The incidence of strokes in low- and middle-income countries has doubled since 1970, surpassing for the first time the rate in rich nations, down by 40 percent over the same period, report two studies released Friday.
The research also reveals huge gaps in stroke-related mortality: in Russia, for example, the condition is nearly ten times more deadly than in Canada, France, the United States or Israel.
The worst-hit regions are eastern Europe, north Asia, central Africa and the South Pacific, while the least affected are north American and western Europe, they report.
Exactly why the gap between developing and wealthy nations is so large, however, remains unclear, according to the studies, both published online in the British journal The Lancet Neurology.
Known risk factors such as diabetes and alcohol consumption are often higher in industrialised countries, and thus cannot account for the gap.
Strokes occur when the brain is deprived of blood and thus oxygen, causing brain cells to die.
In 2002, there were an estimated 15.3 million strokes worldwide. More than a third of them were fatal, accounting for about 10 percent of deaths worldwide from all causes.
A trio of researchers led by Claiborne Johnston of the University of California in San Francisco assessed national differences in stroke mortality and the impact of disease measured in years lost because of premature death and disability.
They found that national income was the best single predictor of stroke burden and mortality, with death rates on average 3.5 times higher in poor and middle-income countries than middle-to-high income nations.
Most risk factors for stroke -- including diabetes, smoking, alcohol consumption, and obesity -- were more prevalent in high-income countries.
"Disparities in stroke rates in low-income countries are unexplained by the metrics that are commonly used in assessing the cardiovascular health of a country," they noted, speculating that other measures such as rheumatic heart disease and HIV/AIDS might be better indicators.
In the second study, Valery Feigin at AUT University in Auckland, New Zealand sifted through thousands of studies from around the world to compile a database for comparing stroke incidence over the last four decades.
Drawing from 56 studies across 28 countries with comparable data, they found that the problem had reached "epidemic levels" in less developed nations, more than doubling since 1970.
Moreover, patients in these regions were disproportionately hit with haemorrhagic strokes -- the most severe kind -- and more often died within one month after an attack.
"The time to decide whether or not stroke is an issue that should be on the governmental agenda in low-to-middle income countries has now passed. Now is the time for action," they conclude.