Stroke hospitalisations among Americans under 45, particularly teenage boys and men under 34, rose dramatically between 1994 and 2007 but fell among older people, said a study.

Researchers said they could not speculate on the cause, since the study only examined the number of hospitalizations across age and gender, but that the results merit further investigation of obesity and high blood pressure.

“I believe this is the first large study to report these findings, stratified by age and gender,” said Xin Tong, a health statistician with the US Centres for Disease Control in Atlanta, Georgia.

“We cannot link anything in particular to the trend in younger patients, but I believe the role of obesity and hypertension will prompt a big discussion. Unfortunately, right now we can’t speculate on the causes.”

CDC analysts found that stroke numbers had declined by 25 per cent in men and by 29 per cent in women over age 45.

But the good news ended there. The number of hospitalizations for ischemic stroke – a stroke caused by a vessel blockage that interferes with blood flow to the brain – rose 51 per cent in boys and men age 15 to 34. The rate among girls and women age 15 to 34 rose 17 per cent in the same period.

When it came to men age 35-44, they saw a 47 percent increase in stroke hospitalisations and a 36 percent increase among women of the same age group.

Dr Tong said medical professionals should take note of the shift when treating stroke patients, keeping in mind that younger people could benefit from a clot-busting tissue plasminogen activator (tPA) within hours after a stroke.

“Acute ischemic stroke is currently considered something that mostly happens to older people, but awareness of rising rates in the young is important or else tPA and other important stroke treatment may be unnecessarily delayed in younger patients,” she said.