Observed stroke attack rates, as measured by a highly specific clinical definition, remained stable from 1980 to 2000 for women, confirmed from 1990 to 2000 by a sensitive and specific neuroimaging definition. For men, rates measured by the same definitions also plateaued from 1990 to 2000 while declining modestly from 1980 to 1990. Consistent with national statistics, incident events constituted three-quarters of all strokes, and trends in incident event rates were parallel to attack rate trends (1
). Comparable to our findings, the Greater Cincinnati/Northern Kentucky Stroke Study reported unchanged stroke incidence when comparing 1993–1994 with 1999 (11
). Two studies using vital statistics databases reported flat rates of hospitalized stroke, declining length of stay, and decreased case fatality in the 1990s, results similar to ours (12
). Brown et al. (14
) also reported a plateauing of stroke rates from 1980 to 1989 in Rochester, Minnesota.
Measured stroke rates varied widely by case definition. Furthermore, although the most specific measures of stroke (Minnesota Stroke Survey and neuroimaging definitions) suggest stable trends, rates as measured by ICD-9–based metrics declined substantially among men and, to a lesser extent, women. We argue that this decline is a consequence of increased neuroimaging use leading to improvement in discharge coding; that is, stroke-related ICD-9 codes have become more specific for stroke. Supporting evidence for this argument is the recent convergence of definitions based purely on ICD-9 coding with the neuroimaging definition (). The decline in the proportion of patients presenting with unconsciousness also supports the influence of neuroimaging on discharge coding (). Unconsciousness at presentation is often not due to stroke, and neuroimaging in recent surveys has likely helped clarify the diagnosis.
We also draw attention to the substantial decline (16% in 1980 to 6% in 2000) in the proportion of definition 1 cases found to have nonstroke etiologies of stroke. We therefore infer that the ICD-9 codes in early surveys contained many misdiagnoses and that most or all of the decline in stroke rates, as measured by definitions based purely on ICD-9 codes, is artifactual and due to the influence of neuroimaging on diagnosis and consequent discharge coding.
While rates assessed by the specific stroke measures appear to be static since 1980 for women and since 1990 for men, a key question remains: have stroke rates genuinely plateaued despite advances in medical therapy, or is the flat trend the result of a true decline masked by a countercurrent of more sensitive case ascertainment due to better diagnostic tools? The following are relevant observations from our data.
Neuroimaging improves the accuracy (sensitivity and specificity) of stroke diagnosis. Across every survey period, imaging use rates either increased substantially (1980 to 1990) or a more technologically advanced modality began to be used. While overall imaging rates were flat from 1990 to 2000, magnetic resonance imaging use increased substantially from 1990 to 2000, and diffusion weighted imaging was introduced only in the 2000 survey.
Have imaging trends changed Minnesota Stroke Survey case ascertainment by detecting smaller strokes or led to reclassification of transient ischemic attacks as strokes? The Minnesota Stroke Survey definition uses specific clinical variables abstracted from medical records that do not depend on imaging findings, although retrospective clinician documentation of stroke signs could be influenced by imaging results. A fraction of transient ischemic attacks with imaging correlates will be coded as strokes on hospital discharge. These cases will not meet the 24-hour requirement for clinical deficit duration required by the Minnesota Stroke Survey definition. Cases with acute strokes on imaging but such short-lived deficits increased from 13% in 1990 to 18% in 2000, reflecting perhaps the high sensitivity of diffusion weighted imaging. The Minnesota Stroke Survey definition excludes cases with alternate explanations for focal symptoms and has therefore always been specific, although it is conceivable that imaging has increased the ease of detection of nonstroke etiologies. Thus, even the Minnesota Stroke Survey definition may have become more specific over time.
We examined improved medical record documentation and its effect on case ascertainment. shows fewer missing data for 3 key clinical variables (level of consciousness, aphasia, and visual field cut). This improvement is due to increased documentation of the absence (vs. presence) of stroke signs and symptoms. For example, missing documentation on results of visual field testing declined from 43% in 1980 to 19% in 2000 and documentation of its absence of abnormality increased from 41% to 64%, whereas documentation of its presence remained unchanged. Improved documentation did not necessarily translate into identification of more cases in our study.
Several markers of stroke severity declined from 1980 to 2000. Median length of stay declined, with the largest drop between 1980 and 1985 coinciding with introduction of diagnosis-related group-based reimbursement in the early 1980s (15
). The proportion of cases with major neurologic deficits declined. Short-term (14-day, 28-day) survival, a reasonable surrogate for stroke severity, improved substantially. While this improvement may be in part have been due to improved acute care, detection of less severe events likely played a role.
The observations described above emphasize that the Minnesota Stroke Survey case definition is by design highly specific and reasonably impervious to technologic advances such as the availability of more accurate neuroimaging. Nevertheless, we cannot dismiss improvements in various markers of stroke severity possibly due to improved detection of smaller strokes. Hence, while the observed Minnesota Stroke Survey rates appear flat, a true decline may be masked.
We also draw attention to the changing risk factor profile of stroke cases (). The distribution of race in the stroke population mirrors the distribution in the general population. Prestroke history of myocardial infarction decreased across survey years, reflecting the declining incidence of coronary heart disease in the general population. There was no significant trend in prestroke atrial fibrillation history. Thus, we do not have evidence to suggest that advances in coronary care and improved post–myocardial infarction survival have led to increased rates of stroke in our population aged 30–74 years. It is possible that increased stroke rates due to improved post–myocardial infarction survival are actually occurring in those aged 75 years or older.
The rising prevalence of diabetes mellitus in the US population is not reflected in our stroke population. Prestroke history of hypertension increased significantly across the survey years, from 53% in 1980 to 68% in 2000. The Minnesota Heart Survey (16
), a parallel study to the Minnesota Stroke Survey, found declining hypertension from the 1980s to 2000–2002 in the general Minneapolis-St. Paul population. During this same time frame, increases were observed in the proportion of hypertensive patients aware of their diagnosis of hypertension. These results parallel those from the National Health and Nutrition Examination Survey, which found increased hypertension awareness, detection, and treatment at a national level (17
). On the basis of Minnesota Heart Survey results, we argue that the increased prevalence of prestroke history of hypertension in Minnesota Stroke Survey patients reflects population trends in increased awareness and detection of hypertension. Although improved hypertension detection and treatment have not apparently led to a decline in stroke rates, it is consistent with our view that stroke rates may have actually declined and that this decline may be masked by increased identification of less severe strokes.
We noted that our data show declining poststroke case fatality (), with most of the gains occurring between 1980 and 1995, although 1-year case fatality continued to decline among women between 1995 and 2000 as well. These observations are consistent with national trends that have shown a rapid decline in mortality in the 1970s and 1980s and a leveling off or slowed decline in the 1990s despite advances in therapies (18
Strengths of our study include the large population base under observation, the longitudinal nature of our data, and the use of multiple measures of stroke rates. A criticism of our study is that it examined rates of stroke hospitalizations only. Hence, acute strokes treated in an outpatient setting or nursing home were not captured. However, data from the Northern Manhattan Stroke Study suggest that hospital surveillance captured 94% of acute strokes and that only an additional 6%–7% of strokes can be identified by community surveillance efforts (19
). The Northern Manhattan Stroke Study conducted its surveillance from 1993 to 1996. It is possible that the proportion of stroke patients hospitalized for acute stroke changed over the 2-decade span of our study because of increased public awareness or changing practice due to the availability of acute treatments such as recombinant tissue-type plasminogen activator.
In conclusion, despite a significant decline in the use of stroke-related ICD-9 codes in the Minneapolis-St. Paul metropolitan area, rates, as measured by highly specific clinical criteria, remained stable among women between 1980 and 2000 and among men between 1990 and 2000. Short-term survival after stroke improved substantially from 1980 to 2000 for both men and women. Although our study used clinical criteria specifically designed to be robust to changing technology such as neuroimaging advances and the improved short-term survival is likely partly explained by improved stroke treatment, there is a strong possibility that less severe strokes are being ascertained, masking a true decline in strokes rates.