Editorial Comment—Explanations for International Trends in Stroke Mortality
Stroke mortality varies greatly from country to country. In 1985, the highest figures in eastern European countries were 6- to 7-fold those of countries with the lowest mortality rates,1 and the same trend has continued in the 1990s.2 The mortality rates have not, however, remained stable during the last decades. In most countries, a significant reduction has occurred, whereas in some countries the opposite is true: during 1970 to 1985, the annual changes ranged from +3.9% to −7.1%, and during 1985 to 1994, from +3.2% to −6.8%.1,2 The obvious causes of the reduced mortality rates have been either a decreased incidence of stroke or the case-fatality rate, or both. The explanations for the growing mortality rates, mainly in the eastern European countries, have been more or less speculative.
In the present study, Sarti and associates have answered these questions. Their impressive patient material consisted of 36 000 young (35 to 64 years) acute stroke victims from the WHO MONICA project. The patient data were collected during 1982 to1995 from 14 centers in 9 countries with as complete case finding as possible. The subtypes of stroke were considered together. The attack rate included all patients, both first-ever and recurrent strokes, and the case-fatality rate was calculated on the basis of patients dying within 28 days of stroke onset.3
Age-standardized (World Standard Population) average attack rate (per 100 000 per year), case fatality (%), and MONICA mortality rate (per 100 000 per year) were calculated, as well as the annual trends. MONICA mortality trends were compared with those of official mortality statistics. Both the rates and trends from different centers varied greatly, the highest age-standardized rates being 2- to 6-fold compared with the lowest rates. In two thirds of the centers mortality declined, whereas in one third it remained unchanged or increased.
A comparison of changes in attack and case-fatality rates with changes in mortality rates revealed that two thirds of a decreasing mortality is attributable to reduced case fatality, and one third to reduced attack rate. The increasing mortality rate is almost exclusively explained by an increasing case-fatality rate.
A few points in this excellent study deserve comments. The most important one is the relatively young age structure of the subjects, which represents one third of all stroke cases and only one tenth of all patients dying of stroke. Thus, the results are certainly applicable to young stroke patients, but I am uncertain whether they can be generalized to older age groups.
One of the MONICA centers (Glostrup, Denmark) collected data during 1982 to 1991 on stroke patients of all ages.4 Both the age-adjusted attack and incidence rates declined significantly in all age groups, and the age-adjusted mortality rates followed the trend of incidence rates. A significantly higher case-fatality rate was, however, observed among recurrent strokes and with increasing age, and the age-adjusted case-fatality rate increased significantly in men and insignificantly in women. On the basis of these results, one can speculate that the inclusion of elderly (>64 years) stroke patients may give the trends of attack and incidence rates a more prominent role in determining mortality trends than in the present study.
Another question is the comparison of MONICA mortality rates with official mortality statistics. The MONICA project is concerned with acute stroke (International Classification of Diseases, 8th and 9th revisions, codes 430 to 434 and 436), and the official statistics include both acute strokes and nonacute cerebrovascular disorders (codes 430 to 438). Furthermore, only deaths within 28 days of stroke onset are counted in the MONICA project. In practice, however, many stroke victims die months or even years after the acute phase of some complication (eg, pneumonia, pulmonary embolism), but stroke appears as the underlying cause on the death certificate. Therefore, it is difficult to comprehend how one MONICA register can obtain, compared with official mortality statistics, a ratio of 1.0 or above in the number of stroke deaths. Obviously, the number of deaths occurring in the study population are extrapolated from the official mortality statistics, but have the nonacute deaths appearing in the official statistics been omitted? An explanation would be needed.
Anyhow, the study by Sarti and colleagues gives important data elucidating the background of stroke mortality trends. Sixty-five percent of the decrease in the mortality rate is explained by a reduced case-fatality rate, and the remaining 35% is attributed to a reduced event rate. Mortality rate increases are almost solely a result of an increased case-fatality rate. The results are valid for young stroke patients. I look forward to a corresponding analysis based on the MONICA centers that have collected data on patients of all ages.
Bonita R, Stewart A, Beaglehole R. International trends in stroke mortality: 1970–1985. Stroke. 1990; 21: 989–992.
Sarti C, Rastenyte D, Cepaitis Z, Tuomilehto J. International trends in mortality from stroke, 1968 to 1994. Stroke. 2000; 31: 1588–1601.
Thorvaldsen P, Asplund K, Kuulasmaa K, Rajakangas A-M, Schroll M, for the WHO MONICA Project. Stroke incidence, case fatality, and mortality in the WHO MONICA project. Stroke. 1995; 26: 361–367.
Thorvaldsen P, Davidsen M, Brönnum-Hansen H, Schroll M, for the Danish MONICA Study Group. Stable stroke occurrence despite incidence reduction in an aging population: stroke trends in the Danish Monitoring Trends and Determinants in Cardiovascular Disease (MONICA) population. Stroke. 1999; 30: 2529–2534.