Age specific temporal trends in stroke incidence in high income countries
Keywords: temporal trends
stroke
epidemiology

Overall stroke incidence has fallen in high-income countries, but data on time trends in incidence of young stroke (i.e. stroke in individuals younger than 55 years) are conflicting. I therefore aimed to: first, clarify and update overall stroke incidence trends in high income countries, second, compare temporal trends in incidence of stroke at younger vs older ages, third, explore if temporal trends in risk factors or ascertainment at younger ages might be associated with incidence trends, and finally, compare temporal trends in young stroke incidence with trends in other vascular events at younger ages.
I analysed data from a population-based study of stroke and vascular events, the Oxford Vascular Study (OXVASC) which has been ongoing since 2002, and compared results with a previous study, the Oxford Community Stroke Project (OCSP) 1981-1986, and with national admission data for England. I created a dataset from the Health Survey for England and the Annual Population Survey to estimate age specific time trends in risk factors and occupation in the underlying population. I also performed three systematic reviews and meta-analyses to summarise existing data on overall stroke trends, stroke trends at younger ages, and the relationship between trends at younger vs older ages, using a novel measure; the Relative Temporal Trend Ratio (RTTR). This thesis includes several interesting and original observations. First, I confirmed that overall trends in stroke incidence in high income countries continued to decline; both in Oxfordshire (between 1981-1986 and 2014-2017), and in my systematic review, with a pooled estimate of a 28% decline in overall stroke incidence (8 additional studies, reporting between 1990s and 2010s). Second, within Oxfordshire, I confirmed an increase in stroke incidence at age <55 years between 2002 and 2018 with an annual percent change of +5.5 (95%CI 3.4-7.7) and a clearly divergent temporal trend in incidence between younger and older people in this population. Third, I showed that incidence of TIA also increased at younger ages in Oxfordshire, pointing away from the idea of diagnostic drift from TIA to stroke as a cause of rising young stroke trends in this population. Fourth, in my systematic review of studies reporting young stroke incidence over time, I found inconsistent trends in young stroke incidence, however when comparing the incidence trends at younger versus older ages within studies, I found a more consistent picture of age-specific divergence, with less favourable stroke trends at younger ages to some extent in almost all studies. Fifth, I ruled out a number of potential hypothesis as to the cause of rising young stroke incidence rates in the OXVASC population, including: sex specific factors, illicit drug use, age specific changes in imaging rates, I did not identify any obvious trends in traditional risk factors or demographics within young stroke patients over time, with the exception of finding that the trend occurred to a higher degree in those in professional occupations compared with those in unskilled jobs. Finally, I found that in contrast to the observed increases in stroke and TIA at younger ages, incidence of other major non-stroke vascular events (acute myocardial infarction, peripheral vascular events and sudden cardiac deaths) at age<55 declined suggesting factors other than atherosclerosis may be at play.