Melanopsin-mediated pupil function is impaired in Parkinson's disease

Digital biomarkers for precision diagnosis and monitoring in Parkinson's disease

Parkinson's disease (PD) is a multifactorial neurodegenerative disorder with high prevalence among the elderly, primarily manifested by progressive decline in motor function. The aging global demographic and increased life expectancy have led to a rapid surge in PD cases, imposing a significant societal burden. PD along with other neurodegenerative diseases has garnered increasing attention from the scientific community. In PD, motor symptoms are recognized when approximately 60% of dopaminergic neurons have been damaged. The irreversible feature of PD and benefits of early intervention underscore the importance of disease onset prediction and prompt diagnosis. The advent of digital health technology in recent years has elevated the role of digital biomarkers in precisely and sensitively detecting early PD clinical symptoms, evaluating treatment effectiveness, and guiding clinical medication, focusing especially on motor function, responsiveness and sleep quality assessments. This review examines prevalent digital biomarkers for PD and highlights the latest advancements.

Melanopsin retinal ganglion cell function in Alzheimer's vs. Parkinson's disease an exploratory meta-analysis and review of pupillometry protocols

BACKGROUND

Neurodegenerative diseases share retinal abnormalities. Chromatic pupillometry allows in vivo assessment of photoreceptor functional integrity, including melanopsin-expressing retinal ganglion cells. This exploratory meta-analysis assesses retinal photoreceptor functionality in Alzheimer's vs. Parkinson's disease and conducts an in-depth review of applied pupillometric protocols.

METHODS

Literature reviews on PubMed and Scopus from 1991 to August 2023 identified chromatic pupillometry studies on Alzheimer's disease (AD; n = 42 patients from 2 studies) and Parkinson's disease (PD; n = 66 from 3 studies). Additionally, a pre-AD study (n = 10) and an isolated REM Sleep Behavior Disorder study (iRBD; n = 10) were found, but their results were not included in the meta-analysis statistics.

RESULTS

Melanopsin-mediated post-illumination pupil response to blue light was not significantly impaired in Alzheimer's (weighted mean difference = -1.54, 95% CI: 4.57 to 1.49, z = -1.00, p = 0.319) but was in Parkinson's (weighted mean difference = -9.14, 95% CI: 14.19 to -4.08, z = -3.54, p < 0.001). Other pupil light reflex metrics showed no significant differences compared to controls. Studies adhered to international standards of pupillometry with moderate to low bias. All studies used full-field stimulation. Alzheimer's studies used direct while Parkinson's studies used consensual measurement. Notably, studies did not control for circadian timing and Parkinson's patients were on dopaminergic treatment.

CONCLUSION AND RELEVANCE

Results affirm chromatic pupillometry as a useful method to assess melanopsin-related retinal cell dysfunction in Parkinson's but not in Alzheimer's disease. While adhering to international standards, future studies may analyze the effects of local field stimulation, dopaminergic treatment, and longitudinal design to elucidate melanopsin dysfunction in Parkinson's disease.

Method to Quickly Map Multifocal Pupillary Response Fields (mPRF) Using Frequency Tagging

We present a method for mapping multifocal Pupillary Response Fields in a short amount of time using a visual stimulus covering 40° of the visual angle divided into nine contiguous sectors simultaneously modulated in luminance at specific, incommensurate, temporal frequencies. We test this multifocal Pupillary Frequency Tagging (mPFT) approach with young healthy participants (N = 36) and show that the spectral power of the sustained pupillary response elicited by 45 s of fixation of this multipartite stimulus reflects the relative contribution of each sector/frequency to the overall pupillary response. We further analyze the phase lag for each temporal frequency as well as several global features related to pupil state. Test/retest performed on a subset of participants indicates good repeatability. We also investigate the existence of structural (RNFL)/functional (mPFT) relationships. We then summarize the results of clinical studies conducted with mPFT on patients with neuropathies and retinopathies and show that the features derived from pupillary signal analyses, the distribution of spectral power in particular, are homologous to disease characteristics and allow for sorting patients from healthy participants with excellent sensitivity and specificity. This method thus appears as a convenient, objective, and fast tool for assessing the integrity of retino-pupillary circuits as well as idiosyncrasies and permits to objectively assess and follow-up retinopathies or neuropathies in a short amount of time.

From pre-processing to advanced dynamic modeling of pupil data

The pupil of the eye provides a rich source of information for cognitive scientists, as it can index a variety of bodily states (e.g., arousal, fatigue) and cognitive processes (e.g., attention, decision-making). As pupillometry becomes a more accessible and popular methodology, researchers have proposed a variety of techniques for analyzing pupil data. Here, we focus on time series-based, signal-to-signal approaches that enable one to relate dynamic changes in pupil size over time with dynamic changes in a stimulus time series, continuous behavioral outcome measures, or other participants' pupil traces. We first introduce pupillometry, its neural underpinnings, and the relation between pupil measurements and other oculomotor behaviors (e.g., blinks, saccades), to stress the importance of understanding what is being measured and what can be inferred from changes in pupillary activity. Next, we discuss possible pre-processing steps, and the contexts in which they may be necessary. Finally, we turn to signal-to-signal analytic techniques, including regression-based approaches, dynamic time-warping, phase clustering, detrended fluctuation analysis, and recurrence quantification analysis. Assumptions of these techniques, and examples of the scientific questions each can address, are outlined, with references to key papers and software packages. Additionally, we provide a detailed code tutorial that steps through the key examples and figures in this paper. Ultimately, we contend that the insights gained from pupillometry are constrained by the analysis techniques used, and that signal-to-signal approaches offer a means to generate novel scientific insights by taking into account understudied spectro-temporal relationships between the pupil signal and other signals of interest.

Targeting sleep and the circadian system as a novel treatment strategy for Parkinson's disease

There is a growing appreciation of the wide range of sleep-wake disturbances that occur frequently in Parkinson's disease. These are known to be associated with a range of motor and non-motor symptoms and significantly impact not only on the quality of life of the patient, but also on their bed partner. The underlying causes for fragmented sleep and daytime somnolence are no doubt multifactorial but there is clear evidence for circadian disruption in Parkinson's disease. This appears to be occurring not only as a result of the neuropathological changes that occur across a distributed neural network, but even down to the cellular level. Such observations indicate that circadian changes may in fact be a driver of neurodegeneration, as well as a cause for some of the sleep-wake symptoms observed in Parkinson's disease. Thus, efforts are now required to evaluate approaches including the prescription of precision medicine to modulate photoreceptor activation ratios that reflect daylight inputs to the circadian pacemaker, the use of small molecules to target clock genes, the manipulation of orexin pathways that could help restore the circadian system, to offer novel symptomatic and novel disease modifying strategies.

Fine-tuning the circadian system with light treatment for Parkinson's disease: an in-depth, critical review

Late in the twentieth century, interest intensified regarding the involvement of the circadian system in the aetiology and treatment of Parkinson's disease (PD). It has been envisaged that this approach might provide relief beyond the limited benefits and severe side effects achieved by dopamine (DA) replacement. In the first clinical article, published in 1996, polychromatic light was used to shift the circadian clock as it is considered to be the most powerful zeitgeber (time keeper) that can be implemented to realign circadian phase. Since that time, 11 additional articles have implemented light treatment (LT) in various forms as an adjuvant to DA replacement. In spite of the growing interest in this area, the systematic exploration of LT in PD has been stymied by several methodological factors. Such factors include time of LT presentation, duration of studies undertaken, frequency of light employed, dose of light prescribed and relevance of experimental design to the prolonged course of the illness. On this basis, it is the purpose of this review to provide an in-depth examination of these papers, and the underlying preclinical work, to provide critique, thereby giving direction for future studies in therapeutic applications of LT for PD. Consideration of this collective work may serve to carve a path for future research and thereby improve the lives of those suffering from this debilitating disorder.

Chromatic pupillometry for evaluating melanopsin retinal ganglion cell function in Alzheimer's disease and other neurodegenerative disorders: a review

The evaluation of pupillary light reflex (PLR) by chromatic pupillometry may provide a unique insight into specific photoreceptor functions. Chromatic pupillometry refers to evaluating PLR to different wavelengths and intensities of light in order to differentiate outer/inner retinal photoreceptor contributions to the PLR. Different protocols have been tested and are now established to assess in-vivo PLR contribution mediated by melanopsin retinal ganglion cells (mRGCs). These intrinsically photosensitive photoreceptors modulate the non-image-forming functions of the eye, which are mainly the circadian photoentrainment and PLR, via projections to the hypothalamic suprachiasmatic and olivary pretectal nucleus, respectively. In this context, chromatic pupillometry has been used as an alternative and non-invasive tool to evaluate the mRGC system in several clinical settings, including hereditary optic neuropathies, glaucoma, and neurodegenerative disorders such as Parkinson's disease (PD), idiopathic/isolated rapid eye movement sleep behavior disorder (iRBD), and Alzheimer's disease (AD). The purpose of this article is to review the key steps of chromatic pupillometry protocols for studying in-vivo mRGC-system functionality and provide the main findings of this technique in the research setting on neurodegeneration. mRGC-dependent pupillary responses are short-wavelength sensitive, have a higher threshold of activation, and are much slower and sustained compared with rod- and cone-mediated responses, driving the tonic component of the PLR during exposure to high-irradiance and continuous light stimulus. Thus, mRGCs contribute mainly to the tonic component of the post-illumination pupil response (PIPR) to bright blue light flash that persists after light stimulation is switched off. Given the role of mRGCs in circadian photoentrainment, the use of chromatic pupillometry to perform a functional evaluation of mRGcs may be proposed as an early biomarker of mRGC-dysfunction in neurodegenerative disorders characterized by circadian and/or sleep dysfunction such as AD, PD, and its prodromal phase iRBD. The evaluation by chromatic pupillometry of mRGC-system functionality may lay the groundwork for a new, easily accessible biomarker that can be exploited also as the starting point for future longitudinal cohort studies aimed at stratifying the risk of conversion in these disorders.

Delayed oculomotor response associates with optic neuritis in youth with demyelinating disorders

INTRODUCTION

Impairment in visual and cognitive functions occur in youth with demyelinating disorders such as multiple sclerosis, neuromyelitis optica spectrum disorder, and myelin oligodendrocyte glycoprotein antibody-associated disease. Quantitative behavioral assessment using eye-tracking and pupillometry can provide functional metrics for important prognostic and clinically relevant information at the bedside.

METHODS

Children and adolescents diagnosed with demyelinating disorders and healthy, age-matched controls completed an interleaved pro- and anti-saccade task using video-based eye-tracking and underwent spectral-domain optical coherence tomography examination for evaluation of retinal nerve fiber layer and ganglion cell inner plexiform layer thickness. Low-contrast visual acuity and Symbol Digit Modalities Test were performed for visual and cognitive functional assessments. We assessed saccade and pupil parameters including saccade reaction time, direction error rate, pupil response latency, peak constriction time, and peak constriction and dilation velocities. Generalized Estimating Equations were used to examine the association of eye-tracking parameters with optic neuritis history, structural metrics, and visual and cognitive scores.

RESULTS

The study included 36 demyelinating disorders patients, aged 8-18 yrs. (75% F; median = 15.22 yrs., SD = 2.8) and 34 age-matched controls (65% F; median = 15.26 yrs., SD = 2.3). Surprisingly, pro- and anti-saccade performance was comparable between patients and controls, whereas pupil control was altered in patients. Oculomotor latency measures were strongly associated with the number of optic neuritis episodes, including saccade reaction time, pupil response latency, and peak constriction time. Peak constriction time was associated with both retinal nerve fiber layer and ganglion cell inner plexiform layer thickness. Pupil response latency and peak constriction time were associated with visual acuity. Pupil velocity for both constriction and dilation was associated with Symbol Digit Modalities Test scores.

CONCLUSION

The strong associations between oculomotor measures with history of optic neuritis, structural, visual, and cognitive assessments in these cohorts demonstrates that quantitative eye-tracking can be useful for probing demyelinating injury of the brain and optic nerve. Future studies should evaluate their utility in discriminating between demyelinating disorders and tracking disease progression.

Retinal dysfunction in Parkinson's disease-results of the extended protocol for photopic negative response (PHNR) full-field electroretinogram (ERG)

BACKGROUND

We investigated whether the photopic negative response (PhNR) in the electroretinogram (ERG) was affected in Parkinson's disease (PD) patients and whether it was associated with retinal changes on optical coherence tomography (OCT).

METHODS

Thirty-two patients with PD and 31 age and sex-matched healthy controls from a single tertiary centre were included in the study. Hoehn and Yahr scale scores and the presence of REM sleep behaviour were recorded. PhNR, a-wave and b-wave responses in photopic ERG (red on blue background) and retinal layer thicknesses in OCT were obtained.

RESULTS

The mean age was 61 ± 10.4 in the PD group (female/male: 18/14) and 60.9 ± 7 in the control group (female/male: 18/13). The amplitudes of the PhNR, a- and b-waves in the ERG were significantly decreased in the PD group, but the implicit times were not significantly different. BCVA was significantly correlated with Hoehn and Yahr scores (p < 0.001, r = - 0.596). There was a significant correlation between BCVA and a-wave amplitude (p = 0.047, r = - 0.251). On OCT analysis, the thickness of the nasal INL was increased, and the temporal and inferior OPL and temporal peripapillary RNFL were decreased in the PD group compared to healthy controls (p = 0.032, p = 0.002, p = 0.016 and p = 0.012, respectively).

CONCLUSION

This study demonstrated reduced a-wave, b-wave and PhNR-wave amplitudes on ERG measurements in PD patients. These findings suggest that the whole ERG response, not just the PhNR, is attenuated in patient with PD, suggesting a possible involvement of the visual system in the disease.

Post-Illumination Pupil Response and Sleep Quality in Patients With Glaucoma: The LIGHT Study

Purpose

This study aimed to investigate whether intrinsically photosensitive retinal ganglion cell function evaluated using post-illumination pupil response (PIPR) in patients with glaucoma is associated with sleep quality.

Methods

This cross-sectional study measured the PIPR in 138 patients with glaucoma (mean age, 70.3 years) using pupil diameter after red and blue light exposure. The net PIPR change was classified into three groups according to tertiles (i.e., low, intermediate, and high groups), with lower net PIPR change indicating lower intrinsically photosensitive retinal ganglion cell (ipRGC) function. Subjective and objective sleep qualities were assessed using the Pittsburgh Sleep Quality Index (PSQI) questionnaire and actigraphy, respectively, with a total PSQI score of ≥6 indicating sleep disturbance.

Results

The prevalence of subjective sleep disturbance significantly increased with decreasing tertile groups of net PIPR change (P = 0.036). Subgroup analysis obtained the same results in the severe glaucoma group (P = 0.004) but not in the non-severe glaucoma group. In the severe glaucoma group, multivariable logistic regression analysis adjusted for potential confounders showed a higher odds ratio for subjective sleep disturbance in the low-tertile group of net PIPR compared with the high-tertile group (odds ratio = 6.22; 95% confidence interval, 1.76-21.90; P = 0.004). Significant associations between PIPR and objective sleep quality (total sleep time, sleep efficiency, and wake after sleep onset) were found in the severe glaucoma group (P = 0.015, P = 0.013, and P = 0.015, respectively).

Conclusions

The PIPR in patients with glaucoma was significantly associated with decreased sleep quality, independent of potential confounders.

Alterations of color vision and pupillary light responses in age-related macular degeneration

Introduction

Age-related macular degeneration (AMD) is the leading cause of irreversible central vision loss in developed countries and one of the leading causes of blindness. In this work, we evaluated color vision and the pupil light reflex (PLR) to assess visual function in patients with early and neovascular AMD (NVAMD) compared with the control group.

Methods

We recruited 34 early patients with dry AMD and classified them into two groups following AREDS: 13 patients with NVAMD and 24 healthy controls. Controls and patients with early dry AMD had visual acuity (VA) best or equal to 20/25 (0.098 logMAR). Color vision was assessed in controls and patients with early dry AMD using the Cambridge Color Test (CCT) 2.0 through the Trivector protocol. The PLR was evaluated using a Ganzfeld, controlled by the RETI-port system. The stimuli consisted of 1s blue (470 nm) and red (631 nm) light flashes presented alternately at 2-min intervals. To assess the cone contribution, we used a red flash at 2.4 log cd.m^-2, with a blue background at 0.78 log cd.m^-2. For rods, we used 470-nm flashes at -3 log cd.m^-2, and for the melanopsin function of ipRGCs, we used 470 nm at 2.4 log cd.m^-2.

Results

Patients with early dry AMD had reduced color discrimination in all three axes: protan (p = 0.0087), deutan (p = 0.0180), and tritan (p = 0.0095) when compared with the control group. The PLR has also been affected in patients with early dry AMD and patients with NVAMD. The amplitude for the melanopsin-driven response was smaller in patients with early dry AMD (p = 0.0485) and NVAMD (p = 0.0035) than in the control group. The melanopsin function was lower in patients with NVAMD (p = 0.0290) than the control group. For the rod-driven response, the latency was lower in the NVAMD group (p = 0.0041) than in the control group. No changes were found in cone-driven responses between the control and AMD groups.

Conclusion

Patients with early dry AMD present diffusely acquired color vision alteration detected by CCT. Rods and melanopsin contributions for PLR are affected in NVAMD. The CCT and the PLR may be considered sensitive tests to evaluate and monitor functional changes in patients with AMD.

PyPlr: A versatile, integrated system of hardware and software for researching the human pupillary light reflex

We introduce PyPlr-a versatile, integrated system of hardware and software to support a broad spectrum of research applications concerning the human pupillary light reflex (PLR). PyPlr is a custom Python library for integrating a research-grade video-based eye-tracker system with a light source and streamlining stimulus design, optimisation and delivery, device synchronisation, and extraction, cleaning, and analysis of pupil data. We additionally describe how full-field, homogenous stimulation of the retina can be realised with a low-cost integrating sphere that serves as an alternative to a more complex Maxwellian view setup. Users can integrate their own light source, but we provide full native software support for a high-end, commercial research-grade 10-primary light engine that offers advanced control over the temporal and spectral properties of light stimuli as well as spectral calibration utilities. Here, we describe the hardware and software in detail and demonstrate its capabilities with two example applications: (1) pupillometer-style measurement and parametrisation of the PLR to flashes of white light, and (2) comparing the post-illumination pupil response (PIPR) to flashes of long and short-wavelength light. The system holds promise for researchers who would favour a flexible approach to studying the PLR and the ability to employ a wide range of temporally and spectrally varying stimuli, including simple narrowband stimuli.

Mitochondrial autophagy in the sleeping brain

A significant percentage of the mitochondrial mass is replaced on a daily basis via mechanisms of mitochondrial quality control. Through mitophagy (a selective type of autophagy that promotes mitochondrial proteostasis) cells keep a healthy pool of mitochondria, and prevent oxidative stress and inflammation. Furthermore, mitophagy helps adapting to the metabolic demand of the cells, which changes on a daily basis.

Core components of the mitophagy process are PINK1 and Parkin, which mutations are linked to Parkinson’s Disease. The crucial role of PINK1/Parkin pathway during stress-induced mitophagy has been extensively studied in vitro in different cell types. However, recent advances in the field allowed discovering that mitophagy seems to be only slightly affected in PINK1 KO mice and flies, putting into question the physiological relevance of this pathway in vivo in the whole organism. Indeed, several cell-specific PINK1/Parkin-independent mitophagy pathways have been recently discovered, which appear to be activated under physiological conditions such as those that promote mitochondrial proteome remodeling during differentiation or in response to specific physiological stimuli.

In this Mini Review we want to summarize the recent advances in the field, and add another level of complexity by focusing attention on a potentially important aspect of mitophagy regulation: the implication of the circadian clock. Recent works showed that the circadian clock controls many aspects of mitochondrial physiology, including mitochondrial morphology and dynamic, respiratory activity, and ATP synthesis. Furthermore, one of the essential functions of sleep, which is controlled by the clock, is the clearance of toxic metabolic compounds from the brain, including ROS, via mechanisms of proteostasis. Very little is known about a potential role of the clock in the quality control mechanisms that maintain the mitochondrial repertoire healthy during sleep/wake cycles. More importantly, it remains completely unexplored whether (dys)function of mitochondrial proteostasis feedbacks to the circadian clockwork.

Desynchronosis: Types, Main Mechanisms, Role in the Pathogenesis of Epilepsy and Other Diseases: A Literature Review

Circadian information is stored in mammalian tissues by an autonomous network of transcriptional feedback loops that have evolved to optimally regulate tissue-specific functions. Currently, stable circadian rhythms of the expression of clock genes (Bmal1/Per2/Cry1, etc.), hormones, and metabolic genes (Glut4/leptin, etc.) have been demonstrated. Desynchronoses are disorders of the body's biorhythms, where the direction and degree of shift of various indicators of the oscillatory process are disturbed. Desynchronosis can be caused by natural conditions or man-made causes. The disruption of circadian rhythms is a risk factor for the appearance of physiological and behavioral disorders and the development of diseases, including epilepsy, and metabolic and oncological diseases. Evidence suggests that seizure activity in the epilepsy phenotype is associated with circadian dysfunction. Interactions between epilepsy and circadian rhythms may be mediated through melatonin, sleep-wake cycles, and clock genes. The correction of circadian dysfunction can lead to a decrease in seizure activity and vice versa. Currently, attempts are being made to pharmacologically correct desynchronosis and related psycho-emotional disorders, as well as combined somatic pathology. On the other hand, the normalization of the light regimen, the regulation of sleep-wake times, and phototherapy as additions to standard treatment can speed up the recovery of patients with various diseases.

Melanopsin photoreception differentially modulates rod-mediated and cone-mediated human temporal vision

To evaluate the nature of interactions between visual pathways transmitting the slower melanopsin and faster rod and cone signals, we implement a temporal phase summation paradigm in human observers using photoreceptor-directed stimuli. We show that melanopsin stimulation interacts with and alters both rod-mediated and cone-mediated vision regardless of whether it is perceptually visible or not. Melanopsin-rod interactions result in either inhibitory or facilitatory summation depending on the temporal frequency and photoreceptor pathway contrast sensitivity. Moreover, by isolating rod vision, we reveal a bipartite intensity response property of the rod pathway in photopic lighting that extends its operational range at lower frequencies to beyond its classic saturation limits but at the expense of attenuating sensitivity at higher frequencies. In comparison, melanopsin-cone interactions always lead to facilitation. These interactions can be described by linear or probability summations and potentially involve multiple intraretinal and visual cortical pathways to set human visual contrast sensitivity.

•

Melanopsin ipRGCs support vision independent of the rod and cone signals

•

Rod pathways mediate robust visual responses in daylight

•

Temporal contrast sensitivity is contingent on the melanopsin excitation level

•

Visual performance is collectively regulated by melanopsin, rod and cone pathways

In vivo biomechanical assessment of iridial deformations and muscle contractions in human eyes

The iris is a muscular organ whose deformations can cause primary angle-closure glaucoma (PACG), a leading cause of blindness. PACG risk assessment does not consider iridial biomechanical factors, despite their expected influence on iris deformations. Here, we exploited an existing biometric dataset consisting of near-infrared movies acquired during the pupillary light reflex (PLR) as a unique resource to study iris biomechanics. The PLR caused significant (greater than 100%) and essentially spatially uniform radial strains in the iris in vivo, consistent with previous findings. Inverse finite-element modelling showed that sphincter muscle tractions were ca fivefold greater than iridial stroma stiffness (range 4- to 13-fold, depending on sphincter muscle size). This muscle traction is greater than has been previously estimated, which may be due to methodological differences and/or to different patient populations in our study (European descent) versus previous studies (Asian); the latter possibility is of particular interest due to differential incidence rates of PACG in these populations. Our methodology is fast and inexpensive and may be a useful tool in understanding biomechanical factors contributing to PACG.

Circadian Rhythm Sleep-Wake Disorders in Older Adults

The timing, duration, and consolidation of sleep result from the interaction of the circadian timing system with a sleep-wake homeostatic process. When aligned and functioning optimally, this allows for wakefulness throughout the day and a long consolidated sleep episode at night. Changes to either the sleep regulatory process or how they interact can result in an inability to fall asleep at the desired time, difficulty remaining asleep, waking too early, and/or difficulty remaining awake throughout the day. This mismatch between the desired timing of sleep and the ability to fall asleep and remain asleep is a hallmark of a class of sleep disorders called the circadian rhythm sleep-wake disorders. In this updated article, we discuss typical changes in the circadian regulation of sleep with aging; how age influences the prevalence, diagnosis, and treatment of circadian rhythm sleep disorders; and how neurologic diseases in older patient impact circadian rhythms and sleep.

A Perspective on the Potential of Opsins as an Integral Mechanism of Photobiomodulation: It's Not Just the Eyes

Objective: To investigate the potential relationship between opsins and photobiomodulation. Background: Opsins and other photoreceptors occur in all phyla and are important in light-activated signaling and organism homeostasis. In addition to the visual opsin systems of the retina (OPN1 and OPN2), there are several non-visual opsins found throughout the body tissues, including encephalopsin/panopsin (OPN3), melanopsin (OPN4), and neuropsin (OPN5), as well as other structures that have light-sensitive properties, such as enzymes, ion channels, particularly those located in cell membranes, lysosomes, and neuronal structures such as the nodes of Ranvier. The influence of these structures on exposure to light, including self-generated light within the body (autofluorescence), on circadian oscillators, and circadian and ultradian rhythms have become increasingly reported. The visual and non-visual phototransduction cascade originating from opsins and other structures has potential significant mechanistic effects on tissues and health. Methods: A PubMed and Google Scholar search was made using the search terms "photobiomodulation", "light", "neuron", "opsins", "neuropsin", "melanopsin", "encephalopsin", "rhodopsin", and "chromophore". Results: This review was examined the influence of neuropsin (also known as kallikrein 8), encephalopsin, and melanopsin specifically on ion channel function, and more broadly on the central and peripheral nervous systems. The relationship between opsins 3, 4, and 5 and photobiomodulation mechanisms was evaluated, along with a proposed role of photobiomodulation through opsins and light-sensitive organelles as potential alleviators of symptoms and accelerators of beneficial regenerative processes. The potential clinical implications of this in musculoskeletal conditions, wounds, and in the symptomatic management of neurodegenerative disease was also examined. Conclusions: Systematic research into the pleotropic therapeutic role of photobiomodulation, mediated through its action on opsins and other light-sensitive organelles may assist in the future execution of safe, low-risk precision medicine for a variety of chronic and complex disease conditions, and for health maintenance in aging.

Post-Illumination Pupil Response as a Biomarker for Cognition in α-Synucleinopathies

Neurodegenerative processes in the brain are reflected by structural retinal changes. As a possible biomarker of cognitive state in prodromal α-synucleinopathies, we compared melanopsin-mediated post-illumination pupil response (PIPR) with cognition (CERAD-plus) in 69 patients with isolated REM-sleep behavior disorder. PIPR was significantly correlated with cognitive domains, especially executive functioning (r = 0.417, p < 0.001), which was more pronounced in patients with lower dopamine-transporter density, suggesting advanced neurodegenerative state (n = 26; r = 0.575, p = 0.002). Patients with mild neurocognitive disorder (n = 10) had significantly reduced PIPR (smaller melanopsin-mediated response) compared to those without (p = 0.001). Thus, PIPR may be a functional-possibly monitoring-marker for impaired cognitive state in (prodromal) α-synucleinopathies.

Circadian rhythms in neurodegenerative disorders

Endogenous biological clocks, orchestrated by the suprachiasmatic nucleus, time the circadian rhythms that synchronize physiological and behavioural functions in humans. The circadian system influences most physiological processes, including sleep, alertness and cognitive performance. Disruption of circadian homeostasis has deleterious effects on human health. Neurodegenerative disorders involve a wide range of symptoms, many of which exhibit diurnal variations in frequency and intensity. These disorders also disrupt circadian homeostasis, which in turn has negative effects on symptoms and quality of life. Emerging evidence points to a bidirectional relationship between circadian homeostasis and neurodegeneration, suggesting that circadian function might have an important role in the progression of neurodegenerative disorders. Therefore, the circadian system has become an attractive target for research and clinical care innovations. Studying circadian disruption in neurodegenerative disorders could expand our understanding of the pathophysiology of neurodegeneration and facilitate the development of novel, circadian-based interventions for these disabling disorders. In this Review, we discuss the alterations to the circadian system that occur in movement (Parkinson disease and Huntington disease) and cognitive (Alzheimer disease and frontotemporal dementia) neurodegenerative disorders and provide directions for future investigations in this field.

Dynamic volume-rendered optical coherence tomography pupillometry

PURPOSE

To assess intrapupillary space (IPS) changes in healthy subjects with regard to decreased iris motility in patients with pseudoexfoliation glaucoma (PEXG) or non-arteritic anterior ischaemic optic neuropathy (NAION) in a feasibility study in a clinical environment.

METHODS

Scotopic and photopic IPS measurements using three-dimensionally rendered swept-source optical coherence tomography (SS-OCT) data were obtained and compared for all subjects. Intrapupillary space (IPS) parameters were evaluated such as absolute volumetric differences, relative light response for volumetric ratios and pupillary ejection fraction (PEF) for functional contraction measurements.

RESULTS

From a total of 122 IPS from 66 subjects, 106 IPS were eligible for comparison providing values for 72 normal, 30 PEXG and 4 NAION eyes. In healthy, PEXG and NAION subjects, scotopic overall mean IPS was 8.90, 3.45 and 4.16 mm³ , and photopic overall mean IPS was 0.87, 0.74 and 1.13 mm³ , respectively. Three-dimensional contractility showed a mean absolute difference of 8.03 mm³ for normals (defined as 100% contractility), 2.72 mm³ for PEXG (33.88% of normal) and 3.03 mm³ for NAION (38.50% of normal) with a relative light response ratio between scotopic and photopic volumes of 10.26 (100%), 4.69 (45.70%) and 3.67 (35.78%), respectively. Pupillary ejection fraction (PEF) showed a contractile pupillary emptying of 88.11% for normals, 76.92% for PEXG and 70.91% for NAION patients.

CONCLUSION

This 3D pupillometry OCT assessment allows for quantitative measurements of pupil function, contractility and response to light. More specifically, PEF is presented as a potential (neuro)-pupillary outcome measure that could be useful in the monitoring of ophthalmic disorders that affect pupillary function.

Chromatic Pupillometry in Isolated Rapid Eye Movement Sleep Behavior Disorder

Background

Melanopsin retinal ganglion cell (mRGC)‐mediated pupillary light reflex (PLR) abnormalities have been documented in several neurodegenerative disorders including Parkinson's disease. Overall, isolated rapid eye movement (REM) sleep behavior disorder (iRBD) represents the strongest prodromal risk factor for impending α‐synucleinopathies.

Objectives

To quantitatively compare PLR and mRGC‐mediated contribution to PLR in 16 iRBD patients and 16 healthy controls.

Methods

iRBD and controls underwent extensive neuro‐ophthalmological evaluation and chromatic pupillometry. In iRBD, PLR metrics were correlated with clinical variables and with additional biomarkers including REM atonia index (RAI), DaTscan, and presence of phosphorylated‐α‐synuclein (p‐α‐syn) deposition in skin biopsy.

Results

We documented higher baseline pupil diameter and decreased rod‐transient PLR amplitude in iRBD patients compared to controls. PLR rod‐contribution correlated with RAI. Moreover, only iRBD patients with evidence of p‐α‐syn deposition at skin biopsy showed reduced PLR amplitude compared to controls.

Conclusion

The observed PLR abnormalities in iRBD might be considered as potential biomarkers for the risk stratification of phenoconversion of the disease. © 2021 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society

Defining circadian disruption in neurodegenerative disorders

Neurodegenerative diseases encompass a large group of conditions that are clinically and pathologically diverse yet are linked by a shared pathology of misfolded proteins. The accumulation of insoluble aggregates is accompanied by a progressive loss of vulnerable neurons. For some patients, the symptoms are motor focused (ataxias), while others experience cognitive and psychiatric symptoms (dementias). Among the shared symptoms of neurodegenerative diseases is a disruption of the sleep/wake cycle that occurs early in the trajectory of the disease and may be a risk factor for disease development. In many cases, the disruption in the timing of sleep and other rhythmic physiological markers immediately raises the possibility of neurodegeneration-driven disruption of the circadian timing system. The aim of this Review is to summarize the evidence supporting the hypothesis that circadian disruption is a core symptom within neurodegenerative diseases, including Alzheimer's disease, Huntington's disease, and Parkinson's disease, and to discuss the latest progress in this field. The Review discusses evidence that neurodegenerative processes may disrupt the structure and function of the circadian system and describes circadian-based interventions as well as timed drug treatments that may improve a wide range of symptoms associated with neurodegenerative disorders. It also identifies key gaps in our knowledge.

Intrinsically photosensitive retinal ganglion cell-driven pupil responses in patients with traumatic brain injury

Previous findings regarding intrinsically photosensitive retinal ganglion cell (ipRGC) function after traumatic brain injury (TBI) are conflicting. We examined ipRGC-driven pupil responses in civilian TBI and control participants using two pupillography protocols that assessed transient and adaptive properties: (1) a one second (s) long wavelength "red" stimulus (651 nm, 133 cd/m²) and 10 increasing intensities of 1 s short wavelength "blue" stimuli (456 nm, 0.167 to 167 cd/m²) with a 60 s interstimulus interval, and (2) two minutes of 0.1 Hz red stimuli (33 cd/m²), followed by two minutes of 0.1 Hz blue stimuli (16 cd/m²). For Protocol 1, constriction amplitude and the 6 s post illumination pupil response (PIPR) were calculated. For Protocol 2, amplitudes and peak velocities of pupil constriction and redilation were calculated. For Protocol 1, constriction amplitude and the 6 s PIPR were not significantly different between TBI patients and control subjects for red or blue stimuli. For Protocol 2, pupil constriction amplitude attenuated over time for red stimuli and potentiated over time for blue stimuli across all subjects. Constriction and redilation velocities were similar between groups. Pupil constriction amplitude was significantly less in TBI patients compared to control subjects for red and blue stimuli, which can be attributed to age-related differences in baseline pupil size. While TBI, in addition to age, may have contributed to decreased baseline pupil diameter and constriction amplitude, responses to blue stimulation suggest no selective damage to ipRGCs.

Abnormal Vision-Based Displacement Perception in Parkinson's Disease

In this work, we investigate the effect of Parkinson’s disease (PD), and common corresponding therapies on vision-based perception of motion, a critical perceptual ability required for performing a wide range of activities of daily livings. While PD has been recognized as mainly a motor disorder, sensory manifestation of PD can also play a major role in the resulting disability. In this paper, for the first time, the effect of disease duration and common therapies on vision-based perception of displacement were investigated. The study is conducted in a movement-independent manner, to reject the shadowing effects and isolate the targeted perceptual disorder to the maximum possible extent. Data was collected using a computerized graphical tool on 37 PD patients [6 early-stage de novo, 25 mid-stage using levodopa therapy, six later-stage using deep brain stimulation (DBS)] and 15 control participants. Besides the absolute measurement of perception through a psychometric analysis on two tested position reference magnitudes, we also investigated the linearity in perception using Weber’s fraction. The results showed that individuals with PD displayed significant perceptual impairments compared to controls, though early-stage patients were not impaired. Mid-stage patients displayed impairments at the greater of the two tested reference magnitudes, while late-stage patients were impaired at both reference magnitudes. Levodopa and DBS use did not cause statistically significant differences in absolute displacement perception. The findings suggest abnormal visual processing in PD increasing with disease development, perhaps contributing to sensory-based impairments of PD such as bradykinesia, visuospatial deficits, and abnormal object recognition.

Optimizing methods to isolate melanopsin-directed responses

The intrinsic melanopsin photoresponse may initiate visual signals that differ in spatiotemporal characteristics from the cone-opsin- and rhodopsin-mediated signals. Applying the CIE standard observer functions in silent-substitution methods can require individual differences in photoreceptor spectral sensitivities and pre-receptoral filtering to be corrected; failure to do so can lead to the intrusion of more sensitive cone processes with putative melanopsin-directed stimuli. Here we evaluate heterochromatic flicker photometry (HFP) and photoreceptor-directed temporal white noise as techniques to limit the effect of these individual differences. Individualized luminous efficiency functions (V(λ)) were compared to the CIE standard observer functions. We show that adapting chromaticities used in silent-substitution methods can deviate by up to 54% in luminance when estimated with the individual and standard observer functions. These deviations lead to inadvertent cone intrusions in the visual functions measured with melanopsin-directed stimuli. To eliminate the intrusions, individual HFP corrections are sufficient at low frequencies (∼1Hz) but temporal white noise is also required at higher frequencies to desensitize penumbral cones. We therefore recommend the selective application of individualized observer calibration and/or temporal white noise in silent-substitution paradigms when studying melanopsin-directed photoresponses.

PupilEXT: Flexible Open-Source Platform for High-Resolution Pupillometry in Vision Research

The human pupil behavior has gained increased attention due to the discovery of the intrinsically photosensitive retinal ganglion cells and the afferent pupil control path's role as a biomarker for cognitive processes. Diameter changes in the range of 10-2 mm are of interest, requiring reliable and characterized measurement equipment to accurately detect neurocognitive effects on the pupil. Mostly commercial solutions are used as measurement devices in pupillometry which is associated with high investments. Moreover, commercial systems rely on closed software, restricting conclusions about the used pupil-tracking algorithms. Here, we developed an open-source pupillometry platform consisting of hardware and software competitive with high-end commercial stereo eye-tracking systems. Our goal was to make a professional remote pupil measurement pipeline for laboratory conditions accessible for everyone. This work's core outcome is an integrated cross-platform (macOS, Windows and Linux) pupillometry software called PupilEXT, featuring a user-friendly graphical interface covering the relevant requirements of professional pupil response research. We offer a selection of six state-of-the-art open-source pupil detection algorithms (Starburst, Swirski, ExCuSe, ElSe, PuRe and PuReST) to perform the pupil measurement. A developed 120-fps pupillometry demo system was able to achieve a calibration accuracy of 0.003 mm and an averaged temporal pupil measurement detection accuracy of 0.0059 mm in stereo mode. The PupilEXT software has extended features in pupil detection, measurement validation, image acquisition, data acquisition, offline pupil measurement, camera calibration, stereo vision, data visualization and system independence, all combined in a single open-source interface, available at https://github.com/openPupil/Open-PupilEXT.

Seasonal variation in neurohormones, mood and sleep in patients with primary open angle glaucoma - implications of the ipRGC-system

Primary open angle glaucoma is associated with an increased risk of mood and sleep disorders. These adversities have been suggested to relate to a disrupted function of the intrinsically photosensitive retinal ganglion cells (ipRGCs). The ipRGCs are key components in the nonvisual photoreceptive system that mediates light effects on mood, sleep and circadian rhythm. We assessed the diurnal hormone levels, pupillary responses and mood and sleep under seasons with different photoperiods in 24 patients with glaucoma and 24 age- and sex-matched healthy controls to investigate responses to naturalistic seasonal changes in daylight. The patients had moderate-to-advanced glaucoma with substantial visual field defects and reductions in the ipRGC-mediated pupillary responses (p < .001). In winter, compared with summer, patients with glaucoma had higher daytime melatonin concentration (p < .001) and lower nighttime cortisol (p = .002). In winter, the daytime melatonin level was inversely correlated with the ipRGC-mediated pupillary responses in the control group (p = .04). In the control group, there were no significant changes in hormone levels between seasons or any correlations between neurohormone levels and the ipRGC-mediated responses. The two groups showed a similar response to season with lower depression scores in summer compared with winter. In between-group comparison, the nocturnal melatonin level (area under curve from 20:00 h to 08:00 h) in summer was lower in glaucoma compared with controls (p = .03). In winter, nocturnal cortisol (at 04:00 h) was lower (p = .004) and daytime cortisol (12:00 h and 16:00 h) was higher (p = .007) in glaucoma compared with controls. In conclusion, we found that patients with glaucoma displayed a seasonal variation in diurnal hormone levels that was not present in healthy controls. Such neurohormonal changes may contribute to the increased risk of mood and sleep disorders seen in patients with glaucoma.

Variable abnormality of the melanopsin-derived portion of the pupillary light reflex (PLR) in patients with Parkinson's disease (PD) and parkinsonism features

OBJECTIVES

Ascertain and quantify abnormality of the melanopsin-derived portion of the pupillary light reflex (PLR) in patients with Parkinson's disease (PD) and parkinsonism features based on a statistical predictive modeling strategy for PLR classification.

METHODS

Exploratory cohort analysis of pupillary kinetics in non-disease controls, PD subjects, and subjects with parkinsonism features using chromatic pupillometry. Receiver operating characteristic (ROC) curve interpretation of pupillary changes consistent with abnormality of intrinsically photosensitive retinal ganglion cells (ipRGCs) was employed using a thresholding algorithm to discriminate pupillary abnormality between study groups.

RESULTS

Twenty-eight subjects were enrolled, including 17 PD subjects (age range 64-85, mean 70.65) and nine controls (age range 48-95, mean 63.89). Two subjects were described as demonstrating parkinsonism symptoms due to presumed Lewy body dementia and motor system atrophy (MSA) respectively. On aggregate analysis, PD subjects demonstrated abnormal but variable pupillary dynamics suggestive of ipRGC abnormality. Subjects with parkinsonism features did not demonstrate pupillary changes consistent with ipRGC abnormality. There was no relationship between levodopa equivalent dosage or PD severity and ipRGC abnormality. The pupillary test sensitivity in predicting PD was 0.75 and likelihood ratio was 1.2.

CONCLUSIONS

ipRGC deficit is demonstrated in PD subjects; however, the degree and constancy of abnormality appear variable.

Identification of attention-deficit hyperactivity disorder based on the complexity and symmetricity of pupil diameter

Adult attention-deficit/hyperactivity disorder (ADHD) frequently leads to psychological/social dysfunction if unaddressed. Identifying a reliable biomarker would assist the diagnosis of adult ADHD and ensure that adults with ADHD receive treatment. Pupil diameter can reflect inherent neural activity and deficits of attention or arousal characteristic of ADHD. Furthermore, distinct profiles of the complexity and symmetricity of neural activity are associated with some psychiatric disorders. We hypothesized that analysing the relationship between the size, complexity of temporal patterns, and asymmetricity of pupil diameters will help characterize the nervous systems of adults with ADHD and that an identification method combining these features would ease the diagnosis of adult ADHD. To validate this hypothesis, we evaluated the resting state hippus in adult participants with or without ADHD by examining the pupil diameter and its temporal complexity using sample entropy and the asymmetricity of the left and right pupils using transfer entropy. We found that large pupil diameters and low temporal complexity and symmetry were associated with ADHD. Moreover, the combination of these factors by the classifier enhanced the accuracy of ADHD identification. These findings may contribute to the development of tools to diagnose adult ADHD.

Pupillary light reflex as a diagnostic aid from computational viewpoint: A systematic literature review

This work presents a detailed and complete review of publications on pupillary light reflex (PLR) used to aid diagnoses. These are computational techniques used in the evaluation of pupillometry, as well as their application in computer-aided diagnoses (CAD) of pathologies or physiological conditions that can be studied by observing the movements of miosis and mydriasis of the human pupil. A careful survey was carried out of all studies published over the last 10 years which investigated, electronic devices, recording protocols, image treatment, computational algorithms and the pathologies related to PLR. We present the frontier of existing knowledge regarding methods and techniques used in this field of knowledge, which has been expanding due to the possibility of performing diagnoses with high precision, at a low cost and with a non-invasive method.

Detection of Parkinson's Disease Through Automated Pupil Tracking of the Post-illumination Pupillary Response

Parkinson's disease (PD) is one of the most common neurodegenerative disorders, but it is often diagnosed after the majority of dopaminergic cells are already damaged. It is critical to develop biomarkers to identify the disease as early as possible for early intervention. PD patients appear to have an altered pupillary response consistent with an abnormality in photoreceptive retinal ganglion cells. Tracking the pupil size manually is a tedious process and offline automated systems can be prone to errors that may require intervention; for this reason in this work we describe a system for pupil size estimation with a user interface to allow rapid adjustment of parameters and extraction of pupil parameters of interest for the present study. We implemented a user-friendly system designed for clinicians to automate the process of tracking the pupil diameter to measure the post-illumination pupillary response (PIPR), permit manual corrections when needed, and continue automation after correction. Tracking was automated using a Kalman filter estimating the pupil center and diameter over time. The resulting system was tested on a PD classification task in which PD subjects are known to have similar responses for two wavelengths of light. The pupillary response is measured in the contralateral eye to two different light stimuli (470 and 610 nm) for 19 PD and 10 control subjects. The measured Net PIPR indicating different responsiveness to the wavelengths was 0.13 mm for PD subjects and 0.61 mm for control subjects, demonstrating a highly significant difference (p < 0.001). Net PIPR has the potential to be a biomarker for PD, suggesting further study to determine clinical validity.

Intrinsically Photosensitive Retinal Ganglion Cells of the Human Retina

Light profoundly affects our mental and physical health. In particular, light, when not delivered at the appropriate time, may have detrimental effects. In mammals, light is perceived not only by rods and cones but also by a subset of retinal ganglion cells that express the photopigment melanopsin that renders them intrinsically photosensitive (ipRGCs). ipRGCs participate in contrast detection and play critical roles in non-image-forming vision, a set of light responses that include circadian entrainment, pupillary light reflex (PLR), and the modulation of sleep/alertness, and mood. ipRGCs are also found in the human retina, and their response to light has been characterized indirectly through the suppression of nocturnal melatonin and PLR. However, until recently, human ipRGCs had rarely been investigated directly. This gap is progressively being filled as, over the last years, an increasing number of studies provided descriptions of their morphology, responses to light, and gene expression. Here, I review the progress in our knowledge of human ipRGCs, in particular, the different morphological and functional subtypes described so far and how they match the murine subtypes. I also highlight questions that remain to be addressed. Investigating ipRGCs is critical as these few cells play a major role in our well-being. Additionally, as ipRGCs display increased vulnerability or resilience to certain disorders compared to conventional RGCs, a deeper knowledge of their function could help identify therapeutic approaches or develop diagnostic tools. Overall, a better understanding of how light is perceived by the human eye will help deliver precise light usage recommendations and implement light-based therapeutic interventions to improve cognitive performance, mood, and life quality.

Deep learning-based pupil model predicts time and spectral dependent light responses

Although research has made significant findings in the neurophysiological process behind the pupillary light reflex, the temporal prediction of the pupil diameter triggered by polychromatic or chromatic stimulus spectra is still not possible. State of the art pupil models rested in estimating a static diameter at the equilibrium-state for spectra along the Planckian locus. Neither the temporal receptor-weighting nor the spectral-dependent adaptation behaviour of the afferent pupil control path is mapped in such functions. Here we propose a deep learning-driven concept of a pupil model, which reconstructs the pupil's time course either from photometric and colourimetric or receptor-based stimulus quantities. By merging feed-forward neural networks with a biomechanical differential equation, we predict the temporal pupil light response with a mean absolute error below 0.1 mm from polychromatic (2007 [Formula: see text] 1 K, 4983 [Formula: see text] 3 K, 10,138 [Formula: see text] 22 K) and chromatic spectra (450 nm, 530 nm, 610 nm, 660 nm) at 100.01 ± 0.25 cd/m2. This non-parametric and self-learning concept could open the door to a generalized description of the pupil behaviour.

Retinal Ganglion Cells-Diversity of Cell Types and Clinical Relevance

Retinal ganglion cells (RGCs) are the bridging neurons that connect the retinal input to the visual processing centres within the central nervous system. There is a remarkable diversity of RGCs and the various subtypes have unique morphological features, distinct functions, and characteristic pathways linking the inner retina to the relevant brain areas. A number of psychophysical and electrophysiological tests have been refined to investigate this large and varied population of RGCs. Technological advances, such as high-resolution optical coherence tomography imaging, have provided additional tools to define the pattern of RGC involvement and the chronological sequence of events in both inherited and acquired optic neuropathies. The mechanistic insights gained from these studies, in particular the selective vulnerability and relative resilience of particular RGC subtypes, are of fundamental importance as they are directly relevant to the development of targeted therapies for these invariably progressive blinding diseases. This review provides a comprehensive description of the various types of RGCs, the developments in proposed methods of classification, and the current gaps in our knowledge of how these RGCs are differentially affected depending on the underlying aetiology. The synthesis of the current body of knowledge on the diversity of RGCs and the pathways that are potentially amenable to therapeutic modulation will hopefully lead to much needed effective treatments for patients with optic neuropathies.

The circadian system in Parkinson's disease, multiple system atrophy, and progressive supranuclear palsy

Circadian organization of physiology and behavior is an important biologic process that allows organisms to anticipate and prepare for predictable changes in the environment. Circadian disruptions are associated with a wide range of health issues. In patients with neurodegenerative diseases, alterations of circadian rhythms are among the most common and debilitating symptoms. Although a growing awareness of these symptoms has occurred during the last decade, their underlying neuropathophysiologic circuitry remains poorly understood and, consequently, no effective therapeutic strategies are available to alleviate these health issues. Recent studies have examined the neuropathologic status of the different neural components of the circuitry governing the generation of circadian rhythms in neurodegenerative diseases. In this review, we will dissect the potential contribution of dysfunctions in the different nodes of this circuitry to circadian alterations in patients with parkinsonism-linked neurodegenerative diseases (namely, Parkinson's disease, multiple system atrophy, and progressive supranuclear palsy). A deeper understanding of these mechanisms will provide not only a better understanding of disease neuropathophysiology but also holds promise for the development of more effective and mechanisms-based therapies.

Retina and melanopsin neurons

Melanopsin retinal ganglion cells (mRGCs) are the third class of retinal photoreceptors with unique anatomical, electrophysiological, and biological features. There are different mRGC subtypes with differential projections to the brain. These cells contribute to many nonimage-forming functions of the eye, the most relevant being the photoentrainment of circadian rhythms through the projections to the suprachiasmatic nucleus of the hypothalamus. Other relevant biological functions include the regulation of the pupillary light reflex, mood, alertness, and sleep, as well as a possible role in formed vision. The relevance of the mRGC-related pathways in the brain is highlighted by the role that the dysfunction and/or loss of these cells may play in affecting circadian rhythms and sleep in many neurodegenerative disorders including Alzheimer's, Parkinson's and Huntington's disease and in aging. Moreover, the occurrence of circadian dysfunction is a known risk factor for dementia. In this chapter, the anatomy, physiology, and functions of these cells as well as their resistance to neurodegeneration in mitochondrial optic neuropathies or their predilection to be lost in other neurodegenerative disorders will be discussed.

Melanopsin Cell Dysfunction is Involved in Sleep Disruption in Parkinson’s Disease

Background: Melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs) signal the environmental light to mediate circadian photoentrainment and sleep-wake cycles. There is high prevalence of circadian and sleep disruption in people with Parkinson’s disease, however the underlying mechanisms of these symptoms are not clear. Objective: Based on recent evidence of anatomical and functional loss of melanopsin ganglion cells in Parkinson’s disease, we evaluate the link between melanopsin function, circadian, and sleep behavior. Methods: The pupil light reflex and melanopsin-mediated post-illumination pupil response were measured using chromatic pupillometry in 30 optimally medicated people with Parkinson’s disease and 29 age-matched healthy controls. Circadian health was determined using dim light melatonin onset, sleep questionnaires, and actigraphy. Ophthalmic examination quantified eye health and optical coherence tomography measured retinal thickness. Results: The melanopsin-mediated post-illumination pupil response amplitudes were significantly reduced in Parkinson’s disease (p < 0.0001) and correlated with poor sleep quality (r2 = 33; p < 0.001) and nerve fiber layer thinning (r2 = 0.40; p < 0.001). People with Parkinson’s disease had significantly poorer sleep quality with higher subjective sleep scores (p < 0.05) and earlier melatonin onset (p = 0.01). Pupil light (outer retinal) response metrics, daily light exposure and outer retinal thickness were similar between the groups (p > 0.05). Conclusion: Our evidence-based data identify a mechanism through which inner retinal ipRGC dysfunction contributes to sleep disruption in Parkinson’s disease in the presence of normal outer retinal (rod-cone photoreceptor) function. Our findings provide a rationale for designing new treatment approaches in Parkinson’s disease through melanopsin photoreceptor-targeted light therapies for improving sleep-wake cycles.

Static and Dynamic Pupillary Responses in Patients with Different Stages of Diabetic Retinopathy

This study aimed to investigate pupillary involvement in patients with type 2 diabetes mellitus (DM) and to evaluate whether there is a relationship between severity of diabetic retinopathy (DR) and pupillary responses. The study included 133 individuals in four groups: proliferative DR, non-proliferative DR, DM group without retinal involvement and a control group. Static pupillometry measurements including scotopic pupil diameter (PD), mesopic PD, low photopic PD, high photopic PD, and dynamic pupillometry measurements, including resting diameter, amplitude, latency, velocity, duration of pupil contraction and latency, duration, and velocity of pupil dilatation were taken using an automated quantitative pupillometry system. The correlations between glycosylated haemoglobin values and duration of DM with these parameters were also investigated. The study showed that patients with DR may also have diabetic autonomic neuropathy and pupillometry can be a useful screening tool for detecting diabetic autonomic neuropathy.

The Spectrum of Sleep Disorders in Parkinson Disease: A Review

There is increasing interest in the effects of sleep and sleep disturbances on the brain, particularly in relation to aging and neurodegenerative processes. Parkinson disease (PD) is the second most common neurodegenerative disorder, with growing prevalence worldwide. Sleep disorders, including sleep-disordered breathing (SDB), are among the most frequent non-motor manifestations of PD. They can substantially impair quality of life and possibly affect the course of the disease. This article reviews the etiology, implications, and management of sleep disturbances in PD, such as excessive daytime sleepiness, insomnia, restless legs syndrome, rapid eye movement sleep behavior disorder, and SDB. Also briefly explored is the potential role of sleep disorders, including SDB, in the progression of neurodegeneration.

Circadian alterations in patients with neurodegenerative diseases: Neuropathological basis of underlying network mechanisms

Circadian organization of physiology and behavior is an important biological process that allows organisms to anticipate and prepare for daily changes and demands. Disruptions in this system precipitates a wide range of health issues. In patients with neurodegenerative diseases, alterations of circadian rhythms are among the most common and debilitating symptoms. Although a growing awareness of these symptoms has occurred during the last decade, their underlying neuropathophysiological circuitry remains poorly understood and consequently no effective therapeutic strategies are available to alleviate these health issues. Recent studies have examined the neuropathological status of the different neural components of the circuitry governing the generation of circadian rhythms in neurodegenerative diseases. In this review, we will dissect the potential contribution of dysfunctions in the different nodes of this circuitry to circadian alterations in patients with neurodegenerative diseases. A deeper understanding of these mechanisms will provide not only a better understanding of disease neuro-pathophysiology, but also hold the promise for developing effective and mechanisms-based therapies.

The Evolution-Driven Signature of Parkinson's Disease

In this review, we approach Parkinson's disease (PD) in the context of an evolutionary mismatch of central nervous system functions. The neurons at risk have hyperbranched axons, extensive transmitter release sites, display spontaneous spiking, and elevated mitochondrial stress. They function in networks largely unchanged throughout vertebrate evolution, but now connecting to the expanded human cortex. Their breakdown is favoured by longevity. At the cellular level, mitochondrial dysfunction starts at the synapses, then involves axons and cell bodies. At the behavioural level, network dysfunctions provoke the core motor syndrome of parkinsonism including freezing and failed gait automatization, and non-motor deficits including inactive blindsight and autonomic dysregulation. The proposed evolutionary re-interpretation of PD-prone cellular phenotypes and of prototypical clinical symptoms allows a new conceptual framework for future research.

Bright light improves sleep in patients with Parkinson's disease: possible role of circadian restoration

Parkinson’s disease (PD) is one of the most common neurodegenerative disorders. Among the most common manifestations of PD are sleep problems, which are coupled with the adverse effects of dopaminergic therapies (DT). A non-pharmacological solution for these sleep problems has been sought to avoid additional pharmacological intervention. Here, we show that bright light therapy (BLT) is effective for improving sleep in Japanese PD patients receiving DT. Furthermore, experimental evaluation of peripheral clock gene expression rhythms revealed that most PD patients receiving DT who experienced improved sleep following BLT showed a circadian phase shift, indicating the existence of a correlation between circadian modulation and sleep improvement. Conversely, this result indicates that sleep problems in PD patients receiving DT may arise at least in part as a result of circadian dysfunction. Indeed, we found that chronic dopaminergic stimulation induced a rapid attenuation of autonomous oscillations of clock gene expression in ex vivo cultured mouse suprachiasmatic nucleus (SCN) at the single neuron level. In conclusion, BLT is a promising medical treatment for improving sleep in PD patients receiving DT. This BLT-induced improvement may be due to the restoration of circadian function.

An overview of the neuroendocrine system in Parkinson's disease: what is the impact on diagnosis and treatment?

Introduction: A growing body of evidence indicates that neuroendocrine interactions may occur at all levels of the brain-gut-microbiota axis, which is directly involved in the pathogenesis of Parkinson's disease (PD).Areas covered: The review presents some current and emerging concepts regarding the organization and functioning of the neuroendocrine system as well as the role of neuroendocrine disturbances in the pathophysiology and symptomatology of PD. The concept of the brain-gut-microbiota triad interactions in the neuroendocrine system and PD is proposed. In PD, dysregulation of the main neuroendocrine axes coordinated by the hypothalamus is accompanied by disruptions at the peripheral level, which involve enteroendocrine cells producing numerous neuropeptides. Moreover, the important role of the gut microbiota as a main coordinator of immune and neuroendocrine interactions is discussed. The potential diagnostic and therapeutic implications in the context of the recent developments in the fields of neuroendocrinology and neurodegeneration are also presented.Expert opinion: Unraveling complex neuroendocrine interactions in the course of PD may provide crucial diagnostic implications and novel therapeutic approaches including the application of gut neuropeptides and gut microbiota modification.

Contributions of the Melanopsin-Expressing Ganglion Cells, Cones, and Rods to the Pupillary Light Response in Obstructive Sleep Apnea

Purpose

To investigate the impact of obstructive sleep apnea (OSA) on the contribution of inner and outer retinal photoreceptors to the pupillary light response (PLR).

Methods

Ninety-three eyes from 27 patients with OSA and 25 healthy controls were tested. OSA severity was graded according to the apnea-hypopnea index. PLR was measured monocularly with an eye tracker in a Ganzfeld in response to 1-second blue (470 nm) and red (640 nm) flashes at -3, -2, -1, 0, 1, 2, and 2.4 log cd/m2. Peak pupil constriction amplitude, peak latency, and the postillumination pupil response were measured. The Cambridge Colour Test, standard automatic perimetry, spectral domain optical coherence tomography, polysomnography, and the Pittsburgh Sleep Quality Index were used.

Results

OSA patients have a significantly decreased peak pupil constriction amplitude for blue stimuli at -3, -2, -1, 1 log cd/m2 and at all red flash luminances (P < 0.050), revealing reduction of outer retina contributions to PLR. OSA patients showed reduced peak latency for blue (-2, 0, 2, 2.4 log cd/m2) and red stimuli (-2, 0 log cd/m2; P < 0.040). No significant difference was found in the melanopsin-mediated PLR.

Conclusions

This study is the first to evaluate the inner and outer retinal contributions to PLR in OSA patients. The results showed that the outer retinal photoreceptor contributions to PLR were affected in moderate and severe OSA patients. In contrast, the inner retina contributions to PLR are preserved.

The flicker Pupil Light Response (fPLR)

PURPOSE

The photoreceptor classes driving the flicker pupil light response (fPLR) to monochromatic sinusoidal temporal modulation are largely unknown. Here, we determine the photoreceptor inputs to the fPLR.

METHODS

The 0.5-Hz fPLR was measured in healthy observers using a Maxwellian view (41° diameter) pupillometer at five narrowband wavelengths (short: 409 nm; intermediate: 462, 507, 530 nm; and long: 592 nm) over ∼10 log units of irradiance spanning scotopic to photopic levels (5.6 to 15.6 log quanta·cm-2·s-1; -6.9 to 3.6 log cd·m-2). The relative photoreceptor contributions to the fPLR were then derived from these amplitude-irradiance functions using a criterion fPLR.

RESULTS

The fPLR amplitude is small (≤ 3.9 ± 3.1%; mean ± SEM) below 8.0 log quanta·cm-2·s-1 then increases with retinal irradiance in accordance with a Hill function that asymptotes between 13.0 to 15.0 log quanta·cm-2·s-1 (wavelength dependent). The Hill slope is steepest for the intermediate wavelengths. Further increases in irradiance (>15.0 log quanta·cm-2·s-1) produce a distinct suppression of the fPLR for the intermediate wavelengths. The fPLR phase delay shows a linear decrease with increasing irradiance. The spectral sensitivity of the fPLR is dominated by inner retinal melanopsin ganglion cell and outer retinal rod photoreceptor inputs to the afferent pupil control pathway; the relative melanopsin : rhodopsin weighting decreases with the transition from photopic to scotopic lighting.

CONCLUSIONS

The fPLR can be used as a marker of melanopsin and rod interactions during the flicker stimulation and to quantify their contributions to the post-illumination pupil response (PIPR).

TRANSLATIONAL RELEVANCE

These irradiance and wavelength responses will be useful in standardizing the measurements of the fPLR using chromatic pupillometry.

Photosensitive Melanopsin-Containing Retinal Ganglion Cells in Health and Disease: Implications for Circadian Rhythms

Melanopsin-containing retinal ganglion cells (mRGCs) represent a third class of retinal photoreceptors involved in regulating the pupillary light reflex and circadian photoentrainment, among other things. The functional integrity of the circadian system and melanopsin cells is an essential component of well-being and health, being both impaired in aging and disease. Here we review evidence of melanopsin-expressing cell alterations in aging and neurodegenerative diseases and their correlation with the development of circadian rhythm disorders. In healthy humans, the average density of melanopsin-positive cells falls after age 70, accompanied by age-dependent atrophy of dendritic arborization. In addition to aging, inner and outer retinal diseases also involve progressive deterioration and loss of mRGCs that positively correlates with progressive alterations in circadian rhythms. Among others, mRGC number and plexus complexity are impaired in Parkinson's disease patients; changes that may explain sleep and circadian rhythm disorders in this pathology. The key role of mRGCs in circadian photoentrainment and their loss in age and disease endorse the importance of eye care, even if vision is lost, to preserve melanopsin ganglion cells and their essential functions in the maintenance of an adequate quality of life.

Light-Induced Pupillary Responses in Alzheimer's Disease

The impact of Alzheimer's disease (AD) on the pupillary light response (PLR) is controversial, being dependent on the stage of the disease and on the experimental pupillometric protocols. The main hypothesis driving pupillometry research in AD is based on the concept that the AD-related neurodegeneration affects both the parasympathetic and the sympathetic arms of the PLR (cholinergic and noradrenergic theory), combined with additional alterations of the afferent limb, involving the melanopsin expressing retinal ganglion cells (mRGCs), subserving the PLR. Only a few studies have evaluated the value of pupillometry as a potential biomarker in AD, providing various results compatible with parasympathetic dysfunction, displaying increased latency of pupillary constriction to light, decreased constriction amplitude, faster redilation after light offset, decreased maximum velocity of constriction (MCV) and maximum constriction acceleration (MCA) compared to controls. Decreased MCV and MCA appeared to be the most accurate of all PLR parameters allowing differentiation between AD and healthy controls while increased post-illumination pupillary response was the most consistent feature, however, these results could not be replicated by more recent studies, focusing on early and pre-clinical stages of the disease. Whether static or dynamic pupillometry yields useful biomarkers for AD screening or diagnosis remains unclear. In this review, we synopsize the current knowledge on pupillometric features in AD and other neurodegenerative diseases, and discuss potential roles of pupillometry in AD detection, diagnosis and monitoring, alone or in combination with additional biomarkers.