Now's the time to take extra care of your health. Photographer: Jewel Samad/AFP/Getty Images
Now's the time to take extra care of your health. Photographer: Jewel Samad/AFP/Getty Images

It turns out a cold snap really is bad for your health, but not because the chill gives you a runny nose. The fallout is much worse. Just as Americans are digging out from a brutal winter, the news arrives that sudden drops in temperature increase the incidence of strokes.

This unexpected observation is another bizarre data point in the steady barrage of bewildering public health news that health-conscious people -- not to mention public health officials -- are asked to process. So let’s digest this study before it provokes anyone to pack up and move out of reach of the next polar vortex.

In a review of more than 100,000 stroke cases nationwide, Yale University public health researchers found that large day-to-day temperature fluctuations coincide with small rises in stroke hospitalizations. Higher temperatures have a slight protective effect; with each degree (Fahrenheit) rise, strokes become 0.86 percent less likely, and the risk of deadly strokes falls 1.1 percent. Cooler average temperatures, on the other hand, make things worse.

Further insight into this frightening phenomenon comes from another large study of stroke cases at a single Boston hospital over a nine-year span. These researchers analyzed stroke hospitalizations hour by hour and learned that each 9-degree drop in mean daily temperature led to a spike in strokes 10 to 24 hours later. If the temperature dropped 9 degrees and stayed low for a day, strokes increased 7 percent; if the cold lasted two days, strokes went up 9 percent.

It’s important to understand that these are population-level effects on the rise and fall in the number of stroke patients coming to the hospital. They don’t translate into a particular person's stroke risk jumping by 7 or 9 percent -- far from it. Furthermore, the Boston researchers found that this effect isn’t seasonal, so absolute low temperatures don’t matter; rather, it's the relative change in a short time. Trying to find a place to move that has absolutely stable weather won't be easy.

But that doesn't make people helpless to avoid strokes. After all, they don’t occur in a vacuum where only the weather breaks the seal. The stroke victims in the Boston study had all the expected risk factors: advanced age (average 73), prior stroke (almost 30 percent), atrial fibrillation (about 25 percent) and hypertension (about 70 percent). People who don't have such risk factors needn’t worry about weather fluctuations, and those who do are better off spending their energies managing those risks -- especially high blood pressure.

Cold is known to increase platelets, platelet viscosity, cholesterol and plasma fibrinogen, all of which suggest that cold provokes inflammation -- which is very harmful to folks with already impaired cardiovascular systems. But, probably more important, cold constricts blood vessels and increases the heart rate, both of which push already elevated blood pressure even higher. Higher humidity, which helps transmit cold to the skin, sustains the effect, both stroke studies found.

High blood pressure -- which afflicts up to a third of all adults -- is the single largest factor behind all adult death and disability, mainly because of its role in causing strokes. And that is where public health officials need to focus their attention.

The link between blood pressure and stroke is so strong, a new analysis of studies that have included more than 760,000 patients suggests there is no safe blood pressure above normal. In the range of “prehypertension,” in which blood pressure hovers anywhere from normal (120/80 and below) to hypertensive (140/90 and above), stroke risk is 66 percent higher than normal. Even blood pressure on the low end of this range -- 125/85, say -- carries a 44 percent higher stroke risk than blood pressure at 120/80 or below does. In my own clinical practice, many patients rehabilitating after stroke say that their doctors told them, before their strokes, that their blood pressure was “borderline.” These prehypertensives are typically prescribed diet and exercise. We should also prescribe them blood pressure monitors.

An otherwise healthy 30-something woman who is mid-range hypertensive carries about a 2.5 percent risk of having a stroke in 10 years. If she starts to smoke, her risk can become six times higher. In other words, risk factors are additive. Many cannot be helped, including advancing age or sex (men are a third more likely than women to have strokes). But with hypertension, medication, dietary changes and exercise will usually make a difference. The risk of stroke drops 25 percent for every 2 points the systolic blood pressure (the top number) comes down.

As for the U.S. population as a whole, better management of blood pressure could reduce stroke incidence by more than a third. Given that strokes and related disability in the U.S. cost more than $70 billion a year, universal free hypertension care would be a bargain.

(Ford Vox, a doctor at the Shepherd Center for brain and spinal-cord injury in Atlanta, writes about health-care policy. Follow him on Twitter @fordvox.)

To contact the writer of this article:
Ford Vox at docvox@gmail.com.

To contact the editor responsible for this article:
Mary Duenwald at 1+212-205-0366 or mduenwald@bloomberg.net.