Incidence and Characteristics of Total Stroke in the United States

Stroke is the third leading cause of death in the United States, after heart disease and cancer, and the leading cause of adult disability.[1] In 1994, Matchar and Duncan [2] claimed that each year there are ≈ 550,000 strokes in the US, causing 150,000 deaths and leaving 300,000 survivors disabled. The Heart and Stroke Statistical Update [3,4] of the American Heart Association (1995, 1997) states that ≈ 500,000 Americans suffer a first-ever or recurrent stroke each year. All three reports were based on the predominately white cohort study of Framingham, Massachusetts. In 1998, Broderick et al [5] hypothesized that the figure of approximately half a million strokes substantially underestimates the actual annual stroke burden for the United States. They claimed that there were at least 731,000 first-ever or recurrent strokes during 1996.[5] This estimate was derived by extrapolating from first-ever strokes among whites in the Rochester, Minnesota Stroke Study. The 1999 Heart and Stroke Statistical Update [6] of the American Heart Association adjusted their estimate to ≈ 600,000 first-ever or recurrent strokes each year in the US.

Population-based stroke incidence studies such as those from Framingham, Massachusetts and Rochester, Minnesota have substantially increased the knowledge about stroke trends, subtypes, risk factors and incidence rates in both men and women. However, these studies were conducted among predominately white populations. Recently, epidemiological studies have been focusing on differences in stroke incidence between racial/ethnic groups. Of particular interest are rates for blacks; however, there is little data regarding stroke risk in Hispanics or Asians. Recent data from Northern Manhattan suggest that blacks are not alone in the higher risk category and that Hispanics also appear to be at greater risk than whites.[7]

In order to get a more accurate estimate of occurrences of stroke in the US, Williams et al [8] estimated the 1995 incidence, occurrence and characteristics of total stroke based on a large administrative claims database representative of all 1995 US inpatient discharges. They conservatively claimed that there were ≈ 750,000 first-ever or recurrent strokes during 1995.

The primary goal of this paper was to replicate the recent estimates of Broderick et al [5] and Williams et al, [8] and to examine the trend from 1995 to 1996. This was accomplished by use of a large administrative claims database representing a 20% representative sample of all 1996 US inpatient discharges. The administrative database was supplemented by appropriate adjustments for the International Classification of Diseases, 9th Revision, Clinical Modification (ICD-9-CM) codes 430-438 derived by Williams et al [8] to correct for some of the inaccuracies of the diagnostic codes.

Of the estimated 712,000 (682,000) hospital strokes, 23,700 (23,400) or 3.3% (3.4%) were subarachnoid hemorrhages, 74,100 (71,600) or 10.4% (10.5%) were intracerebral hemorrhages, and the remaining 615,000 (587,000) or 86.4% (86.1%) were ischemic strokes.

The overall incidence rate for total stroke (first-ever and recurrent) was 269 (259) per 100,000 population (age and sex-adjusted to the 1996 (1995) U.S. population). The average annual age and sex-adjusted incidence rate for first-ever stroke was estimated to be 208 (200) per 100,000. Total stroke incidence rates increased exponentially with age for both men and women. In addition, men had higher age-specific total stroke incidence rates than did women, as illustrated in Figure 1.

Table 2 also presents data on resource utilization. For the index hospitalization, the stroke patient population had a mean length of stay of 9.0 (9.8) days (median 5 [5] days) and a mean total charge of $18,022 (17,711) (median $8,845 [$8,735]). Patients with subarachnoid and intracerebral hemorrhages had much longer length of stays, averaging 13.9 (14.0) and 10.3 (10.5) days, respectively. Their mean total charges were also higher, at $51,193 ($46,711) and $24,409 ($23,097), respectively. The majority of stroke patients had a routine discharge or were discharged to a skilled nursing facility with rates of 39% (39%) and 18% (17%), respectively.

Williams et al [8] estimated that there were 750,000 first-ever or recurrent strokes in 1995. We conservatively estimate that there were 783,000 first-ever or recurrent strokes in 1996. The 33,000 (4.4%) increase in first-ever or recurrent strokes may be due to chance, but it is probably due to the population gain (262,803,000 in 1995 versus 265,229,000 in 1996, a 0.9% increase), and the increase in aging (≥ 65 years old) population from 1995 to 1996 (33,619,000 in 1995 versus 33,957,000 in 1996, a 1.0% increase).[16] The aging population effect is more pronounced in the group aged 75+ years who are at greater risk of stroke (14,863,000 in 1995 versus 15,266,000 in 1996, a 2.7% increase).[16] We also estimate that American hospitals charged $12.4 billion for stroke treatment and management during 1996, which translates to a society cost of approximately $7 billion.

It is worthwhile to notice the remarkable consistency in stroke patient characteristics between 1995 and 1996, as shown in Table 2. The length of stay, however, was shorter in 1996 than in 1995 for stroke patients, although the total charge was higher in 1996 than in 1995. It may be due to a combination of inflation and inpatient healthcare practice changes or healthcare reimbursement regulation changes.

The present study may be associated with several limitations. First, the validity of conclusions drawn from analyses of large administrative databases depends on the accuracy of case-defining diagnostic codes. Therefore, the validity of the present study is highly dependent on the accuracy of the positive predictive values of the ICD-9-CM codes, which has been addressed.[8]

The impact of the uncertainty in the PPV pooled estimates was examined by constructing a 95% confidence interval around the number of hospital strokes. The bounds of this confidence interval were tight (688,000, 737,000), indicating that the point estimate had reasonable precision.

Another limitation of the present study is the lack of documented information on the rate of nonhospital stroke. Additional data are needed to produce a more reliable estimate of the proportion of strokes without hospitalization. By intentionally choosing a low percentage, we were confident that our estimate of the total annual stroke burden was not inflated. We used sensitivity analyses to illustrate the potential impact of a different true percentage. In addition, race-specific information was not available, which limited our ability to adjust for race.

The methodology used in the present study was the same as the one used in Williams et al, [8] but different from other studies published on the incidence, occurrence and characteristics of stroke. All those studies used state-of-the-art stroke registries based in relatively small geographical areas (Framingham, Massachusetts; Rochester, Minnesota; Rochester, New York; Northern Manhattan, New York; Greater Cincinnati/Northern Kentucky). Our approach might have slightly reduced internal validity, but it should have far greater external validity, although this might be somewhat compromised by the reduced internal validity.

In summary, this study supports the findings of Broderick et al [5] and Williams et al [8] by conservatively estimating that there are approximately three-quarters of a million strokes each year. In addition, we observed that there is a slight increase, although not statistically significant, in occurrences of stroke from 1995 to 1996. This is likely due to a combination of the population gain and the aging of the population from 1995 to 1996.

In conclusion, stroke is a significant problem in the United States. The importance of preventive measures for a disease that has identifiable and modifiable risk factors must be emphasized. The reduction of morbidity and mortality among stroke patients must remain a public health priority.