Early in the pandemic, a loss of sense of smell and taste became a hallmark of Covid-19 infection. This sparked concerns that the virus may damage the brain, and scientists have been doing research ever since to understand how SARS-Cov-2 may affect brain cells and cause neurological disorders.
A pre-print (a study that has not yet been peer reviewed) analysing brain images reported a loss in grey matter in some people after infection with SARS-Cov-2. Emerging evidence is suggesting that the coronavirus may either directly or indirectly cause numerous different neurological symptoms including “brain fog”, memory loss, depression and even psychosis. One study showed that as many as 82% of 3743 people hospitalised with Covid-19 reported neurological symptoms.
It remains unclear how the virus might break into the brain. Indeed, some studies have indicated that the virus has difficulty crossing the blood-brain barrier, which serves as a line of defense against infection. However, another pre-print shows that SARS-Cov-2 can infect brain cells in the laboratory, with 66% of affected cells being astrocytes (a type of brain cell). This is further supported by a study reporting evidence of SARS-Cov-2 infection in brain cells from people who have died with Covid-19.
Some of the neurological symptoms associated with Covid-19 including fatigue, depression and “brain fog” could be explained by infected astrocytes. However, SARS-Cov-2 does not necessarily need to directly infect brain cells to cause neurological problems. A study has reported that SARS-Cov-2 could infect cells, called pericytes, which are found on small blood vessels throughout the body, including the brain. Reducing blood flow to the brain may impair neuronal function, ultimately killing brain cells.
There is also growing evidence that immune malfunction, or an overreaction of the immune system, results in neurological symptoms. This is also the reason why immunologists dislike the term often used by marketing campaigns selling products to “boost your immune system”, because an unregulated immune response is harmful and sometimes causes unwanted autoimmune conditions.
The coronavirus may inadvertently lead some people to make “autoantibodies” that attack their own cells, which can lead to neurological damage including loss of vision and weakness in limbs. There is evidence that autoantibodies can pass through the blood-brain barrier and contribute to neurological disorders, making this pathway a possibility for Covid-19 induced brain damage.
Infected astrocytes, pericytes and autoantibodies are probably not the only way Covid-19 infection can cause neurological symptoms and may or may not happen all at the same time. Researchers are hopeful that a better understanding of the underlying mechanisms will help improve treatments to prevent long-term problems.
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Angela Marley, PhD Immunology & Infectious Disease
