Although extremely rare, Parkinson’s-like symptoms have occurred in a few people with COVID-19. This phenomenon has researchers investigating whether there is a link between SARS-CoV-2 and Parkinson’s disease.

Since the start of the COVID-19 pandemic, scientists have continued to search for information about how SARS-CoV-2 affects the body.

At this point, researchers and healthcare professionals know that the effects extend beyond the respiratory system. SARS-CoV-2 can impact other organs, including the heart, brain, kidneys, and skin.

In November 2020, an article published in The Lancet Neurology reported that up to 65% of people with COVID-19 have experienced hyposmia, a loss or change in their sense of smell, which is also a symptom of Parkinson’s disease.

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The same article reported three cases of people experiencing Parkinson’s-like symptoms after a SARS-CoV-2 infection, though they had no known risk factors for the condition.

These incidents have scientists questioning whether there is a link between SARS-CoV-2 and Parkinson’s disease or Parkinson’s-like symptoms.

In this Special Feature, we take a closer look at this phenomenon to investigate what scientists know about the possible association between Parkinson’s disease and COVID-19.

What is Parkinson’s disease?

Parkinson’s disease is a neurological condition. Its symptoms appear slowly and progress as time goes on. Symptoms include shaking or tremors, stiffness, and difficulties with balance, walking, talking, and coordination.

Because the disease affects the brain, people with Parkinson’s also experience behavioral changes, memory problems, sleep issues, and fatigue.

The condition results from the impairment of the nerve cells responsible for controlling movement. Other factors thought to contribute include low dopamine or norepinephrine levels and possibly the presence of Lewy bodies in the brain. According to scientists, genetic and environmental factors appear to set off these changes, causing the disease.

A distinct condition is called parkinsonism. People with parkinsonism have symptoms similar to those of Parkinson’s disease, but the symptoms are somewhat atypical.

This condition often accompanies another disorder, and there are different forms, including vascular parkinsonism and drug-induced parkinsonism.

Parkinsonism and viral infections: A possible link?

Reports of Parkinson’s-like symptoms in people with influenza have some scientists speculating whether there could be a viral cause of some types of parkinsonism.

To explore the possible connection, researchers have reviewed the reported instances of a viral illness leading to Parkinson’s-like symptoms. Their findings appear in the journal Frontiers in Neurology.

The scientists evaluated data about viruses including influenza, herpes simplex virus 1, Epstein-Barr, varicella zoster, hepatitis C, Japanese encephalitis, West Nile virus, and HIV.

The team also noted that parkinsonism had developed in some people during the 1918 influenza pandemic. Although these two events are temporally coincidental, a direct cause between influenza and parkinsonism has not been established.

Since then, some symptoms of parkinsonism, including tremors and walking disturbances, have been reported in people with influenza, most often within the first few weeks of the infection. Scientists are particularly interested in the role that influenza A may play in contributing to Parkinson’s-like symptoms.

Researchers speculate that parkinsonism may, rarely, occur in severe influenza infections because of the inflammatory process associated with the body’s immune response to viral threats.

This leads some to believe that there could be a link between parkinsonism and infections with other viruses, including SARS-CoV-2.

Parkinson’s-like symptoms in COVID-19

According to the most recent data, published in The Lancet Neurology on November 27, 2020, three people with COVID-19 have also experienced Parkinson’s-like symptoms.

Two men, aged 45 and 58 years, and one woman, aged 35 years, reported slowness of movement accompanied by muscle stiffness, muscle spasms, irregular eye movement, and tremor.

All three showed reduced function of the brain’s dopamine pathway system on imaging tests. Two of the three responded positively to medication and one recovered spontaneously.

None had a family history or clinical signs of Parkinson’s disease before their illness.

Why would Parkinson’s-like symptoms occur with COVID-19?

Scientists have developed three theories about mechanisms that could be involved in the appearance of parkinsonism following a SARS-CoV-2 infection. They describe their hypotheses in the journal Trends in Neurosciences.

First, SARS-CoV-2 is known to cause vascular complications in the brain and other organs, and the scientists suggest that this process could harm brain pathways. This damage is similar to what happens during the progression of vascular parkinsonism.

Second, because there is a known association between inflammation and an increased risk of Parkinson’s disease, inflammation caused by the immune response to a SARS-CoV-2 infection could potentially trigger parkinsonism.

Studies have also shown that some people with COVID-19 have elevated levels of interleukin-6, an immune system protein, as well as disruptions in the kynurenine pathway. Both are mechanisms associated with Parkinson’s disease.

In addition, the neuroinvasive nature of SARS-CoV-2 may contribute to a possible association between COVID-19 and parkinsonism. Researchers have discovered viral RNA in the brain tissue of people who have died from COVID-19, indicating that the virus may invade brain cells and pathways.

Meanwhile, some research suggests that the progression of Parkinson’s disease may begin in the olfactory system, where the sense of smell originates. Because COVID-19 can present with a loss of smell and taste, scientists wonder whether SARS-CoV-2 can gain access to the same brain pathways associated with Parkinson’s disease.

Researchers also hypothesize that a SARS-CoV-2 infection may reveal Parkinson’s disease that has not yet become symptomatic. Alternately, the infection might initiate the progression of the disease in people who are genetically prone to it.

Closely following further developments

While experiencing parkinsonism during a SARS-CoV-2 infection is currently quite rare, scientists say that the appearance of these symptoms in relation to COVID-19 merits further exploration.

They recommend close monitoring for Parkinson’s-like symptoms in a large cohort of people with COVID-19. Determining whether a link between parkinsonism and COVID-19 exists could help scientists better understand both health issues and develop more effective treatments.

At present, there is still much to learn about a possible connection between COVID-19 and parkinsonism, and scientists are just beginning to investigate this rare and poorly understood phenomenon.

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