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. Author manuscript; available in PMC: 2017 Aug 1.
Published in final edited form as: Transl Stroke Res. 2016 Feb 1;7(4):261–273. doi: 10.1007/s12975-016-0450-1

The Importance of Considering Sex Differences in Translational Stroke Research

Hilda Ahnstedt 1, Louise D McCullough 2, Marilyn J Cipolla 1,3
PMCID: PMC4929018  NIHMSID: NIHMS756608  PMID: 26830778

Abstract

Stroke is the second leading cause of death worldwide and differences between men and women have been documented in incidence, prevalence and outcome. Here, we reviewed the literature on sex differences in stroke severity, mortality, functional outcome and response to therapies after ischemic stroke. Many of the sex differences in stroke severity and mortality are explained by differences in baseline demographics such as older age in women. However, women account for more stroke deaths, consistently suffer from worse stroke outcomes and are more often institutionalized and permanently disabled than men. These sex differences in functional outcome are equalized after treatment with tissue plasminogen activator (tPA) and women may benefit more from treatment than men. However this may depend on race, as African American women have less of a response to tPA than other groups. Regarding endovascular treatments, the few existing studies that have investigated sex differences in stroke outcome point to equal benefit in both sexes, however, many clinical trials are relatively underpowered to detect sex differences. Further, we considered sex-specific effects in animal models of stroke and present recommendations for the performance of stroke studies in female animals. The male-biased use of research animals is distinguished from the clinical situation where there is a disproportionate and growing female stroke population. Stroke in women is greatly understudied and including both sexes is especially important in both preclinical and clinical studies that evaluate potential stroke therapies.

Keywords: Sex differences, stroke, translational, estrogen, tissue plasminogen activator

Introduction

Stroke affects nearly 17 million men and women every year and accounts for close to 6 million deaths globally [1]. Among stroke survivors a large proportion are permanently disabled [1, 2]. Stroke is a sexually dimorphic disease with well-known sex differences in incidence, prevalence and outcome [2, 3]. Women have worse outcomes after stroke and account for a greater number of stroke deaths compared to men [2, 4], mostly due to longer life-expectancy in women and an older age at the time of stroke. The purpose of this review is to evaluate the importance of considering sex differences in translational stroke research. We present and discuss findings on sex differences in clinical stroke with special emphasis on stroke incidence, severity, mortality, functional outcome and the response to treatments. Considerations were made whether data were adjusted for differences in baseline demographics such as age and risk factors, and social factors. Adjusted analysis is important since the risk factor profile is different in male and female stroke patients with atrial fibrillation and hypertension being more common in women while men more frequently have a history of heart disease, smoking and alcohol use [5-8]. If sex differences remain after demographics and social factors have been controlled this speaks for a true biological difference in stroke outcome between men and women. Lastly, we present some important sex differences in experimental stroke and provide guidance for stroke research in females.

Sex differences in clinical stroke

Stroke incidence

The incidence of stroke is higher in men than in women until an advanced age when the gap starts to narrow and incidence rates in women equal or even surpasses that of men [3, 4, 9-15]. Higher incidence rates have been reported for women at 74-85 years [12] and above 85 years [13, 14]. Interestingly, analysis of data from the Greater Cincinnati/Northern Kentucky study showed higher incidence rates in black and white women under 34 years compared to young black and white men [4, 11]. The higher incidence rates that were observed in younger women in this study may be related to inclusion of subarachnoid hemorrhage patients that are more commonly women [3], and a higher proportion of women of childbearing age that may increase stroke risk due to changes during pregnancy.

Stroke risk in women increases after menopause coinciding with a decline in sex hormones, especially estrogen, pointing to a potentially protective role. This is supported by a study in women that found a significant association between an older age at natural menopause and reduced cumulative stroke incidence [16]. Thus, the more years with estrogen exposure prevents stroke. Attempts to capitalize on the beneficial effects of estrogen for primary or secondary stroke prevention have been largely unsuccessful [17]. In addition, hormonal effects likely cannot fully account for the sex differences in stroke incidence since women are protected until the age of 75-85, well past the menopausal years [11, 12, 14]. Intrinsic biological sex differences and organizational hormonal changes (i.e. permanent effects from previous exposure of sex steroid hormones) are likely major factors in the incidence and response to stroke.

Globally, overall incidence rates have decreased in both men and women during the last two decades [9], although there has been minimal change in elderly patients over 80 [15]. Alarmingly, stroke incidence has increased in younger women aged 30 to 49 years, and a trend was also seen in men [15], which may be due to the increased incidence of obesity and the metabolic syndrome.

Stroke severity

Few studies exist where the primary goal was to investigate whether there are sex differences in initial stroke severity (see Table 1). Two large studies based on the Danish stroke registry of first-ever acute stroke reported that women suffered from more severe strokes than men as assessed by the Scandinavian stroke scale [5, 18]. This sex difference was significant in elderly patients in their early 70s even after age and risk factors were adjusted for [18]. Stroke etiology, marital status and socioeconomic factors were also taken into account [5]. In contrast, Gall et al. did not observe any sex differences in the proportion of severe strokes (National Institutes of Health Stroke Scale, NIHSS >7) after adjusting for confounding factors [19]. Importantly, in this study, pre-stroke function was included in the analysis, and fewer women were living independently prior to their stroke. Higher pre-existing disability in women, especially in elderly women, will influence initial measures of stroke severity. Similar findings were seen in a large Chinese study that included ischemic stroke patients above the age of 75 years [20]. Reid et al. showed a larger proportion of women with severe strokes, that remained when age was adjusted for but not when prestroke handicap and other factors were included in the analysis [21]. Most studies present neurological deficits/stroke severity as part of the unadjusted baseline characteristics. In these unadjusted baseline demographics, no sex differences in stroke severity have been documented in several studies [22-25], although others have reported increased severity in females [26, 27]. One study from Japan reported higher prevalence of severe strokes in women (NIHSS ≥8) [26]. In that study, patients were excluded if they were disabled before the stroke (mRS ≥2), but no age-adjustments were made and women were significantly older. Taken together, it appears that most sex differences in stroke severity are explained by differences in baseline demographics and social factors. Studies show that women are more likely to be disabled, dependent or institutionalized before the stroke [6, 21, 22], even after adjusting for age [28]. Prestroke function is an important predictor of stroke outcome [29] and affects the measurement of stroke severity at admission. Future studies should include not only age and differences in risk factor profiles, but also social factors such as marital and residence status (i.e., living alone) and prestroke function in their analysis.

Table 1.

Sex differences in stroke severity

Patients Inclusion Location Period Sex difference in severity Variables controlled for Reference
79,617 IS, ICH Multicenter, Denmark 2003-2012 ♀> ♂, aged over 70 Multivariate (socioeconomic position, marital status) [5]
26,818 IS Multicenter, Denmark 2000-2007 ♀> ♂, aged over 75 Multivariate [18]
1,316 IS, ICH, SAH, UNDETM Melbourne, Australia 1996-1997, 1997-1999 No Multivariate (occupation, prestroke living situation) [19]
810 IS, ≥75 yrs Tianjin, China 2009-2011 No Multivariate [20]
2,725 IS, ICH Halifax, Canada 1996-2006 No Multivariate (prestroke handicap) [21]
4,046 IS, ICH, SAH, TIA Ontario, Canada 2004-2005 No Unadjusted [22]
3,323 IS, ICH, SAH, UNDETM Multicenter, Canada 2001-2002 No Unadjusted [23]
537 IS, ICH Providence, USA 2010-2012 No Unadjusted [24]
505a IS Multicenter, USA NA No Unadjusted [25]
6,236 IS (independent prestroke) Multicenter, Japan 1999-2013 ♀> ♂ Unadjusted [26]
1,581 IS, ICH, SAH UNDETM Barcelona, Spain 1995-2002 ♀> ♂ Unadjusted [27]

Multivariate analysis commonly include age, stroke severity, stroke subtype and risk factors, other variables of importance that were adjusted for are given in brackets. IS = Ischemic stroke, ICH=Intracerebral hemorrhage, SAH=Subarachnoid hemorrhage, UNDETM=Undetermined

a

prospective

NA=Not available

Stroke mortality

Several studies have assessed sex differences in stroke mortality, either by in-hospital mortality during the acute phase, or more long-term after stroke (see Table 2). Higher in-hospital mortality was found in female stroke patients, but this was not seen after adjusting for confounding variables such as age, race, stroke severity and risk factors [7, 21]. Many studies have found higher acute mortality in women, but most of these did not control for age and stroke severity [24, 27, 30]. The majority of studies examining long-term mortality (month(s) to up to a year after stroke) found no difference between men and women, both in unadjusted analysis [13, 23] or when confounders were controlled for [6, 19, 22, 28]. In addition, sex did not predict stroke death at 100 days or at 1 year follow-up [31, 32]. However, some studies suggest that mortality might be higher in men for stroke patients above 65 [33] or the mid-70s [5, 18, 20]. In contrast, higher case fatality in women at 3 and 12 months was reported in a large nation-wide Swedish study that included 64,746 stroke patients that was independent on activities of daily living (ADL) prior to the stroke; however, no adjustment was made for differences in baseline demographics [8]. Higher case fatality in women was seen at 6 months in patients from the International Stroke Trial, however, in the logistic regression model when covariates were adjusted for, men were more likely to be dead at 6 months [34]. Thus, it appears that sex differences in stroke mortality are mostly explained by differences in age and baseline demographics rather than biological differences between men and women.

Table 2.

Sex differences in stroke mortality

Patients Subtypes Location Period Sex difference in mortality Variables controlled for Reference
79,617 IS, ICH Multicenter, Denmark 2003-2012 ♀< ♂ at1 wk and 1 mo (> 70 yrs) Multivariate (socioeconomic position, marital status) [5]
4,499a IS, ICH, SAH Multicenter, Europe 1993-1994 No, 3 mo Multivariate (living situation, prestroke handicap) [6]
383,318 IS Multicenter, USA 2003-2008 No, at discharge Multivariate [7]
64,746 IS, ICH, UNDETM (independent prestroke) Multicenter, Sweden 2008-2010 ♀> ♂, 3 and 12 mo Unadjusted [8]
1,136b IS, ICH, SAH Framingham, USA 1948-2005 No, 30-, 90-180d Unadjusted [13]
26,818 IS Multicenter, Denmark 2000-2007 ♀< ♂ (> 78 yrs) Multivariate [18]
1,316 IS, ICH, SAH Melbourne, Australia 1996-1997, 1997-1999 No, 28 d Multivariate (occupation, prestroke living situation) [19]
810 IS ≥75 yrs Tianjin, China 2009-2011 ♀< ♂ at 12 mo, no sex diff. at 36 mo Multivariate [20]
2,725 IS, ICH Halifax, Canada 1996-2006 No, at discharge Multivariate (prestroke handicap) [21]
4,046 IS, ICH, SAH, TIA Ontario, Canada 2004-2005 No, 30 d Age, subtype, comorbidity index [22]
3,323 IS, ICH, SAH, UNDETM Multicenter, Canada 2001-2002 No, 6 mo Unadjusted [23]
537 IS, ICH Providence, USA 2010-2012 No, at discharge Unadjusted [24]
1,581 IS, ICH, SAH UNDETM Barcelona, Spain 1995-2002 No, at discharge Unadjusted [27]
19,547 IS, ICH Multicenter, Sweden 2001 No, 7-,28- or 90d Age [28]
2,566 IS, ICH, SAH, TIA Michigan, USA 2002 No, at discharge Multivariate (prestroke ambulatory status) [30]
377 IS, ICH Orebro, Sweden 1999-2000 No, 1 y Multivariate [31]
1754a ISc Multicenter, Germany 1998-2000 No, 100 d Multivariate [32]
44,832 IS, ICH, UNDETM Ontario, Canada 1993-1995 ♀> ♂, 30 d and 1 y (above 65 yrs) Multivariate [33]
17,370 IS Int. Stroke Trial, Multicenter, worldwide 1992-1996 ♀< ♂, 6mo Multivariate (level of consciousness) [34]

Multivariate analysis commonly include age, stroke severity, stroke subtype and risk factors, other variables of importance that were adjusted for are given in brackets. IS = Ischemic stroke, ICH=Intracerebral hemorrhage, SAH=Subarachnoid hemorrhage, UNDETM=Undetermined

a

prospective

b

Number of incident strokes in the Framingham original and offspring cohorts

c

prestroke mRS<4 and non-intubated

Functional outcome

Functional outcome after stroke is generally measured using the modified Rankins Scale (mRS), the Barthel Index (BI), or is based on the performance ADL such as dressing, toileting and mobility. Whether the patient was discharged home or institutionalized after stroke is commonly quantified indicating the level of disability and the functional outcome in patients. Outcome measures at discharge commonly show worse functional status in women even when confounders have been adjusted for [13, 21, 26, 30] (see Table 3). No significant differences during the acute phase have also been reported [23], but more women than men were discharged to long-term care or nursing homes. Similar findings were noted at long-term follow-ups. Female sex was associated with worse functional recovery at 3, 6 and 12 months and these sex differences were not eliminated when age, baseline or clinical variables were adjusted for [13, 28, 29, 32, 35, 36]. Furthermore, female sex was found as a predictor of disability (BI) and handicap (mRS) [6] and women were more dependent [8, 34]. One study reported that women were 3.5 times more likely to be institutionalized than men after stroke [13]. In contrast, Appelros et al. did not find sex-specific outcomes in mRS; however, the sample size of stroke survivors was small [31]. In a nation-wide study from United States of the “Get With The Guidelines – Stroke” (GWTG-Stroke) population consisting of 383,318 ischemic stroke patients, women were less likely to be discharged home [7]. The women in the GWTG-Stroke study were also less likely to receive care based on seven predefined measures e.g. IV tPA in patients who arrive <2 hours and antithrombotic medication within 48 hours of admission (for the complete list of measures see [7]). Sex differences in quality of care remained after adjustment for potential confounding variables like age, race and risk factors, emphasizing the difficulties in evaluating whether differences are true biological sex differences or dependent on social factors. However, a recent study showed that differences in demographics, prestroke and clinical factors only explained 41% of the sex differences in stroke outcome [29], leaving room for unknown factors to be discovered. In summary, women consistently suffer from worse short and long-term functional outcome than men, and this sex-disparity is not fully explained by the older age in women, prestroke function or comorbidities.

Table 3.

Sex differences in functional outcome

Patients Subtypes Location Period Sex differences in functional outcome Variables controlled for Reference
4,499a IS, ICH, SAH Multicenter, Europe 1993-1994 ♀> ♂ disability (BI<15) and handicap (mRS>1) at 3 mo Multivariate (prestroke handicap, level of consciousness) [6]
383,318 IS Multicenter, USA 2003-2008 ♀< ♂ discharged to home Multivariate [7]
64,746 IS, ICH, UNDETM (independent prestroke) Multicenter, Sweden 2008-2010 ♀ predictor of dependency at1 y Multivariate [8]
1,136b IS, ICH, SAH Framingham, USA 1948-2005 ♀> ♂ likely to be institutionalized at 3-6 mo Multivariate (prestroke handicap) [13]
2,725 IS, ICH Halifax, Canada 1996-2006 ♀< ♂ excellent outcome at discharge (BI≥95) Multivariate (prestroke handicap) [21]
3,323 IS, ICH, SAH, UNDETM Multicenter, Canada 2001-2002 ♀> ♂ discharged to long-term care Multivariate (marital status, prestroke living situation, consciousness, comorbidity index) [23]
537 IS, ICH Providence, USA 2010-2012 ♀< ♂ discharged to home Unadjusted [24]
6,236 IS (independent prestroke mRS 0-1) Multicenter, Japan 1999-2013 ♀> ♂ poor functional outcome at discharge Multivariate (poststroke treatments) [26]
19,547 IS, ICH Multicenter, Sweden 2001 ♀> ♂ institutional living at 3 mo Multivariate (level of consciousness) [28]
2,566 IS, ICH, SAH, TIA Michigan, USA 2002 ♀> ♂ poor function at discharge (mRS) Age, prestroke ambulatory status [30]
377 IS, ICH Örebro, Sweden 1999-2000 No sex diff in risk of dependency at 1 yr Lack of effect in univariate [31]
1754a ISc Multicenter, Germany 1998-2000 ♀> ♂ incomplete functional recovery at 100d Multivariate [32]
17,370 IS Multicenter, worldwide 1992-1996 ♀>♂, poor outcome at 6 mo Multivariate (level of consciousness) [34]
373a IS, ICH,SAH, TIAd MI, USA ♀>♂ dependent at 3 mo Multivariate (prestroke ambulatory status) [35]

Multivariate analysis commonly include age, stroke severity, stroke subtype and risk factors, other variables of importance that were adjusted for are given in brackets. IS = Ischemic stroke, ICH=Intracerebral hemorrhage, SAH=Subarachnoid hemorrhage, mRS=modified Rankin Scale, UNDETM=Undetermined

a

prospective

b

Number of incident strokes in the Framingham original and offspring cohorts

c

prestroke mRS<4 and non-intubated

d

life expectancy >6 mo and not discharged to hospice.

Response to treatments

Biological sex may not only influence stroke severity, functional outcome and mortality, but also impact how patients respond to stroke treatments. Preventive medicines such as aspirin appear to have sex-specific effects. A meta-analysis of randomized controlled trials showed that aspirin reduced the risk of myocardial infarction but not stroke in men, while in women, the scenario was the opposite with reduced risk of stroke and no effect on myocardial infarction [37]. Thrombolysis, predominantly with tissue plasminogen activator (tPA), is one of the treatments that have been analyzed for sex-specific effects. Since the approval of tPA for stroke treatment and documented beneficial effects on functional outcome, it was no longer justified not to treat eligible patients. Therefore, few randomized placebo-controlled studies on the effect of tPA on stroke outcome exist. One large pooled analysis of randomized trials from the United States, Europe and Australia (n=2,178), found that women benefited more from treatment with tPA than men [38] as treatment nullified the expected gender gap in outcome. In treated women, no sex differences in functional outcome was seen at 3 months, while untreated women had worse outcomes than untreated men, with lower probability of a return to normal or near-to-normal mRS (≤1). Similar findings of no difference in stroke outcome between men and women were reported in a small randomized study with intra-arterial pro-urokinase [39]. Other studies [40-45], as well as a clinical registry study and systematic review [46], have confirmed that men and women treated with tPA had equal functional outcomes at 3 months, suggesting a potentially greater benefit in treated women. In contrast, one study found that men were more likely to have a better functional outcome at 3 months but higher mortality [47]. The study included patients from the “Glycine Antagonist in Neuroprotection” (GAIN) study and patients with early major neurological improvement (<1 h) were excluded which may have impacted the results. In addition, unknown effects from the glycine antagonist cannot be excluded, although no effect on stroke outcome was seen in the trial.

Differences in coagulation and fibrinolysis [48] and a higher prevalence of cardioembolic fibrin-rich occlusions that might be more easily dissolved by tPA [49] have been speculated as contributing factors to the greater benefit seen in women [41]. While higher recanalization rates during the first 72 h were seen in a small cohort of women (n=39) by Savitz et al. [50], a recent study of patients from the CLOTBUST trial did not find sex as a predictor of recanalization in an adjusted analysis [40]. Two other studies with larger sample size (n=205 and n=81) on intra-arterial administration of tPA or urokinase did not find sex differences in recanalization rates [51, 52]. Of note, recanalization might not always lead to reperfusion and studies suggest that reperfusion may be a better predictor of infarct volume and clinical stroke outcome [53, 54]. Sex-specific effects of tPA have also been demonstrated when it is combined with other treatments. Recently, Llull and collaborators published data on female-specific benefit from the combination therapy of alteplase and uric acid [55], one of the first positive neuroprotection trials. This may be due to sex differences in baseline levels of endogenous uric acid, which were lower in women. Women had better functional outcomes after exogenous uric acid administration than men. It has been suggested that women might benefit more from antioxidants such as uric acid, and this merits further investigation in pre-clinical studies. These investigators also found that the combination of alteplase and uric acid reduced the infarct growth (defined by the difference in brain imaging at baseline and 72 hours) compared to alteplase alone, an effect that was only seen in women [55].

The efficacy of tPA may also be influenced by race. Mandava et al. reported that tPA-treated black women had significantly less likelihood of a good outcome (mRS 0-2) compared to white women 3 months following stroke [56]. It was suggested that this differential response to tPA might be due to a higher proportion of blacks presenting with a prothrombotic genotype of plasma inhibitor-1 (PAI-1), an inhibitor of tPA's enzymatic activity [57]. Previous analysis of the NINDS dataset did not show race as a predictor of functional outcome [58] but the study was not powered for subgroup analysis [59]. Therefore, explicit balancing methods were utilized in the study by Mandava et al. to evaluate the influence on sex and race on tPA outcome independent on differences in baseline characteristics [56]. Taken together, sex differences in response to stroke treatment may be depend on the ethnicity of stroke patients and further studies that carefully consider baseline characteristics are warranted.

Multiple recent trials on endovascular treatments have shown increased rate of functional independence in men and women 90 days after stroke [60-64]. Subgroup analysis by sex performed in two of these studies showed no significant sex differences in the rate of functional independence [64]. A previous study by Lutsep et al. that investigated the effect of sex on early revascularization and outcome at 90 days found similar rates of revascularization in men and women and no differences in functional independence or mortality [65]. In line with those findings, sex was not a predictor of poor neurological outcome (mRS >2) 90 days after endovascular treatment in a small Swedish study [66]. Although studies on sex differences in endovascular treatments are scarce, at this point it does not appear to be a sex-specific response or any reason for treating men and women differently.

Sex differences in experimental stroke

Sex differences are not only a consideration in human stroke, but also are relevant in pre-clinical studies. More than twenty years ago, Hall and authors reported that female gerbils sustained less neuronal damage and better reperfusion cerebral blood flow (CBF) after carotid occlusion compared to males [67]. Similar findings were reported by Alkayed et al. in a study of normotensive and stroke-prone spontaneously hypertensive rats where female animals subjected to transient intraluminal middle cerebral artery occlusion sustained less ischemic brain damage [68]. The sex-dependent effects on infarct volumes were eliminated in ovariectomized (OVX) females indicating a protective effect from female sex hormones. Supporting this hypothesis, OVX female rats treated with estrogen before transient or permanent ischemia restored female protection and led to less ischemic damage [69, 70]. Consistent with a protective role of gonadal steroids, young female mice have smaller infarct than males, while aged reproductively senescent male and female mice have equivalent infarcts after transient focal ischemia [71]. Estrogen's protective effect was also verified in castrated male animals and 16 months old reproductively senescent female rats after transient focal ischemia [72, 73]. However, the beneficial effects of estrogen in stroke seem to be time-sensitive as its neuroprotective and anti-inflammatory effects were lost with delayed estrogen-replacement after ovariectomy [74] or when given to aged animals [75, 76].

The potential beneficial effect of estrogen treatment acutely after stroke has also been investigated (see Table 4). Simpkins et al. demonstrated that delivery of estrogen at the beginning of reperfusion, but not after 50 min, significantly reduced the infarct volume [70], while the time window in permanent ischemia was defined to be ≤3 h [77]. It was later demonstrated that male animals benefited from acute postischemic estrogen treatment resulting in improved early reperfusion CBF and attenuated brain damage [78]. Naturally fluctuating estrogen levels in female animals have been linked to the extent of brain damage [79]. In that study, stroke-prone spontaneously hypertensive rats subjected to ischemic stroke during metestrus had a larger brain damage than rats that were exposed to ischemia and reperfusion during proestrus when estrogen levels are high [79]. Further support for a hormonal origin of sex differences in experimental stroke was elegantly demonstrated by Manwani and colleagues [80]. The four core genotype mice model, in which the testis determining gene Sry has been removed from the Y chromosome to an autosome, was utilized to study the contribution of sex chromosomes to stroke outcome [81]. Gonadal males (XXM and XYM) consistently had larger brain infarct volumes than gonadal females (XXF and XYF), and this difference was eliminated when animals were gonadoectomized suggesting hormone-dependency [80]. Some of estrogen's neuroprotective effects have been linked to its anti-inflammatory, anti-apoptotic and vasodilatory properties and are reviewed in detail elsewhere [82, 83]. It should be noted that although the majority of experimental studies favor the beneficial effects of estrogen, there are contrasting studies [84-86], see Table 4 and detailed previous reviews [83, 87]. In summary, young female animals sustain less ischemic damage than young males when subjected to equal types of stroke, an effect that is mainly due to estrogen. Interestingly, middle-aged female mice have been shown to have greater brain damage compared to middle-aged males and young females [71]. Thus, there is a need for studies on sex differences and the contribution of sex hormones to stroke outcome with aging.

Table 4.

Effects of estrogen and progesterone in experimental stroke

Species/model Sex/age Manipulation Effect on outcome Reference
Rat/pMCAO Female/adult OVX+/−E, replacement at OVX or at stroke onset, OVX 1 wk pre-MCAO ↓Infarct with E replacement during OVX [69]
Rat/MCAO Female/adult OVX+/−E, replacement 24 h pre- or 40-90 min post-MCAO, OVX 1 wk pre-MCAO ↓Infarct with E replacement 24 h pre- or 40 min post-stroke [70]
Rat/MCAO Male, female/16 months Male, female+E or P, replacement7 days pre-MCAO ↓Infarct in E and P treated females. No difference in infarct between males and females. [72]
Rat/MCAO Male/adult Intact;Intact+chronic E (7-10 d pre-MCAO), Intact+acute premarin (30 min prestroke);CAST;CAST+E, ↓Infarct in all E treated animals; CAST no effect on infarct [73]
Mouse/MCAO Female/adult OVX+/−E, immediate or 10 wks delayed replacement ↓Infarct and pro-inflammatory cytokines by immediate E [74]
Mouse/MCAO Female/adult, aged (22 mo) Adult OVX+/−E, Aged+/−E ↓Infarct by E in adult, no change in aged. Larger infarct in aged than in adult. [75]
Mouse/MCAO Male,female/aged 20 mo Intact+chronic E (17-20 mo of age), Intact+acute E at 20 months. MCAO was performed at 20.5 mo ↓Infarct by chronic E, no effect by acute E in females. Benefit in males by chronic and acute E. [76]
Rat/MCAO Female/adult Intact;OVX+/−E, replacement post-MCAO ↓Infarct when E was given ≤3h post-stroke [77]
Rat/MCAO Male/adult Intact+premarin at reperfusion ↓Infarct by premarin. [78]
Rat WKY, SHRSP/pMCAO Female/adult Animals were in metestrus or proestrus at time of MCAO ↓Infarct in SHRSP in proestrus compared to estrus [79]
Rat WKY, SHRSP/pMCAO Female/adult OVX+/− E, MCAO 2 wks later ↑Infarct in WKY by E, no effect in SHRSP [84]
Rat/pMCAO Female/adult OVX+/− E, MCAO 2 wks later ↑Infarct by E, no effect neurological function [85]
Rat/MCAO (ET-1) Female/adult, reproductively senescent (9-11 mo) Intact, OVX+/−E ↑Infarct by E in reproductively senescent, ↓ in adult. Larger infarcts in reproductively senescent than in adult [86]
Mouse/MCAO Male,female/Adult (3 mo) Intact female,OVX+/−E, Male+/−E ↑Neurogenesis and functional outcome by E [95]
Mouse/MCAO Female/adult, aged (12 mo) Adult OVX +/− P, Aged +/− P ↓Infarct by P in aged, no effect in OVX. P improved motor function in OVX, no effect in aged. [100]
Rat/MCAO Female/adult OVX+/−P, pre-MCAO or pre- and post-MCAO ↓Infarct by P treatment pre- and post-MCAO [101]
Mouse/MCAO Male/adult Intact +/−P post-MCAO ↓Infarct by P and improved motor function [102]
Rat/MCAO Male/adult Intact +/− P, pre- or post-MCAO treatment ↓Infarct by P and improved motor function when given pre- or post-MCAO [103]
Rat/pMCAO Male/adult Intact+/−P post-MCAO ↓Infarct by P and improved motor function [104]
Rat/pMCAO Male/aged (24 mo) Intact+/−P post-MCAO ↓Infarct by P and improved motor function [105]
Mouse/MCAO Female/adult OVX+/−P pre-MCAO No effect on infarct by P [106]
Rat/MCAO Female/adult OVX+/chronic P (7-10 d pre-MCAO) or acute P (30 min pre-MCAO) ↑Infarct with chronic P [107]
Rat/MCAO Female/aged (14-18 mo) OVX+/−E, P or E+P; replacement 2 mo pre-MCAO ↓Infarct by E and E+P, not by P alone [136]
Rat/MCAO Male, female/adult OVX+/−E, P or E+P post-MCAO; OVX 2-3 wks pre-MCAO ↓Infarct and improved functional outcome in all hormone-treated groups [137]

MCAO=Middle cerebral artery occlusion, OVX=Ovariectomy, E=17β-estradiol, pMCAO=Permanent middle cerebral artery occlusion, P=Progesterone, WKY=Wistar Kyoto, SHRSP=Spontaneously hypertensive stroke prone, ET-1=Endothelin-1, CEE=Conjugated equine estrogens, MPA=Medroxyprogesterone, CEP=Conjugated equine estrogens and medroxyprogesterone

Estrogen's beneficial effects have unfortunately not successfully translated to human stroke. Several large randomized placebo-controlled clinical trials of chronic hormone therapy in postmenopausal women have shown either no benefit in preventing heart disease and stroke, or even increased risk [17, 88-91]. The differential effect of estrogen in experimental and clinical studies might originate from the timing of hormone therapy, the age of the women and number of years since menopause [92]. To validate the timing hypothesis of hormone therapy in stroke prevention, Kronos Early Estrogen Prevention Study (KEEPS) trial was initiated where younger women were enrolled and hormone therapy was introduced early after menopause [93]. Although some markers of cardiovascular disease risk such as high- and low-density lipoprotein cholesterol were improved after 48 months of hormone therapy with oral conjugated equine estrogens and progesterone, no differences in atherosclerosis progression as measured by carotid artery intima-media thickness or coronary artery calcium were noted [94]. Previous and current clinical trials have focused on the ability of estrogen to reduce the incidence of stroke, which seems unlikely if estrogen treatment is delayed past the peri-menopausal period. It remains to be investigated whether acute administration of estrogen, as demonstrated in experimental studies, can improve stroke outcome in patients by reducing histological injury or enhancing repair as seen in animal studies [95].

Overall, studies on sex differences in ischemic stroke have mostly been performed in rodents. Studies in higher order species and non-rodent models are scarce. One study in rabbits investigated the effect of endogenous estrogen levels in females and the efficacy of an antiplatelet therapy with aspirin on outcome after embolic stroke [96]. Behavioral outcomes at 24 h were improved in aspirin-treated female rabbits with high endogenous estrogen levels compared to females with low estrogen, suggesting that estrogen improve the efficacy of anti-platelet treatment. Platelet aggregation was compared to previous experiments in males and no sex differences were found. A small study of ischemic stroke in Rhesus Macaque (n=3 per group) demonstrated no significant difference in the extent of ischemic injury after middle cerebral artery occlusion between males and females, although the variability was larger in females [97]. Studies in higher order species and non-human primates may be of great use in the transition from experimental studies to clinical trials but have the disadvantage of increased research costs and should be carefully designed to answer specific research questions.

Beneficial effects of female sex steroid hormones are not limited to estrogen. Several experimental studies show neuroprotective effects also from progesterone treatment alone or in combination with estrogen in ischemic stroke (see Table 4) and traumatic brain injury, reviewed in detail elsewhere [98, 99]. Acute administration of progesterone reduces ischemic injury and improves neurological function in ovariectomized female rats [100, 101] as well as in young [102-104] and aged rats and mice of both sex [100, 105]. Few existing studies evaluate the chronic effects of progesterone. These either showed no effect [106], increased [107], or reduced [72] stroke injury. In traumatic brain injury, progesterone emerged as a promising candidate for therapy when four phase II clinical trials showed better functional outcomes and decreased mortality [108]. Unfortunately, the recent completion of two large multicenter phase III trials ProTECT III and SyNAPSe showed no effect on mortality and no clinical benefit at 3 and 6 months [109, 110]. However, the potential of progesterone to reduce brain injury is of continued interest and issues with the failed translation of experimental studies into clinical trials has been recently reviewed [108]. The vast majority of preclinical studies demonstrate that sex steroid hormones like estrogen and progesterone can have neuroprotective effects but the experience from clinical trials has been disappointing. Challenges such as dosing, route of administration and time of treatment from stroke onset are likely key factors. Many clinical trials have very wide enrollment windows, as this increases the opportunity to enroll patients; however, the efficacy of any neuroprotective agent is likely to be maximized with early treatment. In addition, changes that occur in the aging brain and vasculature may influence efficacy. Investigators must recognize that stroke is primarily a disease of the elderly, and model the disease appropriately in the laboratory.

Ischemic cell death pathways

Although sex steroid hormones, predominantly estrogen, have been associated with the sex differences in experimental stroke, hormone-independent processes have also been discovered. Activation of different cell death pathways in males and females has received much attention the past years. One of the first studies to demonstrate sexual dichotomy in cell death pathways was performed by Sampei et al. [111]. Neuronal nitric oxide synthase deficient (nNOS−/−) male mice had smaller infarcts after permanent middle cerebral artery occlusion compared to wildtype mice, while female mutant mice did not benefit from nNOS deficiency [111]. Subsequent studies have investigated this further and found that males may be more sensitive to oxidative stress followed by an activation of poly (ADP-ribose) polymerase-1 (PARP-1) and translocation of apoptosis-inducing factor (AIF) [reviewed by 112, 113, 114]. On the other hand, cell death in females is initiated by activation of caspases and involves the early release of cytochrome C [reviewed by 112, 113, 114]. The sex-disparities in apoptotic pathways have been found in rat or mouse pups when gonadal hormone levels are equivalent between the sexes allowing for the study of hormone-independent mechanisms (although hormonal effects cannot be completely excluded) [113, 115], in intact animals [111, 116, 117] and in ovariectomized females with and without estrogen-supplementation [118], suggesting that some of these sex differences are hormone independent. Using genetic knockouts of PARP-1, reduced infarct volume and protection was seen in male pups [115] and adults [116], while no effect or even increased damage was seen in female animals. The potential that sex-specific cell death pathways exist likely impacts the response to treatments and indeed, reduced infarct volume after treatment with a pan-caspase inhibitor was shown in female animals and not in males [116, 118, 119]. The reverse scenario was achieved with a PARP-1 or nNOS inhibitor, where beneficial effects were seen in male mice but female animals had exacerbated stroke damage [117]. The importance of sex-specific effects and studies of sex differences is further emphasized in the case of minocycline, a neuroprotective agent that inhibits PARP-1 signaling [120]. Improved outcomes after minocycline treatment in ischemic stroke patients were reported in one study [121], although no subgroup analysis by sex was performed and there was a low proportion of females (35.1%). In experimental studies, Li et al. found that minocycline was beneficial in male mice with reduced infarct volume and neurological deficits, but not in females [122]. Recently, similar sex-specific effects of minocycline were seen in a clinical study from Iran [123]. Improved clinical outcome as measured by the NIHSS was seen in males at 30, 60 and 90 days after minocycline treatment while no significant effect was seen in females. When male and female data were grouped together, significance was reached at 90 days and the lack of a protective effect in females was not noticed, again highlighting the importance of sex-specific subgroup analysis and designing trials with adequate power to assess sex differences. It should be noted that the sample size in this study was very small (n=53) and NIHSS in the female minocycline group was significantly worse compared to the female control group at admission. Larger studies adjusting for differences in baseline characteristics are warranted to validate these findings.

Sex-specific effects in experimental stroke have been documented for other treatments/targets, e.g. hypothermia [124], erythropoietin [125], G-protein coupled estrogen receptors [126] and the calcium-permeable transient receptor potential M2 (TRPM2) ion channel [127]. In the case of TRPM2 inhibition, smaller infarcts following middle cerebral artery occlusion were only found in male mice and the authors discuss the possibility of PARP-mediated generation of ADPribose that in turn can activate TRPM2, linking TRPM2 signaling to sex specific cell death pathways.

Studying sex differences in stroke

Emerging data on sex differences and the influence of gonadal hormones in experimental stroke make it important to carefully plan and design studies in which female animals are included. A comprehensive guide for research on sex differences has been published by Becker et al. [128]. The authors present strategic guidance and hands-on instructions on how to monitor and characterize the reproductive cycle in female animals. Rodents have a reproductive cycle of 4 to 5 days with rapid changes in female sex hormone levels [129] that can influence studied traits differently depending on the day and time of the day experiments are performed. The use of intact cycling females randomly selected throughout their reproductive cycle might result in large within-group variations and mask effects if female sex hormones influence the studied traits. There is also a risk of missing sex hormone effects if animals are used for experiments on the same day since female animals tend to cycle together. Since the extent of stroke damage has been linked to hormonal fluctuations during the reproductive cycle [79], depending on the research questions this might impact the results obtained. If the goal is to assess an effect of a potential treatment it is especially important not to have contributing protective effect of estrogen that could skew data if the proportion of females in proestrus (high estradiol levels) is not the same in placebo and treatment groups. One approach is to monitor two to three consecutive reproductive cycles by daily collection of vaginal smears and selecting animals at one or several stages of the reproductive cycle, although this increases the number of animals used. In a study examining a MEK1/2 inhibitor on ischemic stroke outcome, female rats in estrus or diestrus were specifically selected (when estrogen levels are low) and small within-group variations in infarct volume and functional outcome were documented [130]. However, if the intent is to model clinical stroke, the use of reproductively senescent or aged animals is desirable, as stroke outcome is different in young, middle-aged and aged male and female mice and the efficacy of neuroprotective agents differ in the aged brain [71].

Much attention has been given to the effect estrogen in ischemic stroke, but androgens in males also affect stroke incidence and outcome. In men, levels of testosterone decline with aging which has been associated with increased risk of stroke and low levels may worsen stroke outcome [131]. In preclinical studies androgens have a detrimental effect after stroke in young male mice but are beneficial if given to aged animals that have an age-related decline in endogenous sex steroid [131]. Gonadectomy provides a tool for studying the effect of sex steroid hormones in stroke but do not recapitulate the overall aging phenotype as the loss of gonadal hormones is abrupt and complete, and the animals are still young, with young vasculature and brain. Chemical induction of menopause in female animals by 4-vinylcyclohexene diepoxide (VCD) better mimics the natural decline in female hormones and can be useful [132], but the use of aged animals is likely to be the most translationally relevant, despite their high cost. One should also be aware of the possibility of difference in body size between males and females, and between ovariectomized females treated with placebo versus estrogen, that can affect behavioral analysis and all tests should be performed in sham male and female animals and in experimental animals before initiating experiments if possible.

Similar to experimental studies of stroke, it is important to perform subgroup analysis by sex in clinical studies and take into account whether women included in the study are pre- or postmenopausal or are on hormone therapy, which is far less common now compared to a decade ago, due to the findings of the WHI. Most female stroke patients are over 70 years of age making this less relevant, but with the increase in stroke incidence in younger women, or during the peri-partum period, hormonal factors need to be considered. In addition, epidemiological studies of sex differences in stroke outcome should take into account not only differences in basic clinical variables and risk factors, but also pre-stroke function and living situation. This is especially important as social isolation has negative effects on stroke mortality and morbidity [133]. Factors such as depression are also understudied and seem to have a differentially negative impact on women. When the number of variables included in multivariate analysis increases, larger numbers of patients are needed, and unfortunately many studies are underpowered. However epidemiological studies help us define factors that may be important to stroke outcome, which then can be further evaluated in experimental studies where age and risk factors can be controlled and targets can be directly manipulated.

Concluding remarks

Sex differences in stroke have received more attention over the past decade but are still largely understudied. Although existing literature on sex differences in stroke severity and mortality are somewhat contradictory, women consistently suffer from worse functional outcomes and have high levels of long-term disability. Out of the number of fatal strokes in the United States during 2010, 67% were in women, a number that is expected to increase even further in the coming years [4]. Furthermore, sex differences in stroke outcome are not fully explained by differences in baseline demographics, including age. Other contributing factors such as epigenetics, immune responsiveness, inflammation and chromosomal contributions to ischemic sensitivity have yet to be investigated in detail. Studying sex-specific effects is especially important in preclinical and clinical studies of potential stroke therapies. If treatments that only have been evaluated in one sex in experimental studies are to be tested clinically, there is a risk of adverse effects in one sex cancelling out beneficial effects in the other sex. This further highlights the importance of subgroup analysis by sex in clinical stroke studies. Lastly, there is a disconnect between a male-bias in research animals used for experimental studies and the disproportionate large population of women among stroke patients [134]. Not only is the sex of research animals important, but sex matters when studying cell lines in vitro as demonstrated by differential susceptibility of cultured XY and XX neuronal cells to cytotoxic insults [134, 135]. Stroke in females is understudied and given the growing stroke burden in women this is the direction where more research is needed. The importance of studying comorbidities in stroke has already been underscored and it is imperative that this applies to both sexes.

Footnotes

Conflict of interest

The authors declare no conflict of interest.

References

  • 1.Feigin VL, Forouzanfar MH, Krishnamurthi R, Mensah GA, Connor M, Bennett DA, et al. Global and regional burden of stroke during 1990-2010: findings from the Global Burden of Disease Study 2010. Lancet. 2014;383:245–54. doi: 10.1016/s0140-6736(13)61953-4. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 2.Mozaffarian D, Benjamin EJ, Go AS, Arnett DK, Blaha MJ, Cushman M, et al. Heart disease and stroke statistics--2015 update: a report from the American Heart Association. Circulation. 2015;131:e29–322. doi: 10.1161/CIR.0000000000000152. [DOI] [PubMed] [Google Scholar]
  • 3.Appelros P, Stegmayr B, Terent A. Sex differences in stroke epidemiology: a systematic review. Stroke. 2009;40:1082–90. doi: 10.1161/STROKEAHA.108.540781. [DOI] [PubMed] [Google Scholar]
  • 4.Reeves MJ, Bushnell CD, Howard G, Gargano JW, Duncan PW, Lynch G, et al. Sex differences in stroke: epidemiology, clinical presentation, medical care, and outcomes. Lancet Neurol. 2008;7:915–26. doi: 10.1016/S1474-4422(08)70193-5. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 5.Dehlendorff C, Andersen KK, Olsen TS. Sex Disparities in Stroke: Women Have More Severe Strokes but Better Survival Than Men. J Am Heart Assoc. 2015:4. doi: 10.1161/JAHA.115.001967. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 6.Di Carlo A, Lamassa M, Baldereschi M, Pracucci G, Basile AM, Wolfe CD, et al. Sex differences in the clinical presentation, resource use, and 3-month outcome of acute stroke in Europe: data from a multicenter multinational hospital-based registry. Stroke. 2003;34:1114–9. doi: 10.1161/01.STR.0000068410.07397.D7. [DOI] [PubMed] [Google Scholar]
  • 7.Reeves MJ, Fonarow GC, Zhao X, Smith EE, Schwamm LH, Get With The Guidelines-Stroke Steering C et al. Quality of care in women with ischemic stroke in the GWTG program. Stroke. 2009;40:1127–33. doi: 10.1161/STROKEAHA.108.543157. [DOI] [PubMed] [Google Scholar]
  • 8.Ullberg T, Zia E, Petersson J, Norrving B. Changes in functional outcome over the first year after stroke: an observational study from the Swedish stroke register. Stroke. 2015;46:389–94. doi: 10.1161/STROKEAHA.114.006538. [DOI] [PubMed] [Google Scholar]
  • 9.Barker-Collo S, Bennett DA, Krishnamurthi RV, Parmar P, Feigin VL, Naghavi M, et al. Sex Differences in Stroke Incidence, Prevalence, Mortality and Disability-Adjusted Life Years: Results from the Global Burden of Disease Study 2013. Neuroepidemiology. 2015;45:203–14. doi: 10.1159/000441103. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 10.Carandang R, Seshadri S, Beiser A, Kelly-Hayes M, Kase CS, Kannel WB, et al. Trends in incidence, lifetime risk, severity, and 30-day mortality of stroke over the past 50 years. JAMA. 2006;296:2939–46. doi: 10.1001/jama.296.24.2939. [DOI] [PubMed] [Google Scholar]
  • 11.Kissela B, Schneider A, Kleindorfer D, Khoury J, Miller R, Alwell K, et al. Stroke in a biracial population: the excess burden of stroke among blacks. Stroke. 2004;35:426–31. doi: 10.1161/01.STR.0000110982.74967.39. [DOI] [PubMed] [Google Scholar]
  • 12.Lofmark U, Hammarstrom A. Evidence for age-dependent education-related differences in men and women with first-ever stroke. Results from a community-based incidence study in northern Sweden. Neuroepidemiology. 2007;28:135–41. doi: 10.1159/000102141. [DOI] [PubMed] [Google Scholar]
  • 13.Petrea RE, Beiser AS, Seshadri S, Kelly-Hayes M, Kase CS, Wolf PA. Gender differences in stroke incidence and poststroke disability in the Framingham heart study. Stroke. 2009;40:1032–7. doi: 10.1161/STROKEAHA.108.542894. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 14.Rothwell PM, Coull AJ, Silver LE, Fairhead JF, Giles MF, Lovelock CE, et al. Population-based study of event-rate, incidence, case fatality, and mortality for all acute vascular events in all arterial territories (Oxford Vascular Study). Lancet. 2005;366:1773–83. doi: 10.1016/S0140-6736(05)67702-1. [DOI] [PubMed] [Google Scholar]
  • 15.Vangen-Lonne AM, Wilsgaard T, Johnsen SH, Carlsson M, Mathiesen EB. Time trends in incidence and case fatality of ischemic stroke: the tromso study 1977-2010. Stroke. 2015;46:1173–9. doi: 10.1161/STROKEAHA.114.008387. [DOI] [PubMed] [Google Scholar]
  • 16.Lisabeth LD, Beiser AS, Brown DL, Murabito JM, Kelly-Hayes M, Wolf PA. Age at natural menopause and risk of ischemic stroke: the Framingham heart study. Stroke. 2009;40:1044–9. doi: 10.1161/STROKEAHA.108.542993. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 17.Rossouw JE, Anderson GL, Prentice RL, LaCroix AZ, Kooperberg C, Stefanick ML, et al. Risks and benefits of estrogen plus progestin in healthy postmenopausal women: principal results From the Women's Health Initiative randomized controlled trial. JAMA. 2002;288:321–33. doi: 10.1001/jama.288.3.321. [DOI] [PubMed] [Google Scholar]
  • 18.Olsen TS, Andersen ZJ, Andersen KK. Explaining poorer stroke outcomes in women: women surviving 3 months have more severe strokes than men despite a lower 3-month case fatality. Gend Med. 2012;9:147–53. doi: 10.1016/j.genm.2012.03.002. [DOI] [PubMed] [Google Scholar]
  • 19.Gall SL, Donnan G, Dewey HM, Macdonell R, Sturm J, Gilligan A, et al. Sex differences in presentation, severity, and management of stroke in a population-based study. Neurology. 2010;74:975–81. doi: 10.1212/WNL.0b013e3181d5a48f. [DOI] [PubMed] [Google Scholar]
  • 20.Li B, Wang T, Lou Y, Guo X, Gu H, Zhu Y, et al. Sex Differences in Outcomes and Associated Risk Factors After Acute Ischemic Stroke in Elderly Patients: A Prospective Follow-up Study. J Stroke Cerebrovasc Dis. 2015 doi: 10.1016/j.jstrokecerebrovasdis.2015.06.007. [DOI] [PubMed] [Google Scholar]
  • 21.Reid JM, Dai D, Gubitz GJ, Kapral MK, Christian C, Phillips SJ. Gender differences in stroke examined in a 10-year cohort of patients admitted to a Canadian teaching hospital. Stroke. 2008;39:1090–5. doi: 10.1161/STROKEAHA.107.495143. [DOI] [PubMed] [Google Scholar]
  • 22.Kapral MK, Degani N, Hall R, Fang J, Saposnik G, Richards J, et al. Gender differences in stroke care and outcomes in Ontario. Womens Health Issues. 2011;21:171–6. doi: 10.1016/j.whi.2010.10.002. [DOI] [PubMed] [Google Scholar]
  • 23.Kapral MK, Fang J, Hill MD, Silver F, Richards J, Jaigobin C, et al. Sex differences in stroke care and outcomes: results from the Registry of the Canadian Stroke Network. Stroke. 2005;36:809–14. doi: 10.1161/01.STR.0000157662.09551.e5. [DOI] [PubMed] [Google Scholar]
  • 24.Madsen TE, Choo EK, Seigel TA, Palms D, Silver B. Lack of gender disparities in emergency department triage of acute stroke patients. West J Emerg Med. 2015;16:203–9. doi: 10.5811/westjem.2014.11.23063. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 25.Barrett KM, Brott TG, Brown RD, Jr., Frankel MR, Worrall BB, Silliman SL, et al. Sex differences in stroke severity, symptoms, and deficits after first-ever ischemic stroke. J Stroke Cerebrovasc Dis. 2007;16:34–9. doi: 10.1016/j.jstrokecerebrovasdis.2006.11.002. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 26.Irie F, Kamouchi M, Hata J, Matsuo R, Wakisaka Y, Kuroda J, et al. Sex differences in short-term outcomes after acute ischemic stroke: the fukuoka stroke registry. Stroke. 2015;46:471–6. doi: 10.1161/STROKEAHA.114.006739. [DOI] [PubMed] [Google Scholar]
  • 27.Roquer J, Campello AR, Gomis M. Sex differences in first-ever acute stroke. Stroke. 2003;34:1581–5. doi: 10.1161/01.STR.0000078562.82918.F6. [DOI] [PubMed] [Google Scholar]
  • 28.Glader EL, Stegmayr B, Norrving B, Terent A, Hulter-Asberg K, Wester PO, et al. Sex differences in management and outcome after stroke: a Swedish national perspective. Stroke. 2003;34:1970–5. doi: 10.1161/01.STR.0000083534.81284.C5. [DOI] [PubMed] [Google Scholar]
  • 29.Lisabeth LD, Reeves MJ, Baek J, Skolarus LE, Brown DL, Zahuranec DB, et al. Factors influencing sex differences in poststroke functional outcome. Stroke. 2015;46:860–3. doi: 10.1161/STROKEAHA.114.007985. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 30.Gargano JW, Wehner S, Reeves M. Sex differences in acute stroke care in a statewide stroke registry. Stroke. 2008;39:24–9. doi: 10.1161/STROKEAHA.107.493262. [DOI] [PubMed] [Google Scholar]
  • 31.Appelros P, Nydevik I, Viitanen M. Poor outcome after first-ever stroke: predictors for death, dependency, and recurrent stroke within the first year. Stroke. 2003;34:122–6. doi: 10.1161/01.str.0000047852.05842.3c. [DOI] [PubMed] [Google Scholar]
  • 32.Weimar C, Ziegler A, Konig IR, Diener HC. Predicting functional outcome and survival after acute ischemic stroke. J Neurol. 2002;249:888–95. doi: 10.1007/s00415-002-0755-8. [DOI] [PubMed] [Google Scholar]
  • 33.Holroyd-Leduc JM, Kapral MK, Austin PC, Tu JV. Sex differences and similarities in the management and outcome of stroke patients. Stroke. 2000;31:1833–7. doi: 10.1161/01.str.31.8.1833. [DOI] [PubMed] [Google Scholar]
  • 34.Niewada M, Kobayashi A, Sandercock PA, Kaminski B, Czlonkowska A, International Stroke Trial Collaborative G Influence of gender on baseline features and clinical outcomes among 17,370 patients with confirmed ischaemic stroke in the international stroke trial. Neuroepidemiology. 2005;24:123–8. doi: 10.1159/000082999. [DOI] [PubMed] [Google Scholar]
  • 35.Gargano JW, Reeves MJ, Paul Coverdell National Acute Stroke Registry Michigan Prototype I Sex differences in stroke recovery and stroke-specific quality of life: results from a statewide stroke registry. Stroke. 2007;38:2541–8. doi: 10.1161/STROKEAHA.107.485482. [DOI] [PubMed] [Google Scholar]
  • 36.Lai SM, Duncan PW, Dew P, Keighley J. Sex differences in stroke recovery. Prev Chronic Dis. 2005;2:A13. [PMC free article] [PubMed] [Google Scholar]
  • 37.Berger JS, Roncaglioni MC, Avanzini F, Pangrazzi I, Tognoni G, Brown DL. Aspirin for the primary prevention of cardiovascular events in women and men: a sex-specific meta-analysis of randomized controlled trials. JAMA. 2006;295:306–13. doi: 10.1001/jama.295.3.306. [DOI] [PubMed] [Google Scholar]
  • 38.Kent DM, Price LL, Ringleb P, Hill MD, Selker HP. Sex-based differences in response to recombinant tissue plasminogen activator in acute ischemic stroke: a pooled analysis of randomized clinical trials. Stroke. 2005;36:62–5. doi: 10.1161/01.STR.0000150515.15576.29. [DOI] [PubMed] [Google Scholar]
  • 39.Hill MD, Kent DM, Hinchey J, Rowley H, Buchan AM, Wechsler LR, et al. Sex-based differences in the effect of intra-arterial treatment of stroke: analysis of the PROACT-2 study. Stroke. 2006;37:2322–5. doi: 10.1161/01.STR.0000237060.21472.47. [DOI] [PubMed] [Google Scholar]
  • 40.Al-hussain F, Hussain MS, Molina C, Uchino K, Shuaib A, Demchuk AM, et al. Does the sex of acute stroke patients influence the effectiveness of rt-PA? BMC Neurol. 2014;14:60. doi: 10.1186/1471-2377-14-60. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 41.Forster A, Gass A, Kern R, Wolf ME, Ottomeyer C, Zohsel K, et al. Gender differences in acute ischemic stroke: etiology, stroke patterns and response to thrombolysis. Stroke. 2009;40:2428–32. doi: 10.1161/STROKEAHA.109.548750. [DOI] [PubMed] [Google Scholar]
  • 42.Hametner C, Kellert L, Ringleb PA. Impact of sex in stroke thrombolysis: a coarsened exact matching study. BMC Neurol. 2015;15:10. doi: 10.1186/s12883-015-0262-z. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 43.Jovanovic DR, Beslac-Bumbasirevic L, Budimkic M, Pekmezovic T, Zivkovic M, Kostic VS, et al. Do women benefit more from systemic thrombolysis in acute ischemic stroke? A Serbian experience with thrombolysis in ischemic stroke (SETIS) study. Clin Neurol Neurosurg. 2009;111:729–32. doi: 10.1016/j.clineuro.2009.06.014. [DOI] [PubMed] [Google Scholar]
  • 44.Kent DM, Buchan AM, Hill MD. The gender effect in stroke thrombolysis: of CASES, controls, and treatment-effect modification. Neurology. 2008;71:1080–3. doi: 10.1212/01.wnl.0000316191.84334.bd. [DOI] [PubMed] [Google Scholar]
  • 45.Lorenzano S, Ahmed N, Falcou A, Mikulik R, Tatlisumak T, Roffe C, et al. Does sex influence the response to intravenous thrombolysis in ischemic stroke?: answers from safe implementation of treatments in Stroke-International Stroke Thrombolysis Register. Stroke. 2013;44:3401–6. doi: 10.1161/STROKEAHA.113.002908. [DOI] [PubMed] [Google Scholar]
  • 46.Meseguer E, Mazighi M, Labreuche J, Arnaiz C, Cabrejo L, Slaoui T, et al. Outcomes of intravenous recombinant tissue plasminogen activator therapy according to gender: a clinical registry study and systematic review. Stroke. 2009;40:2104–10. doi: 10.1161/STROKEAHA.108.546325. [DOI] [PubMed] [Google Scholar]
  • 47.Elkind MS, Prabhakaran S, Pittman J, Koroshetz W, Jacoby M, Johnston KC, et al. Sex as a predictor of outcomes in patients treated with thrombolysis for acute stroke. Neurology. 2007;68:842–8. doi: 10.1212/01.wnl.0000256748.28281.ad. [DOI] [PubMed] [Google Scholar]
  • 48.Roy-O'Reilly M, McCullough LD. Sex differences in stroke: the contribution of coagulation. Exp Neurol. 2014;259:16–27. doi: 10.1016/j.expneurol.2014.02.011. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 49.Niessen F, Hilger T, Hoehn M, Hossmann KA. Differences in clot preparation determine outcome of recombinant tissue plasminogen activator treatment in experimental thromboembolic stroke. Stroke. 2003;34:2019–24. doi: 10.1161/01.STR.0000080941.73934.30. [DOI] [PubMed] [Google Scholar]
  • 50.Savitz SI, Schlaug G, Caplan L, Selim M. Arterial occlusive lesions recanalize more frequently in women than in men after intravenous tissue plasminogen activator administration for acute stroke. Stroke. 2005;36:1447–51. doi: 10.1161/01.STR.0000170647.42126.a8. [DOI] [PubMed] [Google Scholar]
  • 51.Arnold M, Kappeler L, Nedeltchev K, Brekenfeld C, Fischer U, Keserue B, et al. Recanalization and outcome after intra-arterial thrombolysis in middle cerebral artery and internal carotid artery occlusion: does sex matter? Stroke. 2007;38:1281–5. doi: 10.1161/01.STR.0000259711.13490.23. [DOI] [PubMed] [Google Scholar]
  • 52.Shah SH, Liebeskind DS, Saver JL, Starkman S, Vinuela F, Duckwiler G, et al. Influence of gender on outcomes after intra-arterial thrombolysis for acute ischemic stroke. Neurology. 2006;66:1745–6. doi: 10.1212/01.wnl.0000218208.31305.84. [DOI] [PubMed] [Google Scholar]
  • 53.Cho TH, Nighoghossian N, Mikkelsen IK, Derex L, Hermier M, Pedraza S, et al. Reperfusion within 6 hours outperforms recanalization in predicting penumbra salvage, lesion growth, final infarct, and clinical outcome. Stroke. 2015;46:1582–9. doi: 10.1161/STROKEAHA.114.007964. [DOI] [PubMed] [Google Scholar]
  • 54.Soares BP, Tong E, Hom J, Cheng SC, Bredno J, Boussel L, et al. Reperfusion is a more accurate predictor of follow-up infarct volume than recanalization: a proof of concept using CT in acute ischemic stroke patients. Stroke. 2010;41:e34–40. doi: 10.1161/STROKEAHA.109.568766. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 55.Llull L, Laredo C, Renu A, Perez B, Vila E, Obach V, et al. Uric Acid Therapy Improves Clinical Outcome in Women With Acute Ischemic Stroke. Stroke. 2015 doi: 10.1161/STROKEAHA.115.009960. [DOI] [PubMed] [Google Scholar]
  • 56.Mandava P, Murthy SB, Munoz M, McGuire D, Simon RP, Alexandrov AV, et al. Explicit consideration of baseline factors to assess recombinant tissue-type plasminogen activator response with respect to race and sex. Stroke. 2013;44:1525–31. doi: 10.1161/STROKEAHA.113.001116. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 57.Lanfear DE, Marsh S, Cresci S, Shannon WD, Spertus JA, McLeod HL. Genotypes associated with myocardial infarction risk are more common in African Americans than in European Americans. J Am Coll Cardiol. 2004;44:165–7. doi: 10.1016/j.jacc.2004.03.053. [DOI] [PubMed] [Google Scholar]
  • 58.Generalized efficacy of t-PA for acute stroke Subgroup analysis of the NINDS t-PA Stroke Trial. Stroke. 1997;28:2119–25. doi: 10.1161/01.str.28.11.2119. [DOI] [PubMed] [Google Scholar]
  • 59.Ingall TJ, O'Fallon WM, Asplund K, Goldfrank LR, Hertzberg VS, Louis TA, et al. Findings from the reanalysis of the NINDS tissue plasminogen activator for acute ischemic stroke treatment trial. Stroke. 2004;35:2418–24. doi: 10.1161/01.STR.0000140891.70547.56. [DOI] [PubMed] [Google Scholar]
  • 60.Berkhemer OA, Fransen PS, Beumer D, van den Berg LA, Lingsma HF, Yoo AJ, et al. A randomized trial of intraarterial treatment for acute ischemic stroke. N Engl J Med. 2015;372:11–20. doi: 10.1056/NEJMoa1411587. [DOI] [PubMed] [Google Scholar]
  • 61.Campbell BC, Mitchell PJ, Kleinig TJ, Dewey HM, Churilov L, Yassi N, et al. Endovascular therapy for ischemic stroke with perfusion-imaging selection. N Engl J Med. 2015;372:1009–18. doi: 10.1056/NEJMoa1414792. [DOI] [PubMed] [Google Scholar]
  • 62.Goyal M, Demchuk AM, Menon BK, Eesa M, Rempel JL, Thornton J, et al. Randomized assessment of rapid endovascular treatment of ischemic stroke. N Engl J Med. 2015;372:1019–30. doi: 10.1056/NEJMoa1414905. [DOI] [PubMed] [Google Scholar]
  • 63.Jovin TG, Chamorro A, Cobo E, de Miquel MA, Molina CA, Rovira A, et al. Thrombectomy within 8 hours after symptom onset in ischemic stroke. N Engl J Med. 2015;372:2296–306. doi: 10.1056/NEJMoa1503780. [DOI] [PubMed] [Google Scholar]
  • 64.Saver JL, Goyal M, Bonafe A, Diener HC, Levy EI, Pereira VM, et al. Stent-retriever thrombectomy after intravenous t-PA vs. t-PA alone in stroke. N Engl J Med. 2015;372:2285–95. doi: 10.1056/NEJMoa1415061. [DOI] [PubMed] [Google Scholar]
  • 65.Lutsep HL, Hill MD. Effects of sex on mechanical embolectomy outcome. J Stroke Cerebrovasc Dis. 2012;21:240–2. doi: 10.1016/j.jstrokecerebrovasdis.2010.08.002. [DOI] [PubMed] [Google Scholar]
  • 66.Lowhagen Henden P, Rentzos A, Karlsson JE, Rosengren L, Sundeman H, Reinsfelt B, et al. Hypotension During Endovascular Treatment of Ischemic Stroke Is a Risk Factor for Poor Neurological Outcome. Stroke. 2015 doi: 10.1161/STROKEAHA.115.009808. [DOI] [PubMed] [Google Scholar]
  • 67.Hall ED, Pazara KE, Linseman KL. Sex differences in postischemic neuronal necrosis in gerbils. J Cereb Blood Flow Metab. 1991;11:292–8. doi: 10.1038/jcbfm.1991.61. [DOI] [PubMed] [Google Scholar]
  • 68.Alkayed NJ, Harukuni I, Kimes AS, London ED, Traystman RJ, Hurn PD. Gender-linked brain injury in experimental stroke. Stroke. 1998;29:159–65. doi: 10.1161/01.str.29.1.159. discussion 66. [DOI] [PubMed] [Google Scholar]
  • 69.Dubal DB, Kashon ML, Pettigrew LC, Ren JM, Finklestein SP, Rau SW, et al. Estradiol protects against ischemic injury. J Cereb Blood Flow Metab. 1998;18:1253–8. doi: 10.1097/00004647-199811000-00012. [DOI] [PubMed] [Google Scholar]
  • 70.Simpkins JW, Rajakumar G, Zhang YQ, Simpkins CE, Greenwald D, Yu CJ, et al. Estrogens may reduce mortality and ischemic damage caused by middle cerebral artery occlusion in the female rat. J Neurosurg. 1997;87:724–30. doi: 10.3171/jns.1997.87.5.0724. [DOI] [PubMed] [Google Scholar]
  • 71.Manwani B, Liu F, Scranton V, Hammond MD, Sansing LH, McCullough LD. Differential effects of aging and sex on stroke induced inflammation across the lifespan. Exp Neurol. 2013;249:120–31. doi: 10.1016/j.expneurol.2013.08.011. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 72.Alkayed NJ, Murphy SJ, Traystman RJ, Hurn PD, Miller VM. Neuroprotective effects of female gonadal steroids in reproductively senescent female rats. Stroke. 2000;31:161–8. doi: 10.1161/01.str.31.1.161. [DOI] [PubMed] [Google Scholar]
  • 73.Toung TJ, Traystman RJ, Hurn PD. Estrogen-mediated neuroprotection after experimental stroke in male rats. Stroke. 1998;29:1666–70. doi: 10.1161/01.str.29.8.1666. [DOI] [PubMed] [Google Scholar]
  • 74.Suzuki S, Brown CM, Dela Cruz CD, Yang E, Bridwell DA, Wise PM. Timing of estrogen therapy after ovariectomy dictates the efficacy of its neuroprotective and antiinflammatory actions. Proc Natl Acad Sci U S A. 2007;104:6013–8. doi: 10.1073/pnas.0610394104. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 75.Cai M, Ma YL, Qin P, Li Y, Zhang LX, Nie H, et al. The loss of estrogen efficacy against cerebral ischemia in aged postmenopausal female mice. Neurosci Lett. 2014;558:115–9. doi: 10.1016/j.neulet.2013.11.007. [DOI] [PubMed] [Google Scholar]
  • 76.Liu F, Benashski SE, Xu Y, Siegel M, McCullough LD. Effects of chronic and acute oestrogen replacement therapy in aged animals after experimental stroke. J Neuroendocrinol. 2012;24:319–30. doi: 10.1111/j.1365-2826.2011.02248.x. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 77.Yang SH, Shi J, Day AL, Simpkins JW. Estradiol exerts neuroprotective effects when administered after ischemic insult. Stroke. 2000;31:745–9. doi: 10.1161/01.str.31.3.745. discussion 9-50. [DOI] [PubMed] [Google Scholar]
  • 78.McCullough LD, Alkayed NJ, Traystman RJ, Williams MJ, Hurn PD. Postischemic estrogen reduces hypoperfusion and secondary ischemia after experimental stroke. Stroke. 2001;32:796–802. doi: 10.1161/01.str.32.3.796. [DOI] [PubMed] [Google Scholar]
  • 79.Carswell HV, Dominiczak AF, Macrae IM. Estrogen status affects sensitivity to focal cerebral ischemia in stroke-prone spontaneously hypertensive rats. Am J Physiol Heart Circ Physiol. 2000;278:H290–4. doi: 10.1152/ajpheart.2000.278.1.H290. [DOI] [PubMed] [Google Scholar]
  • 80.Manwani B, Bentivegna K, Benashski SE, Venna VR, Xu Y, Arnold AP, et al. Sex differences in ischemic stroke sensitivity are influenced by gonadal hormones, not by sex chromosome complement. J Cereb Blood Flow Metab. 2015;35:221–9. doi: 10.1038/jcbfm.2014.186. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 81.Arnold AP. Mouse models for evaluating sex chromosome effects that cause sex differences in non-gonadal tissues. J Neuroendocrinol. 2009;21:377–86. doi: 10.1111/j.1365-2826.2009.01831.x. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 82.Ritzel RM, Capozzi LA, McCullough LD. Sex, stroke, and inflammation: the potential for estrogen-mediated immunoprotection in stroke. Horm Behav. 2013;63:238–53. doi: 10.1016/j.yhbeh.2012.04.007. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 83.Strom JO, Theodorsson A, Theodorsson E. Mechanisms of estrogens’ dose-dependent neuroprotective and neurodamaging effects in experimental models of cerebral ischemia. Int J Mol Sci. 2011;12:1533–62. doi: 10.3390/ijms12031533. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 84.Carswell HV, Bingham D, Wallace K, Nilsen M, Graham DI, Dominiczak AF, et al. Differential effects of 17beta-estradiol upon stroke damage in stroke prone and normotensive rats. J Cereb Blood Flow Metab. 2004;24:298–304. doi: 10.1097/01.WCB.0000112322.75217.FD. [DOI] [PubMed] [Google Scholar]
  • 85.Gordon KB, Macrae IM, Carswell HV. Effects of 17beta-oestradiol on cerebral ischaemic damage and lipid peroxidation. Brain Res. 2005;1036:155–62. doi: 10.1016/j.brainres.2004.12.052. [DOI] [PubMed] [Google Scholar]
  • 86.Selvamani A, Sohrabji F. Reproductive age modulates the impact of focal ischemia on the forebrain as well as the effects of estrogen treatment in female rats. Neurobiol Aging. 2010;31:1618–28. doi: 10.1016/j.neurobiolaging.2008.08.014. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 87.Carswell HV, Macrae IM, Farr TD. Complexities of oestrogen in stroke. Clin Sci (Lond) 2010;118:375–89. doi: 10.1042/CS20090018. [DOI] [PubMed] [Google Scholar]
  • 88.Hulley S, Grady D, Bush T, Furberg C, Herrington D, Riggs B, et al. Randomized trial of estrogen plus progestin for secondary prevention of coronary heart disease in postmenopausal women. Heart and Estrogen/progestin Replacement Study (HERS) Research Group. JAMA. 1998;280:605–13. doi: 10.1001/jama.280.7.605. [DOI] [PubMed] [Google Scholar]
  • 89.Simon JA, Hsia J, Cauley JA, Richards C, Harris F, Fong J, et al. Postmenopausal hormone therapy and risk of stroke: The Heart and Estrogen-progestin Replacement Study (HERS). Circulation. 2001;103:638–42. doi: 10.1161/01.cir.103.5.638. [DOI] [PubMed] [Google Scholar]
  • 90.Wassertheil-Smoller S, Hendrix SL, Limacher M, Heiss G, Kooperberg C, Baird A, et al. Effect of estrogen plus progestin on stroke in postmenopausal women: the Women's Health Initiative: a randomized trial. Jama. 2003;289:2673–84. doi: 10.1001/jama.289.20.2673. [DOI] [PubMed] [Google Scholar]
  • 91.Viscoli CM, Brass LM, Kernan WN, Sarrel PM, Suissa S, Horwitz RI. A clinical trial of estrogen-replacement therapy after ischemic stroke. N Engl J Med. 2001;345:1243–9. doi: 10.1056/NEJMoa010534. [DOI] [PubMed] [Google Scholar]
  • 92.Rocca WA, Grossardt BR, Miller VM, Shuster LT, Brown RD., Jr Premature menopause or early menopause and risk of ischemic stroke. Menopause. 2012;19:272–7. doi: 10.1097/gme.0b013e31822a9937. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 93.Harman SM, Brinton EA, Cedars M, Lobo R, Manson JE, Merriam GR, et al. KEEPS: The Kronos Early Estrogen Prevention Study. Climacteric. 2005;8:3–12. doi: 10.1080/13697130500042417. [DOI] [PubMed] [Google Scholar]
  • 94.Harman SM, Black DM, Naftolin F, Brinton EA, Budoff MJ, Cedars MI, et al. Arterial imaging outcomes and cardiovascular risk factors in recently menopausal women: a randomized trial. Ann Intern Med. 2014;161:249–60. doi: 10.7326/M14-0353. [DOI] [PubMed] [Google Scholar]
  • 95.Li J, Siegel M, Yuan M, Zeng Z, Finnucan L, Persky R, et al. Estrogen enhances neurogenesis and behavioral recovery after stroke. J Cereb Blood Flow Metab. 2011;31:413–25. doi: 10.1038/jcbfm.2010.181. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 96.Meyer DM, Eastwood JA, Compton MP, Gylys K, Zivin JA. rLOAD: does sex mediate the effect of acute antiplatelet loading on stroke outcome. Biol Sex Differ. 2014;5:9. doi: 10.1186/2042-6410-5-9. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 97.Murphy SJ, Kirsch JR, Zhang W, Grafe MR, West GA, del Zoppo GJ, et al. Can gender differences be evaluated in a rhesus macaque (Macaca mulatta) model of focal cerebral ischemia? Comp Med. 2008;58:588–96. [PMC free article] [PubMed] [Google Scholar]
  • 98.Herson PS, Koerner IP, Hurn PD. Sex, sex steroids, and brain injury. Semin Reprod Med. 2009;27:229–39. doi: 10.1055/s-0029-1216276. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 99.Wong R, Bath PM, Kendall D, Gibson CL. Progesterone and cerebral ischaemia: the relevance of ageing. J Neuroendocrinol. 2013;25:1088–94. doi: 10.1111/jne.12042. [DOI] [PubMed] [Google Scholar]
  • 100.Gibson CL, Coomber B, Murphy SP. Progesterone is neuroprotective following cerebral ischaemia in reproductively ageing female mice. Brain. 2011;134:2125–33. doi: 10.1093/brain/awr132. [DOI] [PubMed] [Google Scholar]
  • 101.Murphy SJ, Littleton-Kearney MT, Hurn PD. Progesterone administration during reperfusion, but not preischemia alone, reduces injury in ovariectomized rats. J Cereb Blood Flow Metab. 2002;22:1181–8. doi: 10.1097/01.WCB.0000037990.07114.07. [DOI] [PubMed] [Google Scholar]
  • 102.Gibson CL, Murphy SP. Progesterone enhances functional recovery after middle cerebral artery occlusion in male mice. J Cereb Blood Flow Metab. 2004;24:805–13. doi: 10.1097/01.WCB.0000125365.83980.00. [DOI] [PubMed] [Google Scholar]
  • 103.Jiang N, Chopp M, Stein D, Feit H. Progesterone is neuroprotective after transient middle cerebral artery occlusion in male rats. Brain Res. 1996;735:101–7. doi: 10.1016/0006-8993(96)00605-1. [DOI] [PubMed] [Google Scholar]
  • 104.Sayeed I, Wali B, Stein DG. Progesterone inhibits ischemic brain injury in a rat model of permanent middle cerebral artery occlusion. Restor Neurol Neurosci. 2007;25:151–9. [PubMed] [Google Scholar]
  • 105.Wang J, Jiang C, Liu C, Li X, Chen N, Hao Y. Neuroprotective effects of progesterone following stroke in aged rats. Behav Brain Res. 2010;209:119–22. doi: 10.1016/j.bbr.2010.01.026. [DOI] [PubMed] [Google Scholar]
  • 106.Coomber B, Gibson CL. Sustained levels of progesterone prior to the onset of cerebral ischemia are not beneficial to female mice. Brain Res. 2010;1361:124–32. doi: 10.1016/j.brainres.2010.09.037. [DOI] [PubMed] [Google Scholar]
  • 107.Murphy SJ, Traystman RJ, Hurn PD, Duckles SP. Progesterone exacerbates striatal stroke injury in progesterone-deficient female animals. Stroke. 2000;31:1173–8. doi: 10.1161/01.str.31.5.1173. [DOI] [PubMed] [Google Scholar]
  • 108.Schumacher M, Denier C, Oudinet JP, Adams D, Guennoun R. Progesterone neuroprotection: The background of clinical trial failure. J Steroid Biochem Mol Biol. 2015 doi: 10.1016/j.jsbmb.2015.11.010. [DOI] [PubMed] [Google Scholar]
  • 109.Skolnick BE, Maas AI, Narayan RK, van der Hoop RG, MacAllister T, Ward JD, et al. A clinical trial of progesterone for severe traumatic brain injury. N Engl J Med. 2014;371:2467–76. doi: 10.1056/NEJMoa1411090. [DOI] [PubMed] [Google Scholar]
  • 110.Wright DW, Yeatts SD, Silbergleit R, Palesch YY, Hertzberg VS, Frankel M, et al. Very early administration of progesterone for acute traumatic brain injury. N Engl J Med. 2014;371:2457–66. doi: 10.1056/NEJMoa1404304. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 111.Sampei K, Mandir AS, Asano Y, Wong PC, Traystman RJ, Dawson VL, et al. Stroke outcome in double-mutant antioxidant transgenic mice. Stroke. 2000;31:2685–91. doi: 10.1161/01.str.31.11.2685. [DOI] [PubMed] [Google Scholar]
  • 112.Lang JT, McCullough LD. Pathways to ischemic neuronal cell death: are sex differences relevant? J Transl Med. 2008;6:33. doi: 10.1186/1479-5876-6-33. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 113.Renolleau S, Fau S, Charriaut-Marlangue C. Gender-related differences in apoptotic pathways after neonatal cerebral ischemia. Neuroscientist. 2008;14:46–52. doi: 10.1177/1073858407308889. [DOI] [PubMed] [Google Scholar]
  • 114.Turtzo LC, McCullough LD. Sex-specific responses to stroke. Future Neurol. 2010;5:47–59. doi: 10.2217/fnl.09.66. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 115.Hagberg H, Wilson MA, Matsushita H, Zhu C, Lange M, Gustavsson M, et al. PARP-1 gene disruption in mice preferentially protects males from perinatal brain injury. J Neurochem. 2004;90:1068–75. doi: 10.1111/j.1471-4159.2004.02547.x. [DOI] [PubMed] [Google Scholar]
  • 116.Liu F, Lang J, Li J, Benashski SE, Siegel M, Xu Y, et al. Sex differences in the response to poly(ADP-ribose) polymerase-1 deletion and caspase inhibition after stroke. Stroke. 2011;42:1090–6. doi: 10.1161/STROKEAHA.110.594861. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 117.McCullough LD, Zeng Z, Blizzard KK, Debchoudhury I, Hurn PD. Ischemic nitric oxide and poly (ADP-ribose) polymerase-1 in cerebral ischemia: male toxicity, female protection. J Cereb Blood Flow Metab. 2005;25:502–12. doi: 10.1038/sj.jcbfm.9600059. [DOI] [PubMed] [Google Scholar]
  • 118.Liu F, Li Z, Li J, Siegel C, Yuan R, McCullough LD. Sex differences in caspase activation after stroke. Stroke. 2009;40:1842–8. doi: 10.1161/STROKEAHA.108.538686. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 119.Renolleau S, Fau S, Goyenvalle C, Joly LM, Chauvier D, Jacotot E, et al. Specific caspase inhibitor Q-VD-OPh prevents neonatal stroke in P7 rat: a role for gender. J Neurochem. 2007;100:1062–71. doi: 10.1111/j.1471-4159.2006.04269.x. [DOI] [PubMed] [Google Scholar]
  • 120.Alano CC, Kauppinen TM, Valls AV, Swanson RA. Minocycline inhibits poly(ADP-ribose) polymerase-1 at nanomolar concentrations. Proc Natl Acad Sci U S A. 2006;103:9685–90. doi: 10.1073/pnas.0600554103. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 121.Lampl Y, Boaz M, Gilad R, Lorberboym M, Dabby R, Rapoport A, et al. Minocycline treatment in acute stroke: an open-label, evaluator-blinded study. Neurology. 2007;69:1404–10. doi: 10.1212/01.wnl.0000277487.04281.db. [DOI] [PubMed] [Google Scholar]
  • 122.Li J, McCullough LD. Sex differences in minocycline-induced neuroprotection after experimental stroke. J Cereb Blood Flow Metab. 2009;29:670–4. doi: 10.1038/jcbfm.2009.3. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 123.Amiri-Nikpour MR, Nazarbaghi S, Hamdi-Holasou M, Rezaei Y. An open-label evaluator-blinded clinical study of minocycline neuroprotection in ischemic stroke: gender-dependent effect. Acta Neurol Scand. 2015;131:45–50. doi: 10.1111/ane.12296. [DOI] [PubMed] [Google Scholar]
  • 124.Bona E, Hagberg H, Loberg EM, Bagenholm R, Thoresen M. Protective effects of moderate hypothermia after neonatal hypoxia-ischemia: short- and long-term outcome. Pediatr Res. 1998;43:738–45. doi: 10.1203/00006450-199806000-00005. [DOI] [PubMed] [Google Scholar]
  • 125.Wen TC, Rogido M, Peng H, Genetta T, Moore J, Sola A. Gender differences in long-term beneficial effects of erythropoietin given after neonatal stroke in postnatal day-7 rats. Neuroscience. 2006;139:803–11. doi: 10.1016/j.neuroscience.2006.02.057. [DOI] [PubMed] [Google Scholar]
  • 126.Broughton BR, Brait VH, Kim HA, Lee S, Chu HX, Gardiner-Mann CV, et al. Sex-dependent effects of G protein-coupled estrogen receptor activity on outcome after ischemic stroke. Stroke. 2014;45:835–41. doi: 10.1161/STROKEAHA.113.001499. [DOI] [PubMed] [Google Scholar]
  • 127.Jia J, Verma S, Nakayama S, Quillinan N, Grafe MR, Hurn PD, et al. Sex differences in neuroprotection provided by inhibition of TRPM2 channels following experimental stroke. J Cereb Blood Flow Metab. 2011;31:2160–8. doi: 10.1038/jcbfm.2011.77. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 128.Becker JB, Arnold AP, Berkley KJ, Blaustein JD, Eckel LA, Hampson E, et al. Strategies and methods for research on sex differences in brain and behavior. Endocrinology. 2005;146:1650–73. doi: 10.1210/en.2004-1142. [DOI] [PubMed] [Google Scholar]
  • 129.Goldman JM, Murr AS, Cooper RL. The rodent estrous cycle: characterization of vaginal cytology and its utility in toxicological studies. Birth Defects Res B Dev Reprod Toxicol. 2007;80:84–97. doi: 10.1002/bdrb.20106. [DOI] [PubMed] [Google Scholar]
  • 130.Ahnstedt H, Mostajeran M, Blixt FW, Warfvinge K, Ansar S, Krause DN, et al. U0126 attenuates cerebral vasoconstriction and improves long-term neurologic outcome after stroke in female rats. Journal of Cerebral Blood Flow and Metabolism. 2015;35:454–60. doi: 10.1038/jcbfm.2014.217. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 131.Quillinan N, Deng G, Grewal H, Herson PS. Androgens and stroke: good, bad or indifferent? Exp Neurol. 2014;259:10–5. doi: 10.1016/j.expneurol.2014.02.004. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 132.Van Kempen TA, Milner TA, Waters EM. Accelerated ovarian failure: a novel, chemically induced animal model of menopause. Brain Res. 2011;1379:176–87. doi: 10.1016/j.brainres.2010.12.064. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 133.Friedler B, Crapser J, McCullough L. One is the deadliest number: the detrimental effects of social isolation on cerebrovascular diseases and cognition. Acta Neuropathol. 2015;129:493–509. doi: 10.1007/s00401-014-1377-9. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 134.Clayton JA, Collins FS. Policy: NIH to balance sex in cell and animal studies. Nature. 2014;509:282–3. doi: 10.1038/509282a. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 135.Du L, Bayir H, Lai Y, Zhang X, Kochanek PM, Watkins SC, et al. Innate gender-based proclivity in response to cytotoxicity and programmed cell death pathway. J Biol Chem. 2004;279:38563–70. doi: 10.1074/jbc.M405461200. [DOI] [PubMed] [Google Scholar]
  • 136.Toung TJ, Chen TY, Littleton-Kearney MT, Hurn PD, Murphy SJ. Effects of combined estrogen and progesterone on brain infarction in reproductively senescent female rats. J Cereb Blood Flow Metab. 2004;24:1160–6. doi: 10.1097/01.WCB.0000135594.13576.D2. [DOI] [PubMed] [Google Scholar]
  • 137.Dang J, Mitkari B, Kipp M, Beyer C. Gonadal steroids prevent cell damage and stimulate behavioral recovery after transient middle cerebral artery occlusion in male and female rats. Brain Behav Immun. 2011;25:715–26. doi: 10.1016/j.bbi.2011.01.013. [DOI] [PubMed] [Google Scholar]

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