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The increase in stroke among younger people was mainly for ischemic stroke, the most common form. (Pablo Martinez Monsivais/Associated Press)

Strokes are occurring at a younger age, say researchers who call the trend concerning.

Researchers looked at strokes occurring in people aged 20 to 54 in the Greater Cincinnati and Northern Kentucky regions.

"We found trends toward increasing stroke incidence at younger ages," study author Dr. Brett Kissela of the University of Cincinnati College of Medicine in Ohio and his co-authors concluded in Wednesday's online issue of the journal Neurology.

A rise in risk factors such as obesity, Type 2 diabetes and high cholesterol are potential reasons for the trend, Kissela said. Better diagnosis from MRI could also be contributing.

"Regardless, the rising trend found in our study is of great concern for public health because strokes in younger people translate to greater lifetime disability," he said in a release. In the study, researchers looked at first strokes that occurred during three, separate year-long periods between 1993 and 2005.

What's behind stroke trend?

The average age of people who experienced stroke fell from 71 years in 1993 and 1994  to 69 years in 2005, the researchers found.

Among the young stroke patients, more coronary heart disease was found in 1999 and 2005 compared with the first year.

The prevalence of heart disease among the general population didn't change.

"Given the increase in stroke among those less than 55 years old, an important public health message is that younger adults should see a physician regularly to monitor their health and risk for stroke and heart disease," the study's authors said.

Since the researchers did not include body mass index data for those surveyed, they weren't  able to comment about obesity.

The investigators kept the diagnostic criteria for stroke the same throughout the study and tried to account for the introduction of MRI, a journal editorial accompanying the study noted.

Traditional risk factors such as high cholesterol, high blood pressure and smoking are associated with stroke in young adults but it's not clear they can fully explain the increased incidence, Dr. Sally Sultan of the neurology department at Columbia University and her epidemiology colleague Dr. Mitchell Elkind, said.

"Among the young, the increase in incidence suggests an unknown and potentially daunting future trajectory," they wrote.

It is important to repeat the research in other U.S. populations and internationally,  Sultan and Elkind said.

Last year, Canadian researchers said that atherosclerosis — buildup of fat in the walls of arteries — affected a proportion of apparently healthy people aged 18 to 35.