Brain activity affects dynamic but not static autoregulation

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.

Transient Visual Loss in Young Females with Crowded Optic Discs: A Proposed Aetiology

I present four cases of transient visual loss (TVL) in young females with crowded optic discs. One patient had asymmetrical cup-to-disc ratios and only experienced TVL in the eye with the more crowded disc. I review the evidence for blood flow autoregulatory dysfunction within crowded optic discs in combination with reduced ocular perfusion pressure to propose a possible aetiology for both unilateral and bilateral TVL in young females with crowded optic discs.

Longitudinal alterations in the dynamic autoregulation of optic nerve head blood flow revealed in experimental glaucoma

PURPOSE

To use a novel dynamic autoregulation analysis (dAR) to test the hypothesis that the optic nerve head (ONH) blood flow (BF) autoregulation is disrupted during early stages of experimental glaucoma (EG) in nonhuman primates.

METHODS

Retinal nerve fiber layer thickness (RNFLT, assessed by optical coherence tomography) and ONH BF (assessed by laser speckle imaging technique) were measured biweekly before and after unilateral laser treatment to the trabecular meshwork. Each nonhuman primate was followed until reaching either an early stage of damage (RNFLT loss < 20%, n = 6) or moderate to advanced stages of damage (RNFLT loss > 20%, n = 9). At each test, dAR was assessed by characterizing ONH BF changes during the first minute of rapid manometrical intraocular pressure (IOP) elevation from 10 to 40 mm Hg. The dAR analysis extracted the following parameters: baseline BF, average BF 10 seconds before IOP elevation; BFΔmax, maximum BF change from baseline BF; Tr, time from baseline BF to the BFΔmax; Kr, average descending BF rate.

RESULTS

Mean postlaser IOP was 20.2 ± 5.9 and 12.3 ± 2.6 mm Hg in EG and control eyes, respectively (P < 0.0001). Compared with prelaser values, baseline BF was higher in early EG, but lower in moderate to advanced EG (P = 0.01). Tr was increased and Kr was reduced in both stages (P < 0.01). BFΔmax was smaller in the early EG (P = 0.05) and remained low in the moderate to advanced EG (P = 0.15). No changes in the parameters were observed in control eyes.

CONCLUSIONS

Chronic IOP elevation causes ONH autoregulation dysfunction in the early stage of EG, characterized by a disrupted BF response and delayed Tr, revealed by dAR analysis.

Static blood flow autoregulation in the optic nerve head in normal and experimental glaucoma

PURPOSE

To characterize the static blood flow autoregulation in the optic nerve head (ONH), and to investigate its role in hemodynamic changes in experimental glaucoma (EG).

METHODS

Unilateral elevation of intraocular pressure (IOP) was induced in 15 adult rhesus macaques by laser treatment to the trabecular meshwork. Prior to and after laser treatment, retinal nerve fiber layer thickness (RNFLT) was assessed, biweekly, by spectral-domain optical coherence tomography. Optic nerve head static autoregulation was assessed by determining the percentage blood flow (BF) change after the IOP was acutely increased from 10 to 30, 40, or 50 mm Hg manometrically, utilizing a laser speckle flowgraphy device.

RESULTS

Postlaser IOP (measured during average 7.7 ± 2.6 months) was 20.2 ± 5.9 mm Hg in EG eyes and 12.3 ± 2.6 mm Hg in control eyes (P < 0.0001). Retinal nerve fiber layer thickness was reduced by 33 ± 22% of the baseline values (P < 0.001) on average in EG eyes and by 0.4 ± 2.3% in control eyes (P > 0.05). The ONH BF remained at a constant level within a range of ocular perfusion pressure (OPP), 41 mm Hg and above. The autoregulation curves, created by all 723 tests in control and 352 tests in EG, were not significantly different (P = 0.71).

CONCLUSIONS

Optic nerve head BF in normal nonhuman primate (NHP) eyes is effectively regulated within a range of OPP approximately 41 mm Hg and above. Chronic IOP elevation causes no remarkable change to the static autoregulation within the ONH of EG eyes.

Quantification of dynamic blood flow autoregulation in optic nerve head of rhesus monkeys

Autoregulation capacity has been classically assessed with a 'two-point' measurement or static autoregulation (sAR). In such an approach, stabilized hemodynamic parameters are determined before and after a perfusion pressure challenge. Analysis of dynamic autoregulation (dAR), an early phase of blood flow response to a sudden perfusion pressure change is emerging as a preferred approach to assess the capacity of autoregulation in many non-ocular tissues and has developed rapidly in the last decade. The purpose of this study was to develop a method to quantify dAR in the optic nerve head (ONH). In six pentobarbital (6-9 mg/kg/h, IV) anesthetized rhesus monkeys, dAR was elicited by increasing intraocular pressure (IOP) from 10 to 30 or 40 mmHg (IOP(10-30)/IOP(10-40)) manometrically via switch between reservoirs connected to the anterior chamber. Relative blood flow changes during dAR in the ONH, estimated with a laser speckle flowgraph (LSFG), were continuously measured for 1 min. Time-domain parameters of dAR response, including: BF(Deltamax) (maximal blood flow decrease, %), K(r) (descending slope of blood flow from baseline to BF(Deltamax)) and T(r) (descending time of blood flow from baseline to BF(Deltamax)) were extracted and analyzed offline. For each monkey, same procedure was repeated three times during three different visits. The test-retest repeatability and inter-ocular difference of the parameters was statistically evaluated. During IOP(10-30) and IOP(10-40), the mean arterial BP was 89 +/- 7 and 85 +/- 6 mmHg, respectively. Immediately after the reservoir was switched, the blood flow started to decline and reached maximal in approximately 4 s. The blood flow then returned back toward baseline despite continuous IOP increase, which took 8-11 s to reach the level of the raised reservoir. The general pattern of blood flow responses was similar between IOP(10-30) and IOP(10-40) and there was no statistically significant difference for T(r) (P > 0.05). However, IOP(10-40) caused greater BF(Deltamax) and deeper K(r) than IOP(10-30) (P < 0.0001 and P < 0.05, respectively). The blood flow during steady state, 5 min after IOP elevation, showed no statistically significant difference from baseline (P > 0.05). All dAR parameters (T(r), K(r) and BF(Deltamax)) showed no significant difference across the 3 visits (Repeat measures ANOVA, P = 0.7, 0.2 and 0.2, respectively); the corresponding coefficients of variance were 24%, 43% and 34% during IOP(10-30) and 11.8%, 30.3% and 19.0% during IOP(10-40). The mean dAR parameters between the eyes showed no statistically differences (P = 0.6) during both IOP(10-30) and IOP(10-40). The current study showed that a rapid ocular perfusion pressure decrease induced by a sudden IOP step increase evoked a transient and reproducible dAR response in the ONH of non-human primates measured with LSFG. Quantitative analysis of dAR may provide a direct view of vasomotorial activity in the resistant vessels and thus a new approach to assess the autoregulatory capacity in the ONH.