Stroke Incidence on the East Coast of Australia
The North East Melbourne Stroke Incidence Study (NEMESIS)
Background and Purpose—Community-based stroke incidence studies are the most accurate way of explaining mortality trends and developing public health policy. The purpose of this study was to determine the incidence of stroke in a geographically defined region of Melbourne, Australia.
Methods—All suspected strokes occurring in a population of 133 816 residents in suburbs north and east of Melbourne, Australia, during a 12-month period of 1996 and 1997 were found and assessed. Multiple overlapping sources were used to ascertain cases, and standard definitions and criteria for stroke and case fatality were used.
Results—A total of 381 strokes occurred among 353 people during the study period, 276 (72%) of which were first-ever-in-a-lifetime strokes. The crude annual incidence rate (first-ever strokes) was 206 (95% CI, 182 to 231) per 100 000 per year overall, 195 (95% CI, 161 to 229) for males, and 217 (95% CI, 182 to 252) for females. The corresponding rates adjusted to the “world” population were 100 (95% CI, 80 to 119) overall, 113 (95% CI, 92 to 134) for males, and 89 (95% CI, 70 to 107) for females. The 28-day case fatality rate for first-ever strokes was 20% (95% CI, 16% to 25%).
Conclusions—The incidence rate of stroke in our population-based study is similar to that of many European studies but is significantly higher than that observed on the west coast of Australia.
Despite the decline in mortality from stroke, cerebrovascular disease remains a major cause of death and disability in many Western countries.1 Population-based studies are the only accurate way of ascertaining the incidence of a disease. There is also a need to monitor trends in incidence and mortality over time to enable the assessment of current prevention strategies and to assist in policy development. Given our aging population, the sparse resources currently available need to be appropriately allocated to the requirements of the population.
Accurate assessment of the incidence of stroke can only be achieved when several “ideal” criteria have been met.2 3 Only 1 Australian study, performed in Perth (on the west coast; Figure⇓) during 1995–1996, has fulfilled the criteria for an ideal stroke incidence study.4 Data from this study, as well as from previous studies in Perth5 6 and 1 in Melbourne,7 provide some evidence that stroke rates might be considerably higher in Melbourne than in Perth. The earlier Melbourne study,7 however, was undertaken approximately 10 years before the initial Perth incidence study, and the lower incidence rates of the later Perth study may consequently be a reflection of a declining incidence over time. Clearly, it would be of interest to clarify these differences in incidence rates. Furthermore, a recent comparison between the Auckland and Perth stroke registries has shown differences in sex-specific rates of stroke.8 Because Australia is a large country with many diverse population groups, and because the majority of prevention strategies are aimed at the state rather than the national level, it is quite probable that incidence rates may be different in different parts of the country.
The main aim of the study was to obtain an accurate measure of the incidence of stroke in an Australian community. Other aims of the study were to determine outcome and to investigate the costs of stroke.
Subjects and Methods
The North East Melbourne Stroke Incidence Study (NEMESIS) was conducted in a defined area of inner northeast Melbourne between May 1, 1996, and April 30, 1997. The study population comprised an 8–postal code region (Figure⇑). The Yarra River and the Eastern Freeway form a southern boundary to the study region, but there is no clearly defined boundary on its other aspects. Consequently, a significant proportion of people resident within the study area may seek medical attention in surrounding areas.7
A census was conducted in Australia in the seventh month of this 12-month study. According to this census, this region had a total population of 133 816, of which 20 976 were aged 65 years and older (15.7% of the population), compared with 378 110 of 3 158 165 (12.0% of the population) in the Melbourne Statistical Division. The study region also contains a higher proportion of people who were born overseas (30%) in comparison with the rest of Victoria (24%).
Ascertainment of Cases
Before commencement of the study, 2 research nurses were provided with comprehensive training in surveillance procedures, verification of potential cases according to the World Health Organization (WHO) definition of stroke,9 determination of stroke subtype, and neurological examination and interview techniques. A validation study of the neurological examination undertaken by the nurses and 2 neurologists provided good agreement.10
A variety of overlapping sources was used to recruit stroke patients. Although there is a universal healthcare system (Medicare) in Australia that provides public hospital care free of charge to all, there is a substantial private healthcare sector, providing a challenge for complete case ascertainment in a stroke incidence study. The study region contains 2 public hospitals, 1 private hospital, 1 public specialist rehabilitation hospital, and 2 private rehabilitation hospitals. There is some spillover of patients living in this area to 12 other public hospitals, 27 private hospitals, 5 public rehabilitation/specialist aged care hospitals, and 6 private rehabilitation hospitals located outside the study area.
The major sources of case finding were the daily admission lists and stroke unit lists of the major public and private hospitals both within the study region and in the nearby surrounding areas. Patients with a wide range of admitting diagnoses were considered as potential cases. Transient ischemic attacks were followed up because many of these actually turn out to be stroke. The research nurses maintained daily contact with medical staff from stroke units. Where available, the radiology and carotid duplex ultrasound lists of public hospitals within the area were also regularly scrutinized. Computerized hospital discharge lists containing patients with International Classification of Diseases, Ninth Revision11 (ICD-9) codes 430 to 438, 342, and 781 were obtained on a regular basis from public and private hospitals. At public hospitals within the study area, these lists provided a double check for “hot pursuit” procedures. Surveillance at some public hospitals located outside the study area as well as at most small private hospitals was by this means alone.
Because a significant proportion of stroke cases are managed solely in the community and never admitted to a public hospital,6 12 all medical practitioners potentially able to refer patients to the study (general practitioners, physicians, neurologists, geriatricians, and rehabilitation specialists) were contacted regularly by letter, facsimile, and newsletter. The study region and a surrounding rim of approximately 5 km are served by approximately 759 general practitioners working from 277 practices. General practitioners were contacted on approximately a 2-month basis, while specialist physicians were contacted on a 6-month basis. In addition, managers or nursing directors of all 24 nursing homes and 24 hostels located within the study region were telephoned every 2 weeks throughout the study period to ask about potential cases. For half of the study period only, a regional Aged Care Assessment Team made available a list of cases seen for assessment with a diagnosis of stroke (past or current). During the latter half of the study period a different institution was responsible for this team: this institution did not provide approval for the study investigators to access the list. Finally, the NEMESIS project was advertised on numerous occasions in Divisional General Practice newsletters, local newspapers, ethnic newspapers, and once in a major Melbourne daily newspaper. In these advertisements members of the public were invited to contact the study investigators if they had suffered a stroke during the study period.
Cases in which stroke was noted as either a primary or secondary cause of death among people whose “usual” residence was within the postal code region were referred through lists supplied by the Australian Bureau of Statistics. For those patients not already notified to the study, further information was sought from hospital or nursing home medical records, from the certifying medical practitioner, from the state coroner’s office, and/or from the next of kin to determine eligibility according to the study definitions. All participants in the study were also followed up with the National Death Index to determine whether any patients lost to follow-up had died.
After informed consent was obtained, potential cases were interviewed and examined by a trained research nurse as soon as possible after the stroke event. When cases were treated privately within the community, clinical details were obtained from the treating doctor. When potential cases had died or were discharged from the hospital before they could be examined by a nurse, medical records were reviewed and, when necessary, the treating or certifying medical practitioners were contacted so that clinical details could be obtained.
All potential stroke cases were formally reviewed by an expert panel before inclusion or exclusion. The panel comprised between 2 and 4 neurologists and an epidemiologist, each with a particular interest in stroke. Clinical details of all potential cases were presented, and consensus was required between neurologists for inclusion or exclusion of potential cases.
After ascertainment, no attempt was made to modify the usual stroke management practices. Specifically, no brain imaging was requested by the investigators.
Stroke was defined according to the WHO definition, as “rapidly developing clinical signs of focal (or global) disturbance of cerebral function lasting more than 24 hours (unless interrupted by surgery or death) with no apparent cause other than of vascular origin.”9 The definition excludes cases of primary cerebral tumor, cerebral metastases, subdural hematoma, postseizure palsy, brain trauma, and transient ischemic attacks. Transient ischemic attacks were defined as transient episodes of focal cerebral or monocular dysfunction with symptoms lasting <24 hours and with a presumed vascular origin.
First-ever strokes were defined as those strokes occurring in patients without any prior stroke event. Incidence rates were based on “first-ever-in-a-lifetime” strokes. Past history of stroke was determined using all available information from hospital records and general practitioners. The presence of a clinically silent past cerebral infarct or hemorrhage found on CT was not considered to constitute a stroke; these cases are not included in incidence rates for first-ever-in-a-lifetime stroke, nor are they included in the attack rates presented. A recurrent event was defined as an additional episode of stroke occurring at least 28 days after a previous event. Additional events occurring within 28 days of a previously registered event were also counted when they occurred in a different vascular territory.
A “possible stroke” was defined as any episode of neurological disturbance that was suggestive of stroke but for which there was insufficient information available to definitely categorize the case as “stroke” or “not stroke” according to the WHO definition, or when it was insufficiently clear whether the duration of focal neurological disturbance was >24 or <24 hours. All subarachnoid hemorrhages were included whether or not there were focal neurological signs.13
For inclusion, stroke onset was required to be within the study time period, the person in whom the stroke occurred was required to be resident within the defined geographic region of the study at the time of the stroke, and the event must have been detected and diagnosed by a medical practitioner within 28 days of onset. Registration, however, could occur later.
If the patient died during the 12-month period after stroke, all available medical records were reviewed, and occasionally the treating doctor was contacted, to establish the timing and cause of death.
Data Collection and Calculation of Rates
All data on questionnaires were coded and entered into a database specifically designed for this study. Attack (first-ever and recurrent stroke) rates and incidence (first-ever stroke only) rates are reported as crude rates (with the use of a standard formula),14 age-standardized rates, and rates standardized to the “world” population of Segi.15 The latter standardization of rates allows comparison of incidence rates between populations. The data are reported with 95% CIs. Case fatality rates reported are those occurring within 28 days of stroke.
The study was approved by ethics committees at each of the participating institutions. Informed consent was obtained from each participant before any interview or neurological examination was conducted. When the participant was cognitively impaired, dysphasic, or had altered consciousness, consent was obtained from the next of kin.
In total, 1371 patients with “potential stroke” were referred to the study. After careful review of the medical records and review of the results of investigations and, in some cases, clinical assessment, 987 were excluded. The majority of exclusions were due to patients living outside the geographically defined study region (21.6%), having transient ischemic attacks (12.7%), and being outside the study time frame (19.8%); the remainder of events were not considered to be stroke (Table 1⇓).
A final diagnosis of acute stroke was made in 381 events occurring among 353 individuals; 276 (72.4%) of these were first-ever in-a-lifetime strokes, 77 (20.2%) occurred in people with a previous event before the commencement of the study period, and 28 (7.4%) events occurred among patients already registered with another event in the study period. An additional 10 patients had a stroke on CT scan, but the symptoms did not comply with the study definition and could not be included in the study (Table 1⇑). In addition, 3 possible strokes occurred, 2 of which were possible first-ever strokes. All patients with possible stroke had considerable other comorbidities. Other investigators may have excluded these cases, as we did with many others; however, in these instances the investigators were unable to come to an agreement.
A total of 241 stroke events were first detected through active surveillance of public hospital admissions (43.2%), casualty department attendances (6.1%), referrals from hospital doctors and nursing staff (12.9%), and surveillance of radiology lists (1.3%) (Table 2⇓). An additional 13 events (3.4%) were first notified by neurologists, geriatricians, and general practitioners, while 54 events (14.2%) were first notified through public and private hospital discharge records. Twenty-seven patients (7.1%) were first notified via nursing homes, hostels, or retirement villages, while an additional 26 (6.8%) were included after review of death certificates. The remaining 19 patients (5.0%) were first notified through a variety of other sources, including self-report, report by relatives, or prior and subsequent stroke events. The last case was referred on February 25, 2000, >3 years after the stroke.
A total of 45.8% of patients were assessed by the study team within 10 days of their stroke. CT scan was performed in 331 events (86.9%) soon after stroke onset. MRI was performed as the sole form of imaging in 2 patients, while an autopsy was performed on an additional 2 patients who did not have CT. Consequently, imaging or autopsy was performed in a total of 335 events (87.9%). More patients with first-ever stroke (90.6%) had imaging or autopsy than did patients with recurrent stroke (80.0%). The mean age of those with imaging or autopsy was 73.34 years (SD 14) compared with 83.25 years (SD 7) among those without.
A total of 328 people (86%) with stroke events were hospitalized. An additional 5 first-ever cases and 2 recurrent cases (2% of all events) were assessed in the emergency department of a hospital before being discharged back to their usual place of residence. More than two thirds of patients (271) were identified by >1 source, with the remaining 110 (28.9%) being ascertained through only 1 referral source. Only 8 events were notified because of a subsequent stroke, 2 of these occurring in 1 patient who had 2 events while in the hospital before having a third event. One patient was identified only through active follow-up of a prior event.
A total of 310 events (81.4%) were referred from a hospital, nursing home, or other source that was located within the study area, while the remaining 71 (18.6%) were referred from a source located outside the study area.
The annual age- and sex-specific attack rates for stroke within the study area (excluding CT only and possible strokes) are shown in Table 3⇓. Men predominated in all of the groups aged >34 years, although the overall rate of stroke for females (297; 95% CI, 256 to 337) was higher than that for men (272; 95% CI, 232 to 312). If cases managed outside hospital had been excluded from the analysis, the event rate would have been underestimated by 12.1% (95% CI, 8.8% to 15.3%).
The annual incidence rates of first-ever stroke (excluding CT only and possible strokes) are presented in Table 4⇓. The crude annual incidence rate was 206 (95% CI, 182 to 231) per 100 000 population and was 195 (95% CI, 161 to 229) per 100 000 population for males and 217 (95% CI, 182 to 252) per 100 000 population for females. The incidence rates approximately double with each decade of life (Table 4⇓). The corresponding rates standardized to the world population were 100 (95% CI, 80 to 119) for all first-ever strokes, 113 (95% CI, 92 to 134) for males, and 89 (95% CI, 70 to 107) for females.
Of those with first-ever stroke, 20% (95% CI, 16% to 25%) had died by 28 days, and 37% (95% CI, 32% to 43%) had died within 1 year of their stroke. Recurrent strokes had a 28-day case fatality rate (16%; 95% CI, 9% to 23%) similar to that for first-ever strokes, giving an overall case-fatality rate for all events of 19% (95% CI, 15% to 23%).
In ensuring that incidence rates are accurate, every effort must be made to obtain complete ascertainment of all strokes. This requires the use of multiple overlapping sources and hot pursuit techniques. Difficulties arise when some strokes are treated not only outside the hospital system but are treated in smaller private hospitals. In the present study we made every effort to obtain all of these cases, particularly the more difficult to obtain nonhospitalized, nonfatal strokes.
Assessment of our data quality according to the criteria proposed by the WHO Monitoring Trends and Determinants in Cardiovascular Disease (MONICA) investigators16 shows (1) a disproportionately low ratio of stroke death rate in the project when compared with the death rate using the 1997 Australian mortality statistics (0.64); (2) an appropriate number of fatal cases occurring outside the hospital relative to all stroke deaths (23%); (3) a 28-day case fatality rate that is not too high (19%); and (4) a reasonable proportion of out-of-hospital stroke survivors (12%). The disproportionately low ratio of stroke death rate in our cohort compared with the Australian mortality data can be explained in 2 ways. First, mortality statistics for stroke in Australia are provided in 1 large group according to ICD-9 codes 430 to 438. These include 2 major groups that have been excluded from our incidence data: subdural hemorrhage (ICD-9 code 432.1) and transient ischemic attacks (ICD-9 code 435). Mortality figures also include patients who have died from late effects of cerebrovascular disease (ICD-9 code 438). These cases are likely to include patients who have had a stroke some months before death and consequently would not be included in our own 28-day case fatality rates. In our own assessment of referrals through death certificates, we found that 14.8% of cerebrovascular disease deaths included in mortality figures were due to late effects of cerebrovascular disease. The second explanation is that many death certificates have considerable inaccuracies. In our assessment of 175 people who lived in the study area and in whom cerebrovascular disease (ICD-9 codes 430 to 438) was the primary cause of death on the death certificate, 50 (28.6%) were assessed not to have had a stroke, and 29 (16.6%) had a history of stroke at some time in the distant past. An additional 3 (1.7%) of these codes were due to subdural hemorrhage, and the remaining 93 (53%) were due to a recent stroke. If we substitute the proportion of death certificates that were either due to a recent or more distant past history of stroke only (69%) for the reported mortality rates, the ratio of case fatalities to that of Australian mortality statistics improves considerably (91.7%).
Although medical care in Australia is free of charge and the majority of patients with stroke are hospitalized, it is possible that some strokes may have escaped our network of surveillance procedures. When one keeps in mind our own findings of inaccuracies among death certificate diagnoses of stroke and the fact that all of the other quality data indicators developed by the WHO MONICA study investigators were within the acceptable ranges, these data provide some support for the notion that case ascertainment in our study was nearly complete.
Scrutiny of referral sources shows a relatively high proportion of included patients referred from out-of-area medical care, thus providing further support that nearly complete case ascertainment was obtained.
Comparison of our incidence rates with those of other population-based studies is complicated by the fact that different methods are used to report incidence rates. In their international comparison study, Sudlow and Warlow3 have provided age- and sex- standardized rates for the group aged 45 to 84 years adjusted to the European population for a number of incidence studies.6 8 17 18 19 20 21 22 23 24 Our own annual incidence rates per 100 000 population for this age group adjusted to the European population for all first-ever strokes, and for males and females separately, are similar to that for the majority of those reported by Sudlow and Warlow (Table 4⇑).3
Further comparison of incidence rates of stroke from the present study with incidence rates among recent studies conducted in Italy, Germany, and Greece reveals that they are also similar.25 26 27 However, comparison of stroke incidence rates (age- and sex-adjusted to the world population) between the present study in Melbourne (100; 95% CI, 80 to 119) and the 1995–1996 Perth study (76; 95% CI, 65 to 87) demonstrates a considerably higher incidence rate in Melbourne than in Perth.4 This same pattern is demonstrated among both men and women, as well as for overall attack rates.
These data provide further evidence for the heterogeneity of stroke incidence and attack rates both between and within countries. Our findings also highlight the fact that data from one source cannot necessarily be extrapolated to produce countrywide estimates of stroke. The age and sex distribution, together with ethnic mix and lifestyle factors, may also need to be considered before estimations of the overall numbers of strokes occurring countrywide can be accurately made.
This incidence study provides an important baseline from which to assess changes in incidence over time. Such data, when applied to national mortality trends, may provide some clues as to whether the declining mortality from stroke may be a reflection of declining incidence, declining case fatality, or a combination of both. Such information is important for healthcare planning, particularly at a time when our population at highest risk of stroke is growing.
This work was supported by grants from the Victorian Health Promotion Foundation, National Health and Medical Research Council, Foundation for High Blood Pressure Research, and National Stroke Foundation. We would also like to acknowledge Catherine Sharples, Meg Hooton, and Elspeth Freeman, who found and assessed the majority of the patients, and Lichun Quang for computer assistance.
- Received January 20, 2000.
- Revision received June 6, 2000.
- Accepted June 6, 2000.
- Copyright © 2000 by American Heart Association
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