We often associate Covid-19 with respiratory illness, caused by white blood cells activating chemicals that lead to leakage of fluid into the lungs. But can it affect the brain as well?
An intriguing study of 214 Wuhan patients showed that 36.4% of them had signs of nervous system related issues. The most common symptoms were muscle pain, headaches, dizziness, or confusion. A few patients had more severe symptoms such as seizures and brain inflammation encephalitis. In fact, one woman’s brain scan displayed lesions on the brain filled with dead and dying neurons in the regions that relay sensory signals, regulate alertness and access memories. This woman was diagnosed with Acute Necrotising Encephalopathy (ANE). Another case study in the International Journal of Infections Diseases by a team in Japan reported traces of the virus in the cerebrospinal fluid- demonstrating that the virus can penetrate the central nervous system.
But does covid-19 directly affect the brain, or is it just part of the immune response? The main question baffling Scientists currently is the pathway used by the virus to access the brain. Neurologist Sherry Chou speculates that it enters through the nose then upward through the olfactory bulb, a part of the brain which is involved in the sense of smell. This explains one of the key signs of covid-19: a loss of your sense of smell. However, various studies have shown that the virus does not actually affect the olfactory neurons directly.
Coronaviruses enter your cells via molecules called ACE2 receptors. Microbiologist Stanley Perlman conducted an experiment in 2008, where he genetically engineered mice to express ACE2 and squirted a small dose of coronavirus into their noses. What he found was that the virus ‘climbed’ out the nasal cavity and travelled into their brains using olfactory neurons almost like rungs on a ladder. What’s interesting is that scientists didn’t find ACE2 receptors in the olfactory neurons; implying that the virus was infecting other cells.
Another conflicting piece of evidence comes from a 74-year-old Dutch man, who showed the typical cough and fever symptoms. However, the next day he could no longer speak or make eye contact. Nose swabs showed that he had tested positive for covid-19, but a brain scan gave a negative result. So how does the virus cause neurological symptoms?
A team of scientists from the University of Geneva in Switzerland have speculated that the virus targets specialised ‘life-support’ cells that surround olfactory neurons, which help them survive. This means that when infected with the virus, the neurons have impaired interactions with ‘odour molecules’, leading to a lessened sense of smell. This could also be a similar case with neurons from other parts of the brain.
The immune system’s response to infection can also negatively affect the brain. Inflammation of the brain can be a result of ‘cytokine storm’, an overreaction of the immune system or ‘sympathetic storm’, an overreaction of the sympathetic nervous system. This could cause depression of the brain stem reflex that senses hypoxia (low oxygen levels), which could explain why people often don’t know how badly they’re infected until it’s too late.
This presents medical professionals with a new challenge. How do we develop a drug that tackles the virus’ neurological as well as respiratory effects? The problem is that new treatments need to be made as most drugs can’t pass what’s called the ‘blood-brain barrier’, a sort of living border wall around the brain. Right now, doctors are trying a combination of antiviral drugs and steroids to prevent the immune system going into overdrive and to impair viral replication.
Another issue is that neurological symptoms sometimes set in before the usual cough and fever associated with infection. This fact led to the conclusion that doctors need to consider a change in brain function as a cause to test for Covid-19 in order to prevent a delayed diagnosis and prevention of transmission.
Also, if doctors knew that the virus was infecting the patient’s brain, it would be difficult to tell if said patient is sedated/ventilated, and obviously the brain can’t be put on a ventilator.
To conclude, how coronavirus affects the brain remains unclear, and the next task for medical professionals is to develop effective and safe treatments to combat neurological as well as respiratory symptoms to reduce transmission as much as possible.