Downing a few drinks or contracting an infection such as the flu seem to be connected to a short-term spike in the risk of stroke, a new research review finds.

The findings, reported in the journal Stroke, do not prove that alcohol and infections act as stroke “triggers” in some people, but they “strongly support” the notion that they do, the researchers say.

On the other hand, there is insufficient evidence on whether other suspected triggers - like extreme stress or physical exertion - do in fact contribute to stroke, lead researcher Dr. Vincent Guiraud, of Hopital Sainte-Anne in Paris, told Reuters Health in an email.

Triggers refer to behaviors or exposures that set off a temporary increase in a person’s risk of a disease. A number of studies, for example, have suggested that factors like heavy physical exertion, extreme stress and infections may trigger heart attacks in some people.

The question of whether there are stroke triggers, however, has been less studied.

The new review attempted to pull together what is known about potential triggers of ischemic stroke - the most common form of stroke, in which a blood clot disrupts blood flow to the brain.

Guiraud and his colleagues found 26 studies conducted since the 1980s that identified a dozen factors related to a short-term increase in stroke risk. Most of those studies focused on the potential roles of alcohol and infections in triggering a stroke over the following hours to weeks.

Overall, the review found, people who downed 40 to 60 grams of alcohol - equivalent to three or four standard drinks in the U.S. - showed a near-tripling in the risk of stroke over the next 24 hours.

A similar increase was linked to having more than 150 grams of alcohol, or about 10 standard drinks, in the past week.

Meanwhile, contracting any type of infection, such as a cold or flu, was linked to a two- to three-fold increase of suffering a stroke over the next week to one month.

Only a few studies looked at other potential stroke triggers. One UK study of 200 stroke patients did find associations between “negative emotion,” anger and exposure to a “startling event” and an increased risk of stroke in the next two hours. Another study linked psychological distress to a heightened stroke risk over the next three days, while a third - of more than 40,000 Canadian stroke patients - found that people had a higher risk on their birthdays compared with other days.

If alcohol, infections or other factors do serve as stroke triggers, the absolute risk of any one person suffering a stroke because of such an exposure would likely be small, according to Guiraud.

In theory, triggers would have a greater impact on people already at increased risk of stroke, due to factors like established heart disease, smoking, diabetes or high blood pressure. However, Guiraud said his team lacked the data to study that issue.

The study is important in that it offers an appraisal of the current knowledge on potential stroke triggers, according to Dr. Craig Anderson of the George Institute for Global Health in Australia, who was not involved in the research.

“There have been substantial anecdotal case reports and case series suggesting certain triggers for ischemic stroke, but until recently there has been a paucity of good epidemiological studies and sensible (physiological) explanations for the link,” Anderson, who is also a professor of stroke medicine and clinical neuroscience at the Sydney Medical School, told Reuters Health in an email.

Even many of the studies in the current review had limitations that made them prone to error or biased results, Anderson noted.

Among the weaknesses is the fact that most of the studies were so-called case-control studies, where people who had suffered a stroke were interviewed about various exposures before their stroke, and compared with “control” individuals who were asked about those same exposures.

A stronger design, according to Guiraud and his colleagues, would be the “case-crossover” study. In this type of study, each stroke patient would serve as his or her own control, being asked about a particular exposure during a defined period before the stroke, and during a different time period.

Case-crossover studies are not perfect either, but they make it easier for researchers to account for the many lifestyle habits and other factors that vary from person to person.

Still, despite the limitations of studies done so far, Anderson said that there is an “increasing appreciation” that most research on stroke risk factors has focused on more stable factors like average blood pressure. But, he said, daily fluctuations in heart rate, blood pressure, hormonal activity and muscle function could also create fluctuations in short-term stroke risk.

In theory, drinking could, for example, cause heart-rhythm disturbances that dislodge a blood clot from the heart that then travels into an artery supplying the brain, Guiraud and his colleagues note.

Similarly, infections could also contribute to heart-rhythm disruptions or have inflammatory effects in the blood vessels that might lead to a stroke.

For now, the practical implications of the current findings “are still hypothetical,” Guiraud said.

But, he added, it’s possible that for people at heightened risk of stroke, avoidance of any established triggers could complement the treatment of traditional stroke risk factors like high blood pressure.

For example, flu vaccination or antibiotic treatment of bacterial infections might help lower the odds of any short-term spike in stroke risk. Some research, Guiraud’s team notes, has found a link between flu vaccination and decreased stroke risk.

However, studies are still needed to show whether any steps to avoid potential stroke triggers are actually effective, according to Guiraud.

SOURCE:  Stroke, online October 14, 2010.