A woman in her forties has become the latest known case of SARS-CoV-2 infection associated with a "sudden and complete" loss of smell. Known as anosmia, loss of smell is regarded as one of the tell-tale signs of COVID-19.
When an odor enters the nose, neurons in the nasal cavity send messages to the olfactory bulb, a structure in the front part of the brain. These signals are then shipped off to different parts of the brain, leading some medical experts to wonder whether the novel coronavirus could cause adverse neurological reactions.
But how could a virus associated with respiratory infections impact the brain? Researchers are trying to find out. Dr Serena Spudich, a neurologist at the Yale School of Medicine specializing in infectious diseases, says that it is too early to say definitively whether SARS-CoV-2 directly infects the brain but limited data suggests it is a possibility.
SARS, a coronavirus closely related to the one that causes COVID-19, has been connected to brain infections in patients. A 2008 study in mice found that the virus was capable of entering the brain through their olfactory bulbs, spreading rapidly through neurons to connected areas of the brain and making neurons a “highly susceptible target for SARS-CoV”. Similarly, a 2000 analysis characterizing human coronavirus (HCoV) RNA, the body’s messenger for DNA, in human brain autopsy samples found that the viral infection is “consistent with neuroinvasion by these respiratory pathogens,” suggesting a possible link between the pathogens and multiple sclerosis, a chronic disease that affects the central nervous system.
SARS-CoV-2 and other related viruses could invade the brain through the cribriform plate, which is located next to the olfactory bulb, resulting in structural changes that could further impact the brain, suggest findings published in the Journal of the American Medical Association. In addition to respiratory distress, some patients with COVID-19 also show neurological signs like headache, nausea, and vomiting, which suggest that infection may not be confined to the respiratory tract but could also invade the central nervous system, according to a study published in the Journal of Medical Virology
It remains largely unclear exactly how these viruses access the brain, but evidence has found coronavirus pathogens in both the brain and cerebrospinal fluid of both patients and experimental animals that indicate the virus may sometimes target the central nervous system. What scientists don't know is whether neurological presentations of the virus are due to direct viral infections of nervous system cells or if infection serves as a catalyst for underlying conditions.
"It's probably a combination of both," said Spudich. "The question is: when you lose your sense of smell, is it because the cells are being infected and inflamed or the neurons?"
It's not clear whether or not a loss of sense or taste in COVID-19 cases are symptoms specific to the sensory neurons, though other coronaviruses do seem to use the nose as a "direct route" into the brain. The human nose is filled with both epithelial cells for protection and neurons that relay information to the brain, some of which begin in the nose and end in the brain.
Following a diagnosis of COVID-19, a 58-year-old American woman suffering from an “altered level of consciousness” became the first known person to suffer from acute necrotizing encephalitis (ANE), a rare brain disorder characterized by swelling and tissue damage that has also been linked to Influenza A.
There is still much to be determined about the novel coronavirus and scientists are continuing to decipher the many ways in which the virus impacts the human body.
“SARS-CoV-2 is so new and a lot of the places that have a lot of cases or would have had the capacity to study them haven’t had the opportunity to systematically try to understand them," said Spudich, adding that as cases around the world begin to level off, researchers will have a chance to analyze more closely the novel coronavirus.