Key words
coronavirus disease 2019 - retinal vascular structures - optical coherence tomography
angiography - vessel density - retinal microcirculation
Schlüsselwörter
Coronavirus-Krankheit 2019 - Netzhautgefäßstrukturen - Gefäßdichte - retinale Mikrozirkulation
- optische Kohärenztomografieangiografie
Introduction
In December 2019, China reported an outbreak of a severe acute respiratory pneumonia,
caused by a beta coronavirus strain, severe acute respiratory syndrome coronavirus
2 (SARS-CoV-2) [1]. The illness was termed coronavirus disease 2019 (COVID-19) [1]. The disease soon began to spread to other countries, prompting the World Health
Organization to declare COVID-19 a public health emergency and issue recommendations
for the prevention of the transmission of SARS-CoV-2.
To date, it has been shown that humans can be infected by seven types of coronavirus,
with SARS-CoV-2 being the most recent [2]. Most of the clinical manifestations of coronaviruses occur in the respiratory tract
and gastrointestinal system [3], [4]. Coronaviruses have also been associated with conjunctivitis in humans [5]. Moreover, retinal disorders, including retinal vasculitis [6], [7], retinal degeneration [8], [9], and blood-retinal barrier breakdown [10], have been reported in experimental animal models of coronavirus infection. However,
the ophthalmological complications of SARS-CoV-2 infection have not been well described
in human ocular structures.
Optical coherence tomography angiography (OCT-A) is an advance in retinal imaging
technology that allows clinicians to assess retinal blood vessel density (VD) and
the flow of macular capillary plexuses in humans both quantitatively and qualitatively
using a contrast-free technique [11]. Furthermore, OCT-A can provide three-dimensional macular perfusion maps [11], [12]. Recently, retinal vascular abnormalities have been detected with OCT-A in the early
post-recovery phase of COVID-19 [13], [14], [15].
The aim of this study was to assess retinal microcirculation in patients in the late
post-recovery period of confirmed COVID-19 compared to healthy controls, using OCT-A
to better understand the ocular characteristics of COVID-19 patients as well as the
clinical course of COVID-19-related ophthalmic complications.
Methods
This prospective cross-sectional study was conducted in a tertiary hospital. The study
is compatible with the tenets of the Declaration of Helsinki and was approved by the
local ethics committee. All of the study participants received oral and written information
about the study protocol and provided written informed consent.
The study recruited patients in the late post-recovery phase of COVID-19 who had been
discharged from the isolation ward of the hospital and had recovered enough to return
home. All participants had a confirmed diagnosis of COVID-19 (based on a positive
real-time reverse transcriptase-polymerase chain reaction [{RT-PCR}] test of respiratory
specimens) and tested negative after hospitalization. The ophthalmologic examination
was carried out after at least 90 days. A control group was created from age- and
sex-matched healthy subjects attending the ophthalmology clinic for a routine ocular
examination. Those with negative PCR results in the last 3 days were included in the
control group. All of the control subjects were healthy and had no systemic or ocular
diseases. Data was analyzed from only the right eye of each study participant.
Patients with the following conditions were excluded from the study: presence of any
retinal or chorioretinal disease, macular edema, a history of any systemic disorder
such as diabetes mellitus, previous ocular surgery or laser photocoagulation, a history
of ocular trauma, glaucoma, usage of a topical corticosteroid within six months of
study enrollment, anterior segment opacities, or a refractive error of 6 D or more.
Only images with a signal strength index (SSI) of 8.0 or greater were included in
the study. Poor-quality OCT-A images due to eye movements, poor fixation, or media
opacities were also excluded.
All study participants underwent a complete ophthalmological examination prior to
OCT-A examination. This included best corrected visual acuity testing via the Snellen
chart (6 m), intraocular pressure measurements using a pneumotonometer, ocular surface
and anterior segment assessment by slit lamp biomicroscopy, and dilated fundus examination.
OCT-A images were acquired using the AngioVue software (Version 2017.1.0.151) with
the RTVue XR Avanti imaging system (Optovue Inc., Fremont, CA, USA). All OCT-A scans
were carried out by the same experienced clinician (HIA) under the same environmental
conditions and at the same time of day (between 9 : 00 AM and 12 : 00 PM). The scans
were performed following dilation of the pupil with 1% tropicamide eye drops. During
scanning, subjects were asked to give their full attention to an internal fixation
target (after stabilization was ensured with a standard chinrest and forehead supports).
Three successive scans were taken of each eye, and the one with the best quality was
selected for analysis.
The OCT-A system used a split-spectrum amplitude decorrelation angiography algorithm.
To obtain OCT-A volumes including 400 X 400 A-scans, the split-spectrum amplitude
decorrelation angiography algorithm ran at 70.000 A-scans per second. Full eye scans
measuring 6 mm X 6 mm were obtained for the fovea. After each examination, the same
experienced independent grader (YSG) looked over the OCT-A images to ensure that the
correct segmentation was obtained and to identify any images unsuitable for analysis
due to motion artifacts or blurred images. Subjects with the following criteria were
excluded from the analysis: low SSI (< 8.0), corneal and lenticular opacity that obstructed
the view of the vasculature, poor fixation resulting in motion or doubling artifacts,
the presence of one or more blink artifacts, and the existence of cystoid macular
changes leading to disrupted retinal anatomic characteristics and segmentation errors.
The deviceʼs density assessment tool automatically inserted three fovea-centered circles
for the superficial capillary plexus (SCP) and deep capillary plexus (DCP). The slab
extracted from the internal limiting membrane to the inner plexiform layer/inner nuclear
layer interface and the slab extracted from the inner plexiform layer/inner nuclear
layer interface to the outer plexiform layer/outer nuclear layer interface were defined
as the SCP slab and the DCP slab, respectively. Vessel densities within the area of
the small circle (diameter 1 mm), middle circle (diameter 3 mm), and outer circle
(diameter 6 mm) were defined as the foveal zone VD, parafoveal zone VD, and perifoveal
zone VD, respectively. Additionally, all VD values in these zones were evaluated further
at the superior and inferior hemispheres and the temporal, nasal, inferior, and superior
quadrants ([Fig. 1]). The nonflow area in the SCP was determined using the OCT-A deviceʼs
nonflow assessment tool. Moreover, the FAZ assessment tool automatically determined
the FAZ area in the whole retinal vasculature, FAZ perimeter, acircularity index (AI)
of FAZ, and foveal density (FD-300). AI is defined as the ratio of the perimeter of
the FAZ and the perimeter of a circle with an equal area. A vessel density of 300 mm
around the FAZ is known as FD-300.
Fig. 1 Density assessment tool of optical coherence tomography angiography (OCT-A). Superficial
(a) and deep (b) capillary plexuses demonstrated by OCT-A. The zones that were automatically divided
by the analytic software of the device are shown at the right corner of each figure.
The following parameters were compared between the COVID-19 and control groups: FAZ
area of the entire retina, nonflow area in the SCP, FAZ perimeter, AI, FD-300, and
the VD values for the SCP and DCP in the foveal, parafoveal, and perifoveal zones.
Statistical analysis
All data were analyzed using SPSS version 22.0 (IBM, Armonk, NY, USA). Descriptive
statistics are presented as mean ± standard deviations. Both Pearsonʼs chi-squared
test and one sample chi-squared test were used to compare categorical variables between
the COVID-19 and control groups. The Kolmogorov-Smirnov test was used to ensure normal
distribution of the data. Independent sample t-tests were used for normally distributed
data and the Mann-Whitney U test was used for non-normally distributed data to classify
the COVID-19 group and the control group. P-values < 0.05 were considered statistically
significant.
Results
The current study included data from 79 eyes belonging to 79 subjects: 39 in the late
post-recovery phase of COVID-19 and 40 healthy controls. There were no significant
differences between the two study groups with regard to age or sex (p > 0.05 respectively).
Demographic and ocular characteristics of both groups are summarized in [Table 1].
Table 1 Demographic and ocular characteristics of the participants.
|
COVID-19 Group (n = 39)
|
Control Group (n = 40)
|
P value
|
BCVA: best corrected visual acuity, IOP: intraocular pressure, D: diopter. * Independent
samples test. ¥ Chi-square test.
|
Age (years)
|
45.23 ± 13.20
|
47.60 ± 12.14
|
0.540*
|
BCVA (Snellen)
|
0.91 ± 0.07
|
0.92 ± 0.06
|
0.776*
|
IOP, mmHg
|
13.05 ± 2.39
|
13.27 ± 2.84
|
0.707*
|
Sex (male/female)
|
20/19
|
18/22
|
0.736¥
|
Refractive error (D)
|
− 1.11 ± − 0.45
|
− 1.22 ± − 0.33
|
0.782*
|
Comparisons of the FAZ and macular VD parameters between the two study groups are
presented in [Table 2]. Both the nonflow area and the FAZ area of the entire retina were larger in the
COVID-19 group compared to the control group; however, these differences were not
statistically significant (p > 0.05 respectively). As for vessel densities, all superficial
parafoveal VD parameters were significantly higher in the COVID-19 group compared
to the control group, as illustrated in [Fig. 2] (p < 0.05 respectively). The VDs in the remaining zones were lower in the COVID-19
group compared to the control group, but these differences were not statistically
significant (p > 0.05 respectively).
Table 2 Comparison of the FAZ and macular vessel density assessment tool parameters in different
sections.
|
COVID-19 Group (n = 39)
|
Control Group (n = 40)
|
P value*
|
DCP: deep capillary plexus; SCP: superficial capillary plexus; *Independent samples
t-test (differences between COVID-19 group and controls). Bold values indicate statistical
significance, p < 0.05.
|
Nonflow area (mm2) SCP
|
0.51 ± 0.10
|
0.50 ± 0.12
|
0.51
|
FAZ area (mm2) whole retina
|
0.29 ± 0.09
|
0.28 ± 0.11
|
0.70
|
FAZ perimeter (mm)
|
2.06 ± 0.33
|
2.01 ± 0.45
|
0.51
|
Acircularity index
|
1.07 ± 0.15
|
1.09 ± 0.02
|
0.32
|
Foveal density (%)
|
55.86 ± 3.42
|
55.54 ± 4.34
|
0.72
|
Vessel density, SCP flow (%)
|
Whole retina
|
51.18 ± 2.90
|
51.74 ± 2.21
|
0.34
|
|
51.00 ± 2.92
|
51.71 ± 2.12
|
0.21
|
|
51.37 ± 2.93
|
51.76 ± 2.40
|
0.51
|
|
19.54 ± 6.40
|
20.08 ± 6.32
|
0.70
|
|
55.50 ± 3.13
|
53.18 ± 2.59
|
0.04
|
|
55.39 ± 3.36
|
53.04 ± 2.66
|
0.04
|
|
55.60 ± 3.09
|
53.13 ± 2.82
|
0.04
|
|
55.82 ± 3.56
|
53.08 ± 2.76
|
0.03
|
|
56.20 ± 3.62
|
54.03 ± 3.04
|
0.04
|
|
55.94 ± 3.39
|
53.15 ± 2.66
|
0.02
|
|
56.03 ± 3.19
|
54.07 ± 3.25
|
0.03
|
Perifovea
|
51.83 ± 3.07
|
52.43 ± 2.28
|
0.33
|
|
51.62 ± 3.10
|
52.40 ± 2.15
|
0.19
|
|
52.04 ± 3.16
|
52.43 ± 2.54
|
0.54
|
|
48.21 ± 2.83
|
48.68 ± 2.25
|
0.41
|
|
51.23 ± 3.77
|
52.54 ± 2.47
|
0.07
|
|
54.47 ± 8.61
|
55.93 ± 2.38
|
0.30
|
|
51.94 ± 3.81
|
52.56 ± 2.87
|
0.42
|
Vessel density, DCP flow (%)
|
Whole retina
|
52.47 ± 5.01
|
53.53 ± 5.02
|
0.35
|
Superior hemisphere
|
52.52 ± 5.22
|
53.60 ± 5.18
|
0.36
|
Inferior hemisphere
|
52.41 ± 4.94
|
53.53 ± 5.04
|
0.32
|
Fovea
|
37.06 ± 6.82
|
38.14 ± 7.95
|
0.52
|
Parafovea
|
55.80 ± 4.62
|
56.95 ± 3.76
|
0.22
|
|
56.15 ± 4.31
|
57.13 ± 3.82
|
0.29
|
|
56.20 ± 3.96
|
56.81 ± 3.81
|
0.49
|
|
56.78 ± 4.07
|
58.00 ± 3.88
|
0.18
|
|
55.65 ± 4.23
|
56.46 ± 3.95
|
0.38
|
|
57.21 ± 4.17
|
57.70 ± 3.54
|
0.58
|
|
55.58 ± 3.77
|
55.70 ± 4.48
|
0.89
|
Perifovea
|
54.19 ± 5.33
|
55.44 ± 5.32
|
0.30
|
|
54.24 ± 5.49
|
55.54 ± 5.48
|
0.29
|
|
54.13 ± 5.32
|
55.36 ± 5.40
|
0.31
|
|
55.11 ± 8.97
|
57.80 ± 3.90
|
0.08
|
|
53.21 ± 6.22
|
55.01 ± 6.21
|
0.20
|
|
53.27 ± 6.49
|
53.53 ± 6.14
|
0.85
|
|
53.72 ± 5.86
|
55.18 ± 6.26
|
0.28
|
Fig. 2 All superficial parafoveal VD parameters in the late post-recovery period were statistically
considerably higher compared to controls.
Discussion
Even though a long time has passed since the emergence of the COVID-19 pandemic, the
effects of the virus on various organ systems are still being investigated. Although
COVID-19 is most often associated with acute respiratory distress syndrome, its apparent
link to conjunctivitis has sparked the interest of ophthalmologists [16]. The fact that the ACE2 receptor used by SARS-CoV-2 for invasion is present in many
ocular tissues also suggests that the virus may have an impact on all ocular tissues
[17], [18], [19], [20], [21].
In animal models, the ACE2 receptor has been identified in the ciliary body, the vitreous
body, and in the inner nuclear layer of the retina, particularly in the Müller cells
[22]. A recent study in postmortem COVID-19 patients reported the presence of viral RNA
in the retina [23]. Moreover, in an OCT/OCT-A study of 12 patients 11 – 33 days after the onset of
COVID-19 symptoms, Marinho et al. reported cotton-wool spots and microhemorrhages
in 4 patients on OCT, suggestive of an inner retinal ischemic process [24]. On the other hand, the researchers did not observe any changes with OCT-A [24]. However, a subsequent editorial letter indicated that these findings represented
normal vascular landmarks [25]. Overall, the ophthalmologic effects of COVID-19 should be evaluated in further
quantitative studies with a large number of subjects.
OCT-A is a new, non-invasive imaging technique that provides volumetric data and
has the clinical capability to specifically localize and delineate pathologies along
with the ability to show both structural and blood flow information [26]. With this in mind, we compared quantitative FAZ and macular VD parameters obtained
by OCT-A from people who had recovered from COVID-19 and from healthy controls.
The effect of COVID-19 on vascular structures and FAZ parameters in the retina has
been addressed in several studies [13], [14], [15]. Savastano et al. [13] detected reduced perfusion density of the radial peripapillary capillary plexus
in recovered COVID-19 patients compared to age-matched controls using OCT-A analysis.
Abrishami et al. [14] reported reduced vessel density in the SCP and DCP of the foveal and parafoveal
regions in recovered COVID-19 patients within two weeks of seronegativity compared
to healthy controls. Additionally, the recovered COVID-19 patients had larger FAZ
areas, but this was not statistically significant [14]. Turker et al. evaluated vascular changes in the early period after moderate COVID-19
infection and at 6-month follow-up and found lower VD values than for control subjects
in
all parafoveal quadrants of both the SCP and DCP at the initial checkup and in
all parafoveal quadrants of the SCP and in 2 of the parafoveal quadrants of the DCP
at 6-month follow-up [27]. In a recent study, Cennamo et al. showed a significant reduction in the VD of the
SCP in whole images and in the DCP in all sectors when they compared post-SARS-CoV-2
pneumonia patients with healthy subjects [28]. In another study, people with moderate and severe COVID-19 examined within 3 months
of RT-PCR positivity showed decreased central retinal VD compared to people with asymptomatic
COVID-19 and healthy controls [15].
In our study, the nonflow area and the FAZ area situated in the entire retina were
slightly larger in the group of people with COVID-19 compared to the controls, but
these differences were not statistically significant. These results are in line with
the findings of Abirashimi et al. [14] for the early post-recovery phase of COVID-19. The fact that we found consistent
results in the post-recovery period of COVID-19 may indicate that SARS-CoV-2 infection
causes a chronic ischemic process in the retina. Moreover, changes of the FAZ might
only be a transient effect related to the acute phase of the disease. Of course, these
findings should be evaluated in larger cohort studies, as the results reported here
are not statistically significant. It is also possible that the differences in FAZ
we and Abirashimi et al. reported may simply reflect differences in retinal vasculature
among individuals, with no relation to COVID-19.
When we evaluated the VD at both the SCP and DCP, only the parafoveal VD at the SCP
was significantly higher in the COVID-19 group compared to the control group. The
VD in the remaining sections was lower in the COVID-19 group, but these differences
were not statistically significant. Although other studies have reported a decrease
in retinal VD in people with COVID-19 [14], [15], we observed an increase in parafoveal VD at the SCP. These differences could be
explained by inflammation and ischemia triggered by invasion of the virus in the early
post-recovery period. It is known that the parafoveal region plays an important role
in supplying the macula [11], [12], and our reported findings could reflect a compensatory mechanism aimed at protecting
the macular area from the ischemic process of SARS-CoV-2 infection during the late
post-recovery period, thereby
resulting in an increase in parafoveal VD at the SCP. It should be noted that
the participants in other studies were patients who had moderately severe or very
severe disease. Since our study group consisted of people who had mild COVID-19, this
may explain the low but not significant VD values.
This study is the first to quantify macular VD and FAZ parameters in patients with
mild COVID-19 during the late post-recovery period. However, like all other studies,
our study has certain limitations. First, our sample size was relatively small. Accordingly,
these findings need to be confirmed in a much larger cohort study of COVID-19 patients
from multiple centers. Second, our study did not assess the long-term evolution of
macular VD and FAZ parameters, and this should also be evaluated in future studies.
Another drawback of our study was the use of chloroquine and hydroxychloroquine to
treat COVID-19 (400 mg/day). Chloroquine/hydroxychloroquine is associated with an
elevated risk of retinopathy at high doses. However, retinopathy is rarely seen before
10 or more years of chloroquine/hydroxychloroquine usage at doses of < 5 mg/kg [29], [30]. But the use of higher dosages has been reported to affect the retina
after 11 to 17 months [30]. Therefore, given the short-term nature of our study, we believe our findings only
reflect the effect of COVID-19 on the retina. Moreover, the changes detected with
OCT-A are not specific to COVID-19 but can be seen with other viral agents. Therefore,
although this issue is a limited aspect of our study, it should be evaluated in future
studies.
To conclude, the inflammatory process of COVID-19 could affect retinal vascular structures
in the late post-recovery period. While the FAZ parameters were stable among people
with COVID-19 in our study, the VD in the parafoveal area at the SCP was significantly
higher compared to healthy controls. Long-term follow-up studies in a large group
of patients are needed to confirm these results.