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A health care worker wearing protective gear walks past a COVID-19 assessment centre, in Scarborough, Ont., on April 3, 2020.

Nathan Denette/The Canadian Press

While the new coronavirus is known to cause respiratory illness, some scientists suggest it can also potentially lead to brain and nerve damage in certain patients.

Beyond the typical symptoms of COVID-19, including fever, cough and difficulty breathing, doctors around the world have reported cases of infected patients with an array of neurological problems, including stroke, seizures, anosmia, or a loss of smell, and encephalopathy, a broad term used to describe brain damage or dysfunction.

Since these reports have so far been limited to anecdotal case studies, it is still too early to know whether the virus is to blame for these neurological symptoms, said clinical epidemiologist Jose Tellez-Zenteno, a professor of neurology at the University of Saskatchewan. Nevertheless, he said, it’s important for the public and health care providers to know this is a possibility.

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“The virus can go to the brain potentially,” Dr. Tellez-Zenteno said. “And not only for neurologists, but for [front-line] doctors …, they have to be aware that neurological complications can happen and be ready to diagnose and ready to treat, if there is some treatment for them.”

He noted that in one study of 214 hospitalized COVID-19 patients in Wuhan, China, researchers reported more than 35 per cent had neurological complications, including decreased levels of consciousness, stroke and muscle damage. These were more likely to occur among the hospitalized patients who were severely ill with COVID-19.

Dr. Tellez-Zenteno emphasized that the vast majority of individuals who catch COVID-19 have mild or no symptoms.

But the fact that many infected people experience anosmia may point to one of the potential ways in which the virus could affect the brain. In research on mice, the new coronavirus has been shown to enter the brain from the nose through the olfactory system, said Avindra Nath, clinical director of the U.S. National Institute of Neurological Disorders and Stroke, and chief of the section of infections of the nervous system.

In humans, it’s possible that the virus merely causes damage to olfactory nerve endings, which for many may result in nothing more serious than a temporary loss of smell, he said.

"The other possibility is that may be a route of entry into the brain,” he said.

Dr. Nath said it would be useful to follow up with these patients, long after they have recovered from the viral infection, to see whether they develop signs of lasting neurological effects.

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In a recent paper, published in the journal Neurology, he also raised the question of whether respiratory symptoms from COVID-19 could arise from damage to the brain stem, which controls breathing.

While it’s often assumed that people develop neurological complications from hypoxia, or a lack of oxygen, when an illness damages the lungs or other organs, it’s possible that the reverse may occur, Dr. Nath said.

“We really need to think that some of these patients [with respiratory syndrome] might have primary neurological involvement, and we should do CT scans or MRI scans on them” when possible, he said.

Chronic neurological problems have been tied to other types of coronaviruses in the past, said Mady Hornig, associate professor of epidemiology at Columbia University’s Mailman School of Public Health. She noted, for example, that there have been reported cases of myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS), a debilitating condition, in some individuals who have had SARS and MERS (Middle East Respiratory Syndrome).

“That’s one of the questions here,” said Dr. Hornig. “We must really look hard, particularly with these types of coronaviruses, for chronic effects that are in the neurological – and even bordering on neuropsychiatric – category.”

In explaining other potential ways in which the virus could enter the brain and cause damage, psychoneuroimmunology expert Boris Sakic at McMaster University said that normally, our brains are protected by the blood-brain barrier, the densely packed cells around the blood vessels of the brain that prevents pathogens from entering. However, in a “cytokine storm,” or a severe immune reaction in certain individuals, the blood-brain barrier can become permeable.

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When this happens, viruses circulating in the blood can then enter the brain, along with other immune cells, or white blood cells, that normally reside in the body, he said. The presence of viruses in the brain then triggers activation of the brain’s immune cells, such as microglial cells, which can produce certain toxic substances, said Dr. Sakic, an associate professor of psychiatry and neurobehavioural sciences.

But generally, when it comes to viral infections, he said, the biggest cause of damage to the brain is from the other white blood cells entering the brain through the blood-brain barrier, which cause “collateral damage.”

“Let’s say you want to kill the virus, but you also kill all the neighbouring neurons,” he said.

While the COVID-19 pandemic spreads, many frontline health-care workers are forced to isolate as they care for patients infected with the virus. See how Dr. Kanna Vela is coping with being away from her family as she treats patients in Ajax and Scarborough, Ontario hospitals.

Sign up for the Coronavirus Update newsletter to read the day’s essential coronavirus news, features and explainers written by Globe reporters.

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