The effect of optical defocus on the pattern electroretinogram in normal subjects and patients with Alzheimer's disease

Multiscale entropy analysis of retinal signals reveals reduced complexity in a mouse model of Alzheimer's disease

Alzheimer's disease (AD) is one of the most significant health challenges of our time, affecting a growing number of the elderly population. In recent years, the retina has received increased attention as a candidate for AD biomarkers since it appears to manifest the pathological signatures of the disease. Therefore, its electrical activity may hint at AD-related physiological changes. However, it is unclear how AD affects retinal electrophysiology and what tools are more appropriate to detect these possible changes. In this study, we used entropy tools to estimate the complexity of the dynamics of healthy and diseased retinas at different ages. We recorded microelectroretinogram responses to visual stimuli of different nature from retinas of young and adult, wild-type and 5xFAD-an animal model of AD-mice. To estimate the complexity of signals, we used the multiscale entropy approach, which calculates the entropy at several time scales using a coarse graining procedure. We found that young retinas had more complex responses to different visual stimuli. Further, the responses of young, wild-type retinas to natural-like stimuli exhibited significantly higher complexity than young, 5xFAD retinas. Our findings support a theory of complexity-loss with aging and disease and can have significant implications for early AD diagnosis.

The Association Between Acquired Color Deficiency and PET Imaging of Neurodegeneration in Mild Cognitive Impairment and Alzheimer Disease

Purpose

To evaluate color vision changes and retinal processing of chromatic and luminance pathways in subjects with Alzheimer disease (AD) and mild cognitive impairment (MCI) compared with a matched control group and whether such changes are associated with impaired brain glucose metabolism and β-amyloid deposition in the brain.

Methods

We evaluated 13 patients with AD (72.4 ± 7.7 years), 23 patients with MCI (72.5 ± 5.5 years), and 18 controls of comparable age (P = 0.44) using Cambridge color test and the heterochromatic flicker ERG (HF-ERG). The Cambridge color test was performed using the trivector protocol to estimate the protan, deutan and tritan color confusion axes. HF-ERG responses were measured at a frequency of 12 Hz, which ERGs reflect chromatic activity, and at 36 Hz, reflecting luminance pathway. A study subsample was performed using neuropsychological assessments and positron emission tomography.

Results

Patients with AD presented higher mean values indicating poorer color discrimination for protan (P = 0.04) and deutan (P = 0.001) axes compared with the controls. Along the tritan axis, both patients with AD and patients with MCI showed decreased color vision (P = 0.001 and P = 0.001) compared with controls. The analyses from the HF-ERG protocol revealed no differences between the groups (P = 0.31 and P = 0.41). Diffuse color vision loss was found in individuals with signs of neurodegeneration (protan P = 0.002, deutan P = 0.003 and tritan P = 0.01), but not in individuals with signs of β-amyloid deposition only (protan P = 0.39, deutan P = 0.48, tritan P = 0.63), regardless of their clinical classification.

Conclusions

Here, patients with AD and patients with MCI present acquired color vision deficiency that may be linked with impaired brain metabolism.

Retinal Responses to Simulated Optical Blur Using a Novel Dead Leaves ERG Stimulus

Purpose

The purpose of this study was to evaluate retinal responses to different types and magnitudes of simulated optical blur presented at specific retinal eccentricities using naturalistic images.

Methods

Electroretinograms (ERGs) were recorded from 27 adults using 30-degree dead leaves naturalistic images, digitally blurred with one of three types of optical blur (defocus, astigmatism, and spherical aberrations), and one of three magnitudes (0.1, 0.3, or 0.5 µm) of blur. Digitally computed blur was applied to the entire image, or on an area outside the central 6 degrees or 12 degrees of retinal eccentricity.

Results

ERGs were significantly affected by blur type, magnitude, and retinal eccentricity. ERGs were differentially affected by defocus and spherical aberrations; however, astigmatism had no effect on the ERGs. When blur was applied only beyond the central 12 degrees eccentricity, the ERGs were unaffected. However, when blur was applied outside the central 6 degrees, the ERG responses were significantly reduced and were no different from the ERGs recorded with entirely blurred images.

Conclusions

Blur type, magnitude, and location all affect the retinal responses. Our data indicate that the retinal area between 6 and 12 degrees eccentricity has the largest effect on the retinal responses to blur. In addition, certain optical blur types appear to have a more detrimental effect on the ERGs than others. These results cannot be solely explained by changes to image contrast and spatial frequency content, suggesting that retinal neurons might be sensitive to spatial cues in order to differentiate between different blur types.

Retinal thinning of inner sub-layers is associated with cortical atrophy in a mouse model of Alzheimer's disease: a longitudinal multimodal in vivo study

BACKGROUND

It has been claimed that the retina can be used as a window to study brain disorders. However, concerning Alzheimer's disease (AD), it still remains controversial whether changes occurring in the brain and retina are associated. We aim to understand when changes start appearing in the retina and brain, how changes progress, and if they are correlated.

METHODS

We carried out a unique longitudinal study, at 4, 8, 12, and 16 months of age, in a triple transgenic mouse model of AD (3×Tg-AD), which mimics pathological and neurobehavioral features of AD, as we have already shown. Retinal structure and physiology were evaluated in vivo using optical coherence tomography and electroretinography. Brain visual cortex structure was evaluated in vivo using magnetic resonance imaging.

RESULTS

The retinal thickness of 3×Tg-AD decreased, at all time points, except for the outer nuclear layer, where the opposite alteration was observed. Amplitudes in scotopic and photopic responses were increased throughout the study. Similarly, higher amplitude and lower phase values were observed in the photopic flicker response. No differences were found in the activity of retinal ganglion cells. Visual cortex gray matter volume was significantly reduced.

CONCLUSIONS

Our results show that this animal model shows similar neural changes in the retina and brain visual cortex, i.e., retinal and brain thinning. Moreover, since similar changes occur in the retina and brain visual cortex, these observations support the possibility of using the eye as an additional tool (noninvasive) for early AD diagnosis and therapeutic monitoring.

The Retina as a Window or Mirror of the Brain Changes Detected in Alzheimer's Disease: Critical Aspects to Unravel

Alzheimer's disease is the most frequent cause of dementia worldwide, representing a global health challenge, with a massive impact on the quality of life of Alzheimer's disease patients and their relatives. The diagnosis of Alzheimer's disease constitutes a real challenge, because the symptoms manifest years after the first degenerative changes occurring in the brain and the diagnosis is based on invasive and/or expensive techniques. Therefore, there is an urgent need to identify new reliable biomarkers to detect Alzheimer's disease at an early stage. Taking into account the evidence for visual deficits in Alzheimer's disease patients, sometimes even before the appearance of the first disease symptoms, and that the retina is an extension of the brain, the concept of the retina as a window to look into the brain or a mirror of the brain has received increasing interest in recent years. However, only a few studies have assessed the changes occurring in the retina and the brain at the same time points. Unlike previous reviews on this subject, which are mainly focused on brain changes, we organized this review by comprehensively summarizing findings related with structural, functional, cellular, and molecular parameters in the retina reported in both Alzheimer's disease patients and animal models. Moreover, we separated the studies that assessed only the retina, and those that assessed both the retina and brain, which are few but allow establishing correlations between the retina and brain. This review also highlights some inconsistent results in the literature as well as relevant missing gaps in this field.

Protein misfolding and retinal degeneration

The retina is a highly complex and specialized organ that performs preliminary analysis of visual information. Composed of highly metabolically active tissue, the retina requires a precise and well-balanced means of maintaining its functional activity during extended periods of time. Maintenance and regulation of a vast array of different structural and functional proteins is required for normal function of the retina. This process is referred to as protein homeostasis and involves a variety of activities, including protein synthesis, folding, transport, degradation, elimination, and recycling. Deregulation of any of these activities can lead to malfunctioning of the retina, from subtle subclinical signs to severe retinal degenerative diseases leading to blindness. Examples of retinal degenerative diseases caused by disruption of protein homeostasis include retinitis pigmentosa and Stargardt's disease. A detailed discussion of the role of disruption in protein homeostasis in these and other retinal diseases is presented, followed by examples of some existing and potential treatments.

Effect of operator and optical defocus on the variability of pattern electroretinogram optimized for glaucoma detection (PERGLA)

PURPOSE

To evaluate the effect of operator and optical defocus on the variability of pattern electroretinogram optimized for glaucoma detection (PERGLA).

METHODS

Two different operators obtained 2 PERGLA recordings each from 10 healthy participants (5 women, mean age 32.1+/-10.3 y). In addition, one of the operators obtained recordings in which corrective lenses of various diopters (+/-0.5, +/-1, +/-2, and +/-3) were used to generate optical defocus in both eyes. The effect of operator on PERGLA amplitude and phase variability was determined using a single nested variance components' analysis model and by using Bland-Altman plots. One-way analysis of variance (ANOVA) was used to determine the effect of optical defocus on amplitude and phase.

RESULTS

Differences in measurements between operators accounted for approximately 26.6% and 18.2% of the total variance for amplitude and phase, respectively. Results were confirmed by the use of Bland-Altman plots. ANOVA identified a significant effect of defocus on mean amplitude (F=2.65, P=0.01), but not phase (F=1.02, P=0.42).

CONCLUSIONS

Measurements obtained by different operators can result in significant differences in PERGLA amplitude. In addition, although optical defocus leads to a decrease in PERGLA amplitude by reducing visual acuity, this can be avoided by obtaining J1 or better vision before testing.

Correlation between morphological and functional retinal impairment in patients affected by ocular hypertension, glaucoma, demyelinating optic neuritis and Alzheimer's disease

In this article the correlations between the morphological evaluation of the nerve fiber layer (NFL) thickness (by OCT) and retinal functional assessment (by Pattern ERG recordings) performed in patients affected by ocular hypertension (OHT), glaucoma (OAG), demyelinating optic neuritis (MSON), and Alzheimer's disease (AD) are reported. In OHT eyes with ocular hypertension we observed that the inter-individual variation in NFL thickness is correlated with the variability of the PERG responses (the thinner the layer, the worse the visual function). In our OAG, MSON and AD eyes we observed a significant reduction in NFL thickness when compared with controls. In OHT, OAG, MSON and AD eyes abnormal PERG responses with delayed implicit times and reduced amplitudes were found. The impairment in the PERG parameters was significantly correlated to the reduction in NFL thickness. Our results suggest that in patients affected by ocular hypertension, glaucoma, demyelinating optic neuritis, and Alzheimer's Disease there is a reduction of NFL thickness evaluated "in vivo" by OCT, and this morphological involvement is correlated with electrophysiological responses assumed to be originating from the innermost retinal layers.

Morphological and functional retinal impairment in Alzheimer's disease patients

OBJECTIVE

Our study aims to assess the optic nerve fiber layer thickness in vivo, the function of the innermost retinal layer and whether a correlation exists between morphological and functional parameters in patients affected by Alzheimer's Disease (AD).

METHODS

Seventeen AD patients (mean age 70.37+/-6.1 years, best corrected visual acuity >8/10 with refractive error between +/-3 sf, intra-ocular pressure (IOP)<18 mmHg) were enrolled. They were compared to 14 age-matched controls. Nerve fiber layer (NFL) thickness was measured by optical coherence tomography (OCT). Three different measurements in each quadrant (superior, inferior, nasal, and temporal) were taken and averaged. The data in all quadrants (12 values averaged) were identified as NFL Overall. Retinal function was assessed by pattern electroretinogram (PERG) recordings using high-contrast (80%) checkerboard stimuli subtending 15 min of the visual arc and reversed at the rate of two reversals/s.

RESULTS

In AD eyes, there was a significant (P<0.01) reduction in NFL thickness in each quadrant and in the NFL Overall evaluation compared with the values observed in control eyes. PERGs showed a significant (P<0.01) delay in N35, P50 and N95 implicit times, and reduction in N35-P50 and P50-N95 amplitudes. NFL Overall values were significantly correlated (P<0.01) to the PERG P50 and N95 implicit times and P50-N95 amplitude. No correlations (P>0.01) between NFL values and other PERG parameters (N35 implicit time, N35-P50 amplitude) were found.

CONCLUSIONS

Our results suggest that in AD patients, there is a reduction of NFL thickness evaluated in vivo by OCT and this morphological abnormality is related to a retinal dysfunction as revealed by abnormal PERG responses.

Pattern electroretinogram, visual evoked potential and psychophysical functions in maculopathy

To compare pattern electroretinograms and visual evoked potentials with psychophysical examinations, such as visual acuity, static (automated) perimetry and color vision in unilateral maculopathies of various origins, 20 patients with unilateral retinal diseases within the macula and the posterior pole were tested. Pattern electroretinography, visual evoked potential testing and static perimetry (Octopus program M1) were performed with three different test field sizes (20 degrees x 20 degrees, 10 degrees x 10 degrees and 6 degrees x 6 degrees). The best correlation in all three test field sizes was found between visual acuity, static perimetry and visual evoked potential. This result is surprising, since central area defined functions (visual evoked potentials, visual acuity) correlated well with a total area integrating function (mean defect in static perimetry. The pattern electroretinogram, which seems to reflect an area-related function as well, showed a correlation to static perimetry only in the smaller 10 degrees x 10 degrees and 6 degrees x 6 degrees fields and not a significant correlation in the 20 degrees x 20 degrees field. Smaller stimulation fields may therefore produce sharper results in pattern electroretinographic testing. There was no correlation between pattern electroretinograms and visual evoked potentials or visual acuity. The pattern electroretinogram was recorded under monocular and binocular viewing conditions. In 60% of the patients, the amplitude of the affected eye was more reduced in the monocular than the binocular viewing condition; the healthy fellow eye controlled stable fixation of the affected eye more readily during binocular pattern electroretinogram registration. The degree of the color vision disturbance (C-index, desaturated panel D-15 test) did not correlate to any of the other examinations.

The gold foil electrode in pattern electroretinography

A recent study found that the gold foil electrode produces large pattern electroretinogram amplitudes, but the test-retest reliability was low. In a three-center study, we observed that 90% of 29 patients who were tested with gold foil electrodes used three times appeared to have markedly lower amplitudes than when tested with new electrodes during the same session. Across study centers, the mean of the new electrode recordings was 3.78 microV (standard deviation, 1.13 microV), versus 2.93 microV (1.29 microV) for used electrodes. This 0.85-microV reduction (22%) was statistically significant (F = 7.10 p = 0.01). Electrodes used three times demonstrated an average change in the coefficient of variation of 14% (standard deviation/mean = coefficient of variation; new, 1.13/3.78 = 30%; used, 1.29/2.93 = 44%). Two of the study sites (Houston/Indianapolis) conducted test-retest pattern electroretinograms on a total of 18 patients and found the mean evoked potential to be 3.55 microV with new electrodes and 2.82 microV with used electrodes. The coefficient of variation for the test-retest data was 30% and 47% for new and used electrodes, respectively. Light microscopy showed small cracks on the surface of the electrode, with the number and configuration of the cracks varying in each electrode. The presence of cracks is further complicated by their proximity to the tear film. These sources of variation can result in significantly different impedances. We propose that constant flexion, as a result of patient blinking, causes cracks in the thin gold surface of the electrode. Used electrodes will produce lower pattern electroretinogram amplitudes and poor test-retest reliability.(ABSTRACT TRUNCATED AT 250 WORDS)

The pattern visual-evoked potential in former preterm infants with retinopathy of prematurity

PURPOSE

Flash and pattern visual-evoked potentials (VEPs) were recorded in 89 former preterm infants (< or = 1500 g birth weight), in whom retinopathy of prematurity (ROP) developed that spontaneously regressed without macular detachment (stages 2-4a).

METHODS

Linear and stepwise regression analyses were performed to determine the correlations between transient pattern-reversal P1 VEP latency (n = 154 eyes) at a mean of 7.2 years (median, 6.9 years) postnatal age using a check size nominally equivalent to 20/100 (30 minutes) and the following nine parameters: three immaturity parameters (gestational age at birth, birth weight, and ROP zone at 1 month postnatal age); one postnatal insults parameter (worst ROP stage); two structural outcome parameters (macular ectopia and vessel traction); one functional outcome parameter (visual acuity); and two methodological parameters (postnatal age at VEP testing and VEP amplitude).

RESULTS

Linear regression analyses, with P1 VEP latency as a dependent variable, identified the worst ROP stage (r = +0.42; P < 0.0001), macular ectopia (r = +0.42; P < 0.0001), visual acuity (r = -0.40; P < 0.0001), and vessel traction (r = +0.35; P < 0.0001) as significant correlates. Stepwise regression analysis demonstrated that worst ROP stage and macular ectopia accounted for 18% and 4% of the cumulative variance, respectively.

CONCLUSIONS

P1 VEP latency correlates with postnatal insults, structural outcome, and functional outcome parameters in former preterms in whom ROP developed that spontaneously regressed without macular detachment. A permanent arrest in the development of the macula and/or prolonged traction on the incompletely developed macula may alter VEPs.