Oral contraceptives increase rare stroke risk in obese women

By Lucy Piper

Researchers have found that obese women taking oral contraceptives have an increased risk of cerebral venous thrombosis (CVT) and should be educated about this risk accordingly.

"[O]bese women should be informed about the increased risk of thrombosis if they use oral contraceptives, especially if other risk factors are present", the team advises.

"Alternative methods of contraception that are not associated with thrombosis, such as an intrauterine device, might be offered to these women."

The study involved 186 patients with CVT and 6134 individuals without the condition as controls. The patients with CVT were more likely than controls to be younger (40 vs 48 years old), female, have a history of cancer and to have used oral contraceptives.

They were also more likely to have a higher body mass index (BMI), with 22.6% of CVT patients versus 13.2% of controls classed as obese with a BMI of 30 kg/m² or above.

After taking into account gender, age, history of cancer, ethnicity, smoking status and oral contraceptive use, obesity was associated with a 2.63-fold increased risk of CVT.

This association was only seen in women, however, whose risk of CVT increased 3.5-fold if they were obese rather than a healthy weight. And among obese women, a driving factor was oral contraceptive use. There was a 29-fold increased risk of CVT among the 86 obese women taking such contraceptives compared with the 1190 women of a healthy weight not using oral contraceptives, the researchers report in JAMA Neurology.

They also observed a dose-response relationship, with a nearly 12-fold higher risk of CVT in overweight (BMI=25-29.99 kg/m²) women who took oral contraceptives compared with healthy-weight women who did not. By contrast, there was no increased risk for overweight or obese women not taking oral contraceptives.

"This dose-response effect, in combination with the magnitude of the effect size and the evidence that obesity is also associated with [venous thromboembolism], favors a causal association among obesity, oral contraceptive use, and CVT", say researcher Jonathan Coutinho (Academic Medical Centre, Amsterdam, the Netherlands) and colleagues.

But the researchers acknowledge that their study was limited by the small sample size and the need for data imputation due to missing BMI data in 15.9% of cases.

They also point out that "although the relative risks are increased substantially, the absolute risks of CVT are still small."

This fact is highlighted by Chirantan Banerjee (Medical University of South Carolina, Charleston, USA) in a related editorial, who says that given the low absolute risk, obesity should not necessarily preclude oral contraceptive use.

"Better counseling and education of obese women informing them of the increased risk would be prudent, as would be consideration of alternate nonhormonal [oral contraceptive] options", he advises.

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