Changes in the Retinal Blood Flow after an Acute Intraocular Pressure Elevation Observed by Optical Coherence Tomography Angiography


 Background:Although the pathogenesis of glaucoma is not fully understood ,an elevated intraocular pressure (IOP) is a major factor contributing to its development and progression. The aim of this study was to investigate the changes in the retinal blood flows of the macula and optic nerve head (ONH) after an acute elevation in the intraocular pressure (IOP) observed using optical coherence tomography angiography (OCTA).
Methods: This was a prospective comparative study of subjects with narrow anterior chamber angles who underwent laser peripheral iridotomies (LPIs). The IOP was measured before and one hour after the LPI. The retinal blood flows of the macula and ONH were measured using OCTA at the baseline and one hour after the LPI.
Results: A total of 64 eyes of 51 individuals were enrolled in this study, and 58 eyes of 43 individuals finally completed the study with a mean IOP rise of 10.5 ± 7.6 mmHg after the LPI. Based on the magnitude of the rise in the IOP, we divided the subjects into three groups: group A = IOP rise ≤ 10 mmHg, group B = 10 mmHg < IOP rise ≤ 20 mmHg, and group C = IOP rise > 20 mmHg. The vessel density did not differ after the acute IOP elevation in either the macular region or papillary region in group A or group B (p > 0.05), but there was a significant difference in group C (p < 0.05). However, when the subjects were not separated into groups, the vessel densities of the ONH and macular region did not differ between the measurements obtained at the baseline and one hour after the LPI (p>0.05).
Conclusion: In these normal subjects, an acute mild or moderate IOP elevation for one hour after the LPI did not affect the blood flow in the macula or ONH, as examined using OCTA. However, when the IOP rise was greater than 20 mmHg, the macular and papillary blood flows decreased significantly.

3 between the local arterial blood pressure and the IOP. However, there are complex regulatory mechanisms involved in the ONH and retinal microcirculation [6] . Relevant studies have proven that a change in the ocular perfusion pressure does not directly translate into a change in the blood flow in the capillary network of the ONH; however, the ONH microcirculation does have autoregulatory abilities. That is, it can maintain constant blood flow in the ONH during ocular perfusion pressure changes within a certain range [7][8] . However, an elevated IOP may be the cause of decreased autoregulation [7] .
Some of the techniques that have been used previously to detect the retinal blood flow include fluorescein angiography, indocyanine green angiography, laser Doppler flowmetry, color Doppler imaging, color Doppler optical coherence tomography (OCT), magnetic resonance imaging, and retinal photographic oximetry [9][10][11][12][13][14][15] . However, all of these methods have disadvantages associated with them, such as invasiveness, insufficient resolution, a large variation rate, and an inability to show capillaries. OCT angiography (OCTA) is a new noninvasive blood flow imaging technique that is based on split-spectrum amplitude-decorrelation angiography (SSADA) with OCT while acquiring retinal vascular imaging in only a few seconds. It allows one to measure the macula and ONH microcirculation in various retinal layers [16][17][18][19][20] . The stability and repeatability of using OCTA to measure the retinal blood flow have been widely demonstrated [21][22][23] .
In this study, we used OCTA to assess whether there were changes in the retinal and papillary blood flows in patients with an acute IOP rise after undergoing a laser peripheral iridotomy (LPI).

Participants
The study subjects included all of the patients with narrow anterior chamber angles who routinely and consecutively underwent LPIs and exhibited IOP elevations afterwards, from September 2018 to December 2018, at the First People's Hospital of Huzhou, Zhejiang Province, China. The inclusion criteria were as follows: > 18 years old, narrow anterior chamber angles without occlusion, IOP ≤ 21 mmHg before the LPI, transparent refractive media, no medication, no retinal diseases, no optic nerve diseases, no diabetes, no ocular trauma, and OCTA images of sufficient quality (a signal strength index higher than 50 and accurate retinal stratification).

Methods of operation and examination
This was a prospective comparative study of subjects with narrow anterior chamber angles who underwent LPIs. The device used to perform the LPIs was an Nd:YAG laser (Ellex Medical, Adelaide, South Australia) with the following parameters: wavelength = 1,064 nm, energy = 6-12 MJ, and spot diameter = 30 um. Preoperatively, proparacaine hydrochloride eye drops were given once for the topical anesthesia, and an Abraham contact lens was placed in the conjunctival sac. An upper temporal or upper nasal iris treatment site was selected, the single pulse mode was used, and the slit lamp brightness was increased to shrink the pupil. After the LPI, tobramycin and dexamethasone eye drops were applied three times (once every 10 minutes), and then four times a day for one week.
Before the LPI and one hour after the LPI, the IOP was measured using noncontact tonometry (CT-60; Topcon Ltd., Tokyo, Japan). All of the IOP measurements were taken three times, and the mean value of the three measurements was used for the statistical analysis. Shortly after measuring the IOP, the vasculature of the macula and ONH was visualized using OCTA. If the IOP was greater than 30 mmHg, it was decreased with medication immediately following the examination. We used the RTVue XR OCT system (ReVue software, version 2017.1.1.155; Optovue Inc., Fremont, CA, USA) with the Angio Retina mode (6×6 mm) and the Angio Disc (4.5×4.5 mm) mode.
The details of the OCTA techniques have been described previously [24] . In order to evaluate the blood flow in the papillary region, we measured the vessel density (VD) in the radial peripapillary capillary (RPC) layer, which reaches from the upper boundary of the inner limiting membrane to the lower boundary of the nerve fiber layer. The ONH VD included the VD of the whole optic papilla (WVD), the VD inside the disc (IVD), and the peripapillary VD (PVD). To evaluate the blood flow in the macular region, we measured the VD of the superficial retinal layer, which extended from approximately 3 μm below the inner limiting membrane to 15 μm below the inner plexiform layer, and the deep retinal layer, which extended from 15 μm below the inner plexiform layer to 70 μm below the inner plexiform layer ( Figure 1).

Statistical analysis
The statistical analysis was performed using the Statistical Package for the Social Sciences (version 17; SPSS Inc., Chicago, IL, USA). The measurements taken before and after the LPI were compared using the paired Student t test. All of the measurements were described as the mean ± standard error. A p value of less than 0.05 was considered to be statistically significant.

Results
A total of 64 eyes of 51 individuals were enrolled in our study, of which 8 eyes of 8 individuals were excluded due to offset deviations or poor imaging signals (signal strength index lower than 50).
According to the magnitude of rise in the IOP, we divided the subjects into three groups: group A = IOP rise ≤ 10 mmHg, group B = 10 mmHg < IOP rise ≤ 20 mmHg, and group C = IOP rise > 20 mmHg. There were 29 eyes (20 individuals) in group A, with a mean IOP rise of 4.2 ± 1.4 mmHg, there were 18 eyes (12 individuals) in group B, with a mean IOP rise of 12.8 ± 2.9 mmHg, and there were 11 eyes (11 individuals) in group C, with a mean IOP rise of 23.4 ± 2.7 mmHg ( Table 1).

Discussion
Currently, noninvasive diagnostic methods, such as OCTA, have become more and more popular, and they are gradually replacing invasive fundus examinations, like fluorescein angiography and indocyanine green angiography. OCTA takes advantage of the SSADA, which is based on the variations in the reflectance amplitude. It is sensitive to motion and flow in all directions, and it provides high quality three-dimensional angiography. Based on the results of our study, OCTA provides an accurate assessment of the microvasculature in the optic nerve and macula, with a few exceptions, such as the consistency of the scan areas before and after the LPI, pupil size, and poor imaging signals. Occasionally a subject's poor coordination could lead to inconsistencies in the scan areas before and after the LPI, so it is necessary to compare the two images immediately after the postoperative examination, and if necessary, conduct a reexamination or exclude the subject. In order to ensure good quality imaging, we used the slit lamp brightness to reduce the size of the pupils during the operation, instead of preoperatively shrinking the pupils with pilocarpine.
Recent studies using OCTA have demonstrated a reduced retinal blood flow in glaucoma cases when compared to control cases [25][26][27] . However, up until now, whether or not an elevated IOP has an effect on the retinal blood flow has not been determined, and there are not many related studies available in the literature. With the extensive developments that have been made in OCTA, we have a more intuitive and accurate understanding of the retinal blood flow, which is helpful for quantitative research. Animal experiments have shown that with a sharp increase in the IOP, the retinal blood flow decreases significantly [28][29] . Hollo′ found that a large medical IOP reduction may result in a clinically significant increase in the peripapillary capillary perfusion in the retinal nerve fiber layer in young individuals with untreated high IOPs [30] . However, the research by Zhang et al. indicated that healthy eyes with acute and physiological IOP elevations of 10 mmHg or 15 mmHg for two hours did not show significant changes in the capillary VD in either the ONH or macula, as examined using OCTA [31] . The results of our study revealed that mild to moderate IOP increases have little effect on the macular and ONH blood flows, but an IOP elevation that exceeds 20 mmHg causes the blood flow to decrease significantly. The reason for this may be that the retinal autoregulation ability plays an important role, and when the IOP elevation exceeds the retinal autoregulation ability, the retinal blood flow is reduced, which is consistent with the results of the study by Cherecheanu et al. [7] .
Previous IOP elevation models mainly employed prone provocative testing in a dark room [31][32] , while we employed an LPI to create a high IOP model, which was caused by the iris debris and inflammation after the LPI. As far as we know, the IOP after a prone provocative test in a dark room is timesensitive, and its normalization begins to occur quickly; however, the high IOP model after an LPI can overcome these shortcomings. Nevertheless, the anterior chamber opacity may affect the OCTA signal after an LPI. The reason why we chose one hour after the LPI for the relevant examinations was because we observed another 29 patients after LPIs and found that the IOPs peaked about one hour after the LPIs, and that the anterior chamber transparency was acceptable. However, 8 eyes of 8 subjects were excluded due to the influence of the anterior chamber opacity, which led to offset deviations or poor imaging signals.
The limitations of this study are as follows. First, our study included only patients with narrow anterior chamber angles, and we do not know whether the results can be generalized to any other individuals.
Second, in the papillary region, we only observed the RPCs, which made up a vascular network with the retinal nerve fiber layer around the optic disc, and they are considered to be vulnerable to retinopathies, such as retinal vascular occlusion and glaucoma. We did not observe the deep blood flow of the optic papilla [33][34] .Third, the limited number of samples in this study, especially in the cases with IOP increases greater than 20 mmHg after the LPI, may have had an impact on the outcomes. Therefore, further research is necessary.

Conclusions
In conclusion, in these normal subjects, an acute mild or moderate IOP elevation for one hour after an LPI did not affect the blood flow in either the macula or ONH, as examined using OCTA. However, when the IOP increase was more than 20 mmHg, the macular and papillary blood flows decreased significantly.

Funding
No current funding sources for this study.

Availability of data and materials
The datasets used and analyzed during the current study are available from the corresponding author on reasonable request.

Authors' contributions
9 ZWM carried out the entire procedure,including research design, data collection, data analysis, manuscript writing and submission. WHQ and DNZ assisted with the acquisition of the data.WHY,XFP and HC participated in the design of the study.All authors reviewed and approved the final manuscript.

Ethics approval and consent to participate
This study was conducted according to the tenets of the Declaration of Helsinki.All of the patients signed informed consent forms, and the study protocol was approved by the ethics committee of the First People's Hospital of Huzhou, Zhejiang Province, China.

Consent for publication
Not applicable.  The age and IOP values are given as the mean ± standard error. The values are given as the mean ± standard error %. The values are given as the mean ± standard error %.