Imaging retina to study dementia and stroke

Developing retinal biomarkers for the earliest stages of Alzheimer's disease: What we know, what we don't, and how to move forward

The last decade has seen a substantial increase in research focused on the identification, development, and validation of diagnostic and prognostic retinal biomarkers for Alzheimer's disease (AD). Sensitive retinal biomarkers may be advantageous because they are cost and time efficient, non-invasive, and present a minimal degree of patient risk and a high degree of accessibility. Much of the work in this area thus far has focused on distinguishing between symptomatic AD and/or mild cognitive impairment (MCI) and cognitively normal older adults. Minimal work has been done on the detection of preclinical AD, the earliest stage of AD pathogenesis characterized by the accumulation of cerebral amyloid absent clinical symptoms of MCI or dementia. The following review examines retinal structural changes, proteinopathies, and vascular alterations that have been proposed as potential AD biomarkers, with a focus on studies examining the earliest stages of disease pathogenesis. In addition, we present recommendations for future research to move beyond the discovery phase and toward validation of AD risk biomarkers that could potentially be used as a first step in a multistep screening process for AD risk detection.

A deep-learning system for the assessment of cardiovascular disease risk via the measurement of retinal-vessel calibre

Retinal blood vessels provide information on the risk of cardiovascular disease (CVD). Here, we report the development and validation of deep-learning models for the automated measurement of retinal-vessel calibre in retinal photographs, using diverse multiethnic multicountry datasets that comprise more than 70,000 images. Retinal-vessel calibre measured by the models and by expert human graders showed high agreement, with overall intraclass correlation coefficients of between 0.82 and 0.95. The models performed comparably to or better than expert graders in associations between measurements of retinal-vessel calibre and CVD risk factors, including blood pressure, body-mass index, total cholesterol and glycated-haemoglobin levels. In retrospectively measured prospective datasets from a population-based study, baseline measurements performed by the deep-learning system were associated with incident CVD. Our findings motivate the development of clinically applicable explainable end-to-end deep-learning systems for the prediction of CVD on the basis of the features of retinal vessels in retinal photographs.

AVNet: A retinal artery/vein classification network with category-attention weighted fusion

BACKGROUND AND OBJECTIVE

Automatic artery/vein (A/V) classification in retinal images is of great importance in detecting vascular abnormalities, which may provide biomarkers for early diagnosis of many systemic diseases. It is intuitive to apply popular deep semantic segmentation network for A/V classification. However, the model is required to provide powerful representation ability since vessel is much more complex than general objects. Moreover, deep network may lead to inconsistent classification results for the same vessel due to the lack of structured optimization objective.

METHODS

In this paper, we propose a novel segmentation network named AVNet, which effectively enhances the classification ability of the model by integrating category-attention weighted fusion (CWF) module, significantly improving the pixel-level A/V classification results. Then, a graph based vascular structure reconstruction (VSR) algorithm is employed to reduce the segment-wise inconsistency, verifying the effect of the graph model on noisy vessel segmentation results.

RESULTS

The proposed method has been verified on three datasets, i.e. DRIVE, LES-AV and WIDE. AVNet achieves pixel-level accuracies of 90.62%, 90.34%, and 93.16%, respectively, and VSR further improves the performance by 0.19%, 1.85% and 0.64%, achieving the state-of-the-art results on these three datasets.

CONCLUSION

The proposed method achieves competitive performance in A/V classification task.

Retinal changes in Alzheimer's disease- integrated prospects of imaging, functional and molecular advances

Alzheimer's Disease (AD) is a devastating neurodegenerative disorder of the brain, clinically characterised by cognitive deficits that gradually worsen over time. There is, at present, no established cure, or disease-modifying treatments for AD. As life expectancy increases globally, the number of individuals suffering from the disease is projected to increase substantially. Cumulative evidence indicates that AD neuropathological process is initiated several years, if not decades, before clinical signs are evident in patients, and diagnosis made. While several imaging, cognitive, CSF and blood-based biomarkers have been proposed for the early detection of AD; their sensitivity and specificity in the symptomatic stages is highly variable and it is difficult to justify their use in even earlier, pre-clinical stages of the disease. Research has identified potentially measurable functional, structural, metabolic and vascular changes in the retina during early stages of AD. Retina offers a distinctively accessible insight into brain pathology and current and developing ophthalmic technologies have provided us with the possibility of detecting and characterising subtle, disease-related changes. Recent human and animal model studies have further provided mechanistic insights into the biochemical pathways that are altered in the retina in disease, including amyloid and tau deposition. This information coupled with advances in molecular imaging has allowed attempts to monitor biochemical changes and protein aggregation pathology in the retina in AD. This review summarises the existing knowledge that informs our understanding of the impact of AD on the retina and highlights some of the gaps that need to be addressed. Future research will integrate molecular imaging innovation with functional and structural changes to enhance our knowledge of the AD pathophysiological mechanisms and establish the utility of monitoring retinal changes as a potential biomarker for AD.

Multi-Task Siamese Network for Retinal Artery/Vein Separation via Deep Convolution Along Vessel

Vascular tree disentanglement and vessel type classification are two crucial steps of the graph-based method for retinal artery-vein (A/V) separation. Existing approaches treat them as two independent tasks and mostly rely on ad hoc rules (e.g. change of vessel directions) and hand-crafted features (e.g. color, thickness) to handle them respectively. However, we argue that the two tasks are highly correlated and should be handled jointly since knowing the A/V type can unravel those highly entangled vascular trees, which in turn helps to infer the types of connected vessels that are hard to classify based on only appearance. Therefore, designing features and models isolatedly for the two tasks often leads to a suboptimal solution of A/V separation. In view of this, this paper proposes a multi-task siamese network which aims to learn the two tasks jointly and thus yields more robust deep features for accurate A/V separation. Specifically, we first introduce Convolution Along Vessel (CAV) to extract the visual features by convolving a fundus image along vessel segments, and the geometric features by tracking the directions of blood flow in vessels. The siamese network is then trained to learn multiple tasks: i) classifying A/V types of vessel segments using visual features only, and ii) estimating the similarity of every two connected segments by comparing their visual and geometric features in order to disentangle the vasculature into individual vessel trees. Finally, the results of two tasks mutually correct each other to accomplish final A/V separation. Experimental results demonstrate that our method can achieve accuracy values of 94.7%, 96.9%, and 94.5% on three major databases (DRIVE, INSPIRE, WIDE) respectively, which outperforms recent state-of-the-arts.

The eye, the kidney, and cardiovascular disease: old concepts, better tools, and new horizons

Chronic kidney disease (CKD) is common, with hypertension and diabetes mellitus acting as major risk factors for its development. Cardiovascular disease is the leading cause of death worldwide and the most frequent end point of CKD. There is an urgent need for more precise methods to identify patients at risk of CKD and cardiovascular disease. Alterations in microvascular structure and function contribute to the development of hypertension, diabetes, CKD, and their associated cardiovascular disease. Homology between the eye and the kidney suggests that noninvasive imaging of the retinal vessels can detect these microvascular alterations to improve targeting of at-risk patients. Retinal vessel-derived metrics predict incident hypertension, diabetes, CKD, and cardiovascular disease and add to the current renal and cardiovascular risk stratification tools. The advent of optical coherence tomography (OCT) has transformed retinal imaging by capturing the chorioretinal microcirculation and its dependent tissue with near-histological resolution. In hypertension, diabetes, and CKD, OCT has revealed vessel remodeling and chorioretinal thinning. Clinical and preclinical OCT has linked retinal microvascular pathology to circulating and histological markers of injury in the kidney. The advent of OCT angiography allows contrast-free visualization of intraretinal capillary networks to potentially detect early incipient microvascular disease. Combining OCT's deep imaging with the analytical power of deep learning represents the next frontier in defining what the eye can reveal about the kidney and broader cardiovascular health.

Novel Acute Retinal Artery Ischemia and Reperfusion Model in Nonhuman Primates

BACKGROUND AND PURPOSE

The retina, as an externally located neural tissue, offers unique advantages in investigating the effect of therapeutic intervention on the brain. In this study, we put forth a clinically relevant model of retinal ischemia and reperfusion in nonhuman primates.

METHODS

Acute retinal artery ischemia and reperfusion was induced by injecting an autologous clot into the ophthalmic artery of adult rhesus monkeys, and recanalization was achieved by focal thrombolysis with tPA (tissue-type plasminogen activator). Digital subtraction angiography and fluorescein angiography were used to evaluate blood flow in the retina and the choroid. Electroretinogram, optical coherence tomography, and hematoxylin and eosin staining were used to evaluate the structure and function of the retina after ischemia.

RESULTS

Digital subtraction angiography and fluorescein angiography images confirmed occlusion of the ophthalmic and central retinal arteries, as well as recanalization after tPA thrombolysis. Electroretinogram indicated retinal functional damage following ischemia, and thrombolysis partially rescued its impairment. Optical coherence tomography and hematoxylin and eosin staining revealed ischemia-induced changes in the retina, and tPA partially mitigated these damages.

CONCLUSIONS

This novel acute retinal artery ischemia and reperfusion model in rhesus monkeys may closely simulate retinal ischemia/reperfusion in clinical practice and provide an optimal platform for screening neuroprotective strategies.

Potential New Approaches for Diagnosis of Alzheimer's Disease and Related Dementias

Dementia is an umbrella term-caused by a large number of specific diagnoses, including several neurodegenerative disorders. Alzheimer's disease (AD) is now the most common cause of dementia in advanced countries, while dementia due to neurosyphilis was the leading cause a century ago. Many challenges remain for diagnosing dementia definitively. Some of these include variability of early symptoms and overlap with similar disorders, as well as the possibility of combined, or mixed, etiologies in some cases. Newer technologies, including the incorporation of PET neuroimaging and other biomarkers (genomics and proteomics), are being incorporated into revised diagnostic criteria. However, the application of novel diagnostic methods at clinical sites is plagued by many caveats including availability and access. This review surveys new diagnostic methods as well as remaining challenges-for clinical care and clinical research.

Optical Coherence Tomography Angiography in Neurodegenerative Disorders

Retinal microcirculation shares similar features with cerebral small blood vessels. Thus, the retina may be considered an accessible 'window' to detect the microvascular damage occurring in the setting of neurodegenerative disorders. Optical coherence tomography angiography (OCT-A) is a non-invasive imaging modality providing depth resolved images of blood flow in the retina, choroid, and optic nerve. In this review, we summarize the current literature on the application of OCT-A in glaucoma and central nervous system conditions such as Alzheimer's disease, Parkinson's disease, and multiple sclerosis. Future directions aiming at evaluating whether OCT-A can be an additional biomarker for the early diagnosis and monitoring of neurodegenerative disorders are also discussed.

Understanding multifactorial brain changes in type 2 diabetes: a biomarker perspective

People with type 2 diabetes are at an increased risk of cognitive impairment and dementia (including Alzheimer's disease), as well as subtle forms of cognitive dysfunction. Current diabetes guidelines recommend screening for cognitive impairment in groups at high risk and providing guidance for diabetes management in patients with diabetes and cognitive impairment. Yet, no disease-modifying treatment is available and important questions remain about the mechanisms underlying diabetes-associated cognitive dysfunction. These mechanisms are likely to be multifactorial and different for subtle and more severe forms of diabetes-associated cognitive dysfunction. Over the past years, research on dementia, brain ageing, diabetes, and vascular disease has identified novel biomarkers of specific dementia aetiologies, brain parenchymal injury, and cerebral blood flow and metabolism. These markers shed light on the processes underlying diabetes-associated cognitive dysfunction, have clear applications in current research and increasingly in clinical diagnosis, and might ultimately guide targeted treatment.

Characterization of the retinal changes of the 3×Tg-AD mouse model of Alzheimer’s disease

Alzheimer’s disease (AD) is a progressive neurodegenerative disorder whose diagnosis remains a notable challenge. The literature suggests that cerebral changes precede AD symptoms by over two decades, implying a significantly advanced stage of AD by the time it is usually diagnosed. In the study herein, texture analysis was applied to computed optical coherence tomography ocular fundus images to identify differences between a group of the transgenic mouse model of the Alzheimer’s disease (3×Tg-AD) and a group of wild-type mice, at the ages of one and two-months-old. A substantial difference between groups was found at both time-points across all neuroretina’s layers. Here, the inner nuclear layer stands out both in the level of statistically significant differences and on the extension of these differences which span through the imaged area. Also, the progression of AD is suggested to be spotted by texture analysis as demonstrated by the significant difference found in the inner plexiform and the outer nuclear layers from the age of one to the age of two-months-old. These findings demonstrate the potential of the use of the retina and texture analysis to the diagnosis of AD and monitor AD progression. Besides, the differences between groups found in this study suggest that the 3×Tg-AD model may be inappropriate to study early changes associated with the AD and other animal models should be tested following the same path and rationale. Moreover, these results also suggest that the human genes present in these transgenic mice may have an impact on the neurodevelopment of offspring which would justify the significant changes found at the age of one-month-old.

Screening for diabetic retinopathy: new perspectives and challenges

Although the prevalence of all stages of diabetic retinopathy has been declining since 1980 in populations with improved diabetes control, the crude prevalence of visual impairment and blindness caused by diabetic retinopathy worldwide increased between 1990 and 2015, largely because of the increasing prevalence of type 2 diabetes, particularly in low-income and middle-income countries. Screening for diabetic retinopathy is essential to detect referable cases that need timely full ophthalmic examination and treatment to avoid permanent visual loss. In the past few years, personalised screening intervals that take into account several risk factors have been proposed, with good cost-effectiveness ratios. However, resources for nationwide screening programmes are scarce in many countries. New technologies, such as scanning confocal ophthalmology with ultrawide field imaging and handheld mobile devices, teleophthalmology for remote grading, and artificial intelligence for automated detection and classification of diabetic retinopathy, are changing screening strategies and improving cost-effectiveness. Additionally, emerging evidence suggests that retinal imaging could be useful for identifying individuals at risk of cardiovascular disease or cognitive impairment, which could expand the role of diabetic retinopathy screening beyond the prevention of sight-threatening disease.

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.

Cerebral microvascular complications of type 2 diabetes: stroke, cognitive dysfunction, and depression

Adults with type 2 diabetes are at an increased risk of developing certain brain or mental disorders, including stroke, dementia, and depression. Although these disorders are not usually considered classic microvascular complications of diabetes, evidence is growing that microvascular dysfunction is one of the key underlying mechanisms. Microvascular dysfunction is a widespread phenomenon in people with diabetes, including effects on the brain. Cerebral microvascular dysfunction is also apparent in adults with prediabetes, suggesting that cerebral microvascular disease processes start before the onset of diabetes. The microvasculature is involved in the regulation of many cerebral processes that when impaired predispose to lacunar and haemorrhagic stroke, cognitive dysfunction, and depression. Main drivers of diabetes-related cerebral microvascular dysfunction are hyperglycaemia, obesity and insulin resistance, and hypertension. Increasing amounts of data from observational studies suggest that diabetes-related microvascular dysfunction is associated with a higher risk of stroke, cognitive dysfunction, and depression. Cerebral outcomes in diabetes might be improved following treatments targeting the pathways through which diabetes damages the microcirculation. These treatments might include drugs that reduce dicarbonyl compounds, augment cerebral insulin signalling, or improve blood-brain barrier permeability and cerebral vasoreactivity.

Retinal axonal degeneration in Niemann-Pick type C disease

Objective

Niemann–Pick disease type C1 (NPC1) is a rare autosomal-recessive lysosomal storage disorder presenting with a broad clinical spectrum ranging from a severe infantile-onset neurovisceral disorder to late-onset neurodegenerative disease. Optical coherence tomography (OCT) is established to detect retinal degeneration in vivo. We examined NPC1-patients (NPC1-P), clinically asymptomatic NPC1-mutation carriers (NPC1-MC), and healthy controls (HC) to (1) identify retinal degeneration in NPC1-disease and (2) to investigate possible subclinical retinal degeneration in NPC1-MC.

Methods

Fourteen NPC1-P, 17 NPC1-MC, and 31 age-matched HC were examined using spectral-domain OCT. Neurological examinations, clinical scales [modified Disability Rating Scale (mDRS); Scale for the Rating and Assessment of Ataxia (SARA); Spinocerebellar Ataxia Functional Index (SCAFI)], and video-oculography (VOG) were correlated with OCT data.

Results

Macular retinal nerve fiber layer and volumes of combined ganglion cell and inner plexiform layer were significantly lower in NPC1-P compared to HC [mRNFL (µm):0.13 ± 0.01 vs. 0.14 ± 0.02; p = 0.01; GCIPL (mm³):0.60 ± 0.05 vs. 0.62 ± 0.04; p = 0.04]. No significant differences were found in NPC1-MC in comparison to HC. In NPC1-P, the amplitude of upward vertical saccades showed positive associations with peripapillary RNFL (ρ = 0.645; p < 0.05), and thinned GCIP (ρ = 0.609; p < 0.05), but not in NPC1-MC. In NPC1-P correlations between combined outer plexiform layer and outer nuclear layer (OPONL) with mDRS (r = − 0.617; p < 0.05) and GCIP with SARA (r = − 0.622; p < 0.05) were observed. Furthermore, in NPC1-MC, motor scores were negatively associated with pRNFL (ρ = − 0.677; p < 0.01).

Conclusions

Using OCT, we showed retinal degeneration in NPC1-P and significant correlation between retinal neuroaxonal degeneration with clinical measurements. We observed a non-significant trend of retinal degeneration in NPC1-MC correlating with subclinical motor abnormalities. Based on these preliminary data, OCT may be an important marker of neurodegeneration in NPC1-disease after onset of clinical symptoms.

Reduced Retinal Thickness Predicts Age-Related Changes in Cognitive Function

Currently, there is a lack of biomarkers to identify individuals in the early stages of Alzheimer's disease (AD). A preponderance of evidence suggests that neurodegenerative processes that affect the brain, may also affect the retina. Using optical coherence tomography (OCT), a non-invasive approach, many have shown thinning of the retina in AD and the developmental precursor to AD, mild cognitive impairment (MCI). However, the relationship between retinal thickness and cognitive function is not entirely clear. This is likely due to the disparity in diagnostic criteria used to determine MCI that does not fully probe the cognitive domains that are particularly vulnerable to aging. This study used a comprehensive neuropsychological assessment involving multiple domains of cognition to determine if retinal thickness correlates with cognitive performance in a normal aged population. In this study, 20 healthy individuals between 60 and 90 years of age were administered neuropsychological assessments probing various domains of cognitive function, and OCT to measure peripapillary retinal nerve fiber layer (RNFL) thickness. We found that RNFL thickness is correlated with neuropsychological performance in multiple cognitive domains (e.g., working memory, psychomotor speed, and executive function). Our work demonstrates a positive correlation between RNFL thickness and several, but not all, domains of cognitive function in a normative aging population. By determining which cognitive domains retinal thickness can predict, this work can help identify individuals at risk or in preclinical stages of AD and other neurodegenerative diseases.

Focal alteration of the intraretinal layers in neurodegenerative disorders

Focal intraretinal alterations have been studied to advance our understanding of the pathology of neurodegenerative diseases. The current literature involving focal alterations in the intraretinal layers was reviewed through PubMed using the search terms "focal alteration", "region of interest", "optical coherence tomography", "glaucoma", "multiple sclerosis", "Alzheimer's disease", "Parkinson disease", "neurodegenerative diseases" and other related items. It was found that focal alterations of intraretinal layers were different in various neurodegenerative diseases. The typical focal thinning might help differentiate various ocular and cerebral diseases, track disease progression, and evaluate the outcome of clinical trials. Advanced exploration of focal intraretinal alterations will help to further validate their clinical and research utility.

Artificial Intelligence in Ophthalmology: Evolutions in Asia

Artificial intelligence (AI) has been studied in ophthalmology since availability of digital information in ophthalmic care. The significant turning point was availability of commercial digital color fundus photography in the late 1990s, which caused digital screening for diabetic retinopathy (DR) to take off. Automated Retinal Disease Assessment software was then developed using machine learning to detect abnormal lesions in fundus to screen DR. The use of this version of AI had not been generalized because the specificity at 45% was not high enough, although the sensitivity reached 90%. The recent breakthrough in machine learning is the invent of deep learning, which accelerates its performance to be on par with experts. The first 2 breakthrough studies on deep learning for screening DR were conducted in Asia. The first represented collaboration of datasets between Asia and the United States for algorithms development, whereas the second represented algorithms developed in Asia but validated in different populations across the world. Both found accuracy for detecting referable DR of >95%. Diversity and variety are unique strengths of Asia for AI studies. There are many more studies of AI ongoing in Asia not only as prospective deployments in DR but in glaucoma, age-related macular degeneration, cataract, and systemic disease, such as Alzheimer's disease. Some Asian countries have laid out plans for digital health care system using AI as one of the puzzle pieces for solving blindness. More studies on AI and digital health are expected to come from Asia in this new decade.

Central arterial stiffness and retinal vessel calibers: the Atherosclerosis Risk in Communities Study-Neurocognitive Study

BACKGROUND

The retinal microvasculature provides a window to the cerebral vasculature and enables examination of changes in retinal caliber that may mimic those occurring in cerebrovascular disease. The association of central arterial stiffness and retinal vessel caliber in a population sample is not fully understood.

METHODS

In 1706 older adults (mean age 76.3, 58.1% women) from the population-based Atherosclerosis Risk in Communities Study, we examined the cross-sectional association of central arterial stiffness [carotid-femoral pulse wave velocity (cfPWV)] with retinal vessel calibers [central retinal arteriolar equivalent (CRAE) and central retinal vein equivalent (CRVE)]. We estimated the association of cfPWV with CRAE narrowing (<25th percentile) and CRVE widening (>75th percentile) after adjustment for age, sex, race-field center, BMI, smoking, and type 2 diabetes. We tested for effect modification by sex, hypertension, and type 2 diabetes.

RESULTS

Carotid-femoral PWV (m/s) was not associated with the odds of CRAE narrowing [odds ratio (OR): 0.99; 95% CI: 0.95-1.03]. The association of cfPWV with CRVE widening was stronger in those without hypertension (OR: 1.10; 95% CI: 1.01-1.20) versus those with hypertension (OR: 1.01 95% CI: 0.96-1.05) and slightly stronger in those with type 2 diabetes (OR: 1.07; 95% CI: 1.00-1.14) versus without type 2 diabetes (OR: 1.01; 95% CI: 0.96-1.06).

CONCLUSIONS

In older adults, cfPWV was associated with wider retinal venular caliber, particularly in individuals without hypertension. Central arterial stiffening may be associated with cerebral microvascular changes, as exhibited in its retinal vasculature component.

Systematic Review on Fractal Dimension of the Retinal Vasculature in Neurodegeneration and Stroke: Assessment of a Potential Biomarker

Introduction: Ocular manifestations in several neurological pathologies accentuate the strong relationship between the eye and the brain. Retinal alterations in particular can serve as surrogates for cerebral changes. Offering a “window to the brain,” the transparent eye enables non-invasive imaging of these changes in retinal structure and vasculature. Fractal dimension (FD) reflects the overall complexity of the retinal vasculature. Changes in FD could reflect subtle changes in the cerebral vasculature that correspond to preclinical stages of neurodegenerative diseases. In this review, the potential of this retinal vessel metric to serve as a biomarker in neurodegeneration and stroke will be explored.

Methods: A literature search was conducted, following the PRISMA Statement 2009 criteria, in four large bibliographic databases (Pubmed, Embase, Web Of Science and Cochrane Library) up to 12 October 2019. Articles have been included based upon their relevance. Wherever possible, level of evidence (LOE) has been assessed by means of the Oxford Centre for Evidence-based Medicine Level of Evidence classification.

Results: Twenty-one studies were included for qualitative synthesis. We performed a narrative synthesis and produced summary tables of findings of included papers because methodological heterogeneity precluded a meta-analysis. A significant association was found between decreased FD and neurodegenerative disease, mainly addressing cognitive impairment (CI) and dementia. In acute, subacute as well as chronic settings, decreased FD seems to be associated with stroke. Differences in FD between subtypes of ischemic stroke remain unclear.

Conclusions: This review provides a summary of the scientific literature regarding the association between retinal FD and neurodegenerative disease and stroke. Central pathology is associated with a decreased FD, as a measure of microvascular network complexity. As retinal FD reflects the global integrity of the cerebral microvasculature, it is an attractive parameter to explore. Despite obvious concerns, mainly due to a lack of methodological standardization, retinal FD remains a promising non-invasive and low-cost diagnostic biomarker for neurodegenerative and cerebrovascular disease. Before FD can be implemented in clinic as a diagnostic biomarker, the research community should strive for uniformization and standardization.

Quantitative measurements of enlarged perivascular spaces in the brain are associated with retinal microvascular parameters in older community-dwelling subjects

BACKGROUND

Perivascular Spaces (PVS) become increasingly visible with advancing age on brain MRI, yet their relationship to morphological changes in the underlying microvessels remains poorly understood. Retinal and cerebral microvessels share morphological and physiological properties. We compared computationally-derived PVS morphologies with retinal vessel morphologies in older people.

METHODS

We analysed data from community-dwelling individuals who underwent multimodal brain MRI and retinal fundus camera imaging at mean age 72.55 years (SD=0.71). We assessed centrum semiovale PVS computationally to determine PVS total volume and count, and mean per-subject individual PVS length, width and size. We analysed retinal images using the VAMPIRE software suite, obtaining the Central Retinal Artery and Vein Equivalents (CRVE and CRAE), Arteriole-to-Venule ratio (AVR), and fractal dimension (FD) of both eyes. We investigated associations using general linear models, adjusted for age, gender, and major vascular risk factors.

RESULTS

In 381 subjects with all measures, increasing total PVS volume and count were associated with decreased CRAE in the left eye (volume β=-0.170, count β=-0.184, p<0.001). No associations of PVS with CRVE were found. The PVS total volume, individual width and size increased with decreasing FD of the arterioles (a) and venules (v) of the left eye (total volume: FDa β=-0.137, FDv β=-0.139, p<0.01; width: FDa β=-0.144, FDv β=-0.158, p<0.01; size: FDa β=-0.157, FDv β=-0.162, p<0.01).

CONCLUSIONS

Increase in PVS number and size visible on MRI reflect arteriolar narrowing and lower retinal arteriole and venule branching complexity, both markers of impaired microvascular health. Computationally-derived PVS metrics may be an early indicator of failing vascular health and should be tested in longitudinal studies.

Automated Deformation-Based Analysis of 3D Optical Coherence Tomography in Diabetic Retinopathy

Diabetic retinopathy (DR) is a significant microvascular complication of diabetes mellitus and a leading cause of vision impairment in working age adults. Optical coherence tomography (OCT) is a routinely used clinical tool to observe retinal structural and thickness alterations in DR. Pathological changes that alter the normal anatomy of the retina, such as intraretinal edema, pose great challenges for conventional layer-based analysis of OCT images. We present an alternative approach for the automated analysis of OCT volumes in DR research based on nonlinear registration. In this paper, we first obtain an anatomically consistent volume of interest (VOI) in different OCT images via carefully designed masking and affine registration. After that, efficient B-spline transformations are computed using stochastic gradient descent optimization. Using the OCT volumes of normal controls, for which layer-based segmentation works well, we demonstrate the accuracy of our registration-based analysis in aligning layer boundaries. By nonlinearly registering the OCT volumes of DR subjects to an atlas constructed from normal controls and measuring the Jacobian determinant of the deformation, we can simultaneously visualize tissue contraction and expansion due to DR pathology. Tensor-based morphometry (TBM) can also be performed for quantitative analysis of local structural changes. In our experimental results, we apply our method to a dataset of 105 subjects and demonstrate that volumetric OCT registration and TBM analysis can successfully detect local retinal structural alterations due to DR.

The Application of Optical Coherence Tomography Angiography in Cerebral Small Vessel Disease, Ischemic Stroke, and Dementia: A Systematic Review

Objective: To investigate the application of optical coherence tomography angiography (OCTA) in cerebral small vessel disease (SVD), ischemic stroke and dementia. Methods: We conducted a systematic search in MEDLINE (from inception) and EMBASE (from 1980) to end 2019 for human studies that measured retinal parameters in cerebral SVD, ischemic stroke, and dementia using OCTA. Results: Fourteen articles (n = 989) provided relevant data. Ten studies included patients with Alzheimer disease (AD) and mild cognitive impairment (n = 679), two investigated pre-symptomatic AD participants (n = 154), and two investigated monogenic SVD patients with cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (n = 32) and Fabry disease (n = 124). Methods to reduce bias and risk factor adjustment were poorly reported. Substantial methodological variations between studies precluded a formal meta-analysis. Quantitative measurements revealed significant yet inconclusive changes in foveal avascular zone, perfusion density, and vessel density (VD) in AD, presymptomatic AD, and SVD patients. Two (n = 160) of three studies (n = 192) showed association between decreased VD and increased white matter hyperintensities. In three (n = 297) of seven studies (n = 563), better cognitive function was associated with increased VD. One study (n = 52) suggested increased VD was associated with increased ganglion cell-inner plexiform layer thickness in AD yet with no covariate adjustment. Conclusions: Changes in retinal microvasculature identified using OCTA are associated with monogenic SVD and different stages of AD, but data are limited and partly confounded by methodological differences. Larger studies with risk factors adjustment and more consistent OCTA methods are needed to fully exploit this technology. PROSPERO registration number: CRD42020166929.

Retinal Microperimetry: A Useful Tool for Detecting Insulin Resistance-Related Cognitive Impairment in Morbid Obesity

Background: There is clear association between type 2 diabetes (T2D) and cognitive decline. Retinal microperimetry is a useful tool for detecting cognitive impairment in T2D. Morbid obesity (MO) has been associated with cognitive impairment. Insulin resistance (IR) seems a major determinant, but the data are unclear. The aim of this study was to evaluate the cognitive impairment in MO as well as the utility of retinal microperimetry in identifying these alterations. Methods: In total, 50 consecutive patients with MO were matched by age and gender with 30 healthy controls. All patients underwent cognitive evaluation (Montreal Cognitive Assessment Test-MoCA) and retinal microperimetry, using MAIA microperimeter 3rd generation. Retinal sensitivity and gaze fixation parameters were used for the evaluation of the analysis. Results: MO patients showed a significantly lower neurocognitive performance than the controls: MoCA score 24.94 ± 2.74 vs. 28.95 ± 1.05, p < 0.001. Cognitive function inversely correlated with the HOMA-IR (r = −0.402, p = 0.007). The AUROC for cognitive impairment using microperimetry was 0.807, CI 95% (0.592–0.947), p = 0.017. Conclusions: (1) Systemic insulin resistance is a major underlying mechanism accounting for the higher prevalence of cognitive impairment detected in young MO subjects. (2) Retinal microperimetry is a useful tool for identifying MO patients with cognitive impairment.

Measuring the Interaction Between the Macro- and Micro-Vasculature

Structural and functional dysfunction in both the macro- and microvasculature are a feature of essential hypertension. In a healthy cardiovascular system, the elastic properties of the large arteries ensure that pulsations in pressure and flow generated by cyclic left ventricular contraction are dampened, so that less pulsatile pressure and flow are delivered at the microvascular level. However, in response to aging, hypertension, and other disease states, arterial stiffening limits the buffering capacity of the elastic arteries, thus exposing the microvasculature to increased pulsatile stress. This is thought to be particularly pertinent to high flow/low resistance organs such as the brain and kidney, which may be sensitive to excess pressure and flow pulsatility, damaging capillary networks, and resulting in target organ damage. In this review, we describe the clinical relevance of the pulsatile interaction between the macro- and microvasculature and summarize current methods for measuring the transmission of pulsatility between the two sites.

Non-invasive in vivo hyperspectral imaging of the retina for potential biomarker use in Alzheimer's disease

Studies of rodent models of Alzheimer’s disease (AD) and of human tissues suggest that the retinal changes that occur in AD, including the accumulation of amyloid beta (Aβ), may serve as surrogate markers of brain Aβ levels. As Aβ has a wavelength-dependent effect on light scatter, we investigate the potential for in vivo retinal hyperspectral imaging to serve as a biomarker of brain Aβ. Significant differences in the retinal reflectance spectra are found between individuals with high Aβ burden on brain PET imaging and mild cognitive impairment (n = 15), and age-matched PET-negative controls (n = 20). Retinal imaging scores are correlated with brain Aβ loads. The findings are validated in an independent cohort, using a second hyperspectral camera. A similar spectral difference is found between control and 5xFAD transgenic mice that accumulate Aβ in the brain and retina. These findings indicate that retinal hyperspectral imaging may predict brain Aβ load.

The use of PET for detection of Aβ in the brain in AD has limitations; studies also indicate that retinal changes, including Aβ deposition, occur in AD. Here the authors demonstrate the potential to use in vivo retinal hyperspectral imaging as a surrogate for brain accumulation of Aβ.

Adult Goat Retinal Neuronal Culture: Applications in Modeling Hyperglycemia

Culture of adult neurons of the central nervous system (CNS) can provide a unique model system to explore neurodegenerative diseases. The CNS includes neurons and glia of the brain, spinal cord and retina. Neurons in the retina have the advantage of being the most accessible cells of the CNS, and can serve as a reliable mirror to the brain. Typically, primary cultures utilize fetal rodent neurons, but very rarely adult neurons from larger mammals. Here, we cultured primary retinal neurons isolated from adult goat up to 10 days, and established an in vitro model of hyperglycemia for performing morphological and molecular characterization studies. Immunofluorescence staining revealed that approximately 30–40% of cultured cells expressed neuronal markers. Next, we examined the relative expression of cell adhesion molecules (CAMs) in adult goat brain and retina. We also studied the effect of different glucose concentrations and media composition on the growth and expression of CAMs in cultured retinal neurons. Hyperglycemia significantly enhances neurite outgrowth in adult retinal neurons in culture. Expression of CAMs such as Caspr1, Contactin1 and Prion is downregulated in the presence of high glucose. Hyperglycemia downregulates the expression of the transcription factor CCAAT/enhancer binding protein (C/EBP α), predicted to bind CAM gene promoters. Collectively, our study demonstrates that metabolic environment markedly affects transcriptional regulation of CAMs in adult retinal neurons in culture. The effect of hyperglycemia on CAM interactions, as well as related changes in intracellular signaling pathways in adult retinal neurons warrants further investigation.

Retinal Microvasculature in Relation to Central Hemodynamics in a Flemish Population

Arterial stiffness and wave reflection predict cardiovascular mortality and morbidity and are associated with renal microvascular disease. We hypothesized that the retinal microvascular traits might be associated with central hemodynamic properties. In 735 randomly recruited Flemish (mean age, 50.3 years; 47.1% women), we derived central pulse pressure and carotid-femoral pulse wave velocity by applanation tonometry and calculated forward (Pf) and backward (Pb) pulse waves, using an automated pressure-based wave separation algorithm. We measured central retinal arteriolar (CRAE) and venular equivalent and their ratio, using IVAN software (Vasculomatic ala Nicola, version 1.1). Mean values for pulse wave velocity (n=554), Pf and Pb were 7.50 m/s, 32.0 mm Hg, and 21.5 mm Hg, respectively. In multivariable-adjusted analyses, CRAE was 4.62 µm and 1.26 µm smaller (P≤0.034) for a 1-SD increment in central mean arterial pressure (+11.3 mm Hg) and central pulse pressure (+15.2 mm Hg); a 1-SD increment in the augmentation ratio (+7.0%), aortic pulse wave velocity (+1.66 m/s), Pf (+10.0 mm Hg), and Pb (+8.5 mm Hg), was associated with smaller CRAE; the association sizes were -1.91 µm, -1.59 µm, -1.45 µm, and -2.38 µm (P≤0.014), respectively. Associations of arteriole-to-venule diameter ratio with the central hemodynamic traits mirrored those of CRAE. None of the multivariable-adjusted associations of central retinal venular diameter with the central hemodynamic traits reached significance with the exception of central diastolic blood pressure (-1.62 µm; P=0.030). In conclusion, in the general population, higher central pulse pressure, pulse wave velocity, Pf, and Pb were associated with smaller CRAE.

Examining the ocular fundus in neurology

PURPOSE OF REVIEW

The funduscopic examination can be a technically difficult, and often omitted, portion of the neurologic examination, despite its great potential to influence patient care.

RECENT FINDINGS

Medical practitioners are often first taught to examine the ocular fundus using a direct ophthalmoscope, however, this skill requires frequent practice. Nonmydriatic tabletop and portable fundus photography and even smartphone-based photography offer alternative and practical means for approaching examination of the ocular fundus. These alternative tools have been shown to be practical in a variety of settings including ambulatory clinics and emergency departments. Decreased retinal microvascular density detected with fundus photography has been linked to accelerated rates of cognitive decline. Research has also found optic disc pallor and retinopathy detected via fundus photography to be more prevalent in patients with recent stroke or transient ischemic attack.

SUMMARY

Alternative methods of funduscopic examination based on fundus photography have the potential to improve the ease of use, portability, and availability of funduscopy. Recognition of changes in retinal microvasculature has the potential to noninvasively identify patients at the highest risk for cognitive impairment and cerebrovascular disease. However, further research is needed to determine the specific utility of measurements of retinal microvascular changes in clinical care. Innovative funduscopy techniques offer neurologists new approaches to this essential facet of the neurological examination.

Retinal texture biomarkers may help to discriminate between Alzheimer's, Parkinson's, and healthy controls

A top priority in biomarker development for Alzheimer's disease (AD) and Parkinson's disease (PD) is the focus on early diagnosis, where the use of the retina is a promising avenue of research. We computed fundus images from optical coherence tomography (OCT) data and analysed the structural arrangement of the retinal tissue using texture metrics. We built clinical class classification models to distinguish between healthy controls (HC), AD, and PD, using machine learning (support vector machines). Median sensitivity is 88.7%, 79.5% and 77.8%, for HC, AD, and PD eyes, respectively. When the same subject has the same classification for both eyes, 94.4% (median) of the classifications are correct. A significant amount of information discriminating between multiple neurodegenerative states is conveyed by OCT imaging of the human retina, even when differences in thickness are not yet present. This technique may allow for simultaneously diagnosing Alzheimer's and Parkinson's diseases.

Spectral-Domain OCT Measurements in Alzheimer's Disease: A Systematic Review and Meta-analysis

TOPIC

OCT is a noninvasive tool to measure specific retinal layers in the eye. The relationship of retinal spectral-domain (SD) OCT measurements with Alzheimer's disease (AD) and mild cognitive impairment (MCI) remains unclear. Hence, we conducted a systematic review and meta-analysis to examine the SD OCT measurements in AD and MCI.

CLINICAL RELEVANCE

Current methods of diagnosing early AD are expensive and invasive. Retinal measurements of SD OCT, which are noninvasive, technically simple, and inexpensive, are potential biomarkers of AD.

METHODS

We conducted a literature search in PubMed and Excerpta Medica Database to identify studies published before December 31, 2017, that assessed the associations between AD, MCI, and measurements of SD OCT: ganglion cell-inner plexiform layer (GC-IPL), ganglion cell complex (GCC), macular volume, and choroidal thickness, in addition to retinal nerve fiber layer (RNFL) and macular thickness. We used a random-effects model to examine these relationships. We also conducted meta-regression and assessed heterogeneity, publication bias, and study quality.

RESULTS

We identified 30 eligible studies, involving 1257 AD patients, 305 MCI patients, and 1460 controls, all of which were cross-sectional studies. In terms of the macular structure, AD patients showed significant differences in GC-IPL thickness (standardized mean difference [SMD], -0.46; 95% confidence interval [CI], -0.80 to -0.11; I2 = 71%), GCC thickness (SMD, -0.84; 95% CI, -1.10 to -0.57; I2 = 0%), macular volume (SMD, -0.58; 95% CI, -1.03 to -0.14; I2 = 80%), and macular thickness of all inner and outer sectors (SMD range, -0.52 to -0.74; all P < 0.001) when compared with controls. Peripapillary RNFL thickness (SMD, -0.67; 95% CI, -0.95 to -0.38; I2 = 89%) and choroidal thickness (SMD range, -0.88 to -1.03; all P < 0.001) also were thinner in AD patients.

CONCLUSIONS

Our results confirmed the associations between retinal measurements of SD OCT and AD, highlighting the potential usefulness of SD OCT measurements as biomarkers of AD.

Ocular Biomarkers of Alzheimer's Disease: The Role of Anterior Eye and Potential Future Directions

Globally, Alzheimer's disease (AD) is a growing health and economic challenge that has no effective cure. Recent clinical trials indicate that preclinical treatment may be required but a routine screening tool for AD has been elusive. Hence, a simple, yet sensitive biomarker for preclinical AD, when the disease is most likely to be amenable to treatment, is lacking. Due to several features, the eye has been explored for this purpose and, among the ocular tissues, the retina has received the most attention. Currently, major works investigating the potential AD diagnosis by detecting amyloid-β (Aβ) signatures in the retinal tissue are underway, while the anterior eye is more accessible for in vivo imaging and examination. This report provides a concise review of current literature on the anterior eye components, including the crystalline lens, cornea, and aqueous humor, in AD. We also discuss the potential for assessment of the corneal nerve structure and regeneration as well as conjunctival tissue for AD-related alterations. The crystalline lens has received considerable attention, but further research is required to confirm whether Aβ accumulates in the lens and whether it mirrors brain neuropathologic changes, particularly in preclinical AD. The rich corneal neural network and conjunctival vasculature also merit exploration in future studies to shed light on their potential association with AD pathologic changes.

Assessment of Differences in Retinal Microvasculature Using OCT Angiography in Alzheimer's Disease: A Twin Discordance Report

The authors report the optical coherence tomography angiography (OCTA)-based comparative assessment of the retinal microvasculature in a rare pair of 96-year-old female monozygotic twins discordant for Alzheimer's disease (AD). Using automated mapping of the superficial capillary plexus, the authors observed that the twin with advanced AD had a significantly reduced vessel density and a larger foveal avascular zone in the superficial capillary plexus as well as a thinner choroid compared to the twin who was cognitively normal. This unique twin discordance report adds to the evidence supporting the use of retinal microvasculature changes in the superficial capillary plexus on OCTA as a possible noninvasive biomarker for AD. [Ophthalmic Surg Lasers Imaging Retina. 2018;49:440-444.].

Retinal signs and risk of incident dementia in the Atherosclerosis Risk in Communities study

INTRODUCTION

The easily-imaged retinal microvasculature may reflect the brain microvasculature and therefore be related to dementia.

METHODS

In a population-based study of 12,482 adults aged 50-73 years (22% African American), we estimated the relationship of retinal characteristics from fundus photography (1993-1995) with incident all-cause dementia (1993-1995 to 2011-2013) and with etiologic subtype of dementia/mild cognitive impairment (2011-13).

RESULTS

A total of 1259 (10%) participants developed dementia over a mean 15.6 years. Moderate/severe (vs. no) retinopathy (hazard ratio [HR], 1.86; 95% confidence interval [CI]: 1.36-2.55) and central retinal arteriolar equivalent (narrowest quartile vs. widest three quartiles; HR, 1.26; 95% CI: 1.09-1.45) were associated with all-cause dementia. Results were qualitatively stronger (but not statistically significantly different) in participants with diabetes. Retinopathy was associated with a joint outcome of cerebrovascular-related, but not Alzheimer's disease-related, dementia/mild cognitive impairment (HR, 2.29; 95% CI: 1.24-4.23).

DISCUSSION

Exploration of measures in the eye may provide surrogate indices of microvascular lesions relevant to dementia.

Retinal α-synuclein deposits in Parkinson's disease patients and animal models

Despite decades of research, accurate diagnosis of Parkinson's disease remains a challenge, and disease-modifying treatments are still lacking. Research into the early (presymptomatic) stages of Parkinson's disease and the discovery of novel biomarkers is of utmost importance to reduce this burden and to come to a more accurate diagnosis at the very onset of the disease. Many have speculated that non-motor symptoms could provide a breakthrough in the quest for early biomarkers of Parkinson's disease, including the visual disturbances and retinal abnormalities that are seen in the majority of Parkinson's disease patients. An expanding number of clinical studies have investigated the use of in vivo assessments of retinal structure, electrophysiological function, and vision-driven tasks as novel means for identifying patients at risk that need further neurological examination and for longitudinal follow-up of disease progression in Parkinson's disease patients. Often, the results of these studies have been interpreted in relation to α-synuclein deposits and dopamine deficiency in the retina, mirroring the defining pathological features of Parkinson's disease in the brain. To better understand the visual defects seen in Parkinson's disease patients and to propel the use of retinal changes as biomarkers for Parkinson's disease, however, more conclusive neuropathological evidence for the presence of retinal α-synuclein aggregates, and its relation to the cerebral α-synuclein burden, is urgently needed. This review provides a comprehensive and critical overview of the research conducted to unveil α-synuclein aggregates in the retina of Parkinson's disease patients and animal models, and thereby aims to aid the ongoing discussion about the potential use of the retinal changes and/or visual symptoms as biomarkers for Parkinson's disease.

Mesenchymal stem cell-derived extracellular vesicles and retinal ischemia-reperfusion

Retinal ischemia is a major cause of vision loss and impairment and a common underlying mechanism associated with diseases such as glaucoma, diabetic retinopathy, and central retinal artery occlusion. The regenerative capacity of the diseased human retina is limited. Our previous studies have shown the neuroprotective effects of intravitreal injection of mesenchymal stem cells (MSC) and MSC-conditioned medium in retinal ischemia in rats. Based upon the hypothesis that the neuroprotective effects of MSCs and conditioned medium are largely mediated by extracellular vesicles (EVs), MSC derived EVs were tested in an in-vitro oxygen-glucose deprivation (OGD) model of retinal ischemia. Treatment of R28 retinal cells with MSC-derived EVs significantly reduced cell death and attenuated loss of cell proliferation. Mechanistic studies on the mode of EV endocytosis by retinal cells were performed in vitro. EV endocytosis was dose- and temperature-dependent, saturable, and occurred via cell surface heparin sulfate proteoglycans mediated by the caveolar endocytic pathway. The administration of MSC-EVs into the vitreous humor 24 h after retinal ischemia in a rat model significantly enhanced functional recovery, and decreased neuro-inflammation and apoptosis. EVs were taken up by retinal neurons, retinal ganglion cells, and microglia. They were present in the vitreous humor for four weeks after intravitreal administration, with saturable binding to vitreous humor components. Overall, this study highlights the potential of MSC-EV as biomaterials for neuroprotective and regenerative therapy in retinal disorders.

Cannabis use and human retina: The path for the study of brain synaptic transmission dysfunctions in cannabis users

Owing to the difficulty of obtaining direct access to the functioning brain, new approaches are needed for the indirect exploration of brain disorders in neuroscience research. Due to its embryonic origin, the retina is part of the central nervous system and is well suited to the investigation of neurological functions in psychiatric and addictive disorders. In this review, we focus on cannabis use, which is a crucial public health challenge, since cannabis is one of the most widely used addictive drugs in industrialized countries. We first explain why studying retinal function is relevant when exploring the effects of cannabis use on brain function. Next, we describe both the retinal electrophysiological measurements and retinal dysfunctions observed after acute and regular cannabis use. We then discuss how these retinal dysfunctions may inform brain synaptic transmission abnormalities. Finally, we present various directions for future research on the neurotoxic effects of cannabis use.

Usefulness of Eye Fixation Assessment for Identifying Type 2 Diabetic Subjects at Risk of Dementia

Type 2 diabetic (T2D) subjects have a significantly higher risk of developing mild cognitive impairment (MCI) and dementia than age-matched non-diabetic individuals. However, the accurate evaluation of cognitive status is based on complex neuropsychological tests, which makes their incorporation into the current standard of care for the T2D population infeasible. Given that the ability to maintain visual gaze on a single location (fixation) is hampered in Alzheimer’s disease (AD), the aim of the present study was: (1) To assess whether the evaluation of gaze fixation during fundus-driven microperimetry correlated with cognitive status in T2D subjects; (2) to examine whether the addition of fixational parameters to the assessment of retinal sensitivity increased the predictive value of retinal microperimetry in identifying T2D subjects with MCI. For this purpose, fixation parameters and retinal sensitivity were compared in three age-matched groups of T2D subjects: normocognitive (n = 34), MCI (n = 33), and AD (n = 33). Our results showed that fixation is significantly more unstable in MCI subjects than normocognitive subjects, and even more altered in those affected by AD (ANOVA; p < 0.01). Moreover, adding fixation parameters to retinal sensitivity significantly increases the predictive value in identifying those subjects with MCI: ROC (Receiver Operating Characteristic) Area 0.68 with retinal sensitivity alone vs. ROC Area 0.86 when parameters of fixation are added to retinal sensitivity (p < 0.01). In conclusion, our results suggest that fixational eye movement parameters assessed by fundus-microperimetry represent a new tool for identifying T2D subjects at risk of dementia.

Automated Method for Retinal Artery/Vein Separation via Graph Search Metaheuristic Approach

Separation of the vascular tree into arteries and veins is a fundamental prerequisite in the automatic diagnosis of retinal biomarkers associated with systemic and neurodegenerative diseases. In this paper, we present a novel graph search metaheuristic approach for automatic separation of arteries/veins (A/V) from color fundus images. Our method exploits local information to disentangle the complex vascular tree into multiple subtrees, and global information to label these vessel subtrees into arteries and veins. Given a binary vessel map, a graph representation of the vascular network is constructed representing the topological and spatial connectivity of the vascular structures. Based on the anatomical uniqueness at vessel crossing and branching points, the vascular tree is split into multiple subtrees containing arteries and veins. Finally, the identified vessel subtrees are labeled with A/V based on a set of handcrafted features trained with random forest classifier. The proposed method has been tested on four different publicly available retinal datasets with an average accuracy of 94.7%, 93.2%, 96.8% and 90.2% across AV-DRIVE, CT-DRIVE. INSPIRE-AVR and WIDE datasets, respectively. These results demonstrate the superiority of our proposed approach in outperforming state-ofthe- art methods for A/V separation.

Adaptive optics imaging of the human retina

Adaptive Optics (AO) retinal imaging has provided revolutionary tools to scientists and clinicians for studying retinal structure and function in the living eye. From animal models to clinical patients, AO imaging is changing the way scientists are approaching the study of the retina. By providing cellular and subcellular details without the need for histology, it is now possible to perform large scale studies as well as to understand how an individual retina changes over time. Because AO retinal imaging is non-invasive and when performed with near-IR wavelengths both safe and easily tolerated by patients, it holds promise for being incorporated into clinical trials providing cell specific approaches to monitoring diseases and therapeutic interventions. AO is being used to enhance the ability of OCT, fluorescence imaging, and reflectance imaging. By incorporating imaging that is sensitive to differences in the scattering properties of retinal tissue, it is especially sensitive to disease, which can drastically impact retinal tissue properties. This review examines human AO retinal imaging with a concentration on the use of the Adaptive Optics Scanning Laser Ophthalmoscope (AOSLO). It first covers the background and the overall approaches to human AO retinal imaging, and the technology involved, and then concentrates on using AO retinal imaging to study the structure and function of the retina.

Measurement of Retinal Vessels as a Biomarker of Cerebrovascular Aging in Older HIV-Positive Men Compared With Controls

BACKGROUND

To compare retinal vascular measurements, biomarkers of cerebral small vessel disease, in HIV-positive men aged 50 years and older with similarly aged HIV-negative men and younger HIV-positive men.

METHODS

We recruited white, nondiabetic men into a cross-sectional substudy of a larger cohort including 3 demographically matched groups. Optic disc-centered 45-degree color fundus photographs were used to calculate central retinal arterial and venous caliber and the arterial-venous ratio (AVR). We used univariate and multivariable linear regression to compare retinal vessel measurements in the 3 groups and to identify factors associated with AVR.

RESULTS

All HIV-positive men were virologically suppressed. In a multivariable model, study group was not associated with AVR [adjusted β 0.010 for HIV-positive men <50 (n = 39) compared with HIV-positive men aged ≥50 years (n = 120), 95% confidence interval [CI] -0.018 to 0.038, P = 0.47; adjusted β 0.00002 for HIV-negative men ≥50 years (n = 52), 95% CI -0.022 to 0.022, P = 0.99]. Factors associated with lower AVR were systolic blood pressure (adjusted β -0.009 per +10 mm Hg, 95% CI -0.015 to -0.003, P = 0.002), history of stroke or transient ischemic attack (adjusted β -0.070, 95% CI -0.12 to -0.015, P = 0.01), and recent recreational drug use (adjusted β -0.037, 95% CI -0.057 to -0.018, P = 0.0002).

CONCLUSIONS

There were no differences in retinal vascular indices between HIV-positive men aged ≥50 years and HIV-negative men aged ≥50 years or HIV-positive men aged <50 years, suggesting that HIV is not associated with an increased burden of cerebral small vessel disease.

Active contour method for ILM segmentation in ONH volume scans in retinal OCT

The optic nerve head (ONH) is affected by many neurodegenerative and autoimmune inflammatory conditions. Optical coherence tomography can acquire high-resolution 3D ONH scans. However, the ONH's complex anatomy and pathology make image segmentation challenging. This paper proposes a robust approach to segment the inner limiting membrane (ILM) in ONH volume scans based on an active contour method of Chan-Vese type, which can work in challenging topological structures. A local intensity fitting energy is added in order to handle very inhomogeneous image intensities. A suitable boundary potential is introduced to avoid structures belonging to outer retinal layers being detected as part of the segmentation. The average intensities in the inner and outer region are then rescaled locally to account for different brightness values occurring among the ONH center. The appropriate values for the parameters used in the complex computational model are found using an optimization based on the differential evolution algorithm. The evaluation of results showed that the proposed framework significantly improved segmentation results compared to the commercial solution.

Functional optical coherence tomography of neurovascular coupling interactions in the retina

Quantitative evaluation of retinal neurovascular coupling is essential for a better understanding of visual function and early detection of eye diseases. However, there is no established method to monitor coherent interactions between stimulus-evoked neural activity and hemodynamic responses at high resolution. Here, we report a multimodal functional optical coherence tomography (OCT) imaging methodology to enable concurrent intrinsic optical signal (IOS) imaging of stimulus-evoked neural activity and hemodynamic responses at capillary resolution. OCT angiography guided IOS analysis was used to separate neural-IOS and hemodynamic-IOS changes in the same retinal image sequence. Frequency flicker stimuli evoked neural-IOS changes in the outer retina; that is, photoreceptor layer, first and then in the inner retina, including outer plexus layer (OPL), inner plexiform layer (IPL), and ganglion cell layer (GCL), which were followed by hemodynamic-IOS changes primarily in the inner retina; that is, OPL, IPL, and GCL. Different time courses and signal magnitudes of hemodynamic-IOS responses were observed in blood vessels with various diameters.

Retinal image analytics detects white matter hyperintensities in healthy adults

Objective

We investigated whether an automatic retinal image analysis (ARIA) incorporating machine learning approach can identify asymptomatic older adults harboring high burden of white matter hyperintensities (WMH) using MRI as gold standard.

Methods

In this cross-sectional study, we evaluated 180 community-dwelling, stroke-, and dementia-free healthy subjects and performed ARIA by acquiring a nonmydriatic retinal fundus image. The primary outcome was the diagnostic performance of ARIA in detecting significant WMH on MRI brain, defined as age-related white matter changes (ARWMC) grade ≥2. We analyzed both clinical variables and retinal characteristics using logistic regression analysis. We developed a machine learning network model with ARIA to estimate WMH and its classification.

Results

All 180 subjects completed MRI and ARIA. The mean age was 70.3 ± 4.5 years, 70 (39%) were male. Risk factor profiles were: 106 (59%) hypertension, 31 (17%) diabetes, and 47 (26%) hyperlipidemia. Severe WMH (global ARWMC grade ≥2) was found in 56 (31%) subjects. The performance for detecting severe WMH with sensitivity (SN) 0.929 (95% CI from 0.819 to 0.977) and specificity (SP) 0.984 (95% CI from 0.937 to 0.997) was excellent. There was a good correlation between WMH volume (log-transformed) obtained from MRI versus those estimated from retinal images using ARIA with a correlation coefficient of 0.897 (95% CI from 0.864 to 0.922).

Interpretation

We developed a robust algorithm to automatically evaluate retinal fundus image that can identify subjects with high WMH burden. Further community-based prospective studies should be performed for early screening of population at risk of cerebral small vessel disease.