Published online before print June 5, 2008, doi: 10.1161/STROKEAHA.107.505800
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(Stroke. 2008;39:2268.)
© 2008 American Heart Association, Inc.
Original Contributions |
Hemostatic and Inflammatory Risk Factors for Intracerebral Hemorrhage in a Pooled Cohort
From the Division of Epidemiology & Community Health (J.D.S., A.R.F.), University of Minnesota, Minneapolis; the Departments of Neurology and Epidemiology (W.T.L.), University of Washington, Seattle; the Division of Epidemiology & Biostatistics (E.S.), Mel & Enid Zuckerman College of Public Health, University of Arizona, Tucson; the Department of Epidemiology (W.D.R.), University of North Carolina, Chapel Hill; and the Departments of Medicine and Pathology (M.C.), University of Vermont, Colchester.
Correspondence to Aaron R. Folsom, MD, MPH, Division of Epidemiology & Community Health, School of Public Health, University of Minnesota, 1300 S Second St, Suite 300, Minneapolis, MN 55454-1015. E-mail folso001{at}umn.edu
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Abstract |
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Background and Purpose— The purpose of this study was to identify novel risk factors for intracerebral hemorrhagic stroke (ICH).
Methods— Risk factors were assessed at baseline in a pooled cohort of the Atherosclerosis Risk in Communities Study (ARIC) and the Cardiovascular Health Study (CHS) involving 21 680 adults aged 45 or over. Over 263 489 person-years of follow-up, we identified 135 incident ICH events.
Results— In multivariable models, for each SD higher baseline level of fibrinogen, the relative rate of incident ICH increased 35% (95% CI, 17% to 55%). Fibrinogen was more strongly related to ICH in ARIC than in CHS. In multivariable models, those with von Willebrand factor levels above the median were 1.72 (95% CI, 0.97 to 3.03) times more likely to have an incident ICH as those below the median. Factor VIII was significantly positively related to ICH in ARIC (relative rate per standard deviation of 1.31; 95% CI, 1.07 to 1.62), but not in CHS. There was no relation in multivariable models between lipoprotein (a), Factor VII, white blood cell count, or C-reactive protein and ICH.
Conclusions— Greater plasma fibrinogen and, to some degree, von Willebrand factor were associated with increased rates of ICH in these prospective studies, whereas Factor VIII was related to ICH in younger ARIC study participants only.
Key Words: risk factors in epidemiology • intracerebral hemorrhage • cohort studies • incidence studies
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Introduction |
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Stroke is the third leading cause of death in the United States.1 Intracerebral hemorrhage (ICH) is the most common cause of hemorrhagic stroke and has a higher case fatality rate than ischemic stroke. Currently, few effective evidence-based treatments exist for ICH.2–4
ICH has been strongly linked to age, African-American ethnicity, and hypertension status5; however, other risk factors demonstrate less consistent associations. Only a few reports have examined ICH and hemostatic and other novel risk factors for vascular disease, despite strong biological plausibility.
ICHs are frequently seen in populations with bleeding disorders, in particular hemophiliacs,6 suggesting a potential link between hemostatic variables and ICH in the general population. Inflammation is increasingly recognized as an important contributor to the etiology of many vascular diseases7; however, it is unknown if inflammation is associated with ICH.
We studied the associations of fibrinogen, von Willebrand factor (vWF), Factor VII, Factor VIII, white blood cell count, and C-reactive protein with ICH incidence in a pooled study of 2 prospective population-based cohorts, the Atherosclerosis Risk in Communities(ARIC) study and the Cardiovascular Health Study (CHS).
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Methods |
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The ARIC study cohort consists of a population sample of 15 792 individuals from 4 communities: the northwestern suburbs of Minneapolis, Minn; Jackson, Miss; Forsyth County, NC; Washington County, Md. The Jackson sample comprised African-American residents only. ARIC participants were 45 to 64 years old at baseline when recruited between 1987 and 1989.8
CHS is a randomly selected population-based cohort study from 4 communities: Pittsburgh, Pa; Forsyth County, NC; Sacramento County, Calif; and Washington County, Md. CHS recruited 5201 participants from 1989 to 1990 and subsequently added 687 African-Americans from 1992 to 1993. Potential participants 
65 years old were randomly sampled from the Health Care Financing Administration (or Medicare) eligibility lists.9 These studies received approval from relevant human subjects review boards and all participants gave voluntary consent.
ARIC and CHS measured many baseline risk factors, as detailed elsewhere.10–13 Both studies used comparable methods to assess most risk factors allowing for the pooling of the studies.
CHS phlebotomy methods and quality assurance have been published in detail and were adapted from ARIC laboratory methods.9,14 Fasting plasma total cholesterol and triglycerides were measured enzymatically according to Centers for Disease Control and Prevention guidelines. Low-density lipoprotein was calculated using the Friedwald equation; high-density lipoprotein was measured following precipitation of other lipids.
Blood was drawn from ARIC participants after a 12-hour fast. Details on blood collection and hemostatic analytes have been published.15 Blood was drawn into vacuum tubes with sodium citrate (hemostatic factors), silicon (insulin and chemistries), and EDTA (lipids and cell counts). Blood was stored at 4°C and white blood cell count determined by automated particle counters (model dependent on ARIC site) within 24 hours of blood draw. Leukocyte count replicate measurements demonstrated reliability coefficients of between 0.96 and 1.00. Additional aliquots were centrifuged at 4°C at 3000 g for 10 minutes and stored at –70°C for analysis of other plasma and serum measures at the central ARIC laboratory at the University of Texas Health Science Center in Houston. Samples were shipped within 1 week of being drawn, and analysis was conducted within 2 weeks of arrival. Fibrinogen was measured by thrombin time titration with reagents from General Diagnostics (Fibriquick, Organon-Technika Co, Morris Plains, NJ). Factor VII activity was measured by the clotting rate of human Factor VII deficient plasma; Factor VIII activity was assessed with VIII deficient plasma. Materials for Factor VII and VIII measurement came from George King Biomedical Inc, Overland Park, Kan. Factor VII and VIII levels were expressed as a percentage of clotting activity determined from a calibration curve constructed from freeze-dried reference plasma from Pacific Hemostasis (Curtin Matheson, Houston, Texas). vWF was measured using enzyme-linked immunosorbent assay kits (American Bioproducts Co, Parsipanny, NJ). ARIC hemostasis variables were measured repeatedly in certain individuals over weeks to assess reliability. The reliability coefficient was 0.68 for vWF, 0.78 for Factor VII, 0.86 for Factor VIII, and 0.72 for fibrinogen.15,16 Intraassay and interassay coefficients of variation of all hemostatic assays were below 5%.17 Lipoprotein (a) (Lp[a]) was measured in plasma samples using a double-antibody enzyme-linked immunosorbent assay technique for the apo(a) protein component of Lp(a).
CHS measured white blood cell count at each field center using automated particle counting instruments. Lp(a) was measured using a highly specific monoclonal enzyme-linked immunosorbent assay that reacts to the isoforms of Lp(a) (Genentech). Results are reported as the protein component, excluding the cholesterol and phospholipid components.18 Fibrinogen was measured using the Clauss method. Like with ARIC, CHS measures of Factors VII and VIII were coagulant activity measures. Factor VII was measured using Coag-A-Mate X2 (Organon Teknika) with deficient plasma from Baxter-Dade and Thromborel S (Behring Diagnostics, Marburg, Germany) with a coefficient of variation of 5.3%. Factor VIII was measured using Coag-A-Mate, Factor VIII deficient plasma, and partial thromboplastin reagent from Organon Teknika with a coefficient of variation of 9.7%.19,20 C-reactive protein was measured in stored plasma samples using a high-sensitivity immunoassay with an interassay coefficient of variation of 6.3%.21 C-reactive protein was not measured in ARIC.
ARIC outcomes were gathered through annual phone interviews, follow-up examinations, community hospital surveillance, and reported deaths. A reported hospitalization led to screening and, if suitable, to medical record abstraction. Potential acute stroke events were abstracted if the discharge diagnosis included a cerebrovascular disease code (International Classification of Diseases, 9th Revision, codes 430 to 438), if a cerebrovascular procedure was mentioned in the summary, or if the CT or MRI report showed evidence of acute cerebrovascular disease.12 In ARIC, hospitalized strokes and out-of-hospital stroke deaths are included, but not nonfatal, nonhospitalized strokes.
CHS surveillance and cerebrovascular event ascertainment have been described in detail.22,23 CHS participants were called every 6 months and questioned about interim medical events. Self- or proxy-reported potential stroke events were explored and medical records abstracted for verification. CHS also searched Health Care Financing Administration Medicare Utilization files for stroke International Classification of Diseases, 9th Revision codes (430 to 438) and on event identification abstracted records for verification. CHS searched reported deaths for CHS participants. In CHS, fatal and nonfatal hospitalized and nonhospitalized strokes were ascertained.
ARIC adapted the National Survey of Stroke criteria for its stroke definition.24 These criteria require cerebrovascular events to have evidence of sudden or rapid onset of neurological symptoms that last for >24 hours or lead to death, and the event had no other apparent cause such as trauma, tumor, infection, or anticoagulation therapy. A definite ICH must have met one of the following criteria: (1) CT or MRI showing intracerebral hematoma; (2) demonstration at autopsy or surgery of ICH; or (3) (a) at least one major or 2 minor neurological deficits, (b) bloody spinal fluid on lumbar puncture, and (c) cerebral angiography demonstrates an avascular mass effect and no evidence of aneurysm or arteriovenous malformation; and (4) no CT or MRI. A probable ICH met criteria 3a, 3b, and 3c with a decreased level of consciousness or coma lasting 24 hours or until the participant died. In ARIC, 98% of strokes underwent a CT or MRI. In ARIC, stroke criteria were computer-automated and reviewed by a physician blinded to the automated results. A second physician adjudicated disagreements between the computer and the initial physician.
CHS adopted stroke criteria similar to the Systolic Hypertension in the Elderly Program (SHEP).11,25 Potential stroke events in CHS were referred to a Cerebrovascular Adjudication Committee. The committee consisted of a neurologist (or internist) representing the coordinating center, a neurologist from each site, and a neuroradiologist. A suspected event was classified as a stroke if there was a rapid-onset neurological deficit (or subarachnoid hemorrhage) lasting >24 hours or until death. A suspected hemorrhagic stroke was classified as an ICH if (1) there was CT or MRI evidence of ICH; (2) bloody cerebrospinal fluid on lumbar puncture with a focal deficit; or (3) autopsy or surgical evidence indicated ICH. The event could not be attributed to trauma, tumor, or infection, but in contrast to ARIC, a hemorrhagic cerebrovascular event while on anticoagulation therapy did not preclude an ICH classification in CHS. Only 7 participants with an ICH in CHS were potentially taking anticoagulation medication. Their exclusion had little impact on results as would be expected from the small numbers and because anticoagulation was only weakly correlated with exposures of interest. The CHS Committee has assessed its reliability by blindly reviewing 30 stroke cases. They reported a kappa of 0.86 for stroke versus no stroke and a kappa of 1.0 for stroke subtype (ICH versus subarachnoid hemorrhage versus ischemic). In CHS, 86% of stroke events had brain imaging as part of their event workup. In both studies, suspected ICH events describing anticoagulation therapy as a major contributing cause were not classified as ICH.
The pooled cohort had 21 680 participants at baseline with follow-up through June 30, 2002, for CHS and December 31, 2002, for ARIC. Participants reporting a history of stroke at baseline (n=582) or were not African-American or white (n=87) were excluded. Participants who did not fast 8 hours before baseline blood draws were excluded from analysis involving triglycerides (n=560). The outcome of interest was definite or probable incident ICH. In the rare cases of repeat ICH, the analysis was limited to initial ICH.
The association of baseline risk factors with incident ICH was assessed. Relative rates and incident rate estimates were calculated using Poisson regression as implemented in SAS 8.2 (SAS Institute, Cary, NC). Two-way multiplicative interactions between all risk factors and study (ARIC, CHS), age, race (whites versus blacks), and hypertension (continuous systolic pressure and hypertension categories) were examined. Power was 80% to detect a relative risk of 1.6 for a dichotomous exposure with 30% exposed. All variables were tested in crude and age-adjusted models. Variables were tested in a Poisson model with ICH as the outcome and adjusted by potential confounders that demonstrated an independent association with ICH in this combined cohort in a previous report: age, race, blood pressure, low-density lipoprotein cholesterol, and triglycerides. Variables that were significant at the alpha=0.05 level after confounder adjustment were considered associated with ICH.
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Results |
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Table 1 shows selected baseline characteristics of ARIC and CHS. Because of the large sample size and difference in age between cohorts, most baseline variables differed significantly between the studies.
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Over 263 489 person-years, 135 incident ICH events occurred (61 in ARIC, 74 in CHS). The median follow-up time was 13.5 years for participants free of ICH, and median time to event was 8.0 years for participants experiencing an ICH. Table 2
illustrates the crude and age-adjusted relative rates of ICH in categories of novel risk factors. Fibrinogen, Lp(a), and vWF (ARIC only) had statistically significant positive associations (P<0.05) with incident ICH before adjustment for potential confounders or testing for interactions. C-reactive protein (CHS only), Factor VII, Factor VIII, and white blood cell count were not significantly associated with incident ICH (P>0.05).
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vWF was moderately positively associated with ICH with more than 2-fold greater risk for those above the median of 109% compared with those below the median (Table 2
). After adjustment for age, hypertension, and systolic blood pressure, the relation between vWF and ICH was slightly attenuated and of borderline statistical significance with a relative rate of 1.72 (95% CI, 0.97 to 3.03) for those above the median versus those below the median. Adjustment for other potential confounders did not substantially alter this association.
As shown in Table 2, the association of Lp(a) with ICH was of borderline statistical significance in CHS (probability value for trend across quartiles=0.10) and significant in ARIC (probability value for trend=0.02). Adjustment for race fully attenuated this association between Lp(a) and ICH in both studies. In both studies, adjustment for other potential confounders did not meaningfully alter the lack of association between Lp(a) and ICH after adjustment for race.
Fibrinogen was positively related to incident ICH before adjustment for covariates (Table 2). Table 3 illustrates the relation between fibrinogen per SD (66.2 mg/dL) and incident ICH in crude and adjusted models. After adjustment for age, systolic blood pressure, race, and lipids, for each SD greater fibrinogen, there was a 1.35-fold greater risk of ICH (95% CI, 1.17 to 1.55). After multivariable adjustment, participants in the highest 10% of fibrinogen were 2.43 (95% CI, 1.60 to 3.69) times as likely to have had an ICH as those in the lower 90%.
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During analysis, all variables were tested for potential interactions with age, race, systolic blood pressure, and study (CHS, ARIC) in their relation to ICH. Both fibrinogen and Factor VIII demonstrated interactions with age. The association between fibrinogen and ICH was weaker in the older CHS participants than the younger ARIC participants (P<0.01). In CHS, the multivariable adjusted relative rate of ICH was 1.17 for each SD greater fibrinogen (95% CI, 0.94 to 1.47), whereas this was 1.53 (95% CI, 1.28 to 1.83) in ARIC. The age/study interaction for Factor VIII with ICH was more apparent with an adjusted relative rate for each SD higher Factor VIII of 1.31 (95% CI, 1.07 to 1.62) in ARIC compared with 0.85 (95% CI, 0.65 to 1.12) in CHS (interaction probability value <0.001).
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Discussion |
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In this pooled population-based study of potentially novel risk factors for ICH, fibrinogen, vWF (borderline significant), and Factor VIII (ARIC) were positively associated with incident ICH after multivariable adjustment. There was no independent relation between white blood cell count, C-reactive protein, Factor VII, or Lp(a) and ICH.
We previously reported the traditional cardiovascular risk factors levels in ARIC and CHS and their relation to ICH.26 Hypertension, the most important risk factor for ICH, was linearly related to the hemostatic risk factors, white blood cell count, and C-reactive protein. Compared with nonhypertensives, hypertensives had mean values that were 2% higher for white blood cell count, 4% higher for Factor VII and Factor VIII, 5% higher for fibrinogen, 9% higher for vWF, and 11% higher for C-reactive protein.
Few studies have examined ICH in relation to the 3 hemostatic proteins most widely studied in relation to cardiovascular risk: fibrinogen, Factor VIII, and vWF. Because intracranial bleeding is more likely in those with bleeding disorders, low levels of these hemostatic factors in the general population could contribute to a prohemorrhagic state that increases the risk of ICH. However, our results seemingly contradict this general hypothesis, because we observed positive, not inverse associations between these procoagulant variables and ICH. Yet, few participants had clearly "deficient" levels of these factors, even in the lowest quartile.
Factor VIII and vWF are highly correlated (r=0.73 in ARIC).27 Both factors were positively associated with ICH in ARIC after multivariable adjustment. In contrast, a Swedish nested case–control study with an average participant age of 51 reported an inverse association between vWF and ICH (n=39) with a multivariable adjusted OR of 0.27 (95% CI, 0.08 to 0.90) for the highest tertile versus the lowest.28 Our study is the first to report a positive association between Factor VIII and ICH. The strong interaction between Factor VIII and study or age in relation to ICH suggests that the association is meaningful in younger, but not older, participants.
Elevated fibrinogen, which represents inflammation and hemostatic balance, is considered prothrombotic and has been linked to greater atherosclerosis, subclinical vascular disease, heart disease outcomes, and ischemic stroke.29–33 Only recently has fibrinogen been associated with hemorrhagic stroke.29,34,35 Consistent with our results, all of these studies29,34,35 reported greater fibrinogen was associated with a higher risk of ICH. It seems somewhat paradoxical that elevated fibrinogen, if prothrombotic, would be associated with greater risk of hemorrhagic stroke. Previous studies have suggested that higher fibrinogen is associated with a lack of nocturnal declines in blood pressure, a trait that may increase risk of ICH.36,37 It is also possible that fibrinogen represents a component of inflammation that is meaningful in ICH. Yet, we found that neither C-reactive protein nor white blood cell count, other inflammatory markers, was associated with ICH. The fibrinogen by study interaction in association with ICH suggests that any relation with inflammation may weaken with advancing age. Although C-reactive protein was only measured in CHS, white blood cell count was available in both studies and demonstrated no interactions by study nor did it demonstrate any relation with ICH. If the association between ICH and fibrinogen is causal and related to inflammation, we might have expected similar associations with white blood cell count and ICH in this study.
Lp(a) has been linked to increasing risk of coronary heart disease, but no studies have examined the potential association between Lp(a) and ICH. Despite the potential to exert effects through both hemostatic and lipid pathways,38–40 Lp(a) did not demonstrate any association with ICH after adjustment for race. Lp(a) level is strongly hereditary and differs by race.41
Despite the relatively large number of events, this study still had limited power to detect weak associations. Despite the relative similarity in ICH classifications in CHS and ARIC, differences in classification could have affected the results. These potential differences could result in misclassification of the exposures or outcome that would bias the results in unpredictable ways. Although the methods of measurement of baseline risk factors were similar between ARIC and CHS, unidentified differences could exist. This study examined variables measured at baseline that may have changed for participants during follow-up, resulting in exposure misclassification. ICH stroke subtype and hemorrhage location were not assessed in these studies preventing the examination of possible associations between ICH subtypes and potential risk factors. Baseline ages for ARIC and CHS did not overlap, resulting in age and study being confounded. Although study (ARIC versus CHS) was not independently related to ICH, there is the possibility of residual confounding or study interactions due to methodological or population differences. This suggests particular caution in interpreting the observed interactions of age/study with fibrinogen and Factor VIII.
In summary, our study found an increased risk of ICH with a greater fibrinogen level and this relation was somewhat stronger in younger ARIC participants. We also observed a positive association between Factor VIII and ICH in ARIC participants and some degree of association between vWF and ICH.
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Acknowledgments |
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Sources of Funding
The National Heart, Lung, and Blood Institute funds the Atherosclerosis Risk in Communities Study (N01-HC-55015, N01-HC-55016, N01-HC-55018, N01-HC-55019, N01-HC-55020, N01-HC-55021, N01-HC-55022) and the Cardiovascular Health Study (N01-HC-85079 through N01-HC-85086, N01-HC-35129, N01 HC-15103, N01-HC-55222, N01-HC-75150, N01-HC-45133, and U01 HL080295) with additional contribution from the National Institute of Neurological Disorders and Stroke. A full list of participating CHS investigators and institutions can be found at www.chs-nhlbi.org. JDS was funded through National Heart, Lung, and Blood Institute grant T32-HL07779 and National Institutes of Health MSTP grant GM008244.
Disclosures
None.
Received September 28, 2007; revision received December 20, 2007; accepted January 24, 2008.
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