Epidemiology of Stroke in Innherred, Norway, 1994 to 1996
Incidence and 30-Day Case-Fatality Rate
Background and Purpose In Norway, as well as other industrialized countries, mortality from stroke has declined over the past decades. Data on stroke morbidity are lacking. This study was conducted to determine the incidence, case fatality, and risk factors of stroke in a defined Norwegian population.
Methods During the period 1994 to 1996, a population-based stroke registry collected uniform information about all cases of first-ever and recurrent stroke occurring in people aged ≥15 years in the region of Innherred in the central part of Norway (target population 70 000), where the prevalence of cardiovascular risk factors was screened in 1984 to 1986 and 1995 to 1997.
Results During the 2 years of registration (September 1, 1994, to August 31, 1996), 432 first-ever (72.8%) and 161 recurrent (27.2%) strokes were registered. The crude annual incidence rate was 3.12/1000 (2.85/1000 for males and 3.38/1000 for females). Adjusted to the European population, the annual incidence rate of first-ever stroke was 2.21/1000. The annual incidence rate of cerebral infarction was 2.32/1000, intracerebral hemorrhage 0.32/1000, subarachnoid hemorrhage 0.19/1000, and unspecified stroke 0.38/1000. The 30-day case-fatality rate was 10.9% for cerebral infarction, 37.8% for intracerebral hemorrhage, and 50.0% for unspecified stroke. Fourteen percent of the patients were found outside the hospital, and only 50% of the suspected stroke cases in the hospital (at admission or reviewed discharge diagnosis of ICD-9 codes 430 to 438) fitted the final inclusion criteria.
Conclusions This first population-based stroke register in Norway revealed incidence rates of stroke similar to other Scandinavian countries, and comparison between other European countries did not indicate regional variations within Western Europe.
Mortality from stroke has been declining in most industrialized countries.1 The reason for the decrease has not been fully explained; possible causes include decreased incidence, decline in case fatality because of better treatment, or a shift toward milder strokes. Changes in risk factors over time may explain the fall in mortality.2 There are many population-based stroke registers around the world, but few studies are well designed.3 Time trend studies from population-based stroke registers in Scandinavia have provided conflicting results. Some studies suggest an increasing incidence of stroke, while others report a decrease or no change of incidence.4 5 6 Little is known about incidence and time trend in Norway. Therefore, a community-based stroke register was established in Innherred, Norway, on September 1, 1994, to determine incidence, risk factors, and prognosis of stroke; factors of importance for primary prevention of stroke (eg, treatment of hypertension); and treatment and rehabilitation of the stroke patient (regional health service planning). This article presents incidence and case-fatality data for stroke from a population-based stroke registry for 1994 to 1996.
Subjects and Methods
Innherred is a rural area located in the Nord-Trøndelag county in the central part of Norway with a population of 69 295 inhabitants (January 1, 1995) in 10 municipalities aged ≥15 years (Figure⇓). The population of Innherred is stable and has a sex and age distribution similar to Norway as a whole; the proportion of the population aged ≥65 years was 20.3% and 19.8%, respectively (January 1, 1995). During 1984 to 1986 the first Nord-Trøndelag Health Survey, a risk factor screening, took place.7 Eighty-eight percent of the residents aged ≥20 years participated. This survey contributes to baseline data for further studies on the impact of different risk factors on stroke.
All incident cases of stroke (first-ever and recurrent), occurring from September 1, 1994, through August 31, 1996, were registered in permanent residents aged ≥15 years (target population 69 295, January 1, 1995). We identified hospitalized stroke patients by checking admission lists daily, and a specially trained coworker traced the different wards in the hospital twice a week. Discharge records from the hospital (ICD-9 codes 430 to 438) (outpatient clinics included) and death certificates on patients with a stroke noted as either the underlying or a contributary cause of death were reviewed regularly. Discharge records from the two neighboring hospitals outside the study area were also reviewed. The GPs in the area were asked to report nonhospitalized cases. Information about the study was provided by meetings, a special booklet, and a newsletter that was sent to the participating doctors in the hospital, GPs, and collaborators in the district nursing services regularly during the study period. To evaluate the case-finding procedures, an intensified search for nonhospitalized cases was performed in four municipalities selected at random (n=31 853) in a 4-month period. Every week, nursing homes and district nursing services were contacted and asked for new suspected stroke cases. In addition, intensified screening of discharge records of cases with nonstroke diagnoses from the local hospital was performed in the same period.
The patients were examined by the consulting physician (GP or the physician on call at the hospital) within 48 hours after the event. In addition to a standard neurological examination, an estimate of disability before the stroke and 5 to 9 days after onset was made.8 Both GPs and physicians on call in the hospital were asked to refer clinical data on suspected strokes cases by completing a special form including symptoms and signs, duration, earlier cerebrovascular events, and time of onset. The form was given to the physician on call when suspected stroke cases were expected (referral diagnoses of vertigo, disturbances in consciousness, and syncope were included). Before the study started, a booklet that included part of the study protocol was distributed to the participating physicians, and meetings were arranged to explain the procedures of registration of clinical data. The GPs and hospital medical records were reviewed to obtain more information about each case and previous illnesses. Information about fatal strokes occurring outside the hospital was obtained from copies of death certificates, and clinical data were obtained from the certifying physician and nursing home records. Both first-ever stroke (first in a lifetime) and recurrent stroke (>21 days since last event) were recorded. The final diagnosis was made with the use of the World Health Organization criteria, ie, “rapidly developing clinical signs of focal (or global) disturbance of cerebral function, lasting more than 24 hours or leading to death, with no apparent cause other than that of vascular origin.”9 Classification into subgroups (cerebral infarction, intracerebral hemorrhage, and subarachnoid hemorrhage; ICD-9 codes 434, 431, and 430) was based on the result of CT or necropsy. Cerebral infarction was diagnosed if CT within 3 weeks or necropsy showed no signs of intracerebral hemorrhage. Cerebral hemorrhage was diagnosed if CT scan or necropsy showed intracerebral hemorrhage. Acute headache, meningism, and coma were classified as subarachnoid hemorrhage if supported by diagnostic CT, MRI, or necropsy. A diagnosis of unspecified stroke was given to patients who underwent only clinical examination. Excluded cases were categorized as TIA, probable stroke, or other diagnosis.
We examined whether misclassification was a problem in this study by measuring the interobserver agreement between the research registrar, a neurologist, and a specialist in internal and physical medicine. A total of 34 suspected strokes were classified into five categories: first-ever stroke, recurrent stroke, probable stroke, TIA, and “other.”
Before they were recorded, surviving patients received an information letter and provided written consent to personal recording. Consent by next of kin was obtained from patients who were severely ill or unconscious. The Ministry of Health and Social Affairs accepted reports on fatal stroke from the local mortality statistics without having consent by next of kin. The patients who refused to participate were recorded without personal identification. The Stroke in Nord-Trøndelag Study has been approved by the Ethics Committee of health region IV and the National Computer Data Inspection Board.
Sex- and age-specific incidence rates were adjusted to the Norwegian population in 1993 by the direct method, and the comparison between different studies was performed by direct adjustment to the European population.10 The CIs of age-specific rates were calculated according to Schoenberg,11 and CIs of age-adjusted rates were estimated by the method of Armitage.12
A total of 1169 cases of suspected stroke were reported to the stroke register from September 1, 1994, through August 31, 1996, in the study population. According to the standardized criteria, 593 cases (50.7%) were included: 432 first-ever strokes (72.8%) and 161 recurrent strokes (27.2%). CT scan was performed in 87.5% (n=378) of the first-ever strokes, with a mean of 2.4 days from onset of the stroke. The first-ever strokes occurred in 235 women (mean age, 77.7 years [SD, 10.7]) and in 197 men (mean age, 75.3 years [SD, 11.7]). Ninety-one women suffered a recurrent stroke (mean age, 80.9 years [SD, 7.6]) and 70 men (mean age, 76.0 years [SD, 10.9]. A total of 88.7% (n=383) of the first-ever stroke cases occurred in patients aged ≥65 years.
A total of 576 referrals were excluded for various reasons, the most noteworthy being possible stroke (n=56), TIA (n=168), other stroke diagnosis (ICD-9 code 432, subdural hematoma [n=7]; ICD-9 code 433, stenosis of precerebral arteries [n=8]; ICD-9 code 437, cerebral arteriosclerosis [n=5]; and ICD-9 code 438, stroke sequelae [n=126]), vertigo/syncope (n=48), coronary heart disease (n=10), tumor cerebri (n=14), mors subita (n=11), infections (n=9), epilepsy (n=9), intoxication (n=9), and migraine (n=6). The source of referral is presented in Table 1⇓. Out-of-hospital cases (fatal and notfatal) were 13.3% (n=79) of the total number of strokes included.
In the period of intensified case finding, a total of four first-ever stroke cases were found, which gave an estimated number of 28 cases in the target population per year, in addition to those cases found by the standard case-finding procedures. The mean age of these cases was 82.8 years (SD, 3.3; range, 79 to 86 years). Thus, the completeness of the case-finding of first-ever stroke in the target population was 100% in the group aged 15 to 74 years, 87% in the group aged 75 to 84 years, and 76% in the group aged ≥85 years.
The validation of the classification obtained by measurement of interobserver agreement resulted in a κ index of 0.68 between research registrar and neurologist, 0.61 between research registrar and specialist in internal medicine, and 0.65 between neurologist and specialist in internal medicine.
Table 2⇓ shows annual age- and sex-specific incidence rates, which increased by each decade. The increase by age was statistically significant between groups aged ≥55 years in women and between groups aged ≥45 years, except the two oldest groups, in men. The incidence of stroke was higher in males than in females in all age groups, and the relative difference between the sexes increased with each age group, but this was not statistically significant. The incidence rates (male and female), including intensified case finding, were 20.94/1000 (95% CI, 18.18 to 24.10) in the group aged 75 to 84 years and 39.75/1000 (95% CI, 33.03 to 54.39) in the group aged ≥85 years. These rates did not differ significantly from incidence rates based on standard case-finding routines but indicated an underestimation in the oldest groups.
The crude annual incidence rate of first-ever stroke in the population aged ≥15 years in 1994 through 1996 was 3.12/1000 (2.85/1000 in males and 3.38/1000 in females). When age- and sex-adjusted by the direct method to the 1993 Norwegian population, the annual incidence rate was 3.01/1000. By direct adjustment to the European population,10 the annual incidence rate was 2.21/1000.
The different subtypes of first-ever stroke were as follows: 322 (74.5%) suffered a cerebral infarction, 45 (10.5%) had intracerebral hemorrhage, and 13 (3.0%) had subarachnoid hemorrhage. In 52 cases (12.0%), classification into subtypes was impossible because neither CT scan nor necropsy was done. The total annual incidence rates per 1000 of different types of stroke were as follows: cerebral infarction, 2.32; intracerebral hemorrhage, 0.32; subarachnoid hemorrhage, 0.19; and unspecified, 0.38 (Table 3⇓).
The overall 30-day case-fatality rates were 19.2% (83/432) for patients with first-ever stroke and 37.9% (61/161) for patients with recurrent stroke. The 30-day case-fatality rates for the different subtypes of stroke were as follows: cerebral infarction, 10.9%; intracerebral hemorrhage, 37.8%; subarachnoid hemorrhage, 38.5%; and unspecified, 50.0%. The mean ages for the four subgroups, respectively, were 76.2 years (SD, 10.3; range, 22.3 to 98.1), 73.8 years (SD, 14.3; range, 20.3 to 92.8), 65.0 years (SD, 14.8; range, 32.3 to 89.1), and 84.5 years (SD, 7.8; range, 64.9 to 100.9).
This first stroke register in Norway was established in an area with geographic and demographic characteristics fairly representative of Norway as a whole. The population is stable, and there is a close relationship and cooperation between the hospital and GPs. The population is served by one acute hospital with an active stroke unit, which encourages admission of stroke patients. The proportion of all strokes not admitted to the hospital was 13.6%, which is lower than that reported from some studies5 13 14 and higher than that reported from others.4 15 16
Evaluation of the case-finding procedures indicated a complete detection of first-ever stroke in the group aged 15 to 74 years; this was somewhat lower in the oldest groups. Both diagnosis of stroke and case finding are more complicated in the older patient because of disability caused by other diseases (eg, dementia and osteoarthritis) and a lower hospital admission rate for stroke. The variation in stroke incidence for the most elderly among studies could be real because of differences in the detection and management of risk factors such as hypertension, or they could be artificial because of variation in case ascertainment (Table 4⇓). Randomized trials have shown a primary preventive effect of blood pressure–lowering therapy in elderly people,18 19 20 but the impact on stroke incidence in the population is not known.
Another issue of importance for measurement of stroke incidence in the population is the accuracy of the diagnosis. In this study all cases were examined by a standard neurological examination within 48 hours after stroke onset and classified according to the standardized criteria by one of the authors (H.E.). Considering the results of the interobserver agreement between the research registrar, neurologist, and a specialist in internal medicine, we do not see misclassification as a major problem in this study.
The fairly high age- and sex-specific stroke incidence rates we found support results from other Scandinavian studies (Table 4⇑). All studies follow the same pattern of increased incidence by age and a trend toward higher incidence in men compared with women in all age groups, except the oldest. In the group aged ≥85 years, three studies report a higher incidence in women than in men.15 17 21
When age-specific rates from different studies were adjusted to the European population, we found nearly the same incidence rate in five of six studies in the group aged 55 to 84 years (Table 5⇑), which does not indicate regional variations in this age group within Western Europe.
The age-specific incidence of cerebral infarction and intracerebral hemorrhage supports studies from other countries6 16 22 and confirms the increasing incidence of cerebral infarction by age. In comparison to Sweden, the overall 30-day case-fatality rate of first-ever stroke is lower than that reported from Söderhamn21 and Enköping-Habo17 and higher than that reported from Malmö.15 The case-fatality rates of different subgroups show a better outcome for cerebral infarction than for intracerebral hemorrhage and unspecified stroke, supporting results from similar studies.15 16 22
The first population-based stroke register in Norway revealed incidence rates of stroke similar to those of other Scandinavian countries, and comparison between other European countries did not indicate regional variations within Western Europe. We believe that a population-based stroke register is the only way to get accurate measurement of incidence and case-fatality rates and further information on risk factors of stroke.
Selected Abbreviations and Acronyms
|ICD-9||=||International Classification of Diseases, 9th Revision|
|TIA||=||transient ischemic attack|
This study was supported by the Norwegian Research Council and the County Council of Nord-Trøndelag. We wish to thank the GPs in the Innherred region for supporting the local data collection and coworker Marit Holthe for excellent and accurate registration.
- Received April 17, 1997.
- Revision received May 23, 1997.
- Accepted July 2, 1997.
- Copyright © 1997 by American Heart Association
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