Many of the symptoms experienced by people infected with SARS-CoV-2 involve the nervous system. Patients complain of headaches, muscle and joint pain, fatigue and “brain fog,” or loss of taste and smell—all of which can last from weeks to months after infection. In severe cases, COVID-19 can also lead to encephalitis or stroke. The virus has undeniable neurological effects. But the way it actually affects nerve cells still remains a bit of a mystery. Can immune system activation alone produce symptoms? Or does the novel coronavirus directly attack the nervous system?
Some studies—including a recent preprint paper examining mouse and human brain tissue—show evidence that SARS-CoV-2 can get into nerve cells and the brain. The question remains as to whether it does so routinely or only in the most severe cases. Once the immune system kicks into overdrive, the effects can be far-ranging, even leading immune cells to invade the brain, where they can wreak havoc.
Some neurological symptoms are far less serious yet seem, if anything, more perplexing. One symptom—or set of symptoms—that illustrates this puzzle and has gained increasing attention is an imprecise diagnosis called “brain fog.” Even after their main symptoms have abated, it is not uncommon for COVID-19 patients to experience memory loss, confusion and other mental fuzziness. What underlies these experiences is still unclear, although they may also stem from the body-wide inflammation that can go along with COVID-19. Many people, however, develop fatigue and brain fog that lasts for months even after a mild case that does not spur the immune system to rage out of control.
Another widespread symptom called anosmia, or loss of smell, might also originate from changes that happen without nerves themselves getting infected. Olfactory neurons, the cells that transmit odors to the brain, lack the primary docking site, or receptor, for SARS-CoV-2, and they do not seem to get infected. Researchers are still investigating how loss of smell might result from an interaction between the virus and another receptor on the olfactory neurons or from its contact with nonnerve cells that line the nose.
Experts say the virus need not make it inside neurons to cause some of the mysterious neurological symptoms now emerging from the disease. Many pain-related effects could arise from an attack on sensory neurons, the nerves that extend from the spinal cord throughout the body to gather information from the external environment or internal bodily processes. Researchers are now making headway in understanding how SARS-CoV-2 could hijack pain-sensing neurons, called nociceptors, to produce some of COVID-19’s hallmark symptoms.
Neuroscientist Theodore Price, who studies pain at the University of Texas at Dallas, took note of the symptoms reported in the early literature and cited by patients of his wife, a nurse practitioner who sees people with COVID remotely. Those symptoms include sore throat, headaches, body-wide muscle pain and severe cough. (The cough is triggered in part by sensory nerve cells in the lungs.)
Curiously, some patients report a loss of a particular sensation called chemethesis, which