The SARS-COV-2 virus, more commonly referred to as COVID-19, has recently cause a global pandemic, and made the world stand still. Despite the virus primarily affecting the respiratory tract, increasing literature is shedding light on the effects of covid-19 on other areas of the body, especially the brain. This article gives a basic understanding on the Covid-19 virus while emphasizing the effects of this disease on the central nervous system (CNS).
Basics of Covid-19 virology
Viruses have a simple structure, that being of genetic material surrounded by a protein coat. To replicate, viruses must infect a host cell, and hijack its machinery to produce more copies of itself. The newly produced copies of the virus leave the host cell and infect surrounding cells to continue the replicative cycle. However, to initiate this process, viruses must first find a way to enter the host cell. Viruses enter host cells through cells’ outer membrane, which is dotted with numerous receptors. Viruses attach to specific receptors on the cell membrane which enable viral entry into the cells, enabling viruses to replicate. If cells do not possess the specific receptors for a particular virus, then the virus will not be able to enter the cell.
The Covid-19 virus enters host cells by attaching to angiotensin-converting enzyme-2, also known as the ACE-2 receptor. This receptor is abundantly present in the cells lining our respiratory tract, however this receptor is also expressed in other cells throughout the body. This explains how the Covid-19 virus is capable of infecting other body tissues, such as nervous tissue.
How COVID-19 can reach the brain
There are multiple ways COVID-19 can affect the nerves that work together with the brain. The most direct means is by actually infecting the cells. Certain nerves such as those in the nose contain the ACE-2 receptors allowing for direct viral infection. Furthermore the cells that are present in blood vessels can also be disrupted resulting in further damage to the nervous system.
Furthermore, being a viral infection COVID-19 tends to promote a strong immune response. This immune response becomes more and more aggressive to combat the virus. Eventually this response becomes so strong it damages the body itself resulting in a cytokine storm. This response has a wide range of effects on the body, the whole range of which is currently unknown.
Effect of Covid-19 on the CNS
The means by which COVID-19 can affect the brain are essential to keep in mind. A rapidly increasing body of research is demonstrating the effects of the covid-19 virus on the nervous system. Covid-19 is associated with loss of smell and taste, referred to as anosmia and ageusia respectively.This is technically quite normal for respiratory viruses like COVID19 since they tend to result in lots of mucus production (what you perceive as a runny nose.) This runny nose blocks your nose and thus prevents you from smelling anything. However in COVID-19 anosmia can occur without any sign of a runny nose. This lends to the fact that COVID-19 can attack the nerves of the nose and mouth directly the full effects of which remain to be determined
Another good example of the effects of Covid-19 is via strokes. Strokes can be defined as an aberrant overexcitation of certain neurons in the brain. These can be brought about by interfering with neural stimulation either via alcohol intake or even a high fever. The high levels of inflammation caused by COVID-19 are thus the likely cause as it interferes with neural stimulation to result in a seizure.
However this inflammatory damage can also result in generalized damage to the CNS namely in the form of encephalopathy. This is evident in the mood changes and states of confusion found in COVID-19 patients who also present high levels of brain damage.
Furthermore ACE-2 receptors are also present in the blood vessels in the brain. This makes the virus able to directly damage the brain’s blood vessels resulting in an increased likelihood a block forms in the blood vessels. These blockages make the vessel more likely to burst open which is exactly what occurs in a stroke. This is likely responsible for the increased incidence of strokes detected in sufferers of COVID-19.
Zooming out into the broader nervous system where further examples of neuro-COVID interactions can be observed. For instance inflammation in the eye referred to as optic neuritis, this can cause temporary vision loss as well as severe pain when moving the eye. This is likely due to inflammation however interestingly this process has also been linked to the production of specific proteins by the immune system known as antibodies. This means that the immune response to COVID-19 might produce molecules that attack the nervous system.
Whilst these are medically relevant cases of COVID-19 affecting the CNS there are some limitations to consider. For a lot of these conditions such as strokes, seizures and ON these only occur in a small percentage of COVID-19 patients who are already badly infected. Furthermore the actual biological processes that lead to these conditions are still unknown. Whilst it is likely it is some combination of direct infection/inflammation whether one plays a bigger role than the other is unknown. Furthermore it is also possible that the exact cause of the neural injury differs across patients. Whatever the case though, further research is going to be crucial to understand and thus treat COVID-19