The reduction of temporal optic nerve head microcirculation in autosomal dominant optic atrophy

Hereditary Optic Neuropathies: A Systematic Review on the Interplay between Biomaterials and Induced Pluripotent Stem Cells

Hereditary optic neuropathies (HONs) such as dominant optic atrophy (DOA) and Leber Hereditary Optic Neuropathy (LHON) are mitochondrial diseases characterized by a degenerative loss of retinal ganglion cells (RGCs) and are a cause of blindness worldwide. To date, there are only limited disease-modifying treatments for these disorders. The discovery of induced pluripotent stem cell (iPSC) technology has opened several promising opportunities in the field of HON research and the search for therapeutic approaches. This systematic review is focused on the two most frequent HONs (LHON and DOA) and on the recent studies related to the application of human iPSC technology in combination with biomaterials technology for their potential use in the development of RGC replacement therapies with the final aim of the improvement or even the restoration of the vision of HON patients. To this purpose, the combination of natural and synthetic biomaterials modified with peptides, neurotrophic factors, and other low- to medium-molecular weight compounds, mimicking the ocular extracellular matrices, with human iPSC or iPSC-derived cell retinal progenitors holds enormous potential to be exploited in the near future for the generation of transplantable RGC populations.

Optical coherence tomography angiography in the multimodal assessment of the retinal posterior pole in autosomal dominant optic atrophy

PURPOSE

To assess retinal vascular involvement in patients with autosomal dominant optic atrophy (ADOA) genetically confirmed by the presence of the OPA1 (Optic Atrophy 1) gene mutation using a multimodal protocol of investigation of retinal posterior pole.

METHODS

In this cross-sectional, case-control, observational study, both eyes of 13 patients with a genetic diagnosis of ADOA were compared with both eyes of 13 healthy controls (HCs). All subjects underwent full ophthalmological examination, spectral domain-optical coherence tomography (SD-OCT), fundus perimetry (FP) and OCT angiography (OCTA).

RESULTS

Vessel density (VD) of the superficial and deep macular vascular plexi and of the radial peripapillary capillary plexus were significantly decreased (p ≤ 0.001) in ADOA patients compared with HCs. The area under the receiver operating characteristics analysis also revealed high values of sensitivity and specificity of OCTA parameters in distinguish between patients and HCs. A strong correlation (Pearson Coefficient, r = 0.91) emerged between OCTA VD of the superficial retinal plexus and the average Ganglion Cell Layer (GCL) thickness as measured by SD-OCT; a slightly lower correlation (Pearson Coefficient, r = 0.89) was also found between VD of the deep plexus and the average GCL thickness of the same eyes in patients with ADOA. The correlation among values of differential light sensitivity (DLS) measured by FP with VD and GCL thickness parameters was also investigated. The correlation analysis among DLS and the VD parameters showed from low-to-moderate correlation (ranging from r = 0.29 for the deep fovea VD to r = 0.59 for the deep whole image VD). The correlation coefficient between the mean DLS and the average thickness of GCL was more significant (Pearson Coefficient, r = 0.75). A significant correlation emerged also between the VD and the visual acuity, in terms of LogMAR BCVA (best-corrected visual acuity), especially for the VD of the deep capillary plexus (Pearson Coefficient for the Deep whole Image VD and LogMAR BCVA r = -0.75; for the Deep parafovea VD and LogMAR BCVA r = -0.78).

CONCLUSION

Retinal microvascular assessment by OCTA angiography can provide relevant clinical information on retinal involvement in ADOA patients. In patients with genetically confirmed OPA1-related ADOA, there is a decrease in retinal vessel density associated with GCL thinning and DLS reduction.

Dominant optic atrophy: Culprit mitochondria in the optic nerve

Dominant optic atrophy (DOA) is an inherited mitochondrial disease leading to specific degeneration of retinal ganglion cells (RGCs), thus compromising transmission of visual information from the retina to the brain. Usually, DOA starts during childhood and evolves to poor vision or legal blindness, affecting the central vision, whilst sparing the peripheral visual field. In 20% of cases, DOA presents as syndromic disorder, with secondary symptoms affecting neuronal and muscular functions. Twenty years ago, we demonstrated that heterozygous mutations in OPA1 are the most frequent molecular cause of DOA. Since then, variants in additional genes, whose functions in many instances converge with those of OPA1, have been identified by next generation sequencing. OPA1 encodes a dynamin-related GTPase imported into mitochondria and located to the inner membrane and intermembrane space. The many OPA1 isoforms, resulting from alternative splicing of three exons, form complex homopolymers that structure mitochondrial cristae, and contribute to fusion of the outer membrane, thus shaping the whole mitochondrial network. Moreover, OPA1 is required for oxidative phosphorylation, maintenance of mitochondrial genome, calcium homeostasis and regulation of apoptosis, thus making OPA1 the Swiss army-knife of mitochondria. Understanding DOA pathophysiology requires the understanding of RGC peculiarities with respect to OPA1 functions. Besides the tremendous energy requirements of RGCs to relay visual information from the eye to the brain, these neurons present unique features related to their differential environments in the retina, and to the anatomical transition occurring at the lamina cribrosa, which parallel major adaptations of mitochondrial physiology and shape, in the pre- and post-laminar segments of the optic nerve. Three DOA mouse models, with different Opa1 mutations, have been generated to study intrinsic mechanisms responsible for RGC degeneration, and these have further revealed secondary symptoms related to mitochondrial dysfunctions, mirroring the more severe syndromic phenotypes seen in a subgroup of patients. Metabolomics analyses of cells, mouse organs and patient plasma mutated for OPA1 revealed new unexpected pathophysiological mechanisms related to mitochondrial dysfunction, and biomarkers correlated quantitatively to the severity of the disease. Here, we review and synthesize these data, and propose different approaches for embracing possible therapies to fulfil the unmet clinical needs of this disease, and provide hope to affected DOA patients.

OCTA in neurodegenerative optic neuropathies: emerging biomarkers at the eye-brain interface

OCTA imaging in optic neuropathies.

Optic Nerve Head Blood Flow Analysis in Patients with Optic Disc Drusen Using Laser Speckle Flowgraphy

Visual field defects are common in patients with optic disc drusen (ODD). Our aim was to examine whether reduced optic nerve head (ONH) microcirculation is related to visual field defects in ODD patients. Vascular and tissue area mean blur rate (MBR_V and MBR_T), measured using laser speckle flowgraphy (LSFG), was significantly lower in the 32 included ODD eyes when compared with 40 healthy eyes (p <.05). There was a moderate correlation between the difference in MBR_T and the perimetric mean defect (R² = 0.53) in ODD patients. These findings demonstrate the utility of LSFG in examining ONH blood flow in ODD patients.

Mitochondrial disorders and the eye

Mitochondria are cellular organelles that play a key role in energy metabolism and oxidative phosphorylation. Malfunctioning of mitochondria has been implicated as the cause of many disorders with variable inheritance, heterogeneity of systems involved, and varied phenotype. Metabolically active tissues are more likely to be affected, causing an anatomic and physiologic disconnect in the treating physicians' mind between presentation and underlying pathophysiology. We shall focus on disorders of mitochondrial metabolism relevant to an ophthalmologist. These disorders can affect all parts of the visual pathway (crystalline lens, extraocular muscles, retina, optic nerve, and retrochiasm). After the introduction reviewing mitochondrial structure and function, each disorder is reviewed in detail, including approaches to its diagnosis and most current management guidelines.

A Missense Mutation in OPA1 Causes Dominant Optic Atrophy in a Chinese Family

Background

To investigate the genetic causes and clinical characteristics of dominant optic atrophy (DOA) in a Chinese family.

Methods

A 5-generation pedigree of 35 family members including 12 individuals affected with DOA was recruited from Shenzhen Eye Hospital, China. Four affected family members and one unaffected family member were selected for whole exome sequencing. Sanger sequencing was used to confirm and screen the identified mutation in 18 members of the family. The disease-causing mutation was identified by bioinformatics analysis and confirmed by segregation analysis. The clinical characteristics of the family members were analyzed.

Results

A heterozygous missense mutation (c.1313A>G, p.D438G) in optic atrophy 1 (OPA1) was identified in 10 individuals affected with DOA in this family. None of the unaffected family members had the mutation. Patients in this family had vision loss since they were children or adolescence. The visual acuity decreased progressively to hand movement, except for one patient (IV-12) who had relatively good vision of 20/30 and 20/28. The fundus typically manifested as optic disc pallor. The visual fields, optical coherence tomography, and visual evoked potential suggested variable degree of abnormality in patients. Patients who had a history of cigarette smoking and alcohol drinking had more severe clinical manifestations.

Conclusions

Our results suggest that the p.D438G mutation in OPA1 causes optic atrophy in this family. The patients who carried the mutation demonstrated heterogeneous clinical manifestations in this family. This is the first report on the c.1313A>G (p.D438G) mutation of OPA1 in a Chinese family affected with DOA.

Non-invasive measurement techniques for quantitative assessment of optic nerve head blood flow

Diseases of the optic nerve head involving changes in blood flow are common. However, the pathophysiology is not always fully understood. Several non-invasive methods for measuring optic nerve head blood flow are available, but currently no gold standard has been established. Methods for measuring blood flow in optic neuropathies including colour Doppler imaging, retinal function imager, optical coherence tomography angiography and laser speckle flowgraphy are reviewed. Ultrasound colour Doppler imaging is a fast measurement technique where several different parameters, especially the blood flow velocity, can be calculated. Though used for many years in ophthalmology, its use is not standardized and it requires significant observer skills. The retinal function imager is a direct method where the haemoglobin in erythrocytes is visualized and blood flow velocities in retinal vessels are calculated from a series of photos. The technique is not suitable for direct measurement of blood flow within the optic nerve head. Laser speckle flowgraphy uses a laser light which creates a light scatter pattern in the tissue. Particles moving in the area causes changes in the speckle pattern from which a relative blood flow can be estimated. It is, however, not known whether optic nerve head microcirculation is measurable with the technique. Optical coherence tomography angiography uses multiple scans to evaluate blood flow with good reproducibility but often problems with artefacts. The technique is continuously being refined and increasingly used in research as a tool for the study of blood flow in retinopathies and optic neuropathies. Most of the conducted studies are based on small sample sizes, but some of the methods show promising results in an optic nerve head blood flow research setting. Further and larger studies are required to provide standardized and comparable measurements before one or more of the methods can be considered clinical helpful in daily practice.

Evaluation of flicker induced hyperemia in the retina and optic nerve head measured by Laser Speckle Flowgraphy

Purpose

The coupling between neural activity and blood flow is a physiological key principle of ocular blood flow regulation. The current study was performed to investigate whether Laser speckle flowgraphy (LSFG), a commercially available technique for measuring blood flow, is capable to assess flicker-induced haemodynamic changes in the retinal and optic nerve head (ONH) circulation.

Methods

Twenty healthy subjects were included in this cross sectional study. A commercial LSFG instrument was used to measure blood flow at the ONH as well as in retinal vessels before and during stimulation with flickering light. Mean blur rate (MBR), a measure of relative blood flow velocity, was obtained for the ONH and relative flow volume (RFV) a measure of relative blood flow of the respective retinal vessels.

Results

Stimulation with flicker light increased ONH MBR by +17.5%±6.6% (p<0.01). In retinal arteries, flicker stimulation led an increase of +23.8±10.0% (p<0.05) in total RFV. For retinal veins, an increase of +23.1%±11.0 (p<0.05) in total RFV was observed during stimulation. A higher response was observed in nasal RFV compared to temporal RFV in retinal arteries (nasal: +28.9%±20.0%; temporal: +20.4%±17.6%, p<0.05) and veins (nasal: +28.3%±19.6%; temporal +17.8%±18.9%, p<0.05).

Conclusion

As shown previously with other techniques, flicker stimulation leads to an increase in retinal and optic nerve head blood flow. Our results indicate that LSFG is an appropriate method for the quantification of retinal and ONH blood flow during visual stimulation and may be used as a non-invasive, easy to use tool to assess neuro-vascular coupling in humans.

Increased Microcirculation on Optic Nerve Head by Laser Speckle Flowgraphy at Early Stage of Leber Hereditary Optic Neuropathy

Leber hereditary optic neuropathy (LHON) is a mitochondrial disorder predominantly affecting young men. Characteristic features of an early stage of LHON are peripapillary telangiectatic microangiopathy with optic disc hyperaemia and swelling of the retinal nerve fibre layers. We evaluated the microcirculation of the optic nerve head (ONH) by laser speckle flowgraphy (LSFG) in a 79-year-old man and a 36-year-old woman with LHON. The ONH microcirculation of the tissue area was markedly increased in the early stage in both patients. LSFG may be a useful noninvasive method to suspect individuals to have an early stage of LHON.

Association between mitochondrial DNA damage and ocular blood flow in patients with glaucoma

Background/Aims

We determined the relationship between tissue mean blur rate (MT) and mitochondrial dysfunction, represented by the mitochondrial/nuclear DNA (mtDNA/nDNA) ratio. We also investigated the usefulness of these biomarkers.

Methods

We assessed ocular blood flow in 123 eyes of 123 patients with open-angle glaucoma (OAG) and 37 control eyes of 37 healthy subjects by measuring MT in the optic nerve head with laser speckle flowgraphy. We measured mtDNA and nDNA with PCR, calculated the mtDNA/nDNA ratio and compared this ratio with MT using Spearman’s rank test. We used multiple regression analysis to further investigate the association between MT and glaucoma in the most severe group.

Results

The control and the patients with glaucoma had significant differences in the mtDNA/nDNA ratio, circumpapillary retinal nerve fibre layer thickness and MT. There was no significant relationship between the mtDNA/nDNA ratio and MT in patients with OAG overall or the female patients with OAG, but there was a significant relationship between the mtDNA/nDNA ratio and MT, temporal-MT and superior-MT in male patients with severe OAG (r=−0.46, p=0.03; r=−0.51, p=0.02; r=−0.61, p<0.01, respectively). Furthermore, we found that the mtDNA/nDNA ratio was an independent contributor to temporal-MT and superior-MT in these patients (p<0.01 and p=0.03, respectively).

Conclusion

We found that there was a significant relationship between the mtDNA/nDNA ratio and MT in male patients with severe OAG, suggesting that the mtDNA/nDNA ratio may be a new biomarker in glaucoma and may help research on the vulnerability of these patients to mitochondrial dysfunction.

Thickness mapping of individual retinal layers and sectors by Spectralis SD-OCT in Autosomal Dominant Optic Atrophy

PURPOSE

To assess layer- and location-specific retinal thickness deficits in autosomal dominant optic atrophy (ADOA) using Spectralis SD-OCT.

METHODS

This cross-sectional study included 41 ADOA patients with OPA1 exon 28 (2826delT) mutation [age, 8.6-83.5 years; best-corrected visual acuity (BCVA), 8-89 Early Treatment Diabetic Retinopathy Study (ETDRS) letters] and 55 mutation-free first-degree relatives as healthy controls (age, 8.9-68.7; BCVA, 80-99). Participants underwent routine examination and optical coherence tomography (OCT) with segmentation of the whole retina, inner retinal layers (IRL) and outer retinal layers (ORL). Individual segmentation was performed of the perifoveal retinal nerve fibre layer (RNFL), ganglion cell layer (GCL), inner plexiform layer (IPL), inner nuclear layer (INL), outer plexiform layer (OPL), outer nuclear layer (ONL), retinal pigment epithelium (RPE) and the peripapillary RNFL. Combinations of layers and sectors were tested for their diagnostic significance. Only right eye data are presented. Statistical analysis was adjusted for age, gender, spherical equivalent, axial length and family clustering in a mixed model analysis.

RESULTS

The perifoveal RNFL, GCL, IPL and the peripapillary RNFL were all significantly thinner in ADOA patients than in healthy controls (p < 0.0001). No statistical difference was found for other layers. The most prominent and diagnostically most valuable deficit was found in the GCL (-49.9%) in the 'nasal inner macula' (NIM) sector (-63%). Attenuation of the peripapillary RNFL was most significant in the temporal sector (-58.4%).

CONCLUSION

In ADOA, retinal ganglion cells are most prominently reduced in the nasal perifoveal area of the GCL, which together with the temporal peripapillary RNFL area serves as the strongest diagnostic OCT marker.

Mitochondrial disorders of the retinal ganglion cells and the optic nerve

OBJECTIVES

To summarise and discuss recent findings and future perspectives concerning mitochondrial disorders (MIDs) affecting the retinal ganglion cells and the optic nerve (mitochondrial optic neuropathy. MON).

METHOD

Literature review.

RESULTS

MON in MIDs is more frequent than usually anticipated. MON may occur in specific as well as non-specific MIDs. In specific and non-specific MIDs, MON may be a prominent or non-prominent phenotypic feature and due to mutations in genes located either in the mitochondrial DNA (mtDNA) or the nuclear DNA (nDNA). Clinically, MON manifests with painless, bilateral or unilateral, slowly or rapidly progressive visual impairment and visual field defects. In some cases, visual impairment may spontaneously recover. The most frequent MIDs with MON include LHON due to mutations in mtDNA-located genes and autosomal dominant optic atrophy (ADOA) or autosomal recessive optic atrophy (AROA) due to mutations in nuclear genes. Instrumental investigations for diagnosing MON include fundoscopy, measurement of visual acuity, visual fields, and color vision, visually-evoked potentials, optical coherence tomography, fluorescein angiography, electroretinography, and MRI of the orbita and cerebrum. In non-prominent MON, work-up of the muscle biopsy with transmission electron microscopy may indicate mitochondrial destruction. Treatment is mostly supportive but idebenone has been approved for LHON and experimental approaches are promising.

CONCLUSIONS

MON needs to be appreciated, requires extensive diagnostic work-up, and supportive treatment should be applied although loss of vision, as the most severe outcome, can often not be prevented.

Thickness of macular inner retinal layers and peripapillary retinal nerve fibre layer in neuromyelitis optica spectrum optic neuritis and isolated optic neuritis with one episode

PURPOSE

The aim of this study was to evaluate the differences between macular inner retinal layers and peripapillary retinal nerve fibre layer (pRNFL) thickness in Chinese patients with neuromyelitis spectrum optic neuritis (NMOSD-ON) and isolated optic neuritis (ION) with only one episode.

METHODS

This cross-sectional study included 35 patients (35 eyes) with NMOSD-ON (NMO-IgG seropositive) and 46 patients (46 eyes) with ION after one episode. Spectral domain optical coherence tomography (SD-OCT) was used to quantify pRNFL, macular RNFL (mRNFL), ganglion cell and inner plexiform layers (GCIPL) and inner nuclear layer (INL) thickness using an automated algorithm. Differences in OCT parameters between NMOSD-ON and ION were compared after adjusting for age, sex and disease duration.

RESULTS

The pRNFL and mRNFL in some locations (average pRNFL, nasal pRNFL, nasal inferior (NI) pRNFL, nasal/temporal (N/T) ratio pRNFL, average mRNFL, inner temporal mRNFL, outer nasal mRNFL and outer temporal mRNFL) in NMOSD-ON differed significantly from those in ION (all p < 0.05). These parameters had moderate diagnostic accuracy, with area under curves (AUCs) ranging from 0.684 to 0.762 for pRNFL and from 0.660 to 0.700 for mRNF. The thickness of GC-IPL and INL in all sectors was similar in NMOSD-ON and ION (p > 0.05). This study and our meta-analysis of four previous studies obtained consistent results, with pooled mean difference (MD) -10.4 μm (95% CI: -12.4 to -8.4, p < 0.001) for pRNFL, -1.5 μm (95% CI: -3.5 to 0.6, p = 0.158) for mRNFL and 0.2 μm (95% CI: -0.4 to 0.9, p = 0.490) for GC-IPL, respectively.

CONCLUSIONS

Neuromyelitis spectrum optic neuritis (NMOSD-ON) patients had more pRNFL and mRNFL loss compared to ION patients after one episode. Spectral domain optical coherence tomography (SD-OCT) may help to distinguish NMOSD-ON from ION with only moderate diagnostic accuracy.