The Effect of Systemic Hyperoxia on Optic Nerve Head Blood Flow in Primary Open-Angle Glaucoma Patients

The association between intraocular pressure dynamics during dark-room prone testing and intraocular pressure over a relatively long-term follow-up period in primary open-glaucoma patients

PURPOSE

To investigate the relationship between the dynamics of intraocular pressure (IOP) during dark-room prone testing (DRPT) and IOP over a relatively long-term follow-up period.

METHODS

This retrospective study enrolled 84 eyes of 51 primary open-angle glaucoma patients who underwent DRPT for whom at least three IOP measurements made using Goldmann applanation tonometry were available over a maximum follow-up period of two years. We excluded eyes with a history of intraocular surgery or laser treatment and those with changes in topical anti-glaucoma medication during the follow-up period. In DRPT, IOP was measured in the sitting position, and after 60 min in the prone position in a dark room, IOP was measured again. In this study, IOP fluctuation refers to the standard deviation (SD) of IOP, and IOP max indicates the maximum value of IOP during the follow-up. The relationship between these parameters was analyzed with a linear mixed-effects model, adjusting for clinical parameters including age, gender, and axial length.

RESULTS

IOP increased after DRPT with a mean of 6.13 ± 3.55 mmHg. IOP max was significantly associated with IOP after DRPT (β = 0.38; p < 0.001). IOP fluctuation was significantly associated with IOP change in DRPT (β = 0.29; p = 0.007).

CONCLUSION

Our findings suggest that short-term and relatively long-term IOP dynamics are associated. Long-term IOP dynamics can be predicted by DRPT to some extent.

The Relationship Between Kiritsu-Meijin-Derived Autonomic Function Parameters and Visual-Field Defects in Eyes with Open-Angle Glaucoma

PURPOSE

This retrospective cross-sectional study aimed to investigate the association between autonomic parameters measured using the Kiritsu-Meijin device and visual-field defects in patients with open-angle glaucoma.

METHODS

A total of 79 eyes of 42 patients with open-angle glaucoma were enrolled in this study. Kiritsu-Meijin testing comprised three phases: sitting, standing, and sitting again (2 min, 2 min, and 1 min, respectively). Continuous electrocardiograms were recorded for five minutes. Autonomic parameters were extracted from the resulting data and analyzed, including activity, balance, reaction, switchover, and recovery; these are five representative parameters derived from Kiritsu-Meijin testing. Correlations between these parameters and mean deviation from Humphrey visual field testing were determined. Additionally, we used a linear mixed-effects model to observe sectoral differences in the relationship between total deviation and the Kiritsu-Meijin parameters. In this study, we focused on superior, central, and inferior total deviations.

RESULTS

Significant positive correlations were observed between activity, balance, and recovery and mean deviation values (β = 0.29-0.38, p < .05). The β value between activity and inferior total deviation was higher than that between activity and superior total deviation (β = 0.22, p < .05). Balance did not show any sectoral differences (p > .05). Recovery was more strongly associated with central to inferior total deviation than superior total deviation (β = 0.17-0.25, p < .05).

CONCLUSION

Our findings suggest that in patients with open-angle glaucoma, lower activity and recovery are associated with more severe central and/or inferior visual field defects in the superior quadrant. These results imply that measurements of autonomic function made with the Kiritsu-Meijin device may have clinical utility in the management of glaucoma.

Neurovascular dysfunction in glaucoma

Retinal ganglion cells, the neurons that die in glaucoma, are endowed with a high metabolism requiring optimal provision of oxygen and nutrients to sustain their activity. The timely regulation of blood flow is, therefore, essential to supply firing neurons in active areas with the oxygen and glucose they need for energy. Many glaucoma patients suffer from vascular deficits including reduced blood flow, impaired autoregulation, neurovascular coupling dysfunction, and blood-retina/brain-barrier breakdown. These processes are tightly regulated by a community of cells known as the neurovascular unit comprising neurons, endothelial cells, pericytes, Müller cells, astrocytes, and microglia. In this review, the neurovascular unit takes center stage as we examine the ability of its members to regulate neurovascular interactions and how their function might be altered during glaucomatous stress. Pericytes receive special attention based on recent data demonstrating their key role in the regulation of neurovascular coupling in physiological and pathological conditions. Of particular interest is the discovery and characterization of tunneling nanotubes, thin actin-based conduits that connect distal pericytes, which play essential roles in the complex spatial and temporal distribution of blood within the retinal capillary network. We discuss cellular and molecular mechanisms of neurovascular interactions and their pathophysiological implications, while highlighting opportunities to develop strategies for vascular protection and regeneration to improve functional outcomes in glaucoma.

The Association between Vascular Abnormalities and Glaucoma-What Comes First?

Glaucoma is a leading cause of irreversible blindness worldwide. While intraocular pressure (IOP) presents a major risk factor, the underlying pathophysiology still remains largely unclear. The correlation between vascular abnormalities and glaucoma has been deliberated for decades. Evidence for a role played by vascular factors in the pathogenesis of glaucomatous neurodegeneration has already been postulated. In addition, the fact that glaucoma causes both structural and functional changes to retinal blood vessels has been described. This review aims to investigate the published evidence concerning the relationship between vascular abnormalities and glaucoma, and to provide an overview of the "chicken or egg" dilemma in glaucoma. In this study, several biomarkers of glaucoma progression from a vascular perspective, including endothelin-1 (ET-1), nitric oxide, vascular endothelial growth factor (VEGF), and matrix metalloproteinases (MMPs), were identified and subsequently assessed for their potential as pharmacological intervention targets.

Vasoreactivity of the optic nerve head, nailfold, and facial skin in response to cold provocation in normal-tension glaucoma patients

BACKGROUND

The dysfunction of optic nerve head (ONH) hemodynamics has been suggested to be involved in the pathogenesis of normal-tension glaucoma (NTG). The aim of this study was to compare vasoreactivity in the ONH, nailfold, and facial skin in response to cold-water provocation in NTG patients and healthy controls.

METHODS

We performed cold-water provocation in 14 eyes of 14 NTG patients and 15 eyes of 15 age-matched control subjects. Laser speckle flowgraphy-derived tissue-area mean blur rate (MT), skin blood flowmetry-derived pulse wave amplitude (PA), nailfold capillaroscopy-derived nailfold capillary diameter, and other clinical parameters were recorded at baseline and 4 and 6 min after the cold stimulus. We compared changes (as percentages) in these variables in the NTG and control subjects with a linear mixed-effects model and evaluated correlations between these changes with Spearman's rank correlation coefficient.

RESULTS

The interaction term between the NTG group (reference, control group) and the 4-min protocol step (reference, baseline) significantly affected the changes in MT, nailfold capillary diameter and PA (β = -9.51%, P = 0.017, β = -20.32%, P = 0.002; β =  + 18.06%, P = 0.017, respectively). The change in MT was positively correlated with the change in nailfold capillary diameter, and negatively correlated with the change in PA (r = 0.39, P = 0.036; r = -0.40, P = 0.031, respectively).

CONCLUSION

NTG patients showed abnormal vasoconstriction in the ONH and nailfold and vasodilation in the facial skin in response to cold-water provocation.

A Pilot Study Assessing Retinal Blood Flow Dysregulation in Glaucoma Using Erythrocyte Mediated Velocimetry

Purpose

The purpose of this study was to compare autoregulation of retinal arteriolar and venular blood flow in patients with glaucoma, glaucoma suspect participants, and control participants using erythrocyte mediated velocimetry.

Methods

This prospective cohort pilot study included 7 eyes of 5 participants with glaucoma, 15 eyes of 8 glaucoma suspect participants, and 11 eyes of 6 control participants. Mean erythrocyte velocity in retinal arterioles and venules was measured using erythrocyte mediated velocimetry at room air and after oxygen supplementation. Change in erythrocyte velocity was compared among all groups using generalized estimating equations.

Results

In total, 64 vessels (18 with glaucoma, 31 that were glaucoma suspect, and 15 controls) of 33 eyes of 19 participants were analyzed. There was no significant difference in baseline velocities in arterioles or venules among the three groups. With induction of hyperoxia, mean arterial erythrocyte velocity decreased in glaucoma (-7.2 ± 13.7%), which differed from controls and glaucoma suspects where erythrocyte velocity increased with hyperoxia by 4.6 ± 13.3% (P = 0.002) and 7.2 ± 21.7% (P = 0.03), respectively. A higher baseline arteriolar velocity (β = -3.9% per mm/s, P = 0.002), glaucoma diagnosis (β = -21.1%, P = 0.03), and White race (β = -20.0%, P = 0.01) were associated with decreased velocity in response to arterial hyperoxia.

Conclusions

Hyperoxia increased erythrocyte velocity in control and glaucoma suspect participants, but decreased erythrocyte velocity in glaucoma participants, possibly due to impaired autoregulation. Baseline velocity, glaucoma diagnosis, and White race were associated with a decrease in velocity with induction of hyperoxia.

Translational Relevance

The European Medicines Agency (EMA) permits precision measurements of blood flow which may aid in the development of biomarkers of glaucoma-related dysregulation of blood flow.

Histological and Top-Down Proteomic Analyses of the Visual Pathway in the Cuprizone Demyelination Model

A change in visual perception is a frequent early symptom of multiple sclerosis (MS), the pathoaetiology of which remains unclear. Following a slow demyelination process caused by 12 weeks of low-dose (0.1%) cuprizone (CPZ) consumption, histology and proteomics were used to investigate components of the visual pathway in young adult mice. Histological investigation did not identify demyelination or gliosis in the optic tracts, pretectal nuclei, superior colliculi, lateral geniculate nuclei or visual cortices. However, top-down proteomic assessment of the optic nerve/tract revealed a significant change in the abundance of 34 spots in high-resolution two-dimensional (2D) gels. Subsequent liquid chromatography-tandem mass spectrometry (LC-TMS) analysis identified alterations in 75 proteoforms. Literature mining revealed the relevance of these proteoforms in terms of proteins previously implicated in animal models, eye diseases and human MS. Importantly, 24 proteoforms were not previously described in any animal models of MS, eye diseases or MS itself. Bioinformatic analysis indicated involvement of these proteoforms in cytoskeleton organization, metabolic dysregulation, protein aggregation and axonal support. Collectively, these results indicate that continuous CPZ-feeding, which evokes a slow demyelination, results in proteomic changes that precede any clear histological changes in the visual pathway and that these proteoforms may be potential early markers of degenerative demyelinating conditions.

Peripapillary Vascular Reactivity in Primary Open-Angle Glaucoma With High Myopia by Using Optical Coherence Tomography Angiography

Purpose

To evaluate peripapillary vascular reactivity in primary open-angle glaucoma (POAG) with and without high myopia (HM) by using optical coherence tomography angiography (OCTA).

Methods

This prospective study enrolled 48 eyes with POAG, including 16 and 32 eyes with and without HM, respectively. The retinal peripapillary vessel density (VD) was repeatedly assessed using OCTA at baseline and after a hyperoxia test (breathing 80% oxygen). The VD changes between different oxygenation conditions were calculated to reflect the vasoreactivity. Linear regression was performed to determine the relationship between myopia and retinal vascular reactivity in patients with POAG. Systemic hemodynamic characteristics were also evaluated under both conditions.

Results

The VD was significantly reduced after hyperoxia in the whole image (baseline and hyperoxia: 41.4 ± 4.5 and 38.8 ± 4.4, respectively, P < 0.001) and in the peripapillary regions (44.3 ± 5.7 and 41.1 ± 5.4, respectively, P < 0.001) in POAG eyes without HM. However, in eyes with HM, the whole-image VD in hyperoxia was not significantly different from the baseline (baseline and hyperoxia: 40.5 ± 6.2 and 40.2 ± 6.2, respectively, P = 0.481). The VD changes in eyes with HM were significantly smaller than those in eyes without HM in both the whole image (0.3 ± 1.8 and 2.6 ± 2.0, respectively, P < 0.001) and peripapillary regions (1.1 ± 2.0 and 3.2 ± 2.3, respectively, P = 0.003). Linear regression results showed a significant correlation between retinal vascular reactivity and spherical equivalent (SE) (β = 0.28, P < 0.001, R² = 0.31) and axial length (AL) (β = −0.72, P < 0.001, R² = 0.33).

Conclusion

Retinal vasoreactivity of peripapillary capillaries in POAG eyes with HM was significantly impaired in comparison with that in POAG eyes without HM. A lower peripapillary vascular response was significantly associated with worse SE and elongated AL.

Pericyte dysfunction and loss of interpericyte tunneling nanotubes promote neurovascular deficits in glaucoma

Reduced blood flow and impaired neurovascular coupling are recognized features of glaucoma, the leading cause of irreversible blindness worldwide, but the mechanisms underlying these defects are unknown. Retinal pericytes regulate microcirculatory blood flow and coordinate neurovascular coupling through interpericyte tunneling nanotubes (IP-TNTs). Using two-photon microscope live imaging of the mouse retina, we found reduced capillary diameter and impaired blood flow at pericyte locations in eyes with high intraocular pressure, the most important risk factor to develop glaucoma. We show that IP-TNTs are structurally and functionally damaged by ocular hypertension, a response that disrupted light-evoked neurovascular coupling. Pericyte-specific inhibition of excessive Ca²⁺ influx rescued hemodynamic responses, protected IP-TNTs and neurovascular coupling, and enhanced retinal neuronal function as well as survival in glaucomatous retinas. Our study identifies pericytes and IP-TNTs as potential therapeutic targets to counter ocular pressure-related microvascular deficits, and provides preclinical proof of concept that strategies aimed to restore intrapericyte calcium homeostasis rescue autoregulatory blood flow and prevent neuronal dysfunction.

Topical Review: Studies of Ocular Function and Disease Using Hyperspectral Imaging

SIGNIFICANCE

Advances in imaging technology over the last two decades have produced significant innovations in medical imaging. Hyperspectral imaging (HSI) is one of these innovations, enabling powerful new imaging tools for clinical use and greater understanding of tissue optical properties and mechanisms underlying eye disease.Hyperspectral imaging is an important and rapidly growing area in medical imaging, making possible the concurrent collection of spectroscopic and spatial information that is usually obtained from separate optical recordings. In this review, we describe several mainstream techniques used in HSI, along with noteworthy advances in optical technology that enabled modern HSI techniques. Presented also are recent applications of HSI for basic and applied eye research, which include a novel method for assessing dry eye syndrome, clinical slit-lamp examination of corneal injury, measurement of blood oxygen saturation in retinal disease, molecular changes in macular degeneration, and detection of early stages of Alzheimer disease. The review also highlights work resulting from integration of HSI with other imaging tools such as optical coherence tomography and autofluorescence microscopy and discusses the adaptation of HSI for clinical work where eye motion is present. Here, we present the background and main findings from each of these reports along with specific references for additional details.

Retinal Microcirculatory Responses to Hyperoxia in Primary Open-Angle Glaucoma Using Optical Coherence Tomography Angiography

Purpose

To investigate the retinal vascular response to hyperoxia in patients with primary open-angle glaucoma (POAG) using optical coherence tomography angiography (OCTA).

Methods

This prospective study included 27 eyes in 27 patients with POAG and 14 eyes in 14 age- and sex-matched healthy participants. Retinal radial peripapillary capillary (RPC) perfusion was measured by OCTA before and after inhaling oxygen in all participants. Systemic hemodynamic variables were also examined and recorded before and after hyperoxia.

Results

Hyperoxia significantly reduced the perfused vessel density (PVD) of RPCs in both healthy controls (baseline and hyperoxia: 54.2 ± 4.1 and 51.0 ± 4.4, respectively, P < 0.001) and patients with POAG (baseline and hyperoxia: 44.7 ± 6.1 and 43.2 ± 5.4, respectively, P = 0.001). However, the changes in peripapillary PVD between the two gas conditions in patients with POAG were significantly lower than in healthy controls, including both the absolute change (baseline-hyperoxia: 1.5 ± 2.0 and 3.2 ± 1.2, respectively, P = 0.006) and relative change (ratio of absolute change and baseline value: 3.0% ± 4.6% and 6.0% ± 2.4%, respectively, P = 0.04).

Conclusions

Retinal microvasculature responds to hyperoxia by reducing RPC perfusion in both healthy participants and patients with POAG. However, this vasoreactivity capacity was significantly impaired in patients with POAG.

Diagnostic performance of laser speckle flowgraphy in glaucoma: a systematic review and meta-analysis

PURPOSE

To evaluate the diagnostic value of laser speckle flowgraphy (LSFG) in glaucoma by investigating the mean blur rate (MBR) in the optic nerve head.

METHODS

Systematic literature search was performed in the CENTRAL, Web of Science, PubMed, and EMBASE databases to obtain relevant studies published until December 2020 without restrictions. The Newcastle-Ottawa Scale (NOS) was used for study quality assessment. The outcome measures included the MBRs of the entire (MA), vascular (MV), and tissue (MT) areas. Subgroup analyses were performed according to glaucoma type. All data were analyzed using RevMan and Comprehensive Meta-Analysis 3.3 software.

RESULTS

Fifteen studies, including 692 glaucomatous and 386 healthy eyes, were included. Of these, 11 studies reported the MA, MV, and MT, three studies only reported MT, and one study only reported MV. All were classified as case-control studies and had good NOS scores. The meta-analysis showed that the MA and MT were significantly reduced in glaucomatous eyes (mean difference [MD] - 5.59, 95% confidence interval [CI] - 6.19 to - 4.99, p = 0.1; MD - 2.2, 95% CI - 2.49 to - 1.91, p = 0.07, respectively) with moderate heterogeneity (p = 0.1, I² = 38%; p = 0.07, I² = 39%, respectively). There was also a significant difference in the MV between glaucomatous and healthy eyes (MD - 5.92, 95% CI - 7.77 to - 4.07) with significant heterogeneity (p = 0.0003, I² = 69%). The subgroup analyses revealed significant differences in the MBR among different glaucoma types.

CONCLUSIONS

Glaucoma is closely related to ocular blood flow changes. This meta-analysis suggests that LSFG is a feasible diagnostic tool for glaucoma. However, further longitudinal prospective studies are needed.

Time-Course Changes in Optic Nerve Head Blood Flow and Retinal Nerve Fiber Layer Thickness in Eyes with Open-angle Glaucoma

PURPOSE

To determine whether decreased optic nerve head (ONH) blood flow (BF) precedes or follows decreased circumpapillary retinal nerve fiber layer thickness (cpRNFLT) in eyes with open-angle glaucoma (OAG).

DESIGN

Retrospective, longitudinal study.

PARTICIPANTS

This study followed up 350 eyes of 225 OAG patients for at least 2 years and collected data from each patient from at least 5 examinations obtained with laser speckle flowgraphy (LSFG) and OCT.

METHODS

In the superior, temporal, and inferior ONH quadrants, tissue area mean blur rate (MT), representing ONH tissue BF, was measured with LSFG, whereas cpRNFLT was measured with OCT. A multivariate linear mixed-effects model was used to identify potential predictors of faster MT decrease, adjusting for possible confounding factors. Based on these results, each quadrant of each patient was assigned a risk point if the quadrant was the superior or temporal, if patient age was older than the median (61 years), and if patient pulse rate was higher than median (74 beats per minute). The quadrants were then compared with a mixed-effects Cox model for MT and cpRNFLT changes, defined as a difference between the baseline value and the values from the latest 2 consecutive follow-up visits of more than 1.96 × the corresponding coefficient of variation.

MAIN OUTCOME MEASURES

Ophthalmic and systemic variables and MT and cpRNFLT in the superior, temporal, and inferior quadrants.

RESULTS

The multivariate model showed that MT decrease was faster in older patients with higher pulse rate and slower in inferior quadrants (P < 0.05). Quadrants with 0 risk points showed primary cpRNFLT decrease (P = 0.048), 1-risk point quadrants showed simultaneous cpRNFLT and MT decrease (P = 0.260), and 2-risk point and 3-risk point quadrants showed primary MT decrease (P < 0.001).

CONCLUSIONS

Older patients with higher pulse rate are at greater risk of a primary reduction in ONH tissue BF, that is, preceding cpRNFLT decrease, in the superior and temporal quadrants.

Correlation between Blood Flow and Temperature of the Ocular Anterior Segment in Normal Subjects

Purpose: To determine a correlation between temperature and blood flow in the ocular anterior segment, and their effects on corneal temperature. Methods: In experiment 1, we recruited 40 eyes and measured the temperature and blood flow in the ocular anterior-segment (upper/lower eyelid skin, palpebral and bulbar conjunctiva, and cornea) before and after application of warm compresses. In experiment 2, we recruited 20 eyes and measured the same tissues before and during stimulation using water and capsaicin solution in the oral cavity. Results: In experiment 1, the temperatures of the upper/lower eyelid skin and cornea increased significantly until 15 min after the application of the warm compress; the temperatures of the palpebral and bulbar conjunctiva increased significantly until 10 min. The blood flow in the upper/lower eyelid skin and bulbar conjunctiva increased significantly until 10 min, and that of the palpebral conjunctiva increased significantly until 15 min. In experiment 2, the temperatures were correlated significantly with the blood flow in the upper and lower eyelid skin and palpebral and bulbar conjunctiva. The temperature of all locations and palpebral conjunctival blood flow contributed independently to the corneal temperature. Conclusions: In the ocular anterior segment, the temperature and blood flow were correlated significantly, and contributed to the corneal temperature.

The optic nerve head vasoreactive response to systemic hyperoxia and visual field defect progression in open-angle glaucoma, a pilot study

PURPOSE

To investigate the effect of optic nerve head (ONH) tissue vasoreactivity on glaucoma visual field defect progression.

METHODS

This prospective, longitudinal study comprised 28 eyes of 16 patients with open-angle glaucoma. Baseline data were obtained from oxygen inhalation testing, including laser speckle flowgraphy measurements of tissue-area mean blur rate (MT), which represents ONH tissue blood flow, and the mean percentage change in MT (MT-change). Follow-up data were collected for at least 2 years, including at least 5 reliable visual field tests. The average total deviation (TD) was calculated in each sector of the Garway-Heath map; in this study, superior, central and inferior TD corresponded to inferior, temporal and superior MT or MT-change, respectively. MT and MT-change in three sectors per eye were included as explanatory variables in a multivariable linear mixed-effects model, with TD slope set as the response variable.

RESULTS

At baseline, lower MT and higher diastolic blood pressure were associated with lower MT-change (p < 0.05). Additionally, MT-change significantly contributed to TD slope in the corresponding sectors (β = 0.41, p = 0.01).

CONCLUSIONS

Weaker ONH tissue vasoreactivity to systemic hyperoxia, assessed with laser speckle flowgraphy, was associated with lower baseline ONH tissue blood flow, higher diastolic blood pressure and rapid visual field defect progression. Laser speckle flowgraphy variables during oxygen provocation testing may represent potentially useful predictive biomarkers of glaucoma progression.

What is glaucomatous optic neuropathy?

Glaucomatous optic neuropathy (GON) is the pathohistological feature of glaucoma in the optic nerve. The pathogenesis of GON has been hypothesized, to either originate from compromised mechanical conditions at the lamina cribrosa or as associated with pathological vascular involvement. From a historical perspective, glaucoma is the degeneration of retinal ganglion cells (RGC) due to the elevation of intraocular pressure (IOP). The consensus of glaucoma treatment is generally accepted as sufficient IOP reduction. Is there an additional option to treat GON from the perspective of the vascular theory? In this section, two distinguished leaders in glaucoma research advance their views and discuss the current opinions surrounding the two theories regarding the causes of GON in primary open angle glaucoma (POAG) and normal tension glaucoma (NTG).

The Impact of Intraocular Pressure Elevation on Optic Nerve Head and Choroidal Blood Flow

Purpose

To use laser speckle flowgraphy (LSFG) to assess blood flow (BF) in the optic nerve head (ONH) tissue and choroid during elevated intraocular pressure (IOP).

Methods

This prospective study included 20 eyes of 20 healthy volunteers. The testing protocol had a baseline phase, two elevated IOP phases (+10 and +20 mm Hg), and a recovery phase. IOP was elevated by pushing against the eyelid with a novel tubular device attached to the LSFG apparatus. Measurement parameters in each phase included: LSFG-derived mean blur rate (MBR) and flow acceleration index (FAI); systemic parameters, and IOP. The % change against baseline was calculated for each phase. The protocol was repeated five times to calculate the coefficient of variation (CV) for % change MBR and to determine the effect of mydriasis on % change MBR. We compared % change MBR and FAI and evaluated the relationship between % change ocular perfusion pressure (OPP) and MBR in the choroid and ONH tissue.

Results

The % change MBR was highly reproducible (CV: 6.1-8.7%) and not affected by mydriasis (P = 0.57-0.96). The % change MBR and FAI were higher in the ONH tissue than choroid during IOP elevation (P = 0.04). The % change OPP and MBR showed positive linear correlations and two-segmental linear correlations in the choroid and ONH tissue, respectively (P < 0.01).

Conclusion

Hemodynamics during IOP elevation differ in the choroid and ONH tissue. LSFG enables highly reproducible assessment of the dynamic autoregulation of ocular BF in the ONH tissue.

Optic nerve head blood flow regulation during changes in arterial blood pressure in patients with primary open-angle glaucoma

PURPOSE

Abnormal autoregulation of optic nerve head blood flow (ONHBF) has been postulated to play an important role in primary open-angle glaucoma (POAG). We used laser Doppler flowmetry (LDF) to estimate quantitatively the ONHBF and compared ONHBF autoregulation between glaucoma patients and healthy controls during isometric exercise.

METHODS

Forty patients with POAG and 40 healthy age- and sex-matched subjects underwent three periods of isometric exercise, each consisting of 2 min of handgripping. Optic nerve head blood flow (ONHBF) was measured continuously using LDF. Systemic blood pressure, intraocular pressure and ocular perfusion pressure were assessed in all participants.

RESULTS

Isometric exercise was associated with an increase in ocular perfusion pressure during all handgripping periods in both groups (p < 0.001). However, there was no change in ONHBF in either group. Three of the glaucoma patients and two of the healthy subjects showed a consistent 10% decrease in blood flow during isometric exercise, in spite of an increase in their blood pressure. This difference between groups was not significant (p = 0.61). Four other glaucoma subjects showed a consistent increase in blood flow of more than 10% during isometric exercise, whereas this was not seen in healthy subjects (p = 0.035).

CONCLUSION

This study suggests that abnormal ONHBF autoregulation is more often seen in patients with POAG than healthy control subjects. The relationship to the glaucoma disease process is currently unknown and requires further investigation.

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.

Human Optic Nerve: An Enhanced Proteomic Expression Profile

Ophthalmology and visual health are new frontiers for postgenomic research and technologies such as proteomics. In this context, the optic nerve and retina extend as the outgrowth of the brain, wherein the latter receives the optical input and the former relays the information for processing. While efforts to understand the optic nerve proteome have been made earlier, there exists a lacuna in its biochemical composition and molecular functions. We report, in this study, a high-resolution mass spectrometry-based approach using an Orbitrap Fusion Tribrid mass spectrometer to elucidate the human optic nerve proteomic profile. Raw spectra were searched against NCBI Human RefSeq 75 database using SEQUEST HT and MASCOT algorithms. We identified nearly 35,000 peptides in human optic nerve samples, corresponding to 5682 proteins, of which 3222 proteins are being reported for the first time. Label-free quantification using spectral abundance pointed out to neuronal structural proteins such as myelin basic protein, glial fibrillary acidic protein, and proteolipid protein 1 as the most abundant proteins. We also identified several neurotransmitter receptors and postsynaptic density synaptosomal scaffold proteins. Pathway analysis revealed that a majority of the proteins are structural proteins and have catalytic and binding activity. This study is one of the largest proteomic profiles of the human optic nerve and offers the research community an initial baseline optic nerve proteome for further studies. This will also help understand the protein dynamics of the human optic nerve under normal conditions.