How the Coronavirus Sneaks into the Brain
Among the many symptoms of Covid-19 are confusion, memory problems, delirium and what many people describe as long-term “brain fog.” A recent study found a third of people hospitalized with the disease had some sort of altered mental function, ranging from confusion to unresponsiveness.
Scientists have long suspected that the coronavirus may cause these symptoms by infecting the brain, a hypothesis that’s recently been supported by strong evidence that brain cells can indeed be infected. But exactly how it gets there hasn’t been pinned down.
One idea is that the virus crosses the normally formidable blood-brain barrier. Another long-suspected pathway is more direct: through the nose.
New images from autopsies of people who died from Covid-19 found intact SARS-Cov-2 virus particles well into the nasal cavity where smell is detected and passed on to the brain. The findings, detailed in the journal Nature Neuroscience, could also explain the relatively common loss of smell among many people who contract Covid, though it’s not clear whether the findings, which obviously involved the most severe cases, apply to people who suffer only milder symptoms.
The blood-brain barrier uses special blood vessels that filter out unwanted molecules from entering the brain. It’s so effective that scientists struggle to bust through it with drugs aiming to treat brain disorders. Increasingly, the more direct nasal pathway is used for delivering drugs to the brain.
The new electron microscopy images revealed a concentration of intact coronavirus particles in the olfactory mucosa, an organ that senses smell but is otherwise poorly understood by science.
“These data support the notion that SARS-CoV-2 is able to use the olfactory mucosa as a port of entry into the brain,” says study co-leader Frank Heppner, MD, a neuropathologist at the Charité-University Medicine in Berlin. “Once inside the olfactory mucosa, the virus appears to use neuroanatomical connections, such as the olfactory nerve, in order to reach the brain.”
“Our data suggest that the virus moves from nerve cell to nerve cell in order to reach the brain,” says Heppner’s colleague, Helena Radbruch, MD. “It is likely, however, that the virus is also transported via the blood vessels, as evidence of the virus was also found in the walls of blood vessels in the brain.”
The research also found that immune cells in the olfactory mucosa and inside the brain had been activated, and in some of the autopsied patients there was evidence of stroke.
“The presence of SARS-CoV-2 in nerve cells of the olfactory mucosa provides good explanation for the neurologic symptoms found in COVID-19 patients, such as a loss of the sense of smell or taste,” Heppner says. “We also found SARS-CoV-2 in areas of the brain which control vital functions, such as breathing. It cannot be ruled out that, in patients with severe COVID-19, presence of the virus in these areas of the brain will have an exacerbating impact on respiratory function, adding to breathing problems due to SARS-CoV-2 infection of the lungs. Similar problems might arise in relation to cardiovascular function.”