Your brain on covid-19

Covid-19 is an acute respiratory illness, but it often affects many organs in the body.  If you read the newspaper or watch TV you know this.  But is one of those organs the brain?  And does this mean anything for cognitive function?  The answer seems to be yes.

The first organ damaged by the virus is the lungs.  According to several articles I've read, this occurs in part due to an unusually powerful inflammatory response.  Inflammation in the lungs can cause extensive damage to lung tissue.

We have discussed the role of inflammation in various diseases before.  Inflammation begins as the body's response to infection or injury, but it has a way of doing its own damage -- maybe lots of it.  Neuroinflammation, inflammation in the brain, plays a major role in Alzheimer's disease.  Can inflammation spread from the infection site in the lungs to the brain and thereby aggravate Alzheimer's disease?  Or can it simply attack the brain in the way that causes cognitive impairment in the absence of Alzheimer's disease?

When the body mounts an inflammatory response, immune cells release proteins called cytokines.  Cytokines coordinate the body's immune response and promote inflammation.  In the case of a massive inflammatory response, the cells release a "cytokine storm."  The cytokines may leave the infection site and circulate to other parts of the body to encourage inflammation elsewhere.

Does the blood-brain barrier protect the brain from cytokines?  You will recall that we explained how the blood-brain barrier protects the brain from many pathogens.  Pathogens in the blood can't pass through the walls of capillaries in the brain because the walls of these blood vessels limit the size of particles able to enter or leave.  Pathogens (and antibodies) are usually too big, so they're screened out.  But cytokines are at about the size limit, and some can get in.  It's also believed that they can enter the brain by way of the olfactory nerve.

Another problem is that impairment of lung function can deprive the brain of oxygen.  And the first casualty of oxygen deprivation is the hippocampus -- which is often the first casualty of Alzheimer's disease.  So people whose brains have been damaged by oxygen deprivation sometimes display symptoms similar to Alzheimer's disease.  It's also easy to see how covid-19 might aggravate a case of Alzheimer's disease in multiple ways.

This article, written by a South African neuropsychologist and her husband provide a more detailed discussion of how this all works.