Why do my eyes hurt when I am infected with the new crown? The latest research revealed that the new coronavirus causes retinal inflammation and affects depth perception

Covid infections tend to cause symptoms such as fever, cough, muscle aches, etc.
, which usually fade away
with the end of the infection.
However, there are also a small number of cured people who will show mild or severe sequelae, common problems such as
brain fog, smell and taste failure.

At the same time, many COVID-19 patients will have symptoms of eye pain and inflammation when infected, but there is still a lack of scientific understanding
of the impact of SARS-CoV-2 on the eyes.

Recently, a research paper published in Nature Communications entitled: Ocular tropism of SARS-CoV-2 in animal models with retinal inflammation via neuronal invasion following intranasal inoculation.
The adverse effects
of the new coronavirus infection on the eyes were revealed.
The study, which used mice as a model, found that the new coronavirus can infect the brain and eyes through the trigeminal nerve and optic nerve, and cause retinal inflammation and reduced
depth perception.

SARS-CoV-2 infects retinal ganglion cells

To investigate whether SARS-CoV-2 infection causes eye disease, the researchers evaluated ocular tropism for respiratory infection SARS-CoV-2 using K18-hACE2 transgenic mice
.

The researchers inoculated mice with SARS-CoV-2 virus or PBS buffer intranasally, and on day 6 post-inoculation, mice vaccinated with SARS-CoV-2 experienced tearing and increased
eye discharge.
Subsequently, the researchers evaluated the presence of SARS-CoV-2 in the eyes of mice and found that infectious viral titers in the eyes of mice were as high
as viral titers from the lungs.

Next, they examined the viral spike protein (S protein) in retinal slices of infected mice using immunofluorescence staining and found that the S protein was mainly located in the ganglion cell layer of the retina and colocalized with the marker γ-synuclein of retinal ganglion cells, indicating that the infected cells may be retinal ganglion cells
.

SARS-CoV-2 infected mice exhibit retinal inflammation

The researchers performed further examination of eye tissue sections from SARS-CoV-2-infected mice to analyze histopathological and inflammatory changes
after infection.
Compared with the control group, the retinal thickness of mice in the SARS-CoV-2 infected group increased significantly by 1.
62 times
.
The average thickness of the retina in the control group was 46.
27μm, and the average thickness of the retina in the infection group reached 75.
14μm, which induced the accumulation
of a large number of infiltrative inflammatory cells in ganglion cells, core layer and outer nuclear layer.

To identify infiltrated immune cells, the researchers stained the eye with CD3+ T, CD4+ T, and CD8+ T cells, macrophages, and GR1+ neutrophils using immunofluorescent staining
.
It was found that on the sixth day after infection, T cell and neutrophil levels in the eyes of infected mice were higher than in control mice
.
In addition, multiplex immunoassays showed elevated levels of pro-inflammatory cytokines and chemokines, including G-CSF, IP-10, MKC, MCP-1, MIP-2, IL-6, and IL-12
.

These findings suggest that in the eyes of K18-hACE2 mice, infection with SARS-CoV-2 promotes retinal inflammation and the production
of pro-inflammatory cytokines and chemokines.

Next, the researchers assessed the effects of
retinal inflammation on visual function.
SARS-CoV-2-infected mice with ocular symptoms were found to have ocular symptoms for an extended period of time through the plateau, indicating that depth perception was affected
in infected mice.

However, there was no difference in the number of mice that stepped on the edge of the cliff, regardless of eye symptoms, suggesting that retinal inflammation caused by viral infection did not exacerbate retinal degeneration or vision loss
.

SARS-CoV-2 invades the eye and brain through the trigeminal nerve and optic nerve

To study the transmission routes of SARS-CoV-2 infection to the brain and eyes, the researchers infected mice using different inoculation modalities (IN: nasal infection, IT: bronchial infection, IC: intracerebral infection, ED: eye infection, IV: tail vein injection).

Through different inoculation routes, they found that SARS-CoV2 infects the eye and brain through nerve invasion of the trigeminal nerve (TN) and optic nerve (ON) in a mouse model, while eye infection does not cause central infection and lung infection
.

In summary, SARS-CoV-2 was found in mouse models to travel to the brain and eyes via the trigeminal and optic nerves, causing retinal inflammation, pro-inflammatory cytokine production, and decreased
depth perception.

The study's author, Professor Suresh Mahalingam, believes that the virus begins to affect vision
when the optic nerve becomes inflamed, abnormal fluid build-up and immune cell infiltration leads to thickening of the retina.
He also stressed that this is not a permanent degeneration
of eye tissue.
Data reported by animal models and human patients show that when the infection disappears, vision problems disappear with it, leaving no associated sequelae
.