A nationwide longitudinal analysis of data from almost 4.9 million people in Denmark including all 36 million PCR tests shows that having COVID-19 considerably increases your risk of developing a disorder of the nervous system such as Parkinson’s disease, dementia, multiple sclerosis, headache, stroke, myopathy and polyneuropathy. However, this increased risk is not greater than that observed after other similarly severe infections.
When the COVID-19 pandemic engulfed the world, it became clear very quickly that many people who developed COVID-19 subsequently developed various disorders related to the brain – both mental disorders and neurological disorders.
Clinics were also set up in many countries to take care of the many people who experienced post-acute sequelae of COVID-19 – also called long COVID.
A new study now shows that the risk of developing a neurological disorder indeed increased after COVID-19 but that this additional risk was not greater than that associated with other similarly severe infections.
“We previously showed that COVID-19 increases the risk of developing a mental disorder but that the risk is not greater than after other infections of similar severity. We have now shown that this also applies to neurological disorders, with COVID-19 being associated with an increased risk of developing many disorders of the nervous system but not more increased than after other similarly severe infections. We also found that the more severe the infection, whether COVID-19 or another infection, the greater the risk of a subsequent new-onset neurological disorder,” explains a researcher behind the study, Michael Eriksen Benros, Clinical Professor and Consultant, Mental Health Centre Copenhagen and Department of Clinical Medicine, University of Copenhagen.
The research was carried out in collaboration with researchers including Clara Grønkjær, PhD Fellow, Mental Health Centre Copenhagen; Rune Christensen, Senior Statistician, Mental Health Centre Copenhagen; and Daniel Kondziella, Clinical Research Associate Professor and Consultant Neurologist, Department of Neurology, Neuroscience Centre, Rigshospitalet and Department of Clinical Medicine, University of Copenhagen.
Entire population of Denmark
The researchers carried out a large-scale longitudinal analysis of data from 4.89 million people and more than 36 million PCR tests in Denmark from March 2020 to 31 December 2021. The study is thus nationwide and includes the entire population – except for those with a previous hospital contact for neurological disorders before March 2020.
The researchers made several comparisons to determine whether COVID-19 was associated with a higher risk of developing a neurological disorder and whether the severity of COVID-19 was associated with an additional increased risk.
The researchers compared the risk of receiving a diagnosis code for a neurological disorder between people with positive versus negative PCR tests for COVID-19. They also compared receiving a positive PCR test versus not being tested and receiving a positive PCR test versus being treated for an infection other than COVID-19.
“Our study is unique because we have several control groups and can thereby determine whether having COVID-19 is specifically associated with an increased risk of developing a diagnosed neurological disorder versus not having COVID-19 and versus having other infections,” says Michael Eriksen Benros.
The researchers also examined the risk for a new-onset neurological disorder for people hospitalised with a diagnosis of COVID-19, with a diagnosis of another infection and with various diagnosed respiratory infections other than COVID-19. They also investigated whether being in intensive care increased the risk.
Risk of neurological disorders was increased but no more than after other infections
The results show that having a positive COVID-19 PCR test versus not being tested was associated with a 96% increase in the risk of being diagnosed with a new-onset neurological disorder.
Compared with having negative tests, the risk only increased by 11%.
Those who tested positive for COVID-19 had no further increased risk of a later neurological disorder than people who picked up a prescription for drugs used for infections other than COVID-19. According to Michael Eriksen Benros, this indicates that infections other than COVID-19 are associated with an equally high risk of later being diagnosed with a neurological disorder.
The researchers found similar results in the data from people who were hospitalised, although being hospitalised was associated with a higher risk of developing a neurological disorder.
Being hospitalised with COVID-19 was associated with a 192% increased risk of later being diagnosed with a neurological disorder versus individuals that had not been hospitalised during the study period.
Being hospitalised with another respiratory infection than COVID-19 was associated with a 1.72-fold increased risk of developing a neurological disorder. The increased risk of developing a neurological disorder did not differ significantly between having COVID-19 versus another respiratory infection.
The more severe the infection, the greater the risk
The study also showed that the severity of the infection is associated with the risk of a later neurological disorder. Thus, being in intensive care with COVID-19 was associated with a 5.64-fold increased risk of developing a neurological disorder.
Finally, the researchers found that the risk was especially increased for some types of neurological disorders versus others.
Having a positive PCR test versus having a negative PCR test increased the risk of developing myopathy (muscle weakness) 7.37-fold. Being hospitalised for COVID-19 increased the risk 20-fold.
Having a positive PCR test increased the risk of developing polyneuropathy (sensory disturbances) 3.59-fold and 10.71-fold for being hospitalised.
“The take-away of this study is that COVID-19 does increase the risk of developing a neurological disorder, but the risk is not increased more than for other similarly severe infectious diseases. Another message is that the more severe the infection, the greater the risk. Clinics dealing with long COVID may find this knowledge useful. They may need to focus more on the fact that sequelae can not only occur after COVID-19 but also after many other severe infections, and that many people develop neurological disorders or other problems they need help in managing after having been severely ill with infections,” concludes Michael Eriksen Benros.