Some patients develop symptoms of psychosis after getting COVID-19.
It is possible that the stress of the pandemic is causing the psychiatric problems.
But some scientists think it may be the virus causing the body to attack the brain.
In 2020, a day after developing COVID-19 symptoms, a 30-year-old man started thinking he could speak to his dead relatives.
The man, who had no history of mental illness, became convinced that the rapture was imminent, according to a case report published in August.
This psychotic episode went on for more than a month, during which the man knocked a door down, shoved his mother, and thought that he was being experimented on with radiation, the report said.
He was given antipsychotic drugs, but they had little effect. It was only after he was given medication usually used to treat autoimmune conditions that he got better, the case study said.
"Psychosis is one of medicine's big enigmas. We have a fairly poor understanding of what causes it and how it develops," Jonathan Rogers, a clinician and psychiatry researcher from University College London, told Insider.
Research suggests that psychiatric symptoms are common among COVID-19 survivors.
One study on the health records of more than 200,000 US COVID-19 patients found that about 13% received some kind of psychiatric or neurological diagnosis for the first time within six months of infection.
Psychosis - a particularly severe psychiatric condition - affected only 0.42% of that group, according to the study.
But such frequency was about twice that of people in the control group (patients who had the flu), the study said.
This sort of increase could be for an indirect reason: the psychological stress that comes from having COVID-19, two scientists who spoke with Insider said.
But research suggests that something else may be going on: The virus could be causing the body to attack itself, making the brain malfunction.
The theory is that the virus causes so-called anti-NMDA-receptor encephalitis, an autoimmune reaction that causes brain inflammation. That, in turn, can cause psychosis.
Usually, the brain is protected from the immune system because of a structure called the blood-brain barrier.
But COVID-19 might make that barrier "leaky," Benedict Michael, a clinician from Liverpool University, said.
"That then exposes immune cells to brain proteins that they wouldn't otherwise see," Michael, who oversees a registry of neurological complications in patients after COVID-19, said.
Michael said the immune system could then start attacking the cells in the brain, specifically, the NMDA receptors that are carried by neurons.
That, in turn, would make the neurons less sensitive to stimulation, he added. "It's a similar effect to ketamine," he said, referring to the powerful sedative.
The scientists said another virus, HSV-1, could cause similar brain problems.
The good news is that this kind of problem ought to be treatable with anti-inflammatory drugs and antipsychotics.
"We are hopeful that the majority will make a reasonable recovery because there's not been much brain damage," Michael said.
But both Rogers and Michael said the theory should be taken with a grain of salt. There are only a small number of documented psychosis cases after COVID-19, and even fewer where antibody levels have been measured, they said.
The presence of the anti-NMDA-receptor antibodies could be unrelated to the psychosis, Michael said.
"It's possible that there's an immunological basis for these individual psych cases, but I don't think it's proven in terms of the treatment," he said.
"Psychiatry has a history of all kinds of treatments that are good if you give them to just one patient but don't look so good when you do a clinical trial," he said.
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