They found 34% of Covid-19 survivors received a diagnosis for a neurological or psychological condition within six months of their infection, according to the study published Tuesday in the journal Lancet Psychiatry.
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The most common diagnosis was anxiety, found in 17% of those treated for Covid-19, followed by mood disorders, found in 14% of patients.
And while the neurological effects are more severe in hospitalized patients, they are still common in those who were only treated in an outpatient setting, the researchers note.
"That rate increased progressively as the severity of the Covid-19 illness increased. If we look at patients who were hospitalized that rate increased to 39%," said Maxime Taquet, an academic clinical fellow in psychiatry at the University of Oxford, and a co-author of the new study.
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The results help light the way for how the healthcare system ought to continue helping Covid-19 survivors, the researchers said.
"Our results indicate that brain diseases and psychiatric disorders are more common after Covid-19 than after flu or other respiratory infections, even when patients are matched for other risk factors. We now need to see what happens beyond six months," Taquet added.
Covid-19 as a 'brain disease'
It's the largest study of its kind yet and involved the electronic health records of more than 236,000 Covid-19 patients, mostly in the US. The researchers compared their records with those who experienced other respiratory tract infections during the same time frame.
They observed that those with Covid-19 had a 44% increased risk for neurological and psychiatric illness compared to people recovering from flu. And they were 16% more likely to experience those effects compared with people with other respiratory tract infections.
About one in 50 Covid-19 patients had an ischemic stroke, which is a blood clot that affects the brain.
However, Covid-19 didn't necessarily increase the risk for the full spectrum of neurological illness.
"Two important negative findings were related to parkinsonism and Guillain-Barré syndrome," Taquet said. "Both of those conditions are neurological conditions that we know are sometimes associated with viral infection. We did not find that they were more common after Covid-19 and after the other respiratory tract infections that we looked at."
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The study was important, in part, because of the sheer number of patient records the researchers were able to analyze, according to Dr. Musa Sami, a clinical associate professor in psychiatry at the University of Nottingham.
"This is a robust piece of work in a large cohort demonstrating the association between Covid-19 and psychiatric and neurological complications," he said in a statement. "This is a very important topic as there has been considerable consternation regarding Covid-19 as a 'brain disease.'"
Sami, who was not associated with the study, highlighted the need for further investigation into how, exactly, Covid-19 affects the brain and nervous system. "Psychological stress, longer stays in hospital, and characteristics of the illness itself may play a part," he said.
One clue: psychological symptoms are more common than severe neurological complications, according to Masud Husain, a professor of neurology and cognitive science at the University of Oxford, and a co-author of the study.
"It's really the people with very severe illness who are at higher risk of developing the neurological complications, unlike what we see with the mental health complications, which is much more across the board with severity," he said.
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Other, smaller studies have pointed to the result. One study in February followed 381 patients treated for Covid-19 at a hospital in Rome, Italy and found that 30% of them experienced post-traumatic stress disorder after recovery.
A December study in the journal Neurology: Clinical Practice showed that Covid-19 could cause seizures and movement disorders, even in some moderate cases.
Covid-19's longer term burden on the health system
One limitation of the Lancet Psychiatry study is that it uses "routine healthcare data," rather than research data, according to Paul Harrison, a professor of psychiatry at the University of Oxford, and the study's lead author.
That could mean that diagnoses are missing, they haven't been fully investigated, or that they are incorrect.
And just having been diagnosed might make a difference.
"Patients that had Covid-19 might be more likely to have a neurological and psychiatric diagnosis, simply because they received more follow up, more medical attention, compared to patients with the respiratory tract infections. That might account for some of the differences that we've observed in the rates," Taquet told a news briefing.
But still, the study provides a sweeping view of the long-term burden the pandemic will have on those it struck.
"Although the individual risks for most disorders are small, the effect across the whole population may be substantial for health and social care systems due to the scale of the pandemic and that many of these conditions are chronic," Harrison said. "As a result, health care systems need to be resourced to deal with the anticipated need, both within primary and secondary care services."