Neuronal Activity Influences Hemodynamics in the Paraoptic Short Posterior Ciliary Arteries: A Comparison between Healthy and Glaucomatous Subjects

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.

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.

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.

Neural control of choroidal blood flow

The choroid is richly innervated by parasympathetic, sympathetic and trigeminal sensory nerve fibers that regulate choroidal blood flow in birds and mammals, and presumably other vertebrate classes as well. The parasympathetic innervation has been shown to vasodilate and increase choroidal blood flow, the sympathetic input has been shown to vasoconstrict and decrease choroidal blood flow, and the sensory input has been shown to both convey pain and thermal information centrally and act locally to vasodilate and increase choroidal blood flow. As the choroid lies behind the retina and cannot respond readily to retinal metabolic signals, its innervation is important for adjustments in flow required by either retinal activity, by fluctuations in the systemic blood pressure driving choroidal perfusion, and possibly by retinal temperature. The former two appear to be mediated by the sympathetic and parasympathetic nervous systems, via central circuits responsive to retinal activity and systemic blood pressure, but adjustments for ocular perfusion pressure also appear to be influenced by local autoregulatory myogenic mechanisms. Adaptive choroidal responses to temperature may be mediated by trigeminal sensory fibers. Impairments in the neural control of choroidal blood flow occur with aging, and various ocular or systemic diseases such as glaucoma, age-related macular degeneration (AMD), hypertension, and diabetes, and may contribute to retinal pathology and dysfunction in these conditions, or in the case of AMD be a precondition. The present manuscript reviews findings in birds and mammals that contribute to the above-summarized understanding of the roles of the autonomic and sensory innervation of the choroid in controlling choroidal blood flow, and in the importance of such regulation for maintaining retinal health.

Is There Any Role for the Choroid in Glaucoma?

The choroid is part of the uveal tract and is a heavily vascularized bed that also contains connective tissue and melanin pigment. Given the role of the choroidal vasculature in the blood supply of the anterior laminar and prelaminar regions of the optic nerve head, the peripapillary choroid might be a relevant target for investigation in patients with glaucoma. The purpose of this paper is to critically review the current understanding of potential role of the choroid in the pathogenesis of glaucomatous damage.

Retinal neurovascular coupling in patients with glaucoma and ocular hypertension and its association with the level of glaucomatous damage

PURPOSE

To analyze neurovascular coupling in the retina of untreated primary open-angle glaucoma (POAG) and ocular hypertension (OHT) patients.

PATIENTS AND METHODS

Maximal vessel dilation in response to flicker light was analyzed with Retinal Vessel Analyzer (RVA) in temporal superior/inferior arterioles and veins in 51 POAG patients, 46 OHT and 59 control subjects. RVA parameters were compared between groups, between contralateral POAG eyes, and correlated to intraocular pressure, visual field mean defect and retinal nerve fiber layer thickness.

RESULTS

POAG eyes demonstrated generally smaller response of all vessels to flicker light than the other two groups (ANOVA p=0.026; mean arterial flicker response in percent of baseline, averaged superior and inferior was 3.48 ± 2.22 % for controls , 2.35 ± 2.06 % for POAG patients , and 2.97 ± 2.35 % for OHT patients; corresponding values for venules were 3.88 ± 1.98 %, 2.89 ± 1.72 %, 3.45 ± 2.77 %). There was no difference in flicker response between the eye with more and less advanced damage in each patient of the POAG group (ANOVA p=0.79). Correlation of flicker response to intraocular pressure (IOP) was borderline at best, correlations to the level of glaucomatous damage were not significant. Correlation of flicker response of superior and inferior vessels of the same eye was significant for the arteries (Pearson r=0.23, p=0.004), as well as venules (r=0.52, p<0.001).

CONCLUSION

General vessel response to flicker light was decreased in POAG patients, compared to normal controls and OHT patients. In contrast to significant correlation between the two contralateral eyes of the flicker response itself, only its borderline correlation to IOP was seen. There was no correlation to the level of damage, altogether indicating a systemic dysregulation phenomenon. GRANTS: Swiss National Foundation Grant 3200B0-113685, Velux Stiftung Grant, Freie Akademische Gesellschaft (FAG) Grant, Pfizer Inc. Grant CLINICAL TRIAL REGISTRATION REFERENCE NUMBER: ClinicalTrials.gov NCT00430209.

Use of colour Doppler imaging in ocular blood flow research

The main objective of this report is to encourage consistent quality of testing and reporting within and between centres that use colour Doppler imaging (CDI) for assessment of retrobulbar blood flow. The intention of this review is to standardize methods in CDI assessment that are used widely, but not to exclude other approaches or additional tests that individual laboratories may choose or continue to use.