1. Introduction
The coronavirus disease 2019 (COVID-19) has had huge global health implication and continues to ravage healthcare systems around the world [
1]. COVID-19 infection can be followed by multi-organ pathology and vascular damage [
2] that, in turn, increases the risk of fatal vascular occlusive diseases such as thrombosis, myocardial infarction, arrhythmia, and cerebral apoplexy [
3,
4,
5,
6]. Elevation of thrombotic tendencies and increased risk of thrombosis have been found to persist for several months after COVID-19 infection [
7].
Fundus photography (which is fast and noninvasive) and fundoscopic examination of the ocular fundus give ophthalmologists direct access to the retina and its vasculature. The retinal arteries and veins are believed to be representative of the state of the body’s entire microvascular system [
8]. Therefore, it is advisable for ophthalmologists to check for any vascular abnormalities caused by COVID-19 following infection. It has been posited that the increased risk of thrombosis caused by COVID-19 may be associated with the development of retinal vascular occlusive diseases (RVODs).
RVODs include retinal vein occlusion (RVO) and retinal artery occlusion (RAO). Both of these are subdivided into central and branch occlusions, i.e., central retinal vein occlusion (CRVO), central retinal artery occlusion (CRAO), branch retinal vein occlusion (BRVO), and branch retinal artery occlusion (BRAO). After diabetic retinopathy, RVO is the most common retinal vascular disease [
9]. The prevalence of RVOs in the developed world is 5.2 per 1,000, while that of CRVOs is 0.8 per 1,000 [
10].
In CRVO, sudden visual impairment is common; while in BRVO, visual field defects and shape distortion are typical symptoms. RVOs affect visual function and are characterized by retinal hemorrhage, soft exudate, and macula edema. CRVO can be classified as ischemic or nonischemic. The ischemic type of CRVO can cause neovascular glaucoma and blindness. The primary treatment methods are anti-vascular endothelial growth factor (VEGF) antibodies and retinal photocoagulation [
11,
12,
13]
. Since the introduction of anti-VEGF antibodies, there has been a significant improvement in the prognoses for these conditions [
13]
.
RAO is an acute disease in which the occlusion usually causes sudden visual reduction. If retinal blood flow does not return promptly, at least some visual reduction and visual field defects will be permanent. Although many treatment methods are reported [
14,
15], there is not yet an established standardized treatment and RAO remain difficult to treat. Yet, RAO carries a risk of cardiovascular events [
16], such as cerebral stroke and may also be a symptom of a systemic condition. The visual prognosis for CRAO is poor and decimal visual acuity (VA) usually falls below 0.05 [
17].
RVODs are relatively common disorders that can cause severe visual impairment. Risk factors for RVODs include hypercoagulability and thrombotic disorders. In this review, we investigate the relationship between COVID-19 and RVODs.
8. CRAO after Vaccination
CRAO cases after COVID-19 vaccination are shown in
Table 6A-
6D [
86,
87,
88,
89,
90,
91]. The average age was 55.5 ± 18.5 years and the male to female ratio was 3: 3. Of the 91 patients with acute nonarteritic CRAO reported by Lee, 62.6% were male and the average age was 66.4 years [
83]. Thus, the average age among the cases shown in
Table 6A to
6D is younger than that previously reported in CRAO unassociated with COVID-19 [
83]. Also among patients with CRAO unrelated to COVID-19, Kido et al. have reported an incidence rate 1.4 times higher in males than females [
92]. The time from COVID-19 vaccination to CRAO symptom onset ranged from 1 to 21 days, with a mean of 7.3 ± 7.4 days. The time between receipt of the COVID-19 vaccine and CRVO symptom onset was 9.1±7.3 days. There was no significant difference between these times for CRVO and CRAO after vaccination (
p = 0.63, unpaired t-test).
The COVID-19 vaccines used were by Pfizer, Moderna, AstraZeneca, and Bharat Biotech.
Although there were some slightly abnormal blood test results, they were not a characteristic feature of the patients in this subset. VA at the initial visit was poor in most cases, but 20/40 in one case.
Some cases had NLP by their final visit and only one case recovered some of their visual function, which increased to 1.2 in decimal VA.
10. BRAO after COVID-19 Vaccination
Reported cases of BRAO after COVID-19 vaccination are summarized in
Table 8A–
8D [
97,
98,
99]. The average age of this subset was 62.3 ± 21.2 years and the average number of days between vaccination and BRAO symptom onset was 21.0 ± 24.9. As stated above, the average age of patients with CRAO after COVID-19 vaccination was 55.5 ± 18.5 years, which was not significantly different from that of the patients with BRAO after COVID-19 vaccination (
p = 0.57). The time between vaccination and symptom onset was 21.0 ± 24.9 days. This was not significantly different from the time between vaccination and CRAO symptom onset (
p = 0.25).
Although all of the vaccines were made by the multinational corporation, Pfizer-BioNTech, there have been few reports on their relationship with RVODs.
VA at the initial visit averaged −0.025 ± 0.16, which was significantly better than that found among patients with CRAO after COVID-19 vaccination (p < 0.001). No significant difference was observed between BRAO after COVID-19 infection and BRAO after COVID-19 vaccination (p = 0.19).
Because treatment methods and final VA values were not provided in most reports, it was difficult to conduct a comprehensive investigation.
We will now consider the mechanisms involved in the pathogenesis of RVODs. Retinal veins can be occluded by arteriosclerosis. This causes components of the plasma to leak out of the vessel, which results in retinal hemorrhage and retinal edema. If the lesion does not reach the arcade area, no symptoms occur. However, once this region is reached, VA begins to decline rapidly. Risk factors for RVODs are older age, hypertension, diabetes, hyperlipidemia, smoking, and glaucoma [
13]. RAO is frequently a symptom of an underlying disease, such as atrial fibrillation; heart valve diseases; stenosis or plaque formation in the internal carotid artery; or arteriosclerosis in older adults due to hypertension, diabetes, or hyperlipidemia [
17]. However, RAO can also occur in youth among those with antiphospholipid syndrome and, exceptionally, those who take oral contraceptives. RVODs require medical scans and imaging and systemic treatments with internal medicine.
When the COVID-19 pandemic began, the resultant abnormalities of the blood coagulation system were greatly remarked upon. Tang et al. proposed that elevated d-dimer levels were predictive of a poor prognosis in infected patients. In fatal cases, the rate of disseminated intravascular coagulation (DIC) was 71.4% [
100]. Tang et al. indicated that the patient was very unlikely to survive if DIC occurred [
100]. COVID-19-associated coagulopathy is strongly correlated with vascular endothelial cell damage [
20,
21] and increased thrombin production in the veins and arteries. Several previous studies have verified that d-dimer elevation is an independent risk factor for thrombosis and death in COVID-19 patients [
101,
102,
103]. However, the present study found that coagulative abnormalities are an infrequent occurrence in COVID-19 patients. Wang et al. posited that mild or focal coagulation activation could cause retinal vessel occlusions without a significant change in the patient’s d-dimer level [
104].
The COVID-19 vaccine approved for clinical use at the end of 2020, is the primary infection control strategy. The vaccine is a combination of an adenovirus vector vaccine and a messenger RNA vaccine. While adenovirus vector vaccines rarely cause thrombosis or thrombocytopenia syndrome [
105], messenger RNA vaccine can cause venous thrombosis and thrombocytopenia and is likely to be the element of the COVID-19 vaccine responsible for vaccine-related thrombi [
106]. It is apparent that ophthalmologists need to monitor patients closely for thrombosis after COVID-19 infection and COVID-19 vaccination.
The regions vulnerable to arterial thrombosis are arteries of the extremities (39%), cerebral vessels (24%), large vessels (19%), coronary arteries (9%), and the superior mesenteric artery (8%) (the remaining 1% of cases occur in other vessels) [
108]. Malas et al. has reported an overall incidence of COVID-19-induced venous thromboembolism (VTE) of 21% [
108]. Furthermore, the VTE rate was 5% Among general ward patients, the rate of VTE is 5%, while the rate among ICU patients is 31% [
108]. When such thrombosis occurs in the retina, it is classed as an RVOD. It has been shown in this review that the increased risk of RVODs following COVID-19 is unsurprising given their reinforcing effects on each other, particularly in relation to coagulation processes. However, it is very difficult to clarify the connection between COVID-19 infection or vaccine and RVODs. Epidemiological investigations make an important contribution to investigations into comorbid conditions and the triggering of one condition by another.
Modjtahedi et al. have reported an RVO incidence rate of 65 in 43,2515 patients (a crude incidence rate of 12.2 per million) in the 6 months after a COVID-19 diagnosis [
109]. This is a clear increase in the rate of RVO compared to that seen in those not recently infected with COVID-19 [
109]. A study in Spain by Napal et al. also showed an increased RVO incidence during COVID-19 pandemic [
110]. Conversely, Parks et al. reported no increase in RVO after COVID-19 infection [111, 112.]
Al-Moujahed reported a diagnosis of CRVO in 7,261 (2.5%) of the 285,759 new patients seen in the pre–COVID-19 period and in 4,098 (2.7%) of the 156,427 new patients seen during the COVID-19 pandemic [
113]. The percentage of new diagnoses in retina clinics that were CRVO remained stable during the pandemic [
113]. Although the rate of patients with newly-diagnosed CRVO during the COVID-19 pandemic is interesting, these rates have not remained clear. According to our unpublished data, the rate of patients with CRVO remained unchanged after the COVID-19 pandemic compared to before. However, retinal photocoagulation was performed significantly more frequently after the pandemic began (
p < 0.001). This may say something about the possible pathogenesis of CRVO. Hashimoto et al. found low causality between RVO and COVID-19 vaccination [
114]. Rachman et al. could not ascertain differences in the RVO risks of different types of COVID-19 vaccines due to a lack of detailed data on dosages and patient medical histories [
115].
The RAO rate before and after COVID-19 pandemic is fascinating. Park et al. reported the RAO incidence rates per 100,000 people/year for 2018–2019 and 2020–2021 as
11.7 and 12.0, respectively [
111]. Despite the increased incidence of RAO during the COVID-19 pandemic, SARS-CoV-2 infection did not significantly increase RAO incidence [
111]. Al-Moujahed et al. reported an increase in CRAO cases during the first few months of the pandemic [
113]. However, RAO diagnosis rates as percentages of all new diagnoses in retina clinics remained stable for the majority of the pandemic [
113]. We are now investigating this issue and will release our data when the investigation is complete.
We found an increase in Vogt-Koyanagi-Harada disease associated with COVID-19 [
116]. Seventy-three cases per 60 months of Vogt-Koyanagi-Harada disease occurred before pandemic and 53 cases per 33 months occurred after COVID-19 pandemic [
116]. Liang et al. observed an overall reduction in eye injuries and substantial differences in the spectrum of ocular trauma during the COVID-19 pandemic [
117]. There was also an increase in dry eye-related complaints among students during the pandemic. This may have resulted from increased screen time [
118]. Alternatively, Krolo et al. reported that mask-associated dry eye increased during the COVID-19 pandemic [
119]. While the incidence of endophthalmitis after intravitreal injections remained unchanged [
120], the rate of endophthalmitis after vitrectomy increased, especially the variant caused by oral bacteria [
121]. Shi et al. found that the incidence of optic neuritis increased during the COVID-19 pandemic as too did acute primary angle closure, although the reason for the latter was unknown [
123].
At this point, we would like to briefly describe three interesting cases that we have encountered [
124,
125,
126]. The first of these was a male in his early 50s who suffered a recurrence of macular edema (ME) due to BRVO 3 days after administration of the messenger RNA COVID-19 vaccine (Pfizer-BioNTech). He was treated for this with an additional intravitreal aflibercept injection [
124]. However, the patient believed the COVID-19 vaccine could have been the cause of his ME recurrence, so refused any further vaccinations after that [
124]. In the 29 months since his initial visit, no recurrence has been reported. The second case was a 59-year-old female patient who suffered a recurrence of Vogt-Koyanagi-Harada disease following her third dose of the messenger RNA COVID-19 vaccine (Pfizer-BioNTech). This recurrence was 46 years after initial treatment [
125]. The patient’s inflammation was reduced by eye drops and oral corticosteroids. However, it seems that the vaccine may have triggered the recurrence [
125]. The third case was a 21-year-old female who had been using oral contraceptives for 2 years [
126]. Despite having received two doses of an mRNA-based COVID-19 vaccine, she contracted COVID-19 [
126]. She then suffered BRVO with ME 40 days after COVID-19 diagnosis. She was treated with an intravitreal aflibercept injection [
126]. In the 29 months since the patient’s initial visit, there has been no ME recurrence. BRVO with ME does not usually occur in young women but oral contraceptive use, COVID-19 vaccination, and COVID-19 infection are all risk factors for venous thromboembolism. In combination, they could have induced the patients of BRVO with ME [
126]. These cases illustrate some of the ways COVID-19 infection and vaccine can lead to various ocular disease.
With the global dissemination of COVID-19 vaccines, the pandemic has now (in 2024) been brought under some degree of control. Still, both COVID-19 and its vaccines have several unknown factors. Both could potentially have unknown complications or side effects. Because of the urgency, the usually lengthy process required for drug approval was hastened and this has led to social doubts about the safety of the vaccinations. Ophthalmologists are among the healthcare workers who may identify these complications and side effects in patients after their infection with or vaccination against COVID-19.