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Dutch researchers found that obese women on birth control pills were nearly 30 times more likely to develop this rare type of stroke, known as cerebral venous thrombosis (CVT), compared with women of normal weight who didn't take birth control pills.
But the chances of having this type of stroke in one's lifetime remain very low, the researchers added. And the study wasn't designed to prove a cause-and-effect relationship.
CVT affects just over one person per 100,000 each year, said study author Dr. Jonathan Coutinho. He's a stroke neurologist at the Academic Medical Center at University of Amsterdam.
In the United States, that's about 4,200 new patients annually, he said.
Coutinho added that CVT tends to occur mostly in children and young to middle-aged adults.
Dr. Carolyn Westhoff, a professor of obstetrics and gynecology, and epidemiology at Columbia University in New York City, said the findings, while not that surprising, provide "an extra supporting piece of the puzzle."
Obese women have a higher risk of clots, as do oral contraceptive users, she explained.
"When you put those two risk factors together, you get even higher risk of clots," said Westhoff. She's also editor-in-chief of the journal Contraception.
Overall, CVT is less disabling than other types of stroke caused by a blocked blood vessel (ischemic), as more than five of six patients have good long-term recovery and outcomes, said Dr. Chirantan Banerjee, an assistant professor of neurology at the Medical University of South Carolina in Charleston.
"The challenge is that it is harder to diagnose sometimes, because patients can have nonspecific and variable symptoms that are not usually attributed to stroke, like headache, confusion, blurry vision or seizures," he said.
The study was published online March 14 in the journal JAMA Neurology.
The study included 186 adult CVT patients, including men and women. Their strokes occurred from the mid-2000s through Dec. 31, 2014.
The study also included a control group of more than 6,000 healthy adults. These adults had taken part in a large Dutch study conducted from 1999 to 2004. That study looked at risk factors for deep vein thrombosis (DVT) and pulmonary embolism (PE), blood clots that occur in the legs and lungs.
The researchers found that CVT patients were more often younger. Their median age was 40 compared to 48 for the control group, the study noted. They were also more likely to have a history of cancer. And, they were more likely to be female and users of oral contraceptives, the study found.
Obesity was associated with a nearly 30-fold increased risk of CVT, but only among women using birth control pills. Obesity is defined as having a body mass index (BMI) of 30 or more, according to the U.S. Centers for Disease Control and Prevention. BMI is a rough estimate of a person's body fat using height and weight measurements.
"For us, it was especially surprising that obesity is only a risk factor in women who use oral contraceptives," Coutinho said. "We would have expected it to be a risk factor in all patients and perhaps stronger in women on oral contraceptives."
Coutinho and colleagues note that the study includes a relatively small number of CVT patients and relies on control subjects from an earlier time period. These and other limitations could have biased the results.
Still, they concluded that obese women should be informed about the risk of stroke if they use oral contraceptives, especially if they have other stroke risk factors.
In a related editorial, Banerjee said better counseling of obese women, including consideration of non-hormonal contraceptive options, "would be prudent."
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