The brain is the best protected organ in the body.
The skull protects the brain from harm from outside forces, while something called the blood-brain barrier protects it from threats inside our body, such as an infection.
The blood-brain barrier is made up of special cells that select what can get through to our brain – things like oxygen, glucose, water and electrolytes – and keeps out toxins or pathogens that could harm the brain.
This defense works well for most threats to our health. Usually when we are infected with a virus or bacteria, our brains are unaffected. But occasionally – as seems to be the case with the novel coronavirus that causes COVID-19 – an infection can permeate the blood-brain barrier and cause neurological complications.
The brain’s defense
In a healthy person, the blood-brain barrier is quite effective at blocking pathogens that could cause harm.
In a person who is sick, and particularly someone who is critically ill, that system becomes compromised.
“In a sick person – and COVID-19 is a good example of this – your whole body is under stress,” Dr. Nair said.
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There’s the damage caused by the virus itself, and there are also effects of our immune system’s response.
When our body mounts a full-strength immune response, it produces chemicals and inflammation to attack the virus that can also affect our body functions.
“It’s a battle going on throughout your bloodstream, and it’s a battle of chemicals. Some of those chemicals can wreak havoc on the blood-brain barrier,” Dr. Nair said.
In an attempt to fight off an infection, your own body’s immune system can make it harder for your brain to get the nutrients it needs. It also can become easier for harmful pathogens to get through the barrier. That can lead to a condition called metabolic encephalopathy, which can affect cognitive functions like mood and memory and often cause delirium.
How COVID-19 affects the brain
While neurological complications from COVID appear to be rare, evidence suggests that the virus can affect the brain and central nervous system.
However, the exact mechanism through which this happens is a puzzle we are still trying to solve.
“With COVID, we are just starting to understand how this virus works and how to best manage these patients,” Dr. Nair said.
“This is an area I suspect we’ll continue to learn a lot about over the next several years.”
What qualifies as a neurological symptom of COVID-19?
“It kind of depends on how you define it,” Dr. Nair said.
Some complications can be easily categorized as neurological, such as a stroke, which appear to occur in 2-3% of those infected with COVID-19 (usually only among those who are critically ill).
“But if you are talking about all neurological symptoms, it’s much more nebulous, because the brain is responsible for every aspect of our human experience – everything you feel and do and perceive as a human being,” Dr. Nair said.
With that in mind, the two most common complications from COVID-19 that appear to affect the central nervous system are slowed thinking – commonly referred to as “brain fog” – and the loss of taste and smell.
Loss of taste and smell
The loss of taste and smell has come to be known as one of the hallmarks of COVID-19, and again doctors are still trying to understand how the virus disrupts these senses.
While we think of taste and smell as two separate things – the tongue and taste buds are connected to one set of nerves, while the nose is connected to another – these nerves are sending sensory information to the same part of our brain.
Sensory information comes in, such as when a taste touches your tongue. The nerves on your tongue then send signals to the sensory processing part of your brain, which converts that information to an electrical signal. That signal is then sent to a higher part of your brain that compares this information to information you’ve encountered before. It compares this taste to everything you’ve ever tasted and helps you understand what it is and if you like it.
All of that just for a sip of coffee.
“When you start to understand brain function in that way, there’s so many different aspects in that process that could be affected by COVID-19,” Dr. Nair said.
It could be affecting the soft tissues in the nose and mouth that helps collect sensory information; it could be impairing the function of the nerves that send that information to the brain. Or it could be interfering with the brain’s ability to interpret that information.
Because of the brain’s robust defense system, it’s usually one of the last organs to be affected by an illness. That might imply it’s unlikely this symptom is caused by damage to neurological function because it is often an early symptom of COVID-19.
It’s more likely the virus is disrupting the transmission process and preventing sensory information from reaching the brain, though more research is needed to fully understand this.
Our brains are constantly processing huge amounts of sensory information – everything that we think, touch, smell, see and hear. All of these bits of information are being taken in and interpreted by our brains.
“This is happening hundreds of thousands of times per second. Your brain is processing information at a volume and speed greater than the most powerful computer,” Dr. Nair said.
But for some people recovering from COVID-19, this process may not be functioning the way it should.
“Their thinking is not as clear. They are not as quick with thinking and movement as they were,” Dr. Nair said.
Part of what makes it difficult to understand exactly how this occurs is our limited ability to assess the brain and its function.
Imaging tools like a CT or MRI scan can show the shape and structure of the brain, but are limited in their ability to show us how a brain is functioning.
An electroencephalogram (EEG) can measure electrical activity in the brain, but is “still a relatively blunt tool,” Dr. Nair said.
“The most important tool we have as neurologists is the time we spend in front of patients and talking to them. All the fancy tools we have today, and I can get a lot more information by asking questions the right way and watching how a patient moves.”
In the case of brain fog, when neurological symptoms can be observed but imaging tools offer no insights, it’s possible the virus is causing microcellular damage that can’t be seen on a scan.
Protecting the brain from COVID-19
One way to prevent neurological complications from COVID-19 is simply to control the disease in general.
Neurological complications are most common in patients who are critically ill. We could expect to see a decrease in neurological complications by managing the disease and preventing symptoms from reaching a high level of acuity – which is something that the medical community has begun to do.
“Especially when it comes to the neurological aspects of care, I’m cautiously optimistic,” Dr. Nair said.
Another important aspect – understanding how this virus affects our bodies to make people so sick.
There’s some promising research into this.
When they look at the chemicals our bodies produce when fighting COVID-19, researchers are investigating two that Dr. Nair thinks could hold some answers.
One is cytokines, an inflammatory chemical that can sometimes attack not just the pathogen making us sick, but the body itself in a phenomenon known as a cytokine storm.
Dr. Nair is interested in another theory about bradykinins, another chemical produced by the immune system which can cause a cascade of inflammation – including disrupting the function of the blood-brain barrier.
“From a neurology perspective, this is a key factor that can explain how the virus affects brain function. It could explain why we are seeing encephalopathies and clotting,” he said.
While these theories will require extensive research before they are proven, Dr. Nair is encouraged by the amount of progress that’s been made in the medical and scientific communities in the year since the novel coronavirus was first discovered.
And with a better understanding of how this virus works comes better tools for how to control it.
“We have learned a lot – not just in medicine, but as a society at large. We will start to learn our way out of this.”