Does getting COVID make you more likely to develop dementia?

If true, it has great implications both for individuals and for society as a whole. There are around 400,000 Australians are already living with dementia – and that number is expected to reach around 850,000 by 2058.

Even a small increase in dementia risk could mean tens of thousands more Australians are living with the disease – terrible for them and their families, and overwhelming for the country’s healthcare system. “These are huge numbers,” says Woodward.

But other experts say the data is being misinterpreted. “I am very skeptical of these results,” says Associate Professor Stephen Macfarlane, Head of Clinical Services at the Dementia Centre.

Data

In August, the medical journal The Lancet published the results of a two-year study involving 1.4 million people. Measured two years after contracting COVID-19, 4.5% of people over the age of 65 had been diagnosed with dementia, compared to 3.3% in the control group of people with other respiratory infections.

A second article, Posted in Nature in Marchrevealed reduction of brain matter in regions related to decision-making and memory in people who have contracted COVID.

“It would be reasonable to suspect that this contributes to the reduced cognitive function they see in these patients,” says Professor Bryce Vissel, director of neuroscience and regenerative medicine at the University of Technology Sydney.

A third source of evidence comes from the link of COVID with known risk factors for dementia.

Slowing and blurring of thinking for many years often precedes a diagnosis of dementia. Data from China follow-up of 1438 elderly survivors of COVID-19 found they had lower scores on a cognitive telephone test six and 12 months after diagnosis.

“We don’t know yet if this will increase dementia rates. But we know it has long-term effects in subpopulations on memory function, and we know it increases the rate of dementia in older people after having a COVID infection,” says Vissel.

And we know falling into a state of delirium is a high risk factor for later being diagnosed with dementia.

But this raises skepticism from other scientists. Is it possible that COVID-19 is causing delirium in people who are already vulnerable – and So to make doctors more attentive to the signs of dementia?

“Any infection can reveal dementia in a cognitively vulnerable older person,” says Macfarlane of the Dementia Centre.

And patients with undiagnosed dementia may be less likely to take steps to avoid catching the virus. They are more likely to catch the virus, more likely to be seen by a doctor and therefore more likely to be diagnosed with dementia.

Possible mechanisms

Why would a respiratory tract infection cause dementia? Perhaps because the brain is vulnerable to the very symptoms it causes – inflammation and oxygen deprivation.

Inflammation is the body’s natural response to infection. It should go away once the virus is defeated, but in some people it persists far too long. COVID-19 is also known to infect nerves in the nose, which go directly to the brain – and there are evidence of tissue damage in the regions of the brain that connect to these nerves.

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“It’s very clear that inflammation is bad for your brain. Low oxygen levels are bad for the brain. There’s no denying that,” says Lange of Mercy Hospital.

But there are also mysteries here. Changes in the brain leading to common forms of dementia usually occur decades before symptoms appear. That COVID causes dementia within a year “seems biologically implausible,” Macfarlane says.

Those long years before dementia presents also hamper our ability to really know what COVID is doing to the brain. “COVID is too new,” says Professor Lezanne Ooi, head of the Neurodevelopment and Neurodegeneration Laboratory at the University of Wollongong. “I don’t think we can really fix that at this point.”

It remains a mystery – to be confronted with our future selves.

Liam Mannix’s Examine newsletter explains and analyzes the science with a rigorous focus on the evidence. Sign up to get it every week.