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Cindy Albert, Grant Cull, Lin Wang, Brad Fortune, Stuart Keith Gardiner; Differences in optic nerve head capillary blood flow in glaucoma. Invest. Ophthalmol. Vis. Sci. 2019;60(9):5623.
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© ARVO (1962-2015); The Authors (2016-present)
It is known that blood flow within the optic nerve head (ONH) decreases in moderate and severe glaucoma. However in early glaucoma, clinical and animal studies have reported an increase in blood flow using various methods, but a decrease in capillary density using OCTA. To investigate this inconsistency, we used Laser Speckle Flowgraphy (LSFG) to measure pulsatile blood flow within the ONH capillaries, across a wide spectrum of disease severities.
75 eyes of 75 healthy volunteers and 101 eyes of 52 glaucoma patients were tested using LSFG and standard automated perimetry. Each 4-second LSFG image series was used to generate a composite mean blur rate (MBR) map representing total blood flow distribution within the ONH. After eliminating large blood vessels, capillary flow within the remaining ONH area was averaged into one heartbeat cycle, and parameterized as detailed in Figure 1. Each parameter was age-corrected to the equivalent value for a 60-year-old, based on linear regression among the healthy eyes. These age-corrected values were then compared against the disease status of the eye from functional testing.
Figure 1 shows plots of the age-corrected values against perimetric Mean Deviation (MD). Average ONH capillary blood flow (as assessed by MBR) increased in early glaucoma compared with healthy eyes, but decreased again with more severe damage. MBR was also more variable across the cardiac cycle in early glaucoma: flow increased more rapidly at pulse onset, and the difference between maximum and minimum flow was larger as a proportion of the average. This is illustrated by the age-matched examples shown in Figure 2. The groups of glaucoma eyes with or without functional loss are formally compared against normal eyes in Figure 2.
Basal blood flow through ONH capillary beds is increased at an early stage of glaucoma, presumably due to increased metabolic demand and/or disruption of autoregulatory control. However, OCTA studies have not shown any increase in capillary density in early glaucoma. Our results are therefore consistent with an increase in the biomechanical stiffness of the vessel walls. Increased basal flow through stiffer vessels would cause increased acceleration of flow at the start of the pulsatile cycle. Later in the disease process, capillary volume, total blood flow, and possibly also flow velocity, decrease with loss of neural tissue and metabolic demand.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.
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