retinal function was analyzed using electroretinography. For morphological analysis, retinal tissues were processed for immunochemistry by using antibodies against the calcium-sensing receptor and calcium-binding proteins. Apoptosis was analyzed using the TUNEL method and electron microscopy. Amplitudes of a- and b-wave in scotopic and photopic responses were found to be reduced in all glaucomatous retinas. Photopic negative response for ganglion cell function significantly reduced from 1-day and more significantly reduced in 2-week glaucoma. Calcium-sensing receptor immunoreactivity in ganglion cells remarkably reduced at 8 weeks; conversely, protein amounts increased significantly. Calcium-binding proteins immunoreactivity in amacrine cells clearly reduced at 8 weeks, despite of uneven changes in protein amounts. Apoptosis appeared in both photoreceptors and ganglion cells in 8-week glaucomatous retina. Apoptotic feature of photoreceptors was typical, whereas that of ganglion cells was necrotic in nature. These findings suggest that elevated intraocular pressure affects the sensitivity of photoreceptors and retinal ganglion cells, and leads to apoptotic death. The calcium-sensing receptor may be a useful detector for alteration of extracellular calcium levels surrounding the ganglion cells.

HSP27 immunization reinforces AII amacrine cell and synapse damage induced by S100 in an autoimmune glaucoma model

Previous studies have revealed a loss of retinal ganglion cells (RGCs) and optic nerve fibers after immunization with the S100B protein. Addition of heat shock protein 27 (HSP27) also leads to a decrease of RGCs. Our present aim has been to analyze various retinal cell types after immunization with S100B or S100B + HSP27 (S100 + HSP). After 28 days, retinas were processed for immunohistology and Western blot. RGCs, immunostained for NeuN, were significantly decreased in the S100 and the S100 + HSP groups. Significantly fewer ChAT⁺ cells were noted in both groups, whereas parvalbumin⁺ cells were only affected in the S100 + HSP group. Western blot results also revealed fewer ChAT signals in both immunized groups. No changes were noted with regard to PKCα⁺ rod bipolar cells, whereas a significant loss of recoverin⁺ cone bipolar cells was observed in both groups via immunohistology and Western blot. The presynaptic marker Bassoon and the postsynaptic marker PSD95 were significantly reduced in the S100 + HSP group. Opsin⁺ and rhodopsin⁺ photoreceptors revealed no changes in either group. Thus, the inner retinal layers are affected by immunization. However, the combination of S100 and HSP27 has a stronger additive effect on the retinal synapses and AII amacrine cells.

Relationship between Retinal Inner Nuclear Layer Thickness and Severity of Visual Field Loss in Glaucoma

Glaucoma is a disease characterized by pathologic changes in inner retinal layers, which are comprised of retinal ganglion cells (RGCs). As retinal ganglion cells (RGCs) cross over other retinal neurons that are connected by synapses, it is meaningful to investigate the outer retinal changes in glaucoma. We evaluated the association between thicknesses of segmented retinal layers in macular region and severity of visual field loss in open-angle glaucoma (OAG). This study involved 103 glaucomatous eyes. Retinal nerve fiber layer (RNFL), ganglion cell layer (GCL), inner plexiform layer (IPL), inner nuclear layer (INL), outer plexiform layer (OPL), and outer nuclear layer (ONL) thicknesses were measured at the macular level using the Spectral-domain optical coherence tomography with segmentation software. The functional losses were measured using 24-2 standard automated perimetry. Macular structure losses were positively correlated with functional loss for RNFL, GCL, and IPL (R = 0.550, 0.637, and 0.649, respectively, P < 0.001) and negatively correlated with INL (R = −0.295, P = 0.041). By multivariate regression analysis, INL thickness was significantly associated with visual field mean deviation (dB) and optic disc hemorrhage. These finding carefully suggest reactive responses of neuronal or glial cells located in the INL occur during glaucoma progression.

Ultra-high resolution profiles of macular intra-retinal layer thicknesses and associations with visual field defects in primary open angle glaucoma

The structural characteristics of the outer retinal layers in primary open angle glaucoma (POAG) are still controversial, and these changes, along with those in the inner retinal layers, could have clinical and/or pathophysiological significance. A custom-built ultra-high resolution optical coherence tomography (UHR-OCT) combined with an automated segmentation algorithm can image and measure the eight intra-retinal layers. The purpose of this study is to determine the thickness characteristics of the macular intra-retinal layers, especially the outer layers, in POAG patients. Thirty-four POAG patients (56 eyes) and 33 normal subjects (63 eyes) were enrolled. Thickness profiles of the eight intra-retinal layers along a 6-mm length centred on the fovea at the horizontal and vertical meridians were obtained and the regional thicknesses were compared between two groups. The associations between the thicknesses of each intra-retinal layer and the macular visual field (VF) sensitivity were then analysed. POAG affected not only the inner retinal layers but also the photoreceptor layers and retinal pigment epithelium of the outer retina. However, the VF loss was correlated mainly with the damage of the inner retinal layers. UHR-OCT with automated algorithm is a useful tool in detecting microstructural changes of macula with respect to the progression of glaucoma.

Genetic and Biochemical Biomarkers in Canine Glaucoma

In many health-related fields, there is great interest in the identification of biomarkers that distinguish diseased from healthy individuals. In addition to identifying the diseased state, biomarkers have potential use in predicting disease risk, monitoring disease progression, evaluating treatment efficacy, and informing pathogenesis. This review details the genetic and biochemical markers associated with canine primary glaucoma. While there are numerous molecular markers (biochemical and genetic) associated with glaucoma in dogs, there is no ideal biomarker that allows early diagnosis and/or identification of disease progression. Genetic mutations associated with canine glaucoma include those affecting ADAMTS10, ADAMTS17, Myocilin, Nebulin, COL1A2, RAB22A, and SRBD1. With the exception of Myocilin, there is very limited crossover in genetic biomarkers identified between human and canine glaucomas. Mutations associated with canine glaucoma vary between and within canine breeds, and gene discoveries therefore have limited overall effects as a screening tool in the general canine population. Biochemical markers of glaucoma include indicators of inflammation, oxidative stress, serum autoantibodies, matrix metalloproteinases, tumor necrosis factor–α, and transforming growth factor–β. These markers include those that indicate an adaptive or protective response, as well as those that reflect the damage arising from oxidative stress.

Quadrant Field Pupillometry Detects Melanopsin Dysfunction in Glaucoma Suspects and Early Glaucoma

It is difficult to detect visual function deficits in patients at risk for glaucoma (glaucoma suspects) and at early disease stages with conventional ophthalmic tests such as perimetry. To this end, we introduce a novel quadrant field measure of the melanopsin retinal ganglion cell mediated pupil light response corresponding with typical glaucomatous arcuate visual field defects. The melanopsin-mediated post-illumination pupil response (PIPR) was measured in 46 patients with different stages of glaucoma including glaucoma suspects and compared to a healthy group of 21 participants with no disease. We demonstrate that the superonasal quadrant PIPR differentiated glaucoma suspects and early glaucoma patients from controls with fair (AUC = 0.74) and excellent (AUC = 0.94) diagnostic accuracy, respectively. The superonasal PIPR provides a linear functional correlate of structural retinal nerve fibre thinning in glaucoma suspects and early glaucoma patients. This first report that quadrant PIPR stimulation detects melanopsin dysfunction in patients with early glaucoma and at pre-perimetric stages may have future implications in treatment decisions of glaucoma suspects.

Macular Structure and Function in Nonhuman Primate Experimental Glaucoma

Purpose

To evaluate structure and function of macular retinal layers in nonhuman primate (NHP) experimental glaucoma (EG).

Methods

Twenty-one NHP had longitudinal imaging of macular structure by SDOCT, 16 also had recordings of function by multifocal ERG. The average thickness over 15° was derived for seven individual SDOCT layers: macular nerve fiber layer (m-NFL), retinal ganglion cell layer (RGCL), inner plexiform layer (IPL), inner nuclear layer (INL), outer plexiform layer (OPL), outer nuclear layer+inner segments combined (ONL+IS), and outer segments (OS). Peripapillary RNFL thickness (ppRNFLT) was measured from a single circular B-scan with 12° diameter. Responses to a slow-sequence multifocal ERG (mfERG) stimulus (7F) were filtered (at 75 Hz) into low- and high-frequency components (LFC, HFC).

Results

At final follow-up, significant structural loss occurred only in EG eyes and only for ppRNFLT (−29 ± 23%), m-NFL (−17 ± 16%), RGCL (−22 ± 15%), and IPL (−19 ± 14%); though there was also a small increase in OPL (+6 ± 7%) and ONL+IS (4 ± 4%) and a similar tendency for INL. Macular structural loss was correlated with ppRNFLT only for the NFL, RGCL and IPL (R = 0.95, 0.93 and 0.95, respectively, P < 0.0001). Significant functional loss occurred only for HFC and N2 in EG eyes. Significant longitudinal structure–function correlations (P < 0.01) were observed only in EG eyes and only for mfERG HFC and N2: HFC was correlated with ppRNFLT (R = 0.69), macular NFL (R = 0.67), RGCL (R = 0.74), and IPL (R = 0.72); N2 was correlated with RGCL (R = 0.54) and IPL (R = 0.48). High-frequency components amplitude change was inversely correlated with outer retinal thickness change (= −0.66).

Conclusions

Macular structural and functional losses are correlated and specific to ganglion cells over a wide range of EG severity. Outer retinal changes are likely due to inner retinal loss.

The pro-inflammatory role of high-mobility group box 1 protein (HMGB-1) in photoreceptors and retinal explants exposed to elevated pressure

To determine the role of high-mobility group box 1 protein (HMGB-1) in cellular and tissue models of elevated pressure-induced neurodegeneration, regeneration, and inflammation. Mouse retinal photoreceptor-derived cells (661W) and retinal explants were incubated either under elevated pressure or in the presence of recombinant HMGB-1 (rHMGB-1) to investigate the mechanisms of response of photoreceptors. Immunohistochemistry, western blotting, and the quantitative real-time PCR were used to examine the expression levels of immunological factors (eg, HMGB-1, receptor for advanced glycation end products (RAGE)), Toll-like receptors 2 and 4 (TLR-2, TLR-4), apoptosis-related factors (eg, B-cell lymphoma 2 (Bcl-2), Bcl-2-associated death promoter (Bad)) as well as cytokine expression (eg, tumor necrosis factor alpha (TNF-α), interleukin (IL)-4, IL-6, and vascular endothelial growth factor (VEGF)). The data revealed increased the expression of HMGB-1 and its receptors RAGE, TLR-2, and TLR-4, and TNF-α as well as pro-apoptotic factors (eg, Bad) as well as apoptosis in 661W cells exposed to elevated pressure. Co-cultivation of 661W cells with rHMGB-1 increased the expression levels of pro-apoptotic Bad and cleaved Caspase-3 resulting in apoptosis. Cytokine array studies revealed an increased release of TNF-α, IL-4, IL-6, and VEGF after incubation of 661W cells with rHMGB-1. Upregulation of HMGB-1, TLR-2, and RAGE as well as anti-apoptotic Bcl-2 expression levels was found in the retinal explants exposed to rHMGB-1 or elevated pressure. The results suggest that HMGB-1 promotes an inflammatory response and mediates apoptosis in the pathology of photoreceptors and retinal homeostasis. HMGB-1 may have a key role in ongoing damage of retinal cells under conditions of elevated intraocular pressure.

Long-term effect of laser-induced ocular hypertension on the cone electroretinogram and central macular thickness in monkeys

OBJECTIVE

The purpose of this study was to investigate the long-term effect of laser-induced ocular hypertension on the cone electroretinogram (ERG) and retinal thickness in monkeys.

BACKGROUND DATA

Degeneration of retinal nerve fiber layer (RNFL) and loss of retinal ganglion cells in the primate glaucoma model have been confirmed by histological studies and optical coherence tomography (OCT) images. However, it remains unclear whether the outer retina distal to the RGCs (e.g., photoreceptors) is involved in histological studies and in functional test.

MATERIALS AND METHODS

Subjects were five monkeys with high intraocular pressure (IOP) induced in the right eye by laser. Six years after the laser coagulation of the mid-trabecular meshwork, RNFL, ganglion cell complex (GCC), central macular thickness (CMT), and the thickness of outer retinal layer (ORL) were measured by OCT. The photopic responses of ERG were recorded in response to red flashes on a blue background. The maximum cone amplitude (Rcone) and cone sensitivity (Scone) were calculated.

RESULTS

Enlarged cup-to-disc (C/D) ratio was found in the lasered eyes. RNFL and GCC were significantly thinner in the lasered eyes (p<0.05), but no significant differences were found in CMT and the thickness of ORL compared with fellow eyes (p>0.05). Mean amplitude of the photopic negative response (PhNR), Mean Rcone were significantly lower in the lasered eye (p<0.05), and no significant differences of Scone were found between the two eyes (p>0.05).

CONCLUSIONS

Long-term ocular hypertension induced by laser affects the function of cone photoreceptor in monkeys.

Retinal neurodegeneration in experimental glaucoma

In rats and mice, limbar tissues of the left eye were laser-photocoagulated (LP) and ocular hypertension (OHT) effects were investigated 1 week to 6 months later. To investigate the innermost layers, retinas were examined in wholemounts using tracing from the superior colliculi to identify retinal ganglion cells (RGCs) with intact retrograde axonal transport, melanopsin immunodetection to identify intrinsically photosensitive RGCs (m(+)RGC), Brn3a immunodetection to identify most RGCs but not m(+)RGCs, RECA1 immunodetection to examine the inner retinal vessels, and DAPI staining to detect all nuclei in the GC layer. The outer retinal layers (ORLs) were examined in cross sections analyzed morphometrically or in wholemounts to study S- and L-cones. Innervation of the superior colliculi was examined 10 days to 14 weeks after LP with orthogradely transported cholera toxin subunit B. By 2 weeks, OHT resulted in pie-shaped sectors devoid of FG(+)RGCs or Brn3a(+)RGCs but with large numbers of DAPI(+)nuclei. Brn3a(+)RGCs were significantly greater than FG(+)RGCs, indicating the survival of large numbers of RGCs with their axonal transport impaired. The inner retinal vasculature showed no abnormalities that could account for the sectorial loss of RGCs. m(+)RGCs decreased to approximately 50-51% in a diffuse loss across the retina. Cross sections showed focal areas of degeneration in the ORLs. RGC loss at 1m diminished to 20-25% and did not progress further with time, whereas the S- and L-cone populations diminished progressively up to 6m. The retinotectal projection was reduced by 10 days and did not progress further. LP-induced OHT results in retrograde degeneration of RGCs and m(+)RGCs, severe damage to the ORL, and loss of retinotectal terminals.

Laser-induced ocular hypertension in adult rats does not affect non-RGC neurons in the ganglion cell layer but results in protracted severe loss of cone-photoreceptors

To investigate the long-term effects of laser-photocoagulation (LP)-induced ocular hypertension (OHT) in the innermost and outermost (outer-nuclear and outer segment)-retinal layers (ORL). OHT was induced in the left eye of adult rats. To investigate the ganglion cell layer (GCL) wholemounts were examined at 1, 3 or 6 months using Brn3a-immunodetection to identify retinal ganglion cells (RGCs) and DAPI-staining to detect all nuclei in this layer. To study the effects of LP on the ORL up to 6 months, retinas were: i) fresh extracted to quantify the levels of rod-, S- and L-opsin; ii) cut in cross-sections for morphometric analysis, or; iii) prepared as wholemounts to quantify and study retinal distributions of entire populations of RGCs (retrogradely labeled with fluorogold, FG), S- and L-cones (immunolabeled). OHT resulted in wedge-like sectors with their apex on the optic disc devoid of Brn3a(+)RGCs but with large numbers of DAPI(+)nuclei. The levels of all opsins diminished by 2 weeks and further decreased to 20% of basal-levels by 3 months. Cross-sections revealed focal areas of ORL degeneration. RGC survival at 15 days represented approximately 28% and did not change with time, whereas the S- and L-cone populations diminished to 65% and 80%, or to 20 and 35% at 1 or 6 months, respectively. In conclusion, LP induces in the GCL selective RGCs loss that does not progress after 1 month, and S- and L-cone loss that progresses for up to 6 months. Thus, OHT results in severe damage to both the innermost and the ORL.

Regional choroidal blood flow and multifocal electroretinography in experimental glaucoma in rhesus macaques

PURPOSE

To test a hypothesis of regional variation in the effect of experimental glaucoma on choroidal blood flow (ChBF) and retinal function.

METHODS

Five rhesus macaques underwent laser trabecular destruction (LTD) to induce elevated intraocular pressure (IOP). Intraocular pressures were elevated for 56 to 57 weeks. Multifocal electroretinographic (mfERG) and multifocal visual evoked cortical potential (mfVEP) testing were performed at regular intervals before and during the period of IOP elevation. At euthanasia, the IOP was manometrically controlled at 35 (experimentally glaucomatous eye) and 15 (fellow control eye) mm Hg. Fluorescent microspheres were injected into the left ventricle. Regional ChBF was determined.

RESULTS

All of the experimentally glaucomatous eyes exhibited supranormal first-order kernel (K1) root mean square (RMS) early portions of the mfERG waveforms and decreased amplitudes of the late waveforms. The supranormality was somewhat greater in the central macula. Second-order kernel, first slice (K2.1) RMS mfVEP response was inversely correlated (R(2) = 0.97) with axonal loss. Total ChBF was reduced in the experimentally glaucomatous eyes. The mean blood flow was 893 ± 123 and 481 ± 37 μL/min in the control and glaucomatous eyes, respectively. The ChBF showed regional variability with the greatest proportional decrement most often found in the central macula.

CONCLUSIONS

This is the first demonstration of globally reduced ChBF in chronic experimental glaucoma in the nonhuman primate. Both the alteration of mfERG waveform components associated with outer retinal function and the reduction in ChBF were greatest in the macula, suggesting that there may be a spatial colocalization between ChBF and some outer retinal effects in glaucoma.

Oxygen saturation measurements of the retinal vasculature in treated asymmetrical primary open-angle glaucoma using hyperspectral imaging

PURPOSE

To determine whether there are differences in retinal vascular oxygen saturation measurements, estimated using a hyperspectral fundus camera, between normal eyes and treated eyes of subjects with asymmetrical primary open-angle glaucoma (POAG).

METHODS

A noninvasive hyperspectral fundus camera was used to acquire spectral images of the retina at wavelengths between 556 and 650 nm in 2-nm increments. In total, 14 normal eyes and both eyes of 11 treated POAG subjects were imaged and analyzed using algorithms that use the spectral variation of the optical densities of blood vessels to estimate the oxygen saturation of blood within the retinal vasculature. In the treated POAG group, each of the eyes were categorized, based on the mean deviation of the Humphrey visual-field analyzer result, as either more-advanced or less-advanced, glaucomatous eyes. Unpaired t-tests (two-tailed) with Welch's correction were used to compare the mean oxygen saturation between the normal subjects and the treated POAG subgroups.

RESULTS

In less-advanced and more-advanced-treated POAG eyes, mean retinal venular oxygen saturations (48.2±21.6% and 42.6±18.8%, respectively) were significantly higher than in normal eyes (27.9±9.9%; P=0.03 and 0.01, respectively). Arteriolar oxygen saturation was not significantly different between normal eyes and treated POAG eyes.

CONCLUSIONS

The increased oxygen saturation of the retinal venules in advanced-treated POAG eyes may indicate reduced metabolic consumption of oxygen in the inner retinal tissues.

Taurine: the comeback of a neutraceutical in the prevention of retinal degenerations

Taurine is the most abundant amino acid in the retina. In the 1970s, it was thought to be involved in retinal diseases with photoreceptor degeneration, because cats on a taurine-free diet presented photoreceptor loss. However, with the exception of its introduction into baby milk and parenteral nutrition, taurine has not yet been incorporated into any commercial treatment with the aim of slowing photoreceptor degeneration. Our recent discovery that taurine depletion is involved in the retinal toxicity of the antiepileptic drug vigabatrin has returned taurine to the limelight in the field of neuroprotection. However, although the retinal toxicity of vigabatrin principally involves a deleterious effect on photoreceptors, retinal ganglion cells (RGCs) are also affected. These findings led us to investigate the possible role of taurine depletion in retinal diseases with RGC degeneration, such as glaucoma and diabetic retinopathy. The major antioxidant properties of taurine may influence disease processes. In addition, the efficacy of taurine is dependent on its uptake into retinal cells, microvascular endothelial cells and the retinal pigment epithelium. Disturbances of retinal vascular perfusion in these retinal diseases may therefore affect the retinal uptake of taurine, resulting in local depletion. The low plasma taurine concentrations observed in diabetic patients may further enhance such local decreases in taurine concentration. We here review the evidence for a role of taurine in retinal ganglion cell survival and studies suggesting that this compound may be involved in the pathophysiology of glaucoma or diabetic retinopathy. Along with other antioxidant molecules, taurine should therefore be seriously reconsidered as a potential treatment for such retinal diseases.

Electroretinogram and visual-evoked potential assessment of retinal and central visual function in a rat ocular hypertension model of glaucoma

PURPOSE/AIM

The aim of the study was to investigate the long-term functional changes that may occur in the retina and visual cortex in a rat ocular hypertension (OHT) model of glaucoma, used in our lab for treatment studies, using electroretinogram (ERG) and visual-evoked potential (VEP) cortical recordings in order to test the hypothesis that experimental glaucoma has differential retinal and central effects.

MATERIALS AND METHODS

Experimental glaucoma was induced unilaterally in Dark Agouti rats using hypertonic saline injection into the episcleral veins. After 3, 8, 16 and 26 weeks, ERGs and VEPs were recorded under scotopic conditions using brief full-field white flashes (10 μcd s m(-2) to 10.4 cd s m(-2)) and under photopic conditions using a rod-adapting background and white light flashes (0.13-10.4 cd s m(-2)).

RESULTS

At 16 and 26 weeks after OHT induction, there was a significant reduction in the amplitudes of the a- (50% and 30% of unoperated eye values, respectively) and b-waves (55% and 40%, respectively) of the scotopic ERG and the b-waves of the photopic ERG (55% and 45%, respectively) in the glaucomatous eyes. However, no significant changes in the VEPs simultaneously recorded over the visual cortex were seen at any of the time points.

CONCLUSIONS

The reductions in ERG amplitudes suggest that this model of glaucoma not only causes retinal ganglion cell (RGC) degeneration but also degeneration of the outer retinal cells, and this was confirmed by histology showing a reduction in the outer retinal layers in the glaucomatous eyes. Cortical VEPs did not show detrimental effects suggesting that the retinal damage in this model was not extensive enough to be detected with the VEP methods used or that there could be central compensation in this model of glaucoma.

Critical pathogenic events underlying progression of neurodegeneration in glaucoma

Glaucoma is a common optic neuropathy with a complex etiology often linked to sensitivity to intraocular pressure. Though the precise mechanisms that mediate or transduce this sensitivity are not clear, the axon of the retinal ganglion cell appears to be vulnerable to disease-relevant stressors early in progression. One reason may be because the axon is generally thin for both its unmyelinated and myelinated segment and much longer than the thicker unmyelinated axons of other excitatory retinal neurons. This difference may predispose the axon to metabolic and oxidative injury, especially at distal sites where pre-synaptic terminals form connections in the brain. This idea is consistent with observations of early loss of anterograde transport at central targets and other signs of distal axonopathy that accompany physiological indicators of progression. Outright degeneration of the optic projection ensues after a critical period and, at least in animal models, is highly sensitive to cumulative exposure to elevated pressure in the eye. Stress emanating from the optic nerve head can induce not only distal axonopathy with aspects of dying back neuropathy, but also Wallerian degeneration of the optic nerve and tract and a proximal program involving synaptic and dendritic pruning in the retina. Balance between progressive and acute mechanisms likely varies with the level of stress placed on the unmyelinated axon as it traverses the nerve head, with more acute insult pushing the system toward quicker disassembly. A constellation of signaling factors likely contribute to the transduction of stress to the axon, so that degenerative events along the length of the optic projection progress in retinotopic fashion. This pattern leads to well-defined sectors of functional depletion, even at distal-most sites in the pathway. While ganglion cell somatic drop-out is later in progression, some evidence suggests that synaptic and dendritic pruning in the retina may be a more dynamic process. Structural persistence both in the retina and in central projection sites offers the possibility that intrinsic self-repair pathways counter pathogenic mechanisms to delay as long as possible outright loss of tissue.

Measurement of photoreceptor layer in glaucoma: a spectral-domain optical coherence tomography study

Objective. To measure and compare photoreceptor layer thickness between normal and glaucomatous eyes using spectral-domain optical coherence tomography (OCT). Methods. Thirty-eight healthy normal volunteers and 47 glaucoma patients were included in the analysis. One eye from each participant was randomly selected for macula imaging by a spectral-domain OCT (3D OCT-1000, Topcon, Tokyo, Japan). The foveal and parafoveal (1.5 mm from the fovea) outer nuclear layer (ONL) and inner and outer segments (IS+OS) layer thicknesses were measured by a single masked observer. The measurements were repeated 3 times in a random sample of 30 normal eyes to determine the repeatability coefficient and intraclass correlation coefficient. Results. The measurement variabilities of photoreceptor thickness were low. The respective intraclass correlation coefficients of ONL and IS+OS thicknesses were 0.96 (95% confidence interval: 0.94-0.98) and 0.82 (95% confidence interval 0.70-0.90). While there were no differences in parafoveal ONL and IS+OS thicknesses between normal and glaucoma groups (P ≤ .410), the foveal ONL thickness was greater in glaucomatous eyes (P = .011) than in normal eyes. Conclusions. Glaucomatous damage may involve structural change in the photoreceptor layer.

Specific amacrine cell changes in an induced mouse model of glaucoma

BACKGROUND

To investigate retinal cell population changes under chronic elevated intraocular pressure in an inducible mouse model of glaucoma.

METHODS

Chronic unilateral ocular hypertension was induced in 40 C57BL6/J mice by ablation of the limbal episcleral veins. After 5, 20, 40 and 60 days of elevated intraocular pressure, specific retinal cell types were identified and/or quantified by immunohistochemistry for protein kinase C α, glial fibrillary acidic protein, parvalbumin and calretinin. Apoptotic cells were identified by TUNEL and cleaved caspase-3 immunohistochemistry.

RESULTS

Elevations in intraocular pressure in the range 22-30 mmHg were developed and sustained in mice for up to 60 days. Protein kinase C α immunoreactivity localized to bipolar cells was unchanged. We observed a rapid increase in glial fibrillary acidic protein expression in Müller cells and a progressive loss of parvalbumin-labelled ganglion cells. After 60 days of elevated intraocular pressure, calretinin-immunoreactive cell counts declined by 55.4% and 46.4% in the inner nuclear and ganglion cell layers, respectively. However, at all time points examined, the markers of cell death were only observed in the ganglion cell layer, not in the inner nuclear layer.

CONCLUSIONS

In addition to ganglion cell death and reactive Müller cell changes, chronic experimental elevation of intraocular pressure alters calcium-binding protein immunohistochemistry in amacrine cells. However, these changes are not indicative of amacrine cell loss but may represent early indicators of cellular distress that precede physiological dysfunction or cell death.

Neuroinflammation in advanced canine glaucoma

PURPOSE

The pathophysiological events that occur in advanced glaucoma are not well characterized. The principal purpose of this study is to characterize the gene expression changes that occur in advanced glaucoma.

METHODS

Retinal RNA was obtained from canine eyes with advanced glaucoma as well as from healthy eyes. Global gene expression patterns were determined using oligonucleotide microarrays and confirmed by real-time PCR. The presence of tumor necrosis factor (TNF) and its receptors was evaluated by immunolabeling. Finally, we evaluated the presence of serum autoantibodies directed against retinal epitopes using western blot analyses.

RESULTS

We identified over 500 genes with statistically significant changes in expression level in the glaucomatous retina. Decreased expression levels were detected for large number of functional groups, including synapse and synaptic transmission, cell adhesion, and calcium metabolism. Many of the molecules with decreased expression levels have been previously shown to be components of retinal ganglion cells. Genes with elevated expression in glaucoma are largely associated with inflammation, such as antigen presentation, protein degradation, and innate immunity. In contrast, expression of many other pro-inflammatory genes, such as interferons or interleukins, was not detected at abnormal levels.

CONCLUSIONS

This study characterizes the molecular events that occur in the canine retina with advanced glaucoma. Our data suggest that in the dog this stage of the disease is accompanied by pronounced retinal neuroinflammation.

Tracking longitudinal retinal changes in experimental ocular hypertension using the cSLO and spectral domain-OCT

PURPOSE

Involvement of the outer retina is controversial in glaucoma. The aim of this study was to test, first, whether the outer retina is affected in experimental ocular hypertension (OHT) and, second, whether whole retinal thickness can be used as a surrogate marker of glaucomatous change.

METHODS

OHT was surgically induced in 20 Dark Agouti rats. Animals were imaged using a modified Spectralis OCT (Heidelberg Engineering, Heidelberg, Germany) at baseline and at 3 and 8 weeks after OHT induction. Measurements were recorded for whole and individual retinal layer thickness in four regions-temporal, superior, nasal, and inferior-around the optic nerve head.

RESULTS

Whole retinal thickness in normal eyes was 172.19 ± 5.17 μm, with no significant regional differences. OHT caused a significant reduction in whole retinal thickness and the outer nuclear layer (ONL) at 3 and 8 weeks (P < 0.05), along with the expected thinning of the retinal nerve fiber layer (RNFL). Whole retinal thickness correlated well with RNFL (P = 0.035) and ONL (P ≤ 0.001) changes. Sensitivity of RNFL and ONL to IOP exposure appeared greater at 3 than at 8 weeks. In addition, regional profiles were significantly altered in the ONL and RNFL after OHT induction.

CONCLUSIONS

Adaptation of the Spectralis OCT enables tracking of structural damage in experimental rat OHT. Here the authors show evidence of glaucomatous damage in the outer retinal layers of this model with significant regional changes and highlight whole retinal thickness in the rat as a useful surrogate marker of inner and outer retinal changes. The authors believe that the OCT data can provide useful information with regard to clinical management.

Functional involvement of cone photoreceptors in advanced glaucoma: a multifocal electroretinogram study

The purpose of the study is (1) to demonstrate the anatomical variation of cone photoreceptor density across normal retina as a sectoral amplitude asymmetry of photopic multifocal electroretinogram (mfERG) and (2) to study the potential presence of sequential or differential, functional cone photoreceptor damage in glaucoma using this amplitude asymmetry. A 37-Block scaled mfERG was recorded from 22 controls and 27 glaucoma subjects. The N1 and P1 amplitudes of averaged responses from corresponding zones nasal and temporal to fovea were analyzed for asymmetry in controls and glaucoma subjects. Amplitude asymmetry was demonstrable for both N1 (p < 0.001) and P1 (p < 0.001) parameters in control subjects. Although this amplitude asymmetry was preserved in glaucoma subjects with moderate field defects, it was not demonstrable in patients with advanced field defects. The anatomical variation in cone photoreceptor distribution across normal retina is demonstrated as an amplitude asymmetry in first order kernel responses of mfERG. The cone photoreceptors in the region nasal to fovea appear to be affected only in advanced glaucoma possibly suggesting that photoreceptors could follow a sequential damage like the overlying neuroretinal rim in glaucoma.

Improved retinal function after trabeculectomy in glaucoma patients

PURPOSE

To investigate retinal function after reduction of intraocular pressure (IOP) by filtration surgery in patients with medically uncontrolled glaucoma.

METHODS

Eleven patients (11 eyes) with medically uncontrolled glaucoma underwent trabeculectomy. Clinical investigation, visual field (testing with standard automated perimetry (SAP-Humphrey), optical coherence tomography (OCT), full-field electroretinography (full-field ERG) and multifocal electroretinography (mfERG) were performed preoperatively as well as 2 and 6 months after surgery.

DESIGN

Interventional prospective, consecutive case series.

RESULTS

No significant reduction was seen in mean log MAR visual acuity 2 or 6 months after filtration surgery. The mean preoperative intraocular pressure of 27.1 (+/-6.2) mmHg decreased to 19.0(+/-6.1) mmHg 2 months after surgery and to 17.1 (+/- 3.4) mmHg 6 months after surgery (both p = 0.001). The reduction in IOP significantly decreased the number of anti-glaucoma agents used, from 3.7 +/- 1.6 at baseline to 0.8 +/- 0.9 2 months after surgery and to 1.3 +/- 1.2 6 months after surgery (p = 0.004 and p = 0.008 respectively). The results of SAP, OCT and full-field ERG did not show any significant difference between pre- and postoperative values at any point in time. No significant improvement was found with regard to the first positive peak (P(1)) amplitudes in the macular retina (area 1) or in the perimacular retina/periphery (area 2) when measured with mfERG 2 months after surgery. The mfERG examinations revealed significantly improved P(1) amplitudes 6 months after surgery in both area 1 and area 2, compared with the preoperative values (p = 0.042 and p = 0.014 respectively). The implicit time of P(1) decreased significantly 6 months after surgery in area 2 compared with the preoperative values (p = 0.023).

CONCLUSION

A significant lowering of IOP seems to improve the function of the central retina, as demonstrated by increased amplitudes and reduced implicit times assessed with mfERG.

The photopic negative response of the blue-on-yellow flash-electroretinogram in glaucomas and normal subjects

The photopic negative response of the flash-electroretinogram driven by the middle- and long-wavelength cones has been shown to be reduced in non-human primates with experimental glaucoma and in human patients with glaucoma. The photopic negative response for the blue-sensitive response has been studied using a blue-green silent-substitution-technique on a red background. The aim of this study was to re-evaluate the value of the photopic negative response of the blue-sensitive pathway in glaucoma using a conventional flash-electroretinogram. In 37 eyes of 37 controls (age: 53 +/- 13.6 years) and 37 eyes of 37 patients with open-angle glaucoma of different perimetric visual field defects (age: 58.3 +/- 10 years; MD: 11.7 +/- 6.7 dB) of the Erlangen glaucoma registry Ganzfeld flash-electroretinograms (LKC, UTAS 3000) were recorded using blue Xenon-flashes of increasing photopic luminance (0.013, 0.018, 0.030, and 0.052 cd s/m(2); 440 nm) on a bright yellow background (238 cd/m(2); 550 nm) after 2 min of light adaptation. Amplitudes and implicit times of the photopic negative response and of L&M-cone- and S-cone-driven b-waves were compared between glaucomas and controls for all flash energies (unpaired t-test). The amplitudes of the photopic negative response were significantly reduced in glaucoma patients for all flash energies (P < 0.001). The implicit times of L&M-cone-driven b-wave (0.013, 0.018, 0.030, and 0.052 cd s/m(2)) and S-cone-driven b-wave (0.030 and 0.052 cd s/m(2)) were significantly prolonged in glaucoma patients (P < 0.05). The changes in these implicit times, however, are very small (1.5 ms or less). The other measures did not differ significantly. The amplitude of the photopic negative response and the implicit times of the L&M-cone and S-cone b-wave in the same responses of the blue-on-yellow flash-electroretinogram are potentially useful in the evaluation of inner-retinal function in glaucoma.

Relationship of cerebral blood flow and central visual function in primary open-angle glaucoma

PURPOSE

To investigate the relations between middle cerebral artery (MCA) blood flow velocities and central visual function measured by foveal cone electroretinograms (ERG) and visual field.

METHODS

Fifteen primary open-angle glaucoma patients were recruited. The eye with the more severe visual field defect (full threshold 24-2) and/or optic disc damage was chosen. Measurements included brachial arterial pressure, heart rate, visual acuity (logMAR), contrast sensitivity (CSV-1000), central visual field (Humphrey SITA 10-2), foveal cone ERG, and transcranial Doppler. Pearson correlation coefficients were estimated to assess the strength of the linear relationship between the MCA flow velocity and the other measured parameters.

RESULTS

Visual field was successfully completed in 12 eyes. We were unable to obtain ERG responses and MCA velocity readings for 1 patient. A significant correlation was observed between MCA mean flow velocity and focal cone ERG amplitude [r=0.69, n=13, confidence interval (CI) 0.22, 0.90, P=0.009], but correlation between mean foveal cone ERG implicit time and middle cerebral was not statistically significant (r=0.011, n=13, CI 0.47, 0.63). A significant correlation was also observed between MCA mean flow velocity and mean sensitivity (r=0.76, n=12, CI 0.32, 0.93, P=0.005), and mean defect (r=0.73, n=12, CI 0.28, 0.92, P=0.007) of the central visual field, logMAR visual acuity (r=0.57, n=14, CI 0.05, 0.84, P=0.036), and contrast sensitivity (r=0.61, n=13, CI 0.09, 0.87, P=0.027).

CONCLUSIONS

Our findings suggest that in certain primary open-angle glaucoma patients diminished central visual function may be one manifestation of widespread cerebrovascular insufficiency.

A new experimental model of glaucoma in rats through intracameral injections of hyaluronic acid

An experimental model of pressure-induced optic nerve damage would greatly facilitate the understanding of the cellular events leading to ganglion cell death, and how they are influenced by intraocular pressure and other risk factors associated to glaucoma. The aim of the present report was to study the effect of a long-term increase of intraocular pressure in rats induced by intracameral injections of hyaluronic acid with respect to electroretinographic activity and retinal and optic nerve histology. For this purpose, hyaluronic acid was injected weekly in the rat anterior chamber of one eye, whereas the contralateral eye was injected with saline solution. The results showed a significant decrease of oscillatory potentials and a- and b-wave amplitude of the scotopic electroretinogram after 3 or 6 weeks of hyaluronic acid administration, respectively. These parameters were further reduced after 10 weeks of treatment with hyaluronic acid. No significant changes in anterior chamber angle structures from hyaluronic acid- and vehicle-injected eyes were observed, whereas a significant loss of ganglion cell layer cells and of optic nerve axons were detected in animals that received hyaluronic acid for 10 weeks, as compared to eyes injected with saline solution. In summary, present results indicate that the chronic administration of hyaluronic acid induced a significant decrease in the electroretinographic activity and histological changes in the retina and optic nerve that seem consistent with some features of chronic open-angle glaucoma. Therefore, this could be an experimental model to study the cellular mechanisms by which elevated intraocular pressure damages the optic nerve and the retina.

The multifocal pattern electroretinogram in glaucoma

BACKGROUND

The pattern ERG can be used to detect early glaucomatous change, because the response of cells in the inner retina from (typically) 20 degrees -40 degrees of area is reduced before perimetric abnormality is certain. The multifocal pattern electroretinogram (mfPERG) allows analysis of many local regions within this area. The aim of this study was to investigate whether in patients with presumed glaucoma the mfPERG permits diagnosis and discrimination from normals.

METHODS

Measurements on 25 age-related normal eyes were compared to those on 23 eyes with different stages of glaucoma. A RETIScan system was used to generate a stimulus pattern of 19 hexagons, each consisting of six triangles. The triangles pattern-reversed black to white at 75 Hz. Those 19 hexagons were grouped into three stimulus regions: a central field, a middle, and a peripheral ring. The complete array subtended 48 degrees at the eye. The hexagons alternated between black and white, in a temporal pattern that followed a corrected binary m-sequence (length 512, 10 cycles with 39 s each). The amplitudes and latencies of positive responses at approximately 50 ms (P-50) and negative responses at approximately 95 ms (N-95) were analyzed.

RESULTS

In patients with glaucoma the P-50 and N-95 components of the mfPERG were significantly reduced for the central area and both outer rings compared to normal volunteers (p<0.001, Mann-Whitney-U). The most distinct reduction was observed for N-95 and the central ring. Changes in latencies were not conclusive. The reduction of the components increased with the stage of glaucoma. A predictive model for detecting early glaucomatous changes was designed based on P-50-N-95 with 88% sensitivity and 76% specificity.

CONCLUSION

In glaucoma a marked reduction of components, especially centrally is observed in the mfPERG. This hints to an early involvement of central ganglion cells and may be useful for future functional tests.