“It can accelerate a neurodegenerative disease already in the making,” she said. But perhaps the biggest driver of differences between the groups is the “scientific focus” on COVID patients. “In Denmark, almost everyone who has had COVID-19, especially severe COVID-19, is offered some sort of cognitive testing, and if you hand out MoCAs [Montreal Cognitive Assessments] which is the cognitive test we use, to almost everyone you’re meeting, you’re going to catch these disorders earlier than you might have otherwise.”
As for cerebrovascular disorders, the study showed an increased risk of ischemic stroke in COVID-positive versus COVID-negative subjects at 12 months (RR, 2.87; 95% confidence interval, 2.2-3.2).
The relatively strong inflammatory response associated with COVID-19, which may create a hypercoagulable state, may help explain the increased ischemic stroke risk in COVID patients, said Dr. Zarifkar.
The study did not show an increased risk for subarachnoid hemorrhage in COVID-positive, compared with COVID-negative, subjects but did reveal an increased risk of intracerebral hemorrhage after 12 months (RR, 4.8; 95% CI, 1.8-12.9).
This could be explained by COVID-positive subjects having a higher risk for ischemic stroke and receiving thrombolysis that may increase risk for bleeding in the brain. However, an analysis accounting for medication use found differences in thrombolysis rates didn’t change the result, said Dr. Zarifkar.
It’s also possible that extracorporeal membrane oxygenation and mechanical ventilation – interventions more frequently used in COVID-19 patients – may increase the risk for bleeding in brain, she added.
The researchers did not find an increased risk for multiple sclerosis, myasthenia gravis, Guillain-Barré syndrome, or narcolepsy in COVID patients. However, Dr. Zarifkar noted that it can take years to detect an association with autoimmune disorders.
The investigators did not stratify risk by disease severity, although this would be an important step, she said. “The threshold of being admitted to the hospital with COVID-19 has been much lower than for influenza or bacterial pneumonia where you’re typically quite ill before you’re admitted, so this might actually dilute the findings and underestimate our findings.”
A national, registry-based study that includes the entire Danish population and additional information on vaccination status, virus variants, socioeconomic status, and comorbidities is needed, said Dr. Zarifkar.
The study was supported by Lundbeck Foundation and Novo Nordisk. Dr. Zarifkar reported no relevant financial relationships.
A version of this article first appeared on Medscape.com.