Glaucoma of the brain: a disease model for the study of transsynaptic neural degeneration

A dam for retrograde axonal degeneration in multiple sclerosis?

OBJECTIVE

Trans-synaptic axonal degeneration is a mechanism by which neurodegeneration can spread from a sick to a healthy neuron in the central nervous system. This study investigated to what extent trans-synaptic axonal degeneration takes place within the visual pathway in multiple sclerosis (MS).

METHODS

A single-centre study, including patients with long-standing MS and healthy controls. Structural imaging of the brain (MRI) and retina (spectral-domain optical coherence tomography) were used to quantify the extent of atrophy of individual retinal layers and the primary and secondary visual cortex. Generalised estimation equations and multivariable regression analyses were used for comparisons.

RESULTS

Following rigorous quality control (OSCAR-IB), data from 549 eyes of 293 subjects (230 MS, 63 healthy controls) were included. Compared with control data, there was a significant amount of atrophy of the inner retinal layers in MS following optic neuritis (ON) and also in absence of ON. For both scenarios, atrophy stopped at the level of the inner nuclear layer. In contrast, there was significant localised atrophy of the primary visual cortex and secondary visual cortex in MS following ON, but not in MS in absence of ON.

INTERPRETATION

These data suggest that retrograde (trans-synaptic) axonal degeneration stops at the inner nuclear layer, a neuronal network capable of plasticity. In contrast, there seems to be no neuroplasticity of the primary visual cortex, rendering the structure vulnerable to anterograde (trans-synaptic) degeneration.

Investigation into an outbreak of encephalomyelitis caused by a neuroinvasive porcine sapelovirus in the United Kingdom

An outbreak of neurological disease in grower pigs characterised by ataxia and paraparesis was investigated in this study. The outbreak occurred 3-4 weeks post weaning in grower pigs which displayed signs of spinal cord damage progressing to recumbency. Pathology in the affected spinal cords and to a lesser extent in the brainstem was characterised by pronounced inflammation and neuronophagia in the grey matter. Molecular investigation using a pan-virus microarray identified a virus related to porcine sapelovirus (PSV) in the spinal cord of the two affected pigs examined. Analysis of 802 nucleotides of the virus polymerase gene showed the highest homology with those of viruses in the genus Sapelovirus of Picornaviridae. This PSV, strain G5, shared 91-93%, 67-69% and 63% nucleotide homology with porcine, simian and avian sapeloviruses, respectively. The nucleotide homology to other members of the Picornaviridae ranged from 41% to 62%. Furthermore, viral antigen was detected and co-localised in the spinal cord lesions of affected animals by an antibody known to react with PSV. In conclusion, clinical and laboratory observations of the diseased pigs in this outbreak are consistent with PSV-G5 being the causative agent. To the best of the authors' knowledge, this is the first unequivocal report of polioencephalomyelitis in pigs by a neuroinvasive PSV in the United Kingdom.

DTI analysis in patients with primary open-angle glaucoma: impact of registration on Voxel-Wise statistics

BACKGROUND AND PURPOSE

Tract-based spatial statistics (TBSS) has been used to assess the integrity of the visual pathway in glaucoma patients. TBSS uses the subjects' FA data to create a mean FA skeleton of white matter tracts before running voxel-wise cross-subject statistics. We compared four different approaches of registration of FA maps to create the skeleton and evaluated alignment and subsequently the impact of the chosen registration on voxel-wise statistics.

MATERIAL AND METHODS

Our study comprised 69 subjects, i.e. 46 patients with primary open angle glaucoma (POAG) and a healthy, age-matched control group of 23 subjects. Mean FA skeletons were created using the following registration approaches: registration to a standard template (T), registration to the group mean (GM), registration to a group-wise atlas (GW) and registration to the most typical subject (N). Subsequently, maps of standard deviation of the 4D images were created to assess the alignment. Voxel-wise statistics for each registration approach were performed.

RESULTS

We found distinct differences in voxel-wise statistics depending on the chosen registration approach. Best alignment results were achieved by registration to a study specific template, i.e. to the group mean (GM) or to a group-wise atlas (GW). Overall alignment did not differ between these two approaches. However, voxel-wise statistics showed clusters of significantly decreased FA values in the T and GM approach, which were not significant after GW registration. These voxels of significantly decreased FA values after T and GM registration did not represent white matter tracts and correlated with higher standard deviation in FA maps across subjects, thus implying registration errors, especially in the optic radiation.

CONCLUSION

Registration to a study-specific template, i.e. to the group mean or a group-wise atlas seems to be the method of choice in TBSS-analysis of glaucoma patients as it shows better alignment of the optic radiation and helps to rule out registration errors due to misalignment.

Apolipoprotein E gene ε4ε4 is associated with elevated risk of primary open angle glaucoma in Asians: a meta-analysis

Background

Epidemiological studies have evaluated the association between Apolipoprotein E (APOE) gene ε2/ε3/ε4 polymorphism and glaucoma susceptibility. However, the published data are still inconclusive. The aim of the present study is to evaluate the impact of APOE gene ε2/ε3/ε4 polymorphism on glaucoma risk by using meta-analysis.

Methods

A comprehensive literature search of PubMed, EMBASE, Cochrane, Elsevier Science Direct and CNKI databases was conducted to identify relevant articles, with the last report up to January 5, 2014. Pooled odds ratio (OR) and 95% confidence interval (CI) were used to assess the strength of association by using the fixed or random effect model.

Results

Fifteen separate studies including 2,700 cases and 2,365 controls were included in the meta-analysis. We did not detect a significant association between APOE gene ε2/ε3/ε4 polymorphism and glaucoma in overall population (P > 0.0083). In Asians, we detected an association of the ε4ε4 genotype with elevated risk for glaucoma (OR = 5.22, 95% CI = 1.85-14.68, P = 0.002), mainly for primary open angle glaucoma (OR = 4.98, 95% CI = 1.75-14.20, P = 0.003).

Conclusions

The meta-analysis suggests that APOE gene ε4ε4 may be associated with elevated risk for primary open angle glaucoma in Asians. However, more epidemiologic studies based on larger sample size, case–control design and stratified by ethnicity as well as types of glaucoma are suggested to further clarify the relationship between APOE gene ε2/ε3/ε4 polymorphism and genetic predisposition to glaucoma.

Refined Data Analysis Provides Clinical Evidence for Central Nervous System Control of Chronic Glaucomatous Neurodegeneration

PURPOSE

Refined data analysis was performed to assess binocular visual field conservation in patients with bilateral glaucomatous damage to determine whether unilateral visual field loss is random, anatomically symmetric, or nonrandom in relation to the fellow eye.

METHODS

This was a case-control study of 47 consecutive patients with bilaterally severe glaucoma; each right eye visual field locus was paired with randomly selected coisopteric left eye loci, with 760,000 (10,000 complete sets of 76 loci) such iterations performed per subject. The potential role of anatomic symmetry in bilateral visual field conservation was also assessed by pairing mirror-image loci of the paired fields. The mean values of the random coisopteric and the symmetric mirror pairings were compared with natural point-for-point pairings of the two eyes by paired t-test.

RESULTS

Mean unilateral thresholds across the entire visual field were 18.9 dB left and 19.9 dB right (average 19.4), 4 dB lower than the better of the naturally paired concomitant loci of 23.4 dB (P < 10^-15). A remarkable natural tendency for conservation of the binocular visual field was confirmed, far stronger than explicable by random chance or anatomic symmetry (P < 0.0001), and reaffirmed by subsequent prospective simultaneous binocular visual field retesting of an arbitrary subset (n = 16) of the study population (P < 0.0001).

CONCLUSIONS

Refined data analysis of paired visual fields confirms the existence of a natural optimization of binocular visual function in severe bilateral glaucoma via interlocking fields that could be created only by central nervous system (CNS) involvement.

TRANSLATIONAL RELEVANCE

Integrated bilateral visual field analysis should better define actual visual disability and more accurately reflect the functional efficacy of current ocular and future CNS-oriented therapeutic approaches to the treatment of glaucoma. Glaucomatous eyes provide a highly accessible paired-organ study model for developing therapeutics to optimize conservation of function in neurodegenerative disorders.

Remote neurodegeneration: multiple actors for one play

Remote neurodegeneration significantly influences the clinical outcome in many central nervous system (CNS) pathologies, such as stroke, multiple sclerosis, and traumatic brain and spinal cord injuries. Because these processes develop days or months after injury, they are accompanied by a therapeutic window of opportunity. The complexity and clinical significance of remote damage is prompting many groups to examine the factors of remote degeneration. This research is providing insights into key unanswered questions, opening new avenues for innovative neuroprotective therapies. In this review, we evaluate data from various remote degeneration models to describe the complexity of the systems that are involved and the importance of their interactions in reducing damage and promoting recovery after brain lesions. Specifically, we recapitulate the current data on remote neuronal degeneration, focusing on molecular and cellular events, as studied in stroke and brain and spinal cord injury models. Remote damage is a multifactorial phenomenon in which many components become active in specific time frames. Days, weeks, or months after injury onset, the interplay between key effectors differentially affects neuronal survival and functional outcomes. In particular, we discuss apoptosis, inflammation, oxidative damage, and autophagy-all of which mediate remote degeneration at specific times. We also review current findings on the pharmacological manipulation of remote degeneration mechanisms in reducing damage and sustaining outcomes. These novel treatments differ from those that have been proposed to limit primary lesion site damage, representing new perspectives on neuroprotection.

Trans-synaptic axonal degeneration in the visual pathway in multiple sclerosis

OBJECTIVE

To evaluate the association between the damage to the anterior and posterior visual pathway as evidence of the presence of retrograde and anterograde trans-synaptic degeneration in multiple sclerosis (MS).

METHODS

We performed a longitudinal evaluation on a cohort of 100 patients with MS, acquiring retinal optical coherence tomography to measure anterior visual pathway damage (peripapillary retinal nerve fiber layer [RNFL] thickness and macular volume) and 3T brain magnetic resonance imaging (MRI) for posterior visual pathway damage (volumetry and spectroscopy of visual cortex, lesion volume within optic radiations) at inclusion and after 1 year. Freesurfer and SPM8 software was used for MRI analysis. We evaluated the relationships between the damage in the anterior and posterior visual pathway by voxel-based morphometry (VBM), multiple linear regressions, and general linear models.

RESULTS

VBM analysis showed that RNFL thinning was specifically associated with atrophy of the visual cortex and with lesions in optic radiations at study inclusion (p < 0.05). Visual cortex volume (β = +0.601, 95% confidence interval [CI] = +0.04 to +1.16), N-acetyl aspartate in visual cortex (β = +1.075, 95% CI = +0.190 to +1.961), and lesion volume within optic radiations (β = -2.551, 95% CI = -3.910 to -1.192) significantly influenced average RNFL thinning at study inclusion independently of other confounders, especially optic neuritis (ON). The model indicates that a decrease of 1cm(3) in visual cortex volume predicts a reduction of 0.6μm in RNFL thickness. This association was also observed after 1 year of follow-up. Patients with severe prior ON (adjusted difference = -3.01, 95% CI = -5.08 to -0.95) and mild prior ON (adjusted difference = -1.03, 95% CI = -3.02 to +0.95) had a lower adjusted mean visual cortex volume than patients without ON.

INTERPRETATION

Our results suggest the presence of trans-synaptic degeneration as a contributor to chronic axon damage in MS.

Reduced cortical thickness in primary open-angle glaucoma and its relationship to the retinal nerve fiber layer thickness

Objectives

To examine possible changes in cortical thickness and their relationship to retinal nerve fiber layer (RNFL) thickness in patients with primary open-angle glaucoma (POAG).

Materials and Methods

Thirty-six patients with POAG and 40 matched healthy controls were enrolled in this study. All subjects underwent a comprehensive ophthalmologic examination and a high resolution structural magnetic resonance scan. Cortical thickness analysis was used to assess the changes between patients and controls. Correlations between the thickness of the visual cortex and RNFL thickness were also analyzed. Finally, the relationship between the severity of changes in the visual cortex and RNFL thickness was evaluated by comparing patients with mild and severe groups.

Results

POAG patients showed significant bilateral cortical thinning in the anterior half of the visual cortex around the calcarine sulci (left BA 17 and BA 18, right BA17) and in some smaller regions located in the left middle temporal gyrus (BA37) and fusiform gyrus (BA19). The thickness of the visual cortex correlated positively with RNFL thickness (left, r = 0.44, p = 0.01; right, r = 0.38, p = 0.03). Significant differences between mild and severe groups were observed with regard to both RNFL thickness and the thickness of bilateral visual cortex (p < 0.05).

Conclusion

Our findings indicate that cortical thickness analysis may be sufficiently sensitive to detect cortical alterations in POAG and that the measurement has great potential for clinical application.

Brimonidine protects against loss of Thy-1 promoter activation following optic nerve crush

BACKGROUND

The loss of RGCs expressing Thy-1 after optic nerve injury has an initial phase of rapid decline followed by a longer phase with slower reduction rate. This study used longitudinal retinal imaging of mice expressing cyan fluorescent protein under control of the Thy-1 promoter (Thy1-CFP mice) to determine how the α2-adrenergic agonist brimonidine influences loss of Thy1 promoter activation.

METHODS

Baseline images of the fluorescent retinal neurons in 30 Thy1-CFP mice were obtained using a modified confocal scanning laser ophthalmoscope. Next, brimonidine (100 ug/kg, IP) was administered either one time immediately after optic nerve crush, or immediately after optic nerve crush and then every 2 days for four weeks. A control group received a single saline injection immediately after optic nerve crush. All animals were imaged weekly for four weeks after optic nerve crush. Loss of fluorescent retinal neurons within specific retinal areas was determined by counting.

RESULTS

At one week after optic nerve crush, the proportion of fluorescent retinal neurons retaining fluorescence was 44±7% of baseline in control mice, 51±6% after one brimonidine treatment, and 55±6% after brimonidine treatment every other day (P<0.05 for both brimonidine treatment groups compared to the control group). Subsequently, the number of fluorescent retinal neurons in the group that received one treatment differed insignificantly from the control group. In contrast, the number of fluorescent retinal neurons in the group that received repeated brimonidine treatments was greater than the control group by 28% at two weeks after crush and by 32% at three weeks after crush (P<0.05 at both time points). Rate analysis showed that brimonidine slowed the initial rate of fluorescent cell decline in the animals that received multiple treatments (P<0.05). Differences in the rate of loss among the treatment groups were insignificant after the second week.

CONCLUSION

Repeated brimonidine treatments protect against loss of fluorescence within fluorescent retinal neurons of Thy1-CFP mice after optic nerve crush. As most of fluorescent retinal neurons in this system are RGCs, these findings indicate that repeated brimonidine treatments may protect RGC health following optic nerve crush.

Correlation between lateral geniculate nucleus atrophy and damage to the optic disc in glaucoma

PURPOSE

To investigate the relationship between morphological changes in the lateral geniculate nucleus (LGN), as measured by magnetic resonance imaging (MRI), and damage to the optic disc in primary open-angle glaucoma (POAG) patients.

METHODS

A total of 23 patients with POAG and 23 age- and gender-matched non-glaucomatous subjects were enrolled. Every patient had structural damage to the optic disc. Cup-to-disc ratio (CDR) and retinal nerve fiber layer thickness (RNFLT) were measured, and 3.0-Tesla MRI examinations performed. Bilateral LGNs were identified and manually extracted, and their maximum heights and volumes compared with the clinical damage to the optic disc.

RESULTS

In POAG patients, morphological changes in LGNs and RNFLT were consistently varied (P<0.05), while a negative correlation between LGN measurements and CDR was observed (P<0.05). LGN height was more significantly correlated with damage to the optic disc than was LGN volume. In contrast, no significant correlation was found between morphological changes in LGNs and age or optic disc parameters in the non-glaucomatous controls (P>0.05).

CONCLUSION

LGN atrophy in POAG patients was altered in a manner consistent with damage to the optic disc. Morphological changes in LGNs as measured by MRI and especially LGN maximum height may be useful ways to detect optic nerve neuropathy in glaucoma.

Kinetics of neurodegeneration based on a risk-related biomarker in animal model of glaucoma

Background

Neurodegenerative diseases including Parkinson’s and Alzheimer’s diseases progress slowly and steadily over years or decades. They show significant between-subject variation in progress and clinical symptoms, which makes it difficult to predict the course of long-term disease progression with or without treatments. Recent technical advances in biomarkers have facilitated earlier, preclinical diagnoses of neurodegeneration by measuring or imaging molecules linked to pathogenesis. However, there is no established “biomarker model” by which one can quantitatively predict the progress of neurodegeneration. Here, we show predictability of a model with risk-based kinetics of neurodegeneration, whereby neurodegeneration proceeds as probabilistic events depending on the risk.

Results

We used five experimental glaucomatous animals, known for causality between the increased intraocular pressure (IOP) and neurodegeneration of visual pathways, and repeatedly measured IOP as well as white matter integrity by diffusion tensor imaging (DTI) as a biomarker of axonal degeneration. The IOP in the glaucomatous eye was significantly increased than in normal and was varied across time and animals; thus we tested whether this measurement is useful to predict kinetics of the integrity. Among four kinds of models of neurodegeneration, constant-rate, constant-risk, variable-risk and heterogeneity models, goodness of fit of the model and F-test for model selection showed that the time course of optic nerve integrity was best explained by the variable-risk model, wherein neurodegeneration kinetics is expressed in an exponential function across cumulative risk based on measured IOP. The heterogeneity model with stretched exponential decay function also fit well to the data, but without statistical superiority to the variable-risk model. The variable-risk model also predicted the number of viable axons in the optic nerve, as assessed by immunohistochemistry, which was also confirmed to be correlated with the pre-mortem integrity of the optic nerve. In addition, the variable-risk model identified the disintegrity in the higher-order visual pathways, known to underlie the transsynaptic degeneration in this disease.

Conclusions

These findings indicate that the variable-risk model, using a risk-related biomarker, could predict the spatiotemporal progression of neurodegeneration. This model, virtually equivalent to survival analysis, may allow us to estimate possible effect of neuroprotection in delaying progress of neurodegeneration.

Anterograde degeneration along the visual pathway after optic nerve injury

PURPOSE

To investigate anterograde degenerative changes along the visual pathway in a rat model of optic nerve axotomy.

METHODS

Optic nerve transection was performed in adult Sprague-Dawley rats. Animals were sacrificed at regular time intervals and tissues harvested. Immunoblotting followed by densitometric analysis was used to determine the phosphorylation profile of Akt in the dorsal lateral geniculate nucleus (dLGN) and the primary visual cortex (V1). The neuronal cell size and cell density were measured in the dLGN and the V1 using Nissl staining. The prevalence of apoptosis was characterized by terminal deoxynucleotidyl-transferase-mediated biotin-dUTP nick end labelling (TUNEL) histochemistry. Caspase-3 antibodies were also used to identify apoptotic cells. Neurons and astrocytes were detected using NeuN and glial fibrillary acidic protein (GFAP), respectively.

RESULTS

An early and sustained loss of Akt phosphorylation was observed after optic nerve transection in both dLGN and V1. At week one, a decrease in the neuronal cell size (50.5±4.9 vs 60.3±5.0 µm(2), P = 0.042) and an increase of TUNEL positive cells (7.9±0.6 vs 1.4±0.5 ×10(2) cells/mm(2), P<0.001) were evident in the dLGN but not in V1. A significant decline in neuronal cell number (14.5±0.1 vs 17.4±1.3 ×10(2) cells/mm(2), P = 0.048), cell size (42.5±4.3 vs 62.1±4.7 µm(2), P = 0.001) and an increase in apoptotic cells (5.6±0.5 vs 2.0±0.4 ×10(2) cells/mm(2), P<0.001) appeared in V1 initially at one month post-transection. The changes in the visual pathway continued through two months. Both neuronal cells and GFAP-positive glial cells were affected in this anterograde degeneration along the visual pathway.

CONCLUSIONS

Anterograde degeneration along the visual pathway takes place in target relay (LGN) and visual cortex following the optic nerve injury. Apoptosis was observed in both neural and adjacent glial cells. Reduction of Akt phosphorylation preceded cellular and apoptotic changes.

Secondary post-geniculate involvement in Leber's hereditary optic neuropathy

Leber’s hereditary optic neuropathy (LHON) is characterized by retinal ganglion cell (RGC) degeneration with the preferential involvement of those forming the papillomacular bundle. The optic nerve is considered the main pathological target for LHON. Our aim was to investigate the possible involvement of the post-geniculate visual pathway in LHON patients. We used diffusion-weighted imaging for in vivo evaluation. Mean diffusivity maps from 22 LHON visually impaired, 11 unaffected LHON mutation carriers and 22 healthy subjects were generated and compared at level of optic radiation (OR). Prefrontal and cerebellar white matter were also analyzed as internal controls. Furthermore, we studied the optic nerve and the lateral geniculate nucleus (LGN) in post-mortem specimens obtained from a severe case of LHON compared to an age-matched control. Mean diffusivity values of affected patients were higher than unaffected mutation carriers (P<0.05) and healthy subjects (P<0.01) in OR and not in the other brain regions. Increased OR diffusivity was associated with both disease duration (B = 0.002; P<0.05) and lack of recovery of visual acuity (B = 0.060; P<0.01). Post-mortem investigation detected atrophy (41.9% decrease of neuron soma size in the magnocellular layers and 44.7% decrease in the parvocellular layers) and, to a lesser extent, degeneration (28.5% decrease of neuron density in the magnocellular layers and 28.7% decrease in the parvocellular layers) in the LHON LGN associated with extremely severe axonal loss (99%) in the optic nerve. The post-geniculate involvement in LHON patients is a downstream post-synaptic secondary phenomenon, reflecting de-afferentation rather than a primary neurodegeneration due to mitochondrial dysfunction of LGN neurons.

Delayed saccadic eye movements in glaucoma

PURPOSE

To determine whether saccadic eye movements are altered in glaucoma patients.

PATIENTS AND METHODS

Sixteen patients with glaucoma and 21 control subjects were prospectively studied. Patients participated in a pro-saccade step task. Saccades were recorded using a noninvasive infrared oculometric device with head-mounted target projection. Medians of saccade reaction time, duration, amplitude, and peak velocity; frequency of express saccades; and percentage of trials with direction error were recorded. t-tests were used to compare the glaucoma and age-matched control groups. A correlation analysis of saccade parameters with visual field loss was also performed.

RESULTS

Median saccade reaction times were significantly prolonged in glaucoma patients compared with controls (220.9 ± 49.02 ms vs 192.1 ± 31.24 ms; t-test: P = 0.036). Median duration, median amplitude, and median peak velocity of saccades did not show significant differences between glaucoma and control groups (P > 0.05). Frequency of express saccades was significantly decreased in glaucoma patients compared with controls (1.75 ± 2.32 vs 7.0 ± 6.99; t-test: P = 0.007). Saccade parameters in glaucoma patients showed no significant correlation with visual field loss.

CONCLUSION

Saccadic eye movements are significantly delayed in patients with early, moderate, or advanced glaucoma. Determination of median saccade reaction time may offer a novel functional test to quantify visual function in glaucoma patients. Further studies are needed to determine pathological processes implicated in delayed initiation of saccades, and to assess whether alteration of saccades affects daily activities in glaucoma patients.

Failure of axonal transport induces a spatially coincident increase in astrocyte BDNF prior to synapse loss in a central target

Failure of anterograde transport to distal targets in the brain is a common feature of neurodegenerative diseases. We have demonstrated in rodent models of glaucoma, the most common optic neuropathy, early loss of anterograde transport along the retinal ganglion cell (RGC) projection to the superior colliculus (SC) is retinotopic and followed by a period of persistence of RGC axon terminals and synapses through unknown molecular pathways. Here we use the DBA/2J mouse model of hereditary glaucoma and an acute rat model to demonstrate that retinotopically focal transport deficits in the SC are accompanied by a spatially coincident increase in brain-derived neurotrophic factor (BDNF), especially in hypertrophic astrocytes. These neurochemical changes occur prior to loss of RGC synapses in the DBA/2J SC. In contrast to BDNF protein, levels of Bdnf mRNA decreased with transport failure, even as mRNA encoding synaptic structures remained unchanged. In situ hybridization signal for Bdnf mRNA was the strongest in SC neurons, and labeling for the immature precursor pro-BDNF was very limited. Subcellular fractionation of SC indicated that membrane-bound BDNF decreased with age in the DBA/2J, while BDNF released from vesicles remained high. These results suggest that in response to diminished axonal function, activated astrocytes in the brain may sequester mature BDNF released from target neurons to counter stressors that otherwise would challenge survival of projection synapses.

Brain involvement in glaucoma: advanced neuroimaging for understanding and monitoring a new target for therapy

On the basis of a large body of experimental data the notion that glaucoma damages retinal ganglion cells and central areas of the visual system has been put forward. The mechanisms underlying glaucomatous involvement of the central areas are not known: the most likely hypothesis is that this event is the result of an anterograde transynaptic neurodegeneration triggered by ganglion cells' death. However, it is possible that in some cases it may be the consequence of a neurodegenerative disease of the central nervous system. In any event, novel mechanisms leading to cell demise might be implicated. The development of powerful neuroimaging techniques in conjunction with sophisticated analysis has recently provided compelling support to the involvement of central stations of the visual pathway in patients suffering of glaucoma. Diffusion Tensor-MRI allows the central damage associated with glaucoma to be assessed and therapeutic efficacy of novel neuroprotective interventions to be quantified.

Diffusion tensor magnetic resonance imaging reveals visual pathway damage that correlates with clinical severity in glaucoma

BACKGROUND

To investigate nerve fibre damage of the visual pathway in patients with primary open-angle glaucoma using tract-based spatial statistical analysis of diffusion tensor imaging and correlate these measures with the clinical severity of glaucoma.

DESIGN

Cross-sectional study.

PARTICIPANTS

Twenty-five individuals with primary open-angle glaucoma and 24 healthy controls were recruited.

METHODS

All subjects underwent detailed ophthalmological examinations, including the cup-to-disc ratio, retinal nerve fibre layer thickness and visual fields test. Diffusion tensor imaging of the visual pathway was performed using a 3.0-T magnetic resonance scanner.

MAIN OUTCOME MEASURES

Diffusivity changes of the nerve fibres in the visual pathway were calculated through tract-based spatial statistical analysis. The mean diffusivity and fractional anisotropy were assessed and compared with ophthalmological measurements.

RESULTS

Compared with controls, bilateral optic tracts and optic radiations in primary open-angle glaucoma patients showed significantly decreased fractional anisotropy and increased mean diffusivity (P < 0.05). In the glaucoma group, the fractional anisotropy of the optic tracts and optic radiations varied consistently with the cup-to-disc ratio, retinal nerve fibre layer thickness and visual function analysis, respectively (P < 0.05). The mean diffusivity of the optic tracts correlated with these ophthalmological measurements (P < 0.05). However, no significant correlation was observed between the mean diffusivity of the optic radiations and the ophthalmological measurements (P > 0.05).

CONCLUSIONS

The optic tracts and optic radiations of primary open-angle glaucoma patients demonstrated radiological evidence of neurodegeneration. This varied with damage to the optic disc and with the loss of visual function. Tract-based spatial statistical analysis of diffusion tensor imaging is an objective and effective tool for detecting the loss of cortical nerve fibres in primary open-angle 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.

Structural and functional abnormalities of retinal ganglion cells measured in vivo at the onset of optic nerve head surface change in experimental glaucoma

PURPOSE

To compare peripapillary retinal nerve fiber layer thickness (RNFLT), RNFL retardance, and retinal function at the onset of optic nerve head (ONH) surface topography change in experimental glaucoma (EG).

METHODS

Thirty-three rhesus macaques had three or more weekly baseline measurements in both eyes of ONH surface topography, peripapillary RNFLT, RNFL retardance, and multifocal electroretinography (mfERG). Laser photocoagulation was then applied to the trabecular meshwork of one eye to induce chronic elevation of IOP and weekly recordings continued alternating between ONH surface topography and RNFLT during one week and RNFL retardance and mfERG the next week. Data were pooled for the group at the onset of ONH surface topography change in each EG eye, which was defined as the first date when either the mean position of the disc (MPD) fell below the 95% confidence limit of each eye's individual baseline range and/or when the topographic change analysis (TCA) map was subjectively judged as having demonstrated change, whichever came first. Analysis of variance with post hoc tests corrected for multiple comparisons were used to assess parameter changes.

RESULTS

At onset of ONH surface topography change, there was no significant difference for RNFLT versus baseline or fellow control eyes. RNFL retardance and mfERG were significantly reduced in the recordings just prior (median of 9 days) to ONH onset (P < 0.01) and had progressed significantly (P < 0.001) an average of 17 days later (median of 7 days after ONH onset). RNFLT did not exhibit significant thinning until 15 days after onset of ONH surface topography change (P < 0.001).

CONCLUSIONS

These results support the hypothesis that during the course of glaucomatous neurodegeneration, axonal cytoskeletal and retinal ganglion cell functional abnormalities exist before thinning of peripapillary RNFL axon bundles begins.

Driving simulation as a performance-based test of visual impairment in glaucoma

The fundamental goal of glaucoma management is to prevent patients from developing visual impairments sufficient to produce disability in their daily lives and impair their quality of life. Ultimately, patients are interested in how their vision will impact their ability to perform daily activities, such as driving. Although technologic advancements such as automated perimetry and devices for optic nerve imaging have resulted in great improvement in our ability to quantify structural and functional damage in glaucoma, the impact on vision-related quality of life of some of the information acquired from these tests remain elusive. In contrast, performance-based measures may be better correlated to traditional measures of vision health and, more importantly, they provide a more direct measure of disability. Driving simulators can be used as a performance-based test for evaluation of functional impairment in glaucoma. Their use can potentially help in the evaluation of driving safety and performance of diseased subjects and provide insight into the different mechanisms involved in causing driving impairment in this disease. The ability to do this in an experimentally controlled and standardized setting enables testing of a much larger number of hypotheses compared with on-road evaluations. Besides evaluating driver fitness, simulators could also potentially be used as a sophisticated test to evaluate cognitive impairment in the context of an everyday task (driving) that has not been available through traditional neuropsychologic assessment.

Cone dysfunctions in retinitis pigmentosa with retinal nerve fiber layer thickening

Purpose

To investigate whether or not thicker retinal nerve fiber layer (RNFL) in retinitis pigmentosa (RP) patients relates to functional abnormalities of the photoreceptors.

Methods

Optical coherence tomography-based RNFL thickness was measured by Stratus-3™ (Zeiss, Basel, Switzerland) optical coherence tomography and electroretinogram (ERG) recordings made using the RETI-port^® system (Roland, Wiesbaden, Germany) in 27 patients with retinitis pigmentosa and in 30 healthy subjects.

Results

Photopic ERG b-wave amplitude, cone ERG b-wave latency, 30 Hz flicker amplitude, and 30 Hz flicker latency had significant correlations to the RNFL-temporal (r = −0.55, P = 0.004, r = 0.68, P = 0.001, r = −0.65, P = 0.001, and r = −0.52, P = 0.007, respectively). Eyes with thicker RNFL (ten eyes) differed significantly from those with thinner RNFL (eight eyes) regarding cone ERG b-wave latency values only (P = 0.001).

Conclusion

Thicker RNFL in patients with retinitis pigmentosa may be associated with functional abnormality of the cone system.

Glaucoma and Alzheimer’s disease in the elderly

Primary open angle glaucoma and Alzheimer’s disease have long been established as two separate pathological entities, primarily affecting the elderly. The progressive, irreversible course of both diseases has significant implications on an aging population. As the complex pathophysiology of the two diseases has progressively unraveled over the past two decades, common pathophysiological changes have also been elucidated. Some of these mechanisms have established a strong grounding, whilst others remain principally speculative. The mutual neuropathological changes in primary open angle glaucoma and Alzheimer’s disease have facilitated the development of neuroprotective strategies. While most of these strategies are still in the preclinical phase, they have shown great promise in experimental animal studies. Further understanding of the common pathophysiology of primary open angle glaucoma and Alzheimer’s disease and their timeline may have great implications on early diagnosis and effective therapeutic targeting.

Trans-lamina cribrosa pressure difference correlated with neuroretinal rim area in glaucoma

BACKGROUND

The aim of this work is to prospectively assess the relationship between trans-laminar cribrosa pressure difference and neuroretinal rim area as morphologic surrogate of glaucomatous optic nerve damage.

METHODS

The study included 22 patients with high-pressure glaucoma, 13 patients with normal-pressure glaucoma, and 17 subjects with ocular hypertension. All participants underwent a standardized ophthalmologic examination including confocal laser scanning tomography of the optic nerve head and computerized perimetry and a neurologic examination including measurement of the lumbar cerebrospinal fluid (CSF) pressure. The trans-lamina cribrosa pressure difference was calculated as difference of intraocular pressure minus lumbar CSF pressure.

RESULTS

Neuroretinal rim area (p = 0.006; correlation coefficient r = -0.38) and mean visual field defect (p = 0.008; r = 0.38) were significantly associated with trans-lamina cribrosa pressure difference. The probability of error was lower (i.e., the p value were lower) and the correlation coefficients were higher for the associations between rim area/visual field defect with trans-lamina cribrosa pressure difference than for the associations between rim area/visual field defect and intraocular pressure or lumbar CSF pressure.

CONCLUSIONS

The trans-lamina cribrosa pressure difference as the difference of intraocular pressure minus the lumbar CSF pressure was the main pressure parameter associated with the amount of glaucomatous optic nerve damage. This may suggest that the CSF pressure as trans-lamina cribrosa counter pressure against the intraocular pressure may play some role in the pathogenesis of glaucomatous optic neuropathy.

Shape analysis of the neostriatum in subtypes of frontotemporal lobar degeneration: neuroanatomically significant regional morphologic change

Frontostriatal circuit mediated cognitive dysfunction has been implicated in frontotemporal lobar degeneration (FTLD) and may differ across subtypes of FTLD. We manually segmented the neostriatum (caudate nucleus and putamen) in FTLD subtypes: behavioral variant frontotemporal dementia, FTD, n=12; semantic dementia, SD, n=13; and progressive non-fluent aphasia, PNFA, n=9); in comparison with controls (n=27). Diagnoses were based on international consensus criteria. Manual bilateral segmentation of the caudate nucleus and putamen was conducted blind to diagnosis by a single analyst, on MRI scans using a standardized protocol. Intracranial volume was calculated via a stereological point counting technique and was used for normalizing the shape analysis. Segmented binaries were analyzed using the Spherical Harmonic (SPHARM) Shape Analysis tools (University of North Carolina) to perform comparisons between FTLD subtypes and controls for global shape difference, local significance maps and mean magnitude maps of shape displacement. Shape analysis revealed that there was significant shape difference between FTLD subtypes and controls, consistent with the predicted frontostriatal dysfunction and of significant magnitude, as measured by displacement maps. These differences were not significant for SD compared to controls; lesser for PNFA compared to controls; whilst FTD showed a more specific pattern in regions relaying fronto- and corticostriatal circuits. Shape analysis shows regional specificity of atrophy, manifest as shape deflation, with a differential between FTLD subtypes, compared to controls.

Involvement of endoplasmic reticulum stress on neuronal cell death in the lateral geniculate nucleus in the monkey glaucoma model

We investigated whether endoplasmic reticulum (ER) stress was involved in the pathophysiological mechanisms underlying neuronal death of the lateral geniculate nucleus (LGN) after intraocular pressure (IOP) elevation. Five cynomolgus monkeys, four with a glaucomatous left eye after laser photocoagulation treatment and one normal monkey, were studied. At 4, 11, 15 and 24 weeks after the laser photocoagulation treatment, the numbers of LGN neurons and atrophy were immunohistochemically evaluated using anti-parvalbumin-antibody, which was used to specifically label relay neurons connecting to the visual cortex. In addition, terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL)-positive cells, polyubiquitin, and production of ER stress-related proteins, such as the phosphorylation of eukaryotic initiation factor 2α (p-eIF2α) and C/EBP-homologous protein (CHOP), were also measured using in situ hybridization and immunostaining. Loss of neurons and/or neuronal atrophy in layers 1, 4 and 6 of the LGN on the contralateral side were observed at 4-24 weeks after the laser photocoagulation treatment. Furthermore, the retinal input from the high IOP eye projected to layers 2 (magnocellular layer), 3 and 5 (parvocellular layer) on the ipsilateral side. Neuronal damage was also confirmed in these layers. In the LGN region, TUNEL-positive cells, polyubiquitin, p-eIF2α and CHOP were also detected at 11-24 weeks after the laser photocoagulation treatment. These findings indicate that ER stress may play a pivotal role in neuronal death of the LGN after IOP elevation.

Kinesin-1 and degenerative changes in optic nerve axons in NMDA-induced neurotoxicity

We examined the histologic findings of optic nerve axons and changes in kinesin-1, which is involved in axonal flow, in N-methyl-d-aspartate (NMDA)-induced neurotoxicity in rats. Substantial degenerative changes visualized as black profiles and pale large axons were observed 72h after NMDA injection, but those degenerative changes were not apparent in axons 12 and 24h after injection. Morphometric analysis showed a significant, approximately 40% reduction in the number of axons 72h after NMDA injection. Immunohistochemical study showed that there was a recognizable loss of neurofilament-immunopositive dots, but myelin basic protein immunostaining was unchanged 72h after NMDA injection. Western blot analysis showed early elevation of kinesin-1 (KIF5B) protein levels in the retina 24 and 72h after NMDA injection. Conversely, significant decreases in KIF5B protein levels in the optic nerve were seen during the same time course. Immunohistochemical study also showed that there was a reduction in KIF5B immunoreactivity in axons, but neurofilament immunostaining was unchanged 24h after NMDA injection. These findings suggest that the intravitreal injection of NMDA causes neurofilament loss without myelin alteration in the early stage. The depletion of kinesin-1 precedes axonal degeneration of the optic nerve in NMDA-induced neurotoxicity.