COVID-19 positive outpatients are at a far increased risk of neurodegenerative disorders compared with those who tested negative for the SARS-CoV-2 virus, according to a study.

The researchers found that those who had tested positive for COVID-19 were at a much higher risk of Alzheimer's disease, Parkinson's disease, and ischaemic stroke.

The study, presented on Sunday at the 8th European Academy of Neurology (EAN) Congress in Vienna, Austria, analysed the health records of over half of the Danish population.

Out of 919,731 individuals that were tested for COVID-19 within the study, researchers found that the 43,375 people who tested positive had a 3.5 times increased risk of being diagnosed with Alzheimer's disease.

They also had 2.6 times increased risk of being diagnosed with Parkinson's disease, 2.7 times with ischaemic stroke, and 4.8 times with intracerebral hemorrhage (bleeding in the brain).

While neuroinflammation may contribute to an accelerated development of neurodegenerative disorders, the researchers also highlighted the implications of the scientific focus on long COVID.

The study analysed in- and outpatients in Denmark between February 2020 and November 2021, as well as influenza patients from the corresponding pre-pandemic period.

Researchers used statistical techniques to calculate relative risk, and results were stratified for hospitalisation status, age, sex, and comorbidities.

"More than two years after the onset of the COVID-19 pandemic, the precise nature and evolution of the effects of COVID-19 on neurological disorders remained uncharacterised," Pardis Zarifkar, lead author from the Department of Neurology, Rigshospitalet, Denmark, explained.

"Previous studies have established an association with neurological syndromes, but until now it is unknown whether COVID-19 also influences the incidence of specific neurological diseases and whether it differs from other respiratory infections," Zarifkar said.

The increased risk of most neurological diseases was, however, no higher in COVID-19-positive patients than in people who had been diagnosed with influenza or other respiratory illnesses.

COVID-19 patients did have a 1.7 times increased risk of ischaemic stroke in comparison to influenza and bacterial pneumonia in patients over 80 years of age.

The frequency of other neurodegenerative illnesses such as multiple sclerosis, Guillain-Barre syndrome, myasthenia gravis, and narcolepsy did not increase after COVID-19, influenza, or pneumonia.

"We found support for an increased risk of being diagnosed with neurodegenerative and cerebrovascular disorders in COVID-19 positive compared to COVID-negative patients, which must be confirmed or refuted by large registry studies in the near future," Zarifkar added.

"Reassuringly, apart from ischemic stroke, most neurological disorders do not appear to be more frequent after COVID-19 than after influenza or community-acquired bacterial pneumonia," he said.

The findings will help to inform our understanding of the long-term effect of COVID-19 on the body and the role that infections play in neurodegenerative diseases and stroke, the researchers added.