Refined Frequency Doubling Perimetry Analysis Reaffirms Central Nervous System Control of Chronic Glaucomatous Neurodegeneration

Contributions of Brain Microstructures and Metabolism to Visual Field Loss Patterns in Glaucoma Using Archetypal and Information Gain Analyses

Purpose

To investigate the contributions of the microstructural and metabolic brain environment to glaucoma and their association with visual field (VF) loss patterns by using advanced diffusion magnetic resonance imaging (dMRI), proton magnetic resonance spectroscopy (MRS), and clinical ophthalmic measures.

Methods

Sixty-nine glaucoma and healthy subjects underwent dMRI and/or MRS at 3 Tesla. Ophthalmic data were collected from VF perimetry and optical coherence tomography. dMRI parameters of microstructural integrity in the optic radiation and MRS-derived neurochemical levels in the visual cortex were compared among early glaucoma, advanced glaucoma, and healthy controls. Multivariate regression was used to correlate neuroimaging metrics with 16 archetypal VF loss patterns. We also ranked neuroimaging, ophthalmic, and demographic attributes in terms of their information gain to determine their importance to glaucoma.

Results

In dMRI, decreasing fractional anisotropy, radial kurtosis, and tortuosity and increasing radial diffusivity correlated with greater overall VF loss bilaterally. Regionally, decreasing intra-axonal space and extra-axonal space diffusivities correlated with greater VF loss in the superior-altitudinal area of the right eye and the inferior-altitudinal area of the left eye. In MRS, both early and advanced glaucoma patients had lower gamma-aminobutyric acid (GABA), glutamate, and choline levels than healthy controls. GABA appeared to associate more with superonasal VF loss, and glutamate and choline more with inferior VF loss. Choline ranked third for importance to early glaucoma, whereas radial kurtosis and GABA ranked fourth and fifth for advanced glaucoma.

Conclusions

Our findings highlight the importance of non-invasive neuroimaging biomarkers and analytical modeling for unveiling glaucomatous neurodegeneration and how they reflect complementary VF loss patterns.

Mechanisms implicated in the contralateral effect in the central nervous system after unilateral injury: focus on the visual system

The retina, as part of the central nervous system is an ideal model to study the response of neurons to injury and disease and to test new treatments. During the last decade is becoming clear that unilateral lesions in bilateral areas of the central nervous system trigger an inflammatory response in the contralateral uninjured site. This effect has been better studied in the visual system where, as a rule, one retina is used as experimental and the other as control. Contralateral retinas in unilateral models of retinal injury show neuronal degeneration and glial activation. The mechanisms by which this adverse response in the central nervous system occurs are discussed in this review, focusing primarily on the visual system.

Neural Conduction Along Postretinal Visual Pathways in Glaucoma

Purpose: This study was conducted in order to evaluate retinal ganglion cell (RCG) function and the neural conduction along the postretinal large and small axons and its correlation with retinal nerve fiber layer thickness (RNFL-T) in open-angle glaucoma (OAG) eyes. Methods: Thirty-seven OAG patients (mean age: 51.68 ± 9.83 years) with 24-2 Humphrey mean deviation (MD) between -2.5 and -20 dB and IOP <21 mmHg on pharmacological treatment (OAG group) and 20 age-matched controls (control group) were enrolled. In both groups, simultaneous pattern electroretinograms (PERG) and visual evoked potentials (VEP), in response to checks stimulating macular or extramacular areas (the check edge subtended 15' and 60' of visual arc, respectively), and RNFL-T (measured in superior, inferior, nasal, and temporal quadrants) were assessed. Results: In the OAG group, a significant (ANOVA, p < 0.01) reduction of 60' and 15' PERG P50-N95 and VEP N75-P100 amplitudes and of RNFL-T [overall (average of all quadrants) or temporal] with respect to controls was found; the values of 60' and 15' PERG P50 and VEP P100 implicit times and of retinocortical time (RCT; difference between VEP P100 and PERG P50 implicit times) were significantly (p < 0.01) increased with respect to control ones. The observed increased RCTs were significantly linearly correlated (Pearson's test, p < 0.01) with the reduced PERG amplitude and MD values, whereas no significant linear correlation (p < 0.01) with RNFL-T (overall or temporal) values was detected. Conclusions: In OAG, there is an impaired postretinal neural conduction along both large and small axons (increased 60' and 15' RCTs) that is related to RGC dysfunction, but independent from the RNFL morphology. This implies that, in OAG, the impairment of postretinal neural structures can be electrophysiologically identified and may contribute to the visual field defects, as suggested by the linear correlation between the increase of RCT and MD reduction.

Diffusion Tensor Imaging of Visual Pathway Abnormalities in Five Glaucoma Animal Models

Purpose

To characterize the visual pathway integrity of five glaucoma animal models using diffusion tensor imaging (DTI).

Methods

Two experimentally induced and three genetically determined models of glaucoma were evaluated. For inducible models, chronic IOP elevation was achieved via intracameral injection of microbeads or laser photocoagulation of the trabecular meshwork in adult rodent eyes. For genetic models, the DBA/2J mouse model of pigmentary glaucoma, the LTBP2 mutant feline model of congenital glaucoma, and the transgenic TBK1 mouse model of normotensive glaucoma were compared with their respective genetically matched healthy controls. DTI parameters, including fractional anisotropy, axial diffusivity, and radial diffusivity, were evaluated along the optic nerve and optic tract.

Results

Significantly elevated IOP relative to controls was observed in each animal model except for the transgenic TBK1 mice. Significantly lower fractional anisotropy and higher radial diffusivity were observed along the visual pathways of the microbead- and laser-induced rodent models, the DBA/2J mice, and the LTBP2-mutant cats compared with their respective healthy controls. The DBA/2J mice also exhibited lower axial diffusivity, which was not observed in the other models examined. No apparent DTI change was observed in the transgenic TBK1 mice compared with controls.

Conclusions

Chronic IOP elevation was accompanied by decreased fractional anisotropy and increased radial diffusivity along the optic nerve or optic tract, suggestive of disrupted microstructural integrity in both inducible and genetic glaucoma animal models. The effects on axial diffusivity differed between models, indicating that this DTI metric may represent different aspects of pathological changes over time and with severity.

Nonpenetrating Deep Sclerectomy for Progressive Glaucoma: Long-term (5-year) Follow-up of Intraocular Pressure Control and Visual Field Survival

Purpose

To monitor 5-year outcomes of nonpenetrating deep sclerectomy (NPDS) with mitomycin C (MMC) in a new consecutive patient cohort.

Materials and methods

All eyes undergoing NPDS surgery between 1/08 and 6/12 were monitored for intraocular pressure (IOP), number of antiglaucoma medications (meds), and visual field indices [mean deviation (MD) and corrected pattern standard deviation (CPSD)], relative to the preoperative baseline using the two-tailed paired Student's t test.

Results

Of 106 eyes undergoing NPDS with MMC, mean IOP was 19.7 ± 0.5 [sem] mm Hg preoperatively, 11.9 ± 0.5 at 3 months, 12.5 ± 0.6 at 6 months, 12.4 ± 0.5 at 12 months, 12.6 ± 0.6 at 18 months, 11.1 ± 0.6 at 2 years, 11.8 ± 0.5 at 2.5 years, 11.0 ± 0.5 at 3 years, 11.7 ± 0.5 at 3.5 years, 10.7 ± 0.7 at 4 years, 11.6 ± 0.5 at 4.5 years, and 12.4 ± 0.7 at 5 years (average IOP reduction of 7.8 mm Hg or 37%; p < 10⁻⁶) at 5 years. About 92% of eyes had stable IOP ≥5 and ≤21 mm Hg at 5 years. Mean preoperative meds 2.7 ± 0.1 was reduced to 0.40 ±0.09 at 3 months, 0.51 ± 0.1 at 6 months, 0.38 ± 0.08 at 12 months, 0.49 ± 0.09 at 18 months, 0.41 ± 0.09 at 2 years, 0.39 ± 0.09 at 2.5 years, 0.49 ± 0.1 at 3 years, 0.58 ± 0.1 at 3.5 years, 0.49 ± 0.1 at 4 years, 0.64 ± 0.1 at 4.5 years, and 0.52 ± 0.1 at 5 years, corresponding to mean reduction of 2.2 meds (81%; p < 10⁻²²) at 5 years. Mean deviation and CPSD were stable relative to baseline at all time intervals (R = 0.83–0.94; p < 0.0001).

Conclusion

With appropriate postoperative management, eyes undergoing NPDS can maintain excellent IOP control with minimal medication use and maintain very stable visual fields over an extended time frame.

How to cite this article

Slagle G, Groth SL, Montelongo M, et al. Nonpenetrating Deep Sclerectomy for Progressive Glaucoma: Long-term (5-year) Follow-up of Intraocular Pressure Control and Visual Field Survival. J Curr Glaucoma Pract 2020;14(1):3–9.

Widespread brain reorganization perturbs visuomotor coordination in early glaucoma

Glaucoma is the world's leading cause of irreversible blindness, and falls are a major public health concern in glaucoma patients. Although recent evidence suggests the involvements of the brain toward advanced glaucoma stages, the early brain changes and their clinical and behavioral consequences remain poorly described. This study aims to determine how glaucoma may impair the brain structurally and functionally within and beyond the visual pathway in the early stages, and whether these changes can explain visuomotor impairments in glaucoma. Using multi-parametric magnetic resonance imaging, glaucoma patients presented compromised white matter integrity along the central visual pathway and around the supramarginal gyrus, as well as reduced functional connectivity between the supramarginal gyrus and the visual occipital and superior sensorimotor areas when compared to healthy controls. Furthermore, decreased functional connectivity between the supramarginal gyrus and the visual brain network may negatively impact postural control measured with dynamic posturography in glaucoma patients. Taken together, this study demonstrates that widespread structural and functional brain reorganization is taking place in areas associated with visuomotor coordination in early glaucoma. These results implicate an important central mechanism by which glaucoma patients may be susceptible to visual impairments and increased risk of falls.

Pattern Electroretinography and Visual Evoked Potentials Provide Clinical Evidence of CNS Modulation of High- and Low-Contrast VEP Latency in Glaucoma

Purpose

Both pattern electroretinography (PERG) and visual evoked potentials (VEP) can be performed using low- (15%; Lc) and high- (85%; Hc) contrast gratings that may preferentially stimulate the magno- and parvocellular pathways. We observed that among glaucomatous patients showing only one VEP latency deficit per eye, there appeared to be a very strong tendency for an Hc delay in one eye and an Lc delay in the other.

Methods

Diopsys NOVA-LX system was used to measure VEP Hc and Lc latency among a clinical glaucoma population to find all individuals with either a single Hc or Lc latency abnormality in each eye (group 1), or with greater than 0 and less than 4 Hc or Lc VEP latency abnormalities in the two eyes (group 2) to determine whether a significant inverse correlation existed for these values in either group. Hc and Lc PERG data were also evaluated to assess associated retinal ganglion cell responses.

Results

A strong inverse correlation (P = 0.0000003) was observed between the Hc and Lc VEP latency values among the 64 eyes in group 1. Group 2 provided a comparable result (n = 143; 286 eyes; P = 0.0005). PERG (n = 81; 162 eyes) also showed strong bilateral symmetry for magnitude values (P < 0.0001 for both Lc and Hc in groups 1 and 2).

Conclusions

Bilateral retention of both low-resolution/high-speed and high-resolution/low-speed function may persist with both eyes open despite symmetrically pathologic retinal ganglion cell PERG waveform asynchrony for Hc and Lc stimuli in the paired eyes.

Translational Relevance

Clinical electrophysiology strongly suggests binocular compensation for dynamic dysfunction operates under central nervous system (CNS) control in glaucoma.

Making Basic Science Studies in Glaucoma More Clinically Relevant: The Need for a Consensus

Glaucoma is a chronic, progressive, and debilitating optic neuropathy that causes retinal damage and visual defects. The pathophysiologic mechanisms of glaucoma remain ill-defined, and there is an indisputable need for contributions from basic science researchers in defining pathways for translational research. However, glaucoma researchers today face significant challenges due to the lack of a map of integrated pathways from bench to bedside and the lack of consensus statements to guide in choosing the right research questions, techniques, and model systems. Here, we present the case for the development of such maps and consensus statements, which are critical for faster development of the most efficacious glaucoma therapy. We underscore that interrogating the preclinical path of both successful and unsuccessful clinical programs is essential to defining future research. One aspect of this is evaluation of available preclinical research tools. To begin this process, we highlight the utility of currently available animal models for glaucoma and emphasize that there is a particular need for models of glaucoma with normal intraocular pressure. In addition, we outline a series of discoveries from cell-based, animal, and translational research that begin to reveal a map of glaucoma from cell biology to physiology to disease pathology. Completion of these maps requires input and consensus from the global glaucoma research community. This article sets the stage by outlining various approaches to such a consensus. Together, these efforts will help accelerate basic science research, leading to discoveries with significant clinical impact for people with glaucoma.

Binocular facilitation of cone-specific visual evoked potentials in colour deficiency

BACKGROUND

Neural compensatory mechanisms have been proposed, which preserve the binocular visual field in glaucoma, as well as cognition in Alzheimer's disease and motor function in Parkinson's disease. It is conceivable that comparable mechanisms operate to preserve function in congenital and/or dystrophic disease. In hereditary colour vision deficiency (CVD), we observed significant facilitation in the amplitude of the binocular cone-specific visual evoked potential (VEP) compared to the monocular amplitude for the cone type corresponding to the CVD. We propose that this finding may reflect preservation of function in hereditary colour vision deficiency.

METHODS

Binocular and monocular L, M and S cone-specific VEPs were recorded from 12 colour vision deficient subjects and 17 with normal colour vision, confirmed to be CVD or normal on a battery of colour vision tests. Binocular VEP amplitudes were compared to monocular amplitudes within subjects and between subject groups.

RESULTS

Subjects with CVDs showed binocular facilitation of VEP amplitude (enhancement more than 2.0 times; mean: 2.8 times, p = 0.0003) for the cone type corresponding to their CVD. Mean facilitation of 2.8 times exceeded binocular enhancement for other cone types within CVDs (2.8 times versus 1.2 times) and compared to colour vision normals (2.8 times versus 1.2 times).

CONCLUSIONS

Hereditary CVDs show binocular facilitation of cone VEP signals for the cone type corresponding to their CVD. As CVD is typically assessed with foveal stimuli, our findings using wider-field binocular stimulation suggest that enhanced colour perception may occur in CVD across a more extensive area of visual field. These results may relate to binocular visual field enhancement in glaucoma and improved colour vision in CVD at supra-threshold levels of stimulation.