The impact of COVID-19 on a person’s risk of a stroke is being intensely researched. But long before the pandemic, it was well-established that viral and bacterial infections can raise a person’s stroke risk, in some cases substantially.
Having even common viral or bacterial infections, such as a UTI (urinary tract infection), can be a co-factor in triggering a stroke. The medical literature commonly refers to influenza-like diseases (ILI) when referencing higher stroke risks, especially among adults who already have underlying health issues such as high blood pressure, diabetes, and heart disease.
“This risk of increased stroke with COVID-19 is not unique to COVID-19,” explains Felipe De Los Rios, M.D. , medical director of the Stroke Program at Baptist Health’s Miami Neuroscience Institute . “We’ve seen this before with a lot of different illnesses. We’ve seen it with just urinary infections, other respiratory tract infections, influenza or influenza-like illnesses.”
Pre-pandemic studies have indicated that even some adults younger than 45 can have a 9-fold higher stroke risk with influenza-like illnesses. Dr. De Los Rios cautions that it’s difficult to pinpoint a cause for stroke in younger people with few if any comorbidities, or underlying health issues.
“It seems likely that there is a stronger association between COVID — as it is with influenza-like illnesses — but it’s difficult to say because you have to correct for lots of other factors,” says Dr. De Los Rios.
Infection and Other Risk Factors
Stroke can be caused either by a clot obstructing the flow of blood to the brain (called an ischemic stroke — the most common) or by a blood vessel rupturing and preventing blood flow to the brain (called a hemorrhagic stroke). Stroke is the No. 5 cause of death and a leading cause of disability in the United States. When a stroke occurs, part of the brain cannot get the blood (and oxygen) it needs, so brain cells die. (May is Stroke Awareness Month).
The top risk factors for stroke are fairly well known: high blood pressure, cigarette smoking, diabetes, high blood cholesterol levels, a diet high in fat (particularly saturated) and salt, and obesity. However, a lesser understood risk factor is infection, which studies have identified as both a potential chronic risk factor and an acute trigger for stroke.
Research presented at the American Stroke Association’s International Stroke Conference  in 2019 indicated that having a flu-like illness increased the odds of having a stroke by nearly 40 percent over the next 15 days. Several studies have reached similar conclusions — long before the COVID pandemic. A separate study in 2019 of over 190,000 stroke patients found that the risk of suffering a stroke was higher in the weeks following any infection that required a trip to an ER or hospitalization. Urinary tract infections (UTIs) showed the strongest link to higher stroke risk.
Infection ‘Pushes Your Body Off Balance’
Why do infections increase a person’s risk for a stroke? There are many proposed cause-and-effect mechanisms behind the infection-stroke connection, but there’s much uncertainty. Researchers suspect a primary mechanism could be excessive inflammation caused by the infection.
Some studies have found that patients with COVID-19 can develop abnormal blood clotting, mostly a result of excessive inflammation. In response to infections, sometimes the body produces an overactive immune response which can lead to increased inflammation. The coronavirus is known to primarily attack the lungs, but it can also affect many other organs, including the blood. This can lead to clot formation.
Dr. De Los Rios says that understanding the impact of an infection on the body comes down to “balance.” “An infection can push your body off balance — that is the main point,” he says. “The more off balance you are, the higher the risk of stroke. Normally, there’s this balance in your body between clotting and bleeding — so that we don’t experience either of them. But if your body is fighting this infection, which is new and may be severe, then your body reacts to it. And in that process of reaction, there can be collateral damage. And part of that collateral damage is a tendency to form clots.”