Effects of water drinking test on ocular blood flow waveform parameters: A laser speckle flowgraphy study

Abstract

The water-drinking test (WDT) is a provocative test used in glaucoma research to assess the effects of elevated intraocular pressure (IOP). Defective autoregulation due to changes in perfusion pressure may play a role in the pathophysiology of several ocular diseases. This study aims to examine the effects of WDT on ocular blood flow (in the form of pulse waveform parameters obtained using laser speckle flowgraphy) to gain insight into the physiology of ocular blood flow and its autoregulation in healthy individuals. Changes in pulse waveform parameters of mean blur rate (MBR) in the entire optic nerve head (ONH), the vasculature of the ONH, the tissue area of the ONH, and the avascular tissue area located outside of the ONH were monitored over time. Significant increases in the falling rate of MBR over the entire ONH and its tissue area and decreases in blowout time (BOT) of the tissue area were observed only at 10 minutes after water intake. Significant increases in the skew of the waveform and the falling rate were observed in the vasculature of the ONH at 40 and 50 minutes after water intake, respectively. In the avascular region of the choroid, the average MBR increased significantly up to 30 minutes after water intake. Furthermore, the rising rate in this region increased significantly at 20 and 40 minutes, and the falling rate and acceleration-time index were both significantly increased at 40 minutes after water intake. Our results indicate the presence of effective autoregulation of blood flow at the ONH after WDT. However, in the choroidal region, outside of the ONH, effective autoregulation was not observed until 30 minutes after water intake in healthy study participants. These pulse waveform parameters could potentially be used in the diagnosis and/or monitoring of patients with glaucoma.

Fig 1. Spatiotemporal divisions of a typical mean blur rate (MBR) waveform.

Regions R₁-R₁₀ indicate the various regions above and below the average pulse waveform, and T₁-T₃ indicate distinct time intervals. T₁ refers to the time taken to complete one heartbeat, T₂ is the time interval where the value of an average pulse wave parameter is greater than the average MBR, and T₃ is the time taken to reach the maximum MBR value [43].

Fig 2. Dynamic localization of optic nerve head (ONH).

(a) Original frames from the CCD camera recording were converted to grayscale. (b) A threshold was applied to generate a binary image. (c) A disk-shaped structuring element was applied after eroding the image and a centroid (‘+’) was obtained. (d) ONH position is localized relative to the location of the centroid.

Fig 3. Timeline for LSFG recordings according to experiment design.

A baseline LSFG recording before drinking 1 liter of water, followed by six further recordings at 10-minute intervals.

Fig 4. Selected regions of interest.

(a) Whole ONH region (‘1’) in the heartbeat image, and the avascular region (‘2’), (b) ONH region extracted from the whole frame for cross-sectional analysis, and (c) Segmentation of the ONH into a vascular (white) and tissue area (black). The white patch in (a) and (b) refers to regions with MBR > 80.

Fig 5. Raw data from one of the participants.

(a) Baseline recording, where waveform parameters for the entire ONH were recorded (‘1’): average MBR = 20.50, skew = 11.27, BOT = 52.53, rising rate = 11.93, falling rate = 12.27, and ATI = 30.23; and from the avascular region (‘2’): average MBR = 4.17, skew = 14.50, BOT = 46.67, rising rate = 10.93, falling rate = 13.63, and ATI = 26.90 (b) Recording taken 10 minutes after water intake, where waveform parameters for the entire ONH were recorded (‘1’): average MBR = 24.57, skew = 11.77, BOT = 54.23, rising rate = 13.17, falling rate = 12.67, and ATI = 29.43; and from the avascular region (‘2’): average MBR = 6.40, skew = 11.30, BOT = 51.33, rising rate = 12.10, falling rate = 13.13, and ATI = 29.83.

Fig 6. Selected mean waveform parameters of the entire ONH region (‘1’).

(a) Heart rate, and (b) falling rate. Asterisk (*) indicates a statistically significant difference compared with baseline values (Wilcoxon signed-rank test: P <0.05) and vertical bars represent the standard errors of means.

Fig 7. Selected mean waveform parameters of the vascular ONH.

(a) Skew, and (b) Falling rate. Asterisk (*) indicates a difference that is considered statistically significant compared with baseline values (Wilcoxon signed-rank test: P <0.05) and vertical bars represent the standard errors of means.

Fig 8. Selected mean waveform parameters of the tissue ONH.

Selected mean waveform parameters of the tissue ONH. (a) BOT, and (b) Falling rate. Asterisk (*) indicates a statistically significant difference compared with baseline values (Wilcoxon signed-rank test: P <0.05) and vertical bars represent the standard errors of means.

Fig 9. Selected mean waveform parameters of the avascular region away from the ONH.

(a) Rising rate, (b) Falling rate, (c) ATI, and (d) Average MBR. Asterisk (*) indicates a statistically significant difference compared with baseline values (Wilcoxon signed-rank test: P < 0.05) and vertical bars represent the standard errors of means.