Dissociation of Pupillary Post-Illumination Responses from Visual Function in Confirmed OPA1 c.983A > G and c.2708_2711delTTAG Autosomal Dominant Optic Atrophy

Basics, benefits, and pitfalls of pupillometers assessing visual function

Numerous commercially and non-commercially available pupillometers are nowadays able to assess various biological functions in humans, by evaluating pupils' dynamics in response to specific stimuli. However, the use of pupillometers for ophthalmic afferent evaluations (i.e., photoreceptoral responses) in real-world settings is relatively limited. Recent scientific and technological advances, coupled with artificial intelligence methods have improved the performance of such devices to objectively detect, quantify, and classify functional disturbances of the retina and the optic nerve. This review aims to summarize the scientific principles, indications, outcomes, and current limitations of pupillometry used for evaluation of afferent pathways in ophthalmic clinical settings.

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.

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.

Melanopsin Retinal Ganglion Cells and Pupil: Clinical Implications for Neuro-Ophthalmology

Melanopsin retinal ganglion cells (mRGCs) are intrinsically photosensitive RGCs that mediate many relevant non-image forming functions of the eye, including the pupillary light reflex, through the projections to the olivary pretectal nucleus. In particular, the post-illumination pupil response (PIPR), as evaluated by chromatic pupillometry, can be used as a reliable marker of mRGC function in vivo. In the last years, pupillometry has become a promising tool to assess mRGC dysfunction in various neurological and neuro-ophthalmological conditions. In this review we will present the most relevant findings of pupillometric studies in glaucoma, hereditary optic neuropathies, ischemic optic neuropathies, idiopathic intracranial hypertension, multiple sclerosis, Parkinson's disease, and mood disorders. The use of PIPR as a marker for mRGC function is also proposed for other neurodegenerative disorders in which circadian dysfunction is documented.

Choroideremia: melanopsin-mediated postillumination pupil relaxation is abnormally slow

PURPOSE

To investigate the rod-cone and melanopsin pupillary light response (PLR) pathways in choroideremia.

METHODS

Eight patients with choroideremia and 18 healthy age-matched controls underwent chromatic pupillometry by applying blue (463 nm) and red light (643 nm) at 100 lux intensity to the right eye while recording pupil diameters. Absolute baseline pupil size (mm), normalized maximal pupil constriction and the early and late postillumination pupillary dilation, from 0 to 10 seconds and 10 to 30 seconds after the end of illumination, respectively, were determined. Postillumination responses to blue light were considered to be primarily driven by melanopsin activation of the intrinsic photosensitive retinal ganglion cells.

RESULTS

Baseline pupil diameters were comparable in patients with choroideremia and control subjects (p = 0.48). The maximum pupil constriction in patients with choroideremia was severely weakened in red light but only mildly weakened in blue light (p < 0.05). Postillumination dilation of the pupil was normal after red illumination but extremely protracted after blue illumination. Also, in contrast to healthy subjects, no abrupt change in the dilation curve was seen in the patients after the end of blue illumination, the early-phase dilation being completely abolished (p < 0.01).

CONCLUSION

Rod-cone-driven pupil responses were decreased as expected in an outer retinal degeneration, and near-normal pupil constriction in blue light supports that the melanopsin system is normal. In contrast, the lack of brisk early-phase dilation after blue illumination in choroideremia is remarkable and may be interpreted to mean that the absence of photoreceptor inhibition promotes a tonic contraction of the pupil.

Pupillometric evaluation of the melanopsin containing retinal ganglion cells in mitochondrial and non-mitochondrial optic neuropathies

In recent years, chromatic pupillometry is used in humans to evaluate the activity of melanopsin expressing intrinsic photosensitive retinal ganglion cells (ipRGCs). Blue light is used to stimulate the ipRGCs and red light activates the rod/cone photoreceptors. The late re-dilation phase of pupillary light reflex is primarily driven by the ipRGCs. Optic neuropathies i.e. Leber hereditary optic neuropathy (LHON), autosomal dominant optic atrophy (ADOA), nonarteritic anterior ischemic optic neuropathy (NAION), glaucoma, optic neuritis and idiopathic intracranial hypertension (IIH) are among the diseases, which have been subject to pupillometric studies. The ipRGCs are differentially affected in these various optic neuropathies. In mitochondrial optic neuropathies, the ipRGCs are protected against degeneration, whereas in glaucoma, NAION, optic neuritis and IIH the ipRGCs are damaged. Here, we will review the results of pupillometric, histopathological and animal studies evaluating the ipRGCs in mitochondrial and non-mitochondrial optic neuropathies.

Dominant Optic Atrophy and Leber's Hereditary Optic Neuropathy: Update on Clinical Features and Current Therapeutic Approaches

Dominant optic atrophy (DOA) and Leber hereditary optic neuropathy (LHON) are the two most common inherited optic neuropathies encountered in clinical practice. This review provides a summary of recent advances in the understanding of the clinical manifestations, current treatments, and ongoing clinical trials of these two optic neuropathies. Substantial progress has been made in the understanding of the clinical, genetic, and pathophysiological basis of DOA and LHON. Pathogenic OPA1 gene mutations in DOA and 3 primary mutations of mitochondrial DNA in LHON-induced mitochondrial dysfunction, which in turn leads to increased reactive oxygen species levels in mitochondria and possibly insufficient ATP production. The pathologic hallmark of these inherited optic neuropathies is primary degeneration of retinal ganglion cells, preferentially in the papillomacular bundle, which results in temporal optic disc pallor and central or cecocentral visual loss. There are no effective treatments for patients with LHON and DOA, although clinical trials are underway for the former. Translational research for these diseases is entering an accelerated phase with the availability of animal models, and a variety of pharmacological and genetic therapies are being developed.

Dominant optic atrophy: updates on the pathophysiology and clinical manifestations of the optic atrophy 1 mutation

PURPOSE OF REVIEW

Review recent advances in clinical and experimental studies of dominant optic atrophy (DOA) to better understand the complexities of pathophysiology caused by the optic atrophy 1 (OPA1) mutation.

RECENT FINDINGS

DOA is the most commonly diagnosed inherited optic atrophy, causing progressive bilateral visual loss that begins early in life. During the past 25 years, there has been substantial progress in the understanding of the clinical, genetic, and pathophysiological basis of this disease. The histopathological hallmark of DOA is the primary degeneration of retinal ganglion cells, preferentially in the papillomacular bundle, which results temporal optic disc pallor and cecocentral scotomata in patients with DOA. Loss of OPA1 protein function by OPA1 gene mutations causes mitochondrial dysfunction because of the loss of mitochondrial fusion, impaired mitochondrial oxidative phosphorylation, increases in reactive oxygen species, and altered calcium homeostasis. These factors lead to apoptosis of retinal ganglion cells by a haploinsufficiency mechanism.

SUMMARY

Improved understanding of the pathophysiology of DOA provides insights that can be used to develop therapeutic approaches to the DOA.

Multiethnic involvement in autosomal-dominant optic atrophy in Singapore

PurposeAutosomal-dominant optic atrophy (ADOA), often associated with mutations in the OPA1 gene (chromosome 3q28-q29) is rarely reported in Asia. Our aim was to identify and describe this condition in an Asian population in Singapore.Patients and methodsPreliminary cross-sectional study at the Singapore National Eye Centre, including patients with clinical suspicion of ADOA, who subsequently underwent genetic testing by direct sequencing of the OPA1 gene.ResultsAmong 12 patients (10 families) with clinically suspected ADOA, 7 patients (5 families) from 3 different ethnic origins (Chinese, Indian, and Malay) carried a heterozygous pathogenic variant in the OPA1 gene. The OPA1 mutations were located on exons 8, 9, 11, and 17: c.869G>A (p.Arg290Glu), c.892A>G (p.Ser298Gly), c.1140G>A (splicing mutation), and c.1669C>T (p.Arg557*), respectively. One splicing mutation (c.871-1G>A) was identified in intron 8. We also identified a novel mutation causing optic atrophy and deafness (c.892A>G (p.Ser298Gly)). Among the phenotypic features, colour pupillometry disclosed a dissociation between low vision and preserved pupillary light reflex in ADOA.ConclusionWe report the first cases of genetically confirmed OPA1-related ADOA from Singapore, including a novel mutation causing 'ADOA plus' syndrome. Further epidemiological studies are needed in order to determine the prevalence of ADOA in South-East Asia.

Pupillary response to direct and consensual chromatic light stimuli

PURPOSE

To assess whether the direct and consensual postillumination (ipRGC-driven) pupil light responses to chromatic light stimuli are equal in healthy subjects.

METHODS

Pupil responses in healthy volunteers were recorded using a prototype binocular chromatic pupillometer (IdeaMedical, Copenhagen), which is capable of both direct and consensual pupillometry measurements. The device uses a pair of dual monochromatic narrow bandwidth LED light sources, red (660 nm) and blue (470 nm). Pupil light responses were recorded with infrared video cameras and analysed using custom-made circuitry and software. Subjects were randomized to receive light stimuli at either the right or left eye after 5 min of dark adaptation. Pupil light responses were recorded in both eyes for 10 seconds before illumination, during illumination and 50 seconds after illumination with red and blue light. Three variables were defined for the recorded pupil responses: the maximal constriction amplitude (CAmax ), the pupil response during illumination and postillumination pupil response (PIPR).

RESULTS

No difference was found in the pupil response to blue light. With red light, the pupil response during illumination was slightly larger during consensual illumination compared to direct illumination (0.54 and 0.52, respectively, p = 0.027, paired Wilcoxon's test, n = 12), while no differences were found for CAmax or the PIPR.

CONCLUSIONS

No difference was found between direct and consensual pupil response to either red or blue light in the postillumination period. Direct and consensual responses can readily be compared when examining the postillumination pupil response to blue light as estimation of photosensitive retinal ganglion cell activation.

Genotype-phenotype heterogeneity of ganglion cell and inner plexiform layer deficit in autosomal-dominant optic atrophy

PURPOSE

To describe the thickness of the combined ganglion cell and inner plexiform layers (GC-IPL) and the peripapillary retinal nerve fibre layer (RNFL) in patients with OPA1 c.983A>G or c.2708_2711delTTAG autosomal-dominant optic atrophy (ADOA).

METHODS

The study included 20 individuals with c.983A>G and nine individuals with c.2708_2711delTTAG. Data for comparison were drawn from 49, previously published, individuals with OPA1 c.2826_2836delinsGGATGCTCCA and 51 individuals with no OPA1 mutation. Subjects underwent refraction, best-corrected visual acuity assessment, axial length measurement and high-definition optical coherence tomography.

RESULTS

There was overlap in GC-IPL thickness in subjects younger than 20-30 years between the two new groups of ADOA patients and controls. Numerical decreases in GC-IPL thickness with age did not reach statistical significance in individuals with c.983A>G (p = 0.18) or in healthy controls (p = 0.22), but it did in individuals with c.2708_2711delTTAG (p = 0.02). Visual acuity decreased with decreasing GC-IPL thickness (p = 0.0006 in c.983A>G and p = 0.0084 in c.2708_2711delTTAG). Unlike c.2826_2836delinsGGATGCTCCA, individuals with c.983A>G or c.2708_2711delTTAG did not show a pattern of maximum GC-IPL deficit inferonasal of the fovea.

CONCLUSION

Genotype-phenotype heterogeneity in OPA1 ADOA is evident when inner retinal atrophy is examined as a function of age. Thus, a pronounced decline with age in GC-IPL thickness is observed in c.2708_2711delTTAG ADOA, an intermediate decline with age is observed in c.983A>G ADOA, whereas little or no change with age is observed in c.2826_2836delinsGGATGCTCCA ADOA. This genotype-phenotype heterogeneity may explain why some patients have progressive visual loss while others have a relatively stable prognosis.