Visual Evoked Potentials in Glaucoma and Alzheimer's Disease

In situ-crosslinked hydrogel-induced experimental glaucoma model with persistent ocular hypertension and neurodegeneration

A reliable animal model providing chronic and persistent ocular hypertension and characteristic neurodegeneration is essential to recapitulate human glaucoma and understand the underlying pathophysiological mechanisms behind this disease. Many approaches have been tried to establish persistently elevated intraocular pressure (IOP), while no efficient model and no systematic evaluation has been widely accepted yet. Herein, we developed a novel approach to reliably induce persistent IOP elevation using an injectable hydrogel formulated by hyperbranched macromolecular poly(ethylene glycol) (HB-PEG) and thiolated hyaluronic acid (HA-SH) under physiological conditions and established a systematic system for model evaluation. By formulation screening, an appropriate hydrogel with proper mechanical property, non-swelling profile and cytocompatibility was selected for further experiment. By intracameral injection, a persistent IOP elevation over 50% above baseline was obtained and it led to progressive retinal ganglion cell loss and ganglion cell complex thickness reduction. The evaluation of the efficacy of the model was thoroughly analyzed by whole-mounts retina immunostaining, optical coherence tomography, and hematoxylin-eosin staining for histological changes and by electroretinography for visual function changes. The N35-P50 amplitude of the pattern electroretinography and the N2-P2 amplitude of the flash visual-evoked potential were decreased, while the scotopic electroretinography showed no statistically significant changes. The in situ-forming HB-PEG/HA-SH hydrogel system could be an appropriate strategy for developing a reliable experimental glaucoma model without any confounding factors. We expect this model would be conducive to the development of neuroprotective and neuro-regenerative therapies.

Evaluation of the Effectiveness of a Chronic Ocular Hypertension Mouse Model Induced by Intracameral Injection of Cross-Linking Hydrogel

Background: Glaucoma is an irreversible and blinding neurodegenerative disease that is characterized by progressive loss of retinal ganglion cells. The current animal models of glaucoma fail to provide a chronic elevated intraocular pressure and cannot maintain the optical media clarity for a long time, which brings some difficulties to the study of glaucoma. Here, we developed a new chronic ocular hypertension model of mice induced by cross-linking hydrogel intracameral injection.

Methods: C57BL/6J mice aged 6–8 weeks were randomly divided into the control group and the operation group. The mice of the operation group were injected with cross-linking hydrogel to induce ocular hypertension. Intraocular pressure was measured preoperatively, 3 days after surgery, and weekly until the end of the study. Flash visual evoked potential (F-VEP) was used to observe optic nerve function at different times (preoperatively and 2, 4, and 6 weeks) after chronic ocular hypertension (COH). Retinal TNF-α, IL-1β, and IL-17A protein expression were measured by western blotting in the control group and in mice at 2, 4, and 6 weeks after COH. Microglial cell activation was evaluated by immunofluorescence staining and western blotting. Apoptosis and loss of retinal ganglion cells after 2, 4, and 6 weeks of intracameral injection of cross-linking hydrogel were observed by the TUNEL assay and Brn3a protein labeling. The loss of optic nerve axons in COH mice was evaluated by neurofilament heavy polypeptide protein labeling.

Results: Intracameral injection of the cross-linking hydrogel induces increased intraocular pressure (IOP) to a mean value of 19.3 ± 4.1 mmHg, which was sustained for at least 8 weeks. A significant difference in IOP was noted between COH mice and sham-operation mice (p < 0.0001). The success rate was 75%. The average amplitude of F-VEP in mice with COH was reduced (p = 0.0149, 0.0012, and 0.0009 at 2, 4, and 6 weeks after COH vs. the control group, respectively), and the average latent period in mice with COH was longer (p = 0.0290, <0.0001, and <0.0001 at 2, 4, and 6 weeks after COH vs. the control group, respectively) compared with that in the control group. TNF-α, IL-1β, IL-17A, Iba-1, and CD68 protein expression increased in COH mice. During the processing of COH, the number of microglial cells increased along with cellular morphological changes of rounder bodies and thicker processes compared with the control group. Apoptosis of retinal ganglion cells (RGCs) was clearly observed in mice at 2, 4, and 6 weeks after COH (p = 0.0061, 0.0012, <0.0001, and 0.0371 at 2, 4, and 6 weeks after COH vs. the control group, respectively). The RGC density decreased significantly in the COH mice compared with the control group (p = 0.0042, 0.0036, and <0.0001 at 2, 4, and 6 weeks after COH vs. the control group, respectively). There was a significant loss of optic nerve axons in mice after intracameral injection of cross-linking hydrogel (p = 0.0095, 0.0002, and <0.0001 at 2, 4, and 6 weeks after COH vs. the control group, respectively).

Conclusions: A single intracameral injection of cross-linking hydrogel can effectively induce chronic ocular hypertension in mice, which causes progressive loss of retinal ganglion cells, increased expression levels of inflammatory cytokines and microglial cell activation, and deterioration of optic nerve function.

Variability, repeatability and test-retest reliability of equine flash visual evoked potentials (FVEPs)

Background

Visual evoked potentials (VEPs) are electrical potentials generated by neurons in the central nervous system in response to visual stimuli. A series of positive and negative wavelets in response to flash-stimuli (flash-VEP; FVEP) or reversing, iso-luminant patterns (pattern-VEP; PVEP) are recorded. Pathological conditions affecting the post-retinal pathways can alter overall waveform morphology, and also affect wavelet peak times and amplitudes. FVEPs have recently been described in horses, but more data on the variability within and between subjects is required, to adequately interpret results from clinical equine patients. Therefore, the purpose of this study was to describe the variability, repeatability and test-retest reliability of equine FVEPs in normal, adult horses.

Results

Equine FVEPs were recorded from one randomly selected eye in 17 horses, from both eyes in eight of these horses, and also at two separate recording sessions in six horses. N1, P2, N2 and P4 wavelets were present in 100% of the recordings in all horses, while P1, N2a, P3 and P5 were only present in some recordings. Coefficients of variation (CVs) were low for P2, N2 and P4 peak times, but higher for all amplitudes. There were no statistically significant differences comparing peak times and amplitudes between eyes or between sessions. Coefficients of repeatability (CRs) are reported for P2, N2 and P4 peak times between eyes (P2; 5 ms, N2; 18 ms, P4; 18 ms) and also between sessions (P2; 5 ms, N2; 16 ms, P4; 39 ms). Intraclass correlation coefficients (ICCs), as an estimate of test-retest reliability, was assessed to be fair to excellent for most parameters.

Conclusions

This study provides important data on variability, repeatability and test-retest reliability of FVEPs in normal, adult horses. We conclude that P2, N2 and P4 peak times should be included in the evaluation of equine FVEPs. The large inherent variability of FVEP amplitudes is likely to make them less suitable and useful for establishing a diagnosis on their own in most clinical patients, but they may occasionally provide support to a clinical diagnosis.

Use of ocular biomarkers as a potential tool for early diagnosis of Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative disease worldwide which unfortunately has no known effective cure to date. Despite many clinical trials indicating the effectiveness of preclinical treatment, a sensitive tool for screening of AD is yet to be developed. Due to multiple similarities between ocular and the brain tissue, the eye is being explored by researchers for this purpose, with utmost attention focused on the retinal tissue. Besides visual functional impairment, neuronal degeneration and apoptosis, retinal nerve fiber degeneration, increase in the cup-to-disc ratio, and retinal vascular thinning and tortuosity are the changes observed in the retinal tissue which are related to AD. Studies have shown that targeting these changes in the retina is an effective way of reducing the degeneration of retinal neuronal tissue. Similar mechanisms of neurodegeneration have been demonstrated in the brain and the eyes of AD patients. Multiple studies are underway to investigate the potential of diagnosing AD and detection of amyloid-β (Aβ) levels in the retinal tissue. Since the tissues in the anterior segment of the eye are more accessible for in vivo imaging and examination, they have more potential as screening biomarkers. This article provides a concise review of available literature on the ocular biomarkers in anterior and posterior segments of the eye including the cornea, aqueous humour (AH), crystalline lens, and retina in AD. This review will also highlight the newer technological tools available for the detection of potential biomarkers in the eye for early diagnosis of AD.

Pattern-Reversal Visual Evoked Potential on Smart Glasses

OBJECTIVE

This paper presents an integrated device, based on smart glasses, for the pattern-reversal visual evoked potential (PR-VEP) clinical test.

METHODS

Smart glasses are used to generate the checkerboard changing pattern, with its related red fixation point through an Android application. Electroencephalographic signals, for monitoring the stimulus generated by PR-VEP, were amplified close to the scalp and then transmitted wirelessly to a PC. A MATLAB real-time algorithm processed the incoming signals to extract the final PR-VEP signal.

METHODS

In total, 40 eyes (from 20 subjects, 12 males and 8 females between 24 and 28 years old) were tested and results were compared, with a commercial device for VEP clinical exam, to test the reproducibility and the efficacy of the proposed solution.

RESULTS

PR-VEPs generated by smart glasses showed typical triphasic waveforms: We observed promising results and components in moderate agreement with those obtained using commercial PR-VEP recorder, with potential for improvements after further refinement works.

SIGNIFICANCE

The proposed device leads the way for a portable and low-cost solution.

Ocular Manifestations of Alzheimer's and Other Neurodegenerative Diseases: The Prospect of the Eye as a Tool for the Early Diagnosis of Alzheimer's Disease

Dementia, including Alzheimer's disease (AD), is a major disorder, leading to several ocular manifestations amongst the elderly population. These visual disorders may be due to retinal nerve degenerative changes, including nerve fibre layer thinning, degeneration of retinal ganglion cells, and changes to vascular parameters. There is no cure for Alzheimer's, but medicines can slow down the development of many of the classic symptoms, such as loss of memory and communication skills, mood swings, and depression. The disease diagnosis is difficult, and it is only possible through PET scans of the brain, detecting evidence of the accumulation of amyloid and tau. PET is expensive and invasive, requiring the injection of radioactive tracers, which bind with these proteins and glow during scanning. Recently, scientists developed promising eye-scan techniques that may detect Alzheimer's disease at its earliest stage, before major symptoms appear, leading to improved management of the disease symptoms. In this review, we are discussing the visual abnormalities of Alzheimer's and other neurodegenerative diseases, focused on ocular functional-visual-structural biomarkers, retinal pathology, and potential novel diagnostic tools.