Discussion
This study demonstrated differences in the distribution of
stroke severity between the OXVASC study population and
the Dijon population, and the 3 previously reported studies,
attributable mainly to a major disparity in the reported incidence
of minor ischemic stroke. Despite the use of multiple
overlapping sources of case ascertainment in all 5 studies, in
strict accordance with the published core criteria for conducting
ideal stroke incidence studies,4,5 residual differences in
ascertainment procedures probably contributed to the divergent
findings (Table 1). First, some cases of minor stroke not
hospitalized and not referred to the research teams may have
been missed in the other studies, none of which had access
to primary care computer systems to systematically identify
all patients coded with a cerebrovascular diagnosis. However,
only 4% of cases of ischemic stroke with an NIHSS score ≤2
in OXVASC were identified by such searches alone. Second,
and more important, the provision of a dedicated TIA clinic
for the OXVASC study population identified the majority of
minor ischemic strokes.
It has been reported previously that ≤25% of patients initially
referred by general practitioners to TIA clinics have evidence
of neurological deficits on the neurologist’s assessment and
symptoms persisting >24 hours.1,7,8,16 Assessment in TIA clinic
also allows the identification of those with persistent discrete
symptoms without signs that are coded in the NIHSS score.
The high proportion of patients with ischemic stroke with an
NIHSS score of 0 in OXVASC (22.1% versus 4.5% in Dijon)
minor strokes were misclassified as TIAs in Dijon. Although
we cannot totally exclude a real higher incidence of both TIA
and minor ischemic stroke in OXVASC, the magnitude of
the observed differences allows us to rather assume that TIA/
minor stroke clinics lead to better ascertainment of both TIA
and minor strokes.
Some limitations of our study must be acknowledged. First,
the evaluation of the NIHSS score was made by different
study doctors from 2 distinct teams, with a potential risk of
heterogeneity in the way of coding and, in some cases, a retrospective
estimation was performed from information obtained
in medical records. However, given the small numbers of
cases involved and the good interrater reliability,17 and validity
of the retrospective assessment with this scale,11 it can be
assumed that this limitation did not significantly influence our
results. Second, we did not find a difference between the studies
in the incidence of minor intracerebral hemorrhage. This
perhaps reflects the fact that diagnosis requires brain imaging,
and that both studies reviewed all requests for brain imaging,
or simply the fact that the numbers of cases were small. Third,
it could be argued that our findings are attributable to the fact
that the proportion of minor strokes ascertained in the Dijon
study was unusually low. However, in the European Registers
of Stroke Study, several proxies were used to compare stroke
severity among patients recruited from 6 European population–
based registries, including Dijon, and severity of stroke
was found to be lowest in Dijon.18 We also found no difference
in severity between the Dijon study and the only other 3 studies
in which the authors reported the distribution of NIHSS
score.13–15 Of note, despite the population-based design of the
Melbourne study, only NIHSS scores of hospitalized patients
were reported by the authors that make difficult comparisons
with the others. Fourth, although our aim was not observer
agreement but to address potential bias across incidence
stroke studies, the 90 out of 100 clinic-referred cases from
the OXVASC Study confirmed by the Dijon study investigator
could be an overestimation because he was aware of the diagnosis
made by OXVASC investigators. Finally, differences in
diagnostic procedures for identifying ischemic stroke pathogenesis
between centers may exist, but we think that they have
probably accounted for only limited discrepancies observed in
our stratified analyses.
To conclude, this study suggests that the proportion of
patients with minor stroke is higher than that reported in
many high-quality stroke incidence studies. Indeed, underestimation
is likely to be greater still in studies from low- to
middle-income countries, in which patients with minor events
may be less likely to seek medical attention at all.19–23 Our
findings have 2 main implications. First, we suggest that the
core criteria for ideal stroke incidence studies (defined by
Malmgren et al4 and refined by Sudlow and Warlow5) are further
refined in 2 ways. Presentation of incidence rates should
include stratification by stroke severity, ideally assessed by
the NIHSS to aid comparison with existing studies. This will
allow more reliable geographical and temporal comparisons
of stroke incidence and possible differences in stroke severity.
In addition, where possible, studies should incorporate
assessment of all TIAs, ideally by face-to-face assessment
by study clinicians. Second, any systematic underestimation
of the incidence of minor ischemic stroke in previous incidence
studies is important to consider in assessing the clinical
burden of cerebrovascular disease, given the high risk of
recurrent stroke,1 the impact on cognitive function,2 fatigue
and depression,3 and the implications for clinical service
provision.
สนทนาการศึกษานี้แสดงให้เห็นว่าความแตกต่างในการกระจายของโรคหลอดเลือดสมองความรุนแรงระหว่างประชากรศึกษาของ OXVASC และประชากรในดิจอง และ 3 ศึกษารายงานไปก่อนหน้านี้disparity สำคัญในอุบัติการณ์รายงานรวมส่วนใหญ่ของจังหวะสำรอกรอง แม้ มีการใช้หลายแหล่งที่มาของ ascertainment ในการศึกษาทั้งหมด 5 กรณีที่ทับซ้อนในสามัคคีที่เข้มงวดกับเงื่อนไขหลักเผยแพร่สำหรับการทำศึกษาจังหวะเหมาะเกิด 4, 5 เหลือแตกต่างกันขั้นตอน ascertainment คงส่วนในขันติธรรม(ตารางที่ 1) ผลการวิจัย ครั้งแรก บางกรณีของจำนวนจังหวะไม่พัก และไม่เรียกว่าการวิจัยอาจมีทีมงานการพลาดในการศึกษาอื่น ไม่มีผู้ใดได้เข้าระบบคอมพิวเตอร์หลักระบบระบุผู้ป่วยทั้งหมดที่เข้ารหัส ด้วยการวินิจฉัย cerebrovascular อย่างไรก็ตามเท่านั้น 4% ของกรณีของจังหวะสำรอกด้วย NIHSS เป็นคะแนน ≤2ใน OXVASC ที่ระบุ โดยการค้นหาเช่นคนเดียว วินาทีและที่ สำคัญ การจัดคลินิกเตี้ยทุ่มเทOXVASC การศึกษา ประชากรระบุส่วนใหญ่จังหวะที่สำรอกรองมีการรายงานก่อนหน้านี้%ที่ ≤25 ของผู้ป่วยเริ่มอ้างอิง โดยทั่วไปผู้ให้เตี้ย มีหลักฐานของระบบประสาทการขาดดุลในการประเมินของ neurologist และอาการ persisting > 24 hours.1,7,8,16 ประเมินในคลินิกเตี้ยนอกจากนี้ยัง ให้หมายเลขของผู้ที่มีแบบเดี่ยว ๆsymptoms without signs that are coded in the NIHSS score.The high proportion of patients with ischemic stroke with anNIHSS score of 0 in OXVASC (22.1% versus 4.5% in Dijon)minor strokes were misclassified as TIAs in Dijon. Althoughwe cannot totally exclude a real higher incidence of both TIAand minor ischemic stroke in OXVASC, the magnitude ofthe observed differences allows us to rather assume that TIA/minor stroke clinics lead to better ascertainment of both TIAand minor strokes.Some limitations of our study must be acknowledged. First,the evaluation of the NIHSS score was made by differentstudy doctors from 2 distinct teams, with a potential risk ofheterogeneity in the way of coding and, in some cases, a retrospectiveestimation was performed from information obtainedin medical records. However, given the small numbers ofcases involved and the good interrater reliability,17 and validityof the retrospective assessment with this scale,11 it can beassumed that this limitation did not significantly influence ourresults. Second, we did not find a difference between the studiesin the incidence of minor intracerebral hemorrhage. Thisperhaps reflects the fact that diagnosis requires brain imaging,and that both studies reviewed all requests for brain imaging,or simply the fact that the numbers of cases were small. Third,it could be argued that our findings are attributable to the factthat the proportion of minor strokes ascertained in the Dijonstudy was unusually low. However, in the European Registersof Stroke Study, several proxies were used to compare strokeseverity among patients recruited from 6 European population–based registries, including Dijon, and severity of strokewas found to be lowest in Dijon.18 We also found no differencein severity between the Dijon study and the only other 3 studiesin which the authors reported the distribution of NIHSSscore.13–15 Of note, despite the population-based design of theMelbourne study, only NIHSS scores of hospitalized patientswere reported by the authors that make difficult comparisonswith the others. Fourth, although our aim was not observeragreement but to address potential bias across incidencestroke studies, the 90 out of 100 clinic-referred cases fromthe OXVASC Study confirmed by the Dijon study investigatorcould be an overestimation because he was aware of the diagnosismade by OXVASC investigators. Finally, differences indiagnostic procedures for identifying ischemic stroke pathogenesisbetween centers may exist, but we think that they haveprobably accounted for only limited discrepancies observed inour stratified analyses.To conclude, this study suggests that the proportion ofpatients with minor stroke is higher than that reported inmany high-quality stroke incidence studies. Indeed, underestimationis likely to be greater still in studies from low- tomiddle-income countries, in which patients with minor eventsmay be less likely to seek medical attention at all.19–23 Ourfindings have 2 main implications. First, we suggest that thecore criteria for ideal stroke incidence studies (defined byMalmgren et al4 and refined by Sudlow and Warlow5) are furtherrefined in 2 ways. Presentation of incidence rates shouldinclude stratification by stroke severity, ideally assessed bythe NIHSS to aid comparison with existing studies. This willallow more reliable geographical and temporal comparisonsof stroke incidence and possible differences in stroke severity.In addition, where possible, studies should incorporateassessment of all TIAs, ideally by face-to-face assessmentby study clinicians. Second, any systematic underestimationof the incidence of minor ischemic stroke in previous incidencestudies is important to consider in assessing the clinicalburden of cerebrovascular disease, given the high risk ofrecurrent stroke,1 the impact on cognitive function,2 fatigueand depression,3 and the implications for clinical serviceprovision.
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