Overall stroke incidence is declining in the U.S., but among younger people the trend may be the opposite. According to one analysis, the average age for a first-ever stroke dropped by two years and the proportion of younger people experiencing strokes grew over time. Changes in the use of diagnostic imaging may explain some of the findings.

In 2006, a report based on the Framingham Study concluded that incidence of stroke in the U.S. had decreased over the past five decades (JAMA 2006;296[24]:2939-2946). More recently, a data brief issued by the Centers for Disease Control and Prevention’s National Center for Health Statistics pointed to promising trajectories in stroke, with hospitalization rates, in-hospital mortality and days in the hospital all declining between 1989 and 2009.

But a study, published Oct. 10 in Neurology, paints a different picture for younger people. Brett M. Kissela, MD, of the University of Cincinnati College of Medicine, and colleagues reported that that the trends weren’t as rosy for people between the ages of 24 and 55. Their analysis was based on the Greater Cincinnati/Northern Kentucky Stroke Study, a retrospective, population-based study designed to track trends in stroke incidence and mortality.

The researchers identified first-ever stroke events in that patient population between July 1, 1993, and June 30, 1994; in the calendar year 1999; and in the calendar year 2005. They included only first-ever strokes with onset of symptoms within the study periods as well as imaging data. Among residents 20 years and older, they recorded 1,942 strokes in 1993-1994, 2,034 in 1999 and 1,961 in 2005.

They noted that MRI rates for first-ever strokes also increased during the study periods, from 18 percent in 1993/1994 to 27 percent in 1999 and 58 percent in 2005. Younger patients had higher rates of receiving MRIs than other age groups and they were more likely to receive an MRI in 2005 than in the previous study periods.

The authors found that among the older residents, stroke rates declined. But they reported significant increases over time in the proportion of strokes in residents between the ages of 20 and 44, growing from of 4.1 percent to 6.4 percent between 1993/1994 and 2005. For residents between 20 and 54, the proportion rose from 12.9 percent to 18.6 percent during that time period.

The median age decreased from 71.2 years in 1993/1994 to 69.2 in 2005. The prevalence of risk factors such as hypertension, diabetes, coronary heart disease and current smoking was higher in younger residents.

“Any decline in stroke incidence is positive from a public health prospective, but reduced incidence in older ages is counterbalanced by the worrisome trend of younger strokes with substantial productive life years lost and immense healthcare expenses over time,” Kissela et al wrote.

They listed changes in the prevalence of risk factors as one possible explanation for the trend. Physicians may target their elderly patients for stroke interventions but not consider younger patients, they suggested. “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,” they wrote. “Furthermore, physicians seeing young adults must identify and treat risk factors for macrovascular events such as stroke.”

They pointed to MRI data for another possible explanation. MRI use increased over time, with younger patients more likely than elderly patients to receive imaging. Consequently, it may be that more strokes were detected in younger patients in the later time periods.

Two editorial writers from Columbia University in New York City also made a case that changes in the use of MRI between the study time periods contributed to the findings. “The progressive adoption of MRI as a diagnostic tool during the study period challenges the validity of comparison of acute stroke diagnoses between these epochs,” wrote Sally Sultan, MD, of the College of Physicians and Surgeons, and Mitchell S.V. Elkind, MD, of the Mailman School of Public Health, in an accompanying editorial. “MRI is substantially more sensitive for the diagnosis of minor and self-resolving clinical ischemic stroke than CT.”

Sultan and Elkind pointed out MRI use among younger people increased over time and that imaging findings likely influenced  diagnosis. They postulated that with a sensitivity of approximately 40 percent, CT might have missed half of minor strokes in 1993-1994, when CT was the dominant modality. By 2005, with the use of MRI, missed cases would have been reduced to only a quarter.

“The effect of improved diagnosis of small strokes using MRI might even be enhanced in the young population, in which alternate diagnoses for transient and minor neurologic symptoms are more likely,” they wrote. “Preferentially improved diagnosis in the young will inflate the discrepancy in incidence between young and old.”

Both the study authors and editorial writers emphasized that the trends identified among younger people are a public health concern. Kissela et al wrote that they studied stroke incidence every five years and not continuously, and that the years chosen may not be typical. Sultan and Elkind also called for future studies based on other populations.