Ischemic core and penumbra in human stroke

Brain extracellular fluid protein changes in acute stroke patients

In vivo human brain extracellular fluids (ECF) of acute stroke patients were investigated to assess the changes in protein levels associated with ischemic damages. Microdialysates (MDs) from the infarct core (IC), the penumbra (P), and the unaffected contralateral (CT) brain regions of patients suffering an ischemic stroke (n = 6) were compared using a shotgun proteomic approach based on isobaric tagging and mass spectrometry. Quantitative analysis showed 53 proteins with increased amounts in the IC or P with respect to the CT samples. Glutathione S-transferase P (GSTP1), peroxiredoxin-1 (PRDX1), and protein S100-B (S100B) were further assessed with ELISA on the blood of unrelated control (n = 14) and stroke (n = 14) patients. Significant increases of 8- (p = 0.0002), 20- (p = 0.0001), and 11-fold (p = 0.0093) were found, respectively. This study highlights the value of ECF as an efficient source to further discover blood stroke markers.

Astroglial proteins as diagnostic markers of acute intracerebral hemorrhage-pathophysiological background and clinical findings

The time span from symptom onset to treatment initiation remains a critical variable determining the efficacy of thrombolysis in acute ischemic stroke. To date, performing a brain scan is indispensable prior to therapy in order to differentiate between patients with ischemic stroke and those with intracerebral hemorrhage (ICH). This causes substantial treatment delay, as thrombolysis cannot be applied prior to hospital admission at much earlier time points. Recently, brain-specific astroglial proteins (i.e., glial fibrillary acidic protein (GFAP), S100B) were identified to be released rapidly from the cytoplasm of destroyed cells in case of acute ICH. Elevated serum concentrations were found within the first 6 h after ICH onset. In contrast, in ischemic stroke, these proteins are released with delay, mirroring the more gradual occurrence of necrotic cell death and blood brain barrier disruption. S100B and GFAP may qualify as candidate serum biomarkers which are able to differentiate between ischemic stroke and ICH in the emergency phase of stroke. This minireview enlightens the pathophysiological background of this finding and provides an overview on currently available clinical data.

Prolyl hydroxylase domain inhibitors: a route to HIF activation and neuroprotection

Abstract Ischemic stroke is a major cause of death worldwide, and current therapeutic options are very limited. Preconditioning with an ischemic or hypoxic insult is beneficial in experimental models of ischemic stroke. Ischemia/hypoxia results in activation of numerous transcription factors, including hypoxia inducible factor (HIF), which is a master regulator of oxygen homeostasis. HIF activation induces a diverse range of target genes, encompassing a wide variety of cellular processes; including angiogenesis, energy metabolism, cell survival, radical production/scavenging, iron metabolism, stem cell homing, and differentiation. Inhibition of HIF prolyl hydroxylase domain (PHD) enzymes results in activation of HIF and is likely to mimic, at least in part, the effects of hypoxia preconditioning. A caveat is that not all consequences of HIF activation will be beneficial and some could even be deleterious. Nevertheless, PHD inhibitors may be therapeutically useful in the treatment of stroke. Prototype PHD inhibitors have shown promising results in preclinical models.

Penumbra, the basis of neuroimaging in acute stroke treatment: current evidence

In modern medicine brain imaging is an essential prerequisite not only to acute stroke triage but also to determining the specific therapy indicated. This article reviews the need for imaging the brain in acute stroke, penumbral pathophysiology, penumbral imaging techniques, as well as current status of various imaging modalities that are being employed to select patients for specific therapeutic approaches.

Intracranial vertebrobasilar artery dissection associated with postpartum angiopathy

Background. Cervicocephalic arterial dissection (CCAD) is rare in the postpartum period. To our knowledge this is the first reported case of postpartum angiopathy (PPA) presenting with ischemic stroke due to intracranial arterial dissection. Case. A 41-year-old woman presented with blurred vision, headache, and generalized seizures 5 days after delivering twins. She was treated with magnesium for eclampsia. MRI identified multiple posterior circulation infarcts. Angiography identified a complex dissection extending from both intradural vertebral arteries, through the basilar artery, and into both posterior cerebral arteries. Multiple segments of arterial dilatation and narrowing consistent with PPA were present. Xenon enhanced CT (Xe-CT) showed reduced regional cerebral blood flow that is improved with elevation in blood pressure. Conclusion. Intracranial vertebrobasilar dissection causing stroke is a rare complication of pregnancy. Eclampsia and PPA may play a role in its pathogenesis. Blood pressure management may be tailored using quantitative blood flow studies, such as Xe-CT.

Hyperemia beneath evacuated acute subdural hematoma is frequent and prolonged in patients with an unfavorable outcome: a xe-computed tomographic study

OBJECTIVE

To verify the values and the time course of regional cerebral blood flow (rCBF) in the cortex located beneath an evacuated acute subdural hematoma (SDH) and their relationship with neurological outcome.

METHODS

rCBF levels were measured in multiple regions of interest, by means of a Xe-computed tomographic technique, in the cortex underlying an evacuated SDH and contralaterally in 20 patients with moderate or severe traumatic brain injury and an evacuated acute SDH. Twenty-three patients with moderate or severe traumatic brain injury and an evacuated extradural hematoma or diffuse injury served as the control group. Outcome was evaluated by means of the Glasgow Outcome Scale at 12 months.

RESULTS

Values for the maximum (rCBFmax) and the mean of all rCBF levels in the cortex beneath the evacuated SDH were more frequently consistent with hyperemia. The side-to-side differences in the mean of all rCBF and rCBFmax levels between lesioned and nonlesioned hemispheres were greater in patients with evacuated SDH than in controls (P = 0.0013 and P = 0.0018, respectively). The side-to-side difference in the maximum rCBF value was higher in SDH patients with unfavorable outcomes than in controls at 24 to 96 hours and at 4 to 7 days and higher than in patients with favorable outcomes at 4 to 7 days. The widest side-to-side difference in rCBFmax value was more elevated in patients with an evacuated SDH with unfavorable outcome than in patients with a favorable outcome (P = 0.047), whereas no differences were found in controls. The SDH thickness and the associated midline shift were greater in patients with unfavorable outcomes than in those with favorable outcomes.

CONCLUSION

On average, hyperemic long-lasting rCBF values frequently occur in the cortex located beneath an evacuated SDH and seem to be associated with unfavorable outcome.

Transplanted human embryonic neural stem cells survive, migrate, differentiate and increase endogenous nestin expression in adult rat cortical peri-infarction zone

Transplantation of stem cells is a potential therapeutic strategy for stroke damage. The survival, migration, and differentiation of transplanted human embryonic neural stem cells in the acute post-ischemic environment were characterized and endogenous nestin expression after transplantation was investigated. Human embryonic neural stem cells obtained from the temporal lobe cortex were cultured and labeled with fluorescent 1,1'-dioctadecy-6,6'-di (4-sulfopheyl)-3,3,3',3'-tetramethylindocarbocyanin (DiI) in vitro. Labeled cells were transplanted into cortical peri-infarction zones of adult rats 24 h after permanent middle cerebral artery occlusion. Survival, migration, and differentiation of grafted cells were quantified in immunofluorescence-stained sections from rats sacrificed at 7, 14, and 28 days after transplantation. Endogenous nestin-positive cells in the cortical peri-infarction zone were counted at serial time points. The cells transplanted into the cortical peri-infarction zone displayed the morphology of living cells and became widely located around the ischemic area. Moreover, some of the transplanted cells expressed nestin, GFAP, or NeuN in the peri-infarction zone. Furthermore, compared with the control group, endogenous nestin-positive cells in the peri-infarction zone had increased significantly 7 days after cell transplantation. These results confirm the survival, migration, and differentiation of transplanted cells in the acute post-ischemic environment and enhanced endogenous nestin expression within a brief time window. These findings indicate that transplantation of neural stem cells into the peri-infarction zone may be performed as early as 24 h after ischemia.

Two region-dependent pathways of eosinophilic neuronal death after transient cerebral ischemia

Various types of eosinophilic neurons (ENs) are found in the post-ischemic brain. We examined the temporal profile of ENs in the core and peripheral regions of the ischemic cortex, and analyzed the relationship to the expression of various cell death-related factors. Unilateral forebrain ischemia was induced in Mongolian gerbils by transient common carotid artery occlusions, and the brains from 3 h to 2 weeks post-ischemia were prepared for morphometric and immunohistochemical analysis of ENs. ENs with minimally abnormal nuclei and swollen cell bodies appeared at 3 h in the ischemic core and at 12 h in the periphery. In both locations multiple cell death-related factors including calcium, micro-calpain, cathepsin D, 78 kDa glucose-regulated protein (GRP78) and ubiquitin were activated. In the ischemic core, pyknosis and irregularly atrophic cytoplasm peaked at 12 h, which was associated with significant increases in staining for calcium and micro-calpain. ENs with pyknosis and scant cytoplasm peaked at 4 days and were positive for TUNEL and calcium staining. In the ischemic periphery, ENs had slightly atrophic cytoplasm and sequentially developed pyknosis, karyorrhexis and karyolysis over 1 week. These cells were positive for TUNEL and calcium staining. All types of EN were negative for caspase 3. There may be two region-dependent pathways of EN changes in the post-ischemic brain: pyknosis with cytoplasmic shrinkage in the core, and nuclear disintegration with slightly atrophic cytoplasm in the periphery. This difference coordinates different activation patterns of cell death-related factors in ENs.

Hyperbaric oxygen preconditioning induces tolerance against brain ischemia-reperfusion injury by upregulation of antioxidant enzymes in rats

The present study examined the hypothesis that cerebral ischemic tolerance induced by hyperbaric oxygen preconditioning (HBO-PC) is associated with an increase of antioxidant enzyme activity. Male Sprague-Dawley rats (250-280 g, n=74) were divided into sham, middle cerebral artery occlusion (MCAO) for 90 min, and MCAO plus HBO-PC groups. HBO-PC was conducted four times by given 100% oxygen at 2.5 atmosphere absolute (ATA), for 1 h at every 12 h interval for 2 days. At 24 h after the last HBO-PC, MCAO was performed and at 24 h after MCAO, neurological function and Nissl Staining were performed to evaluate the effect of HBO-PC. Malondialdehyde (MDA) content, activity of catalase (CAT), superoxide dismutase (SOD) and glutathione peroxidase (GSH-px) sampled from the hippocampus, ischemic penumbra or core of cortex were measured. HBO-PC decreased mortality rate, improved neurological recovery, lessened neuronal injury, reduced the level of MDA and increased the antioxidant activity of CAT and SOD. These observations demonstrated that an upregulation of the antioxidant enzyme activity by HBO preconditioning plays an important role in the generation of tolerance against brain ischemia-reperfusion injury.

Evidence for neuroprotective properties of human umbilical cord blood cells after neuronal hypoxia in vitro

Background

One of the most promising options for treatment of stroke using adult stem cells are human umbilical cord blood (HUCB) cells that were already approved for therapeutic efficacy in vivo. However, complexity of animal models has thus far limited the understanding of beneficial cellular mechanisms. To address the influence of HUCB cells on neuronal tissue after stroke we established and employed a human in vitro model of neuronal hypoxia using fully differentiated vulnerable SH-SY5Y cells. These cells were incubated under an oxygen-reduced atmosphere (O₂< 1%) for 48 hours. Subsequently, HUCB mononuclear cells (MNC) were added to post-hypoxic neuronal cultures. These cultures were characterized regarding to the development of apoptosis and necrosis over three days. Based on this we investigated the therapeutic influence of HUCB MNC on the progression of apoptotic cell death. The impact of HUCB cells and hypoxia on secretion of neuroprotective and inflammatory cytokines, chemokines and expression of adhesion molecules was proved.

Results

Hypoxic cultivation of neurons initially induced a rate of 26% ± 13% of apoptosis. Hypoxia also caused an enhanced expression of Caspase-3 and cleaved poly(ADP-ribose) polymerase (PARP). Necrosis was only detected in low amounts. Within the next three days rate of apoptosis in untreated hypoxic cultures cumulated to 85% ± 11% (p ≤ 0.001). Specific cytokine (VEGF) patterns also suggest anti-apoptotic strategies of neuronal cells. Remarkably, the administration of MNC showed a noticeable reduction of apoptosis rates to levels of normoxic control cultures (7% ± 3%; p ≤ 0.001). In parallel, clustering of administered MNC next to axons and somata of neuronal cells was observed. Furthermore, MNC caused a pronounced increase of chemokines (CCL5; CCL3 and CXCL10).

Conclusion

We established an in vitro model of neuronal hypoxia that affords the possibility to investigate both, apoptotic neuronal cell death and neuroprotective therapies. Here we employed the therapeutic model to study neuroprotective properties of HUCB cells.

We hypothesize that the neuroprotective effect of MNC was due to anti-apoptotic mechanisms related to direct cell-cell contacts with injured neuronal cells and distinct changes in neuroprotective, inflammatory cytokines as well as to the upregulation of chemokines within the co-cultures.

Brain perfusion CT: principles, technique and clinical applications

The imaging of brain haemodynamics and its applications are generating growing interest. By providing quantitative measurements of cerebral blood flow (CBF) and cerebral blood volume (CBV), dynamic perfusion computed tomography (p-CT) allows visualisation of cerebral autoregulation mechanisms and represents a fast, available and reliable imaging option for assessing cerebral perfusion. Thanks to its feasibility in emergency settings, p-CT is considered most useful, in combination with CT angiography, in acute ischaemic patients, as it is able to provide a fast and noninvasive assessment of cerebral perfusion impairment. In addition, p-CT can play a diagnostic role in other types of cerebrovascular disease to assess functional reserve, and in intracranial neoplasms, where it has a role in diagnosis, grading, biopsy guidance, and follow-up during treatment. This article illustrates the principles, technique and clinical applications of p-CT cerebral perfusion studies.

The Management of Blood Pressure After Stroke

BACKGROUND

Control of hypertension is a well-established goal of primary prevention of stroke, but management of blood pressure in patients with a previous stroke or in the setting of acute stroke is complicated by the effect blood pressure changes may have on cerebral perfusion.

REVIEW SUMMARY

For patients with previous transient ischemic attack or chronic stroke, blood pressure reduction appears to be a safe and important facet of the secondary prevention of recurrent stroke. Less information is available concerning blood pressure management in acute stroke. Current protocols require strict blood pressure control in patients who are treated with thrombolytic therapy, to reduce the risk of hemorrhagic complications. In patients presenting with acute intracerebral hemorrhage, blood pressure reduction does not appear to cause significant reduction of cerebral blood flow, but at this time there are no studies to determine if there is a clinical benefit of acute blood pressure reduction in these patients. Finally, blood pressure reduction is not routinely recommended in patients with acute ischemic stroke, as it may precipitate further cerebral ischemia. Preliminary studies suggest, in fact, that there may be a role in the future for blood pressure elevation in the treatment of patients with acute ischemic stroke.

CONCLUSIONS

Current data support the use of blood pressure reduction in the secondary prevention of stroke in patients with cerebrovascular disease. In the setting of acute stroke, however, data are limited and blood pressure management must be tailored to the specific clinical situation.

NEUROPROTECTIVE EFFECTS OF DAURICINE AGAINST APOPTOSIS INDUCED BY TRANSIENT FOCAL CEREBRAL ISCHAEMIA IN RATS VIA A MITOCHONDRIAL PATHWAY

1. Previous experimental studies have shown that dauricine can protect the brain against ischaemic damage, but the underlying mechanisms remain unknown. In the present study, we examined whether dauricine inhibits neuronal apoptosis in the penumbra in a rat model of transient focal cerebral ischaemia. 2. Male Wistar rats underwent a 90 min temporary occlusion of the middle cerebral artery. Dauricine (21, 42 and 84 mg/kg) was administered by intragastric gavage twice a day for 3 days before ischaemia. Rats were killed and brain samples were collected 24 h after ischaemia. Histopathological outcome was evaluated by haematoxylin-eosin staining. Apoptotic changes were evaluated by terminal deoxyribonucleotidyl transferase-mediated dUTP-digoxigenin nick end-labelling (TUNEL) for DNA fragmentation. The mitochondrial pathway was explored using immunohistochemistry for cytochrome c release, caspase 9 and caspase 3 activation, as well as by reverse transcription-polymerase chain reaction for determination of caspase 9 and caspase 3 mRNA expression. 3. Cytochrome c release, activation of caspase 9 and caspase 3 and DNA fragmentation were detected 24 h after ischaemia. Dauricine (42 and 84 mg/kg) pretreatment improved histopathological recovery, diminished DNA fragmentation and reduced cytochrome c release and activation of caspase 9 and caspase 3 in the penumbra at 24 h. 4. These findings suggest that dauricine attenuates apoptosis in the penumbra after transient focal cerebral ischaemia. The infarct-reducing effects of dauricine may be due, in part, to the inhibition of apoptotic cell death via a mitochondrial pathway in the penumbra.

Regional cerebral blood flow levels as measured by xenon-CT in vascular territorial low-density areas after subarachnoid hemorrhage are not always ischemic

INTRODUCTION

The aim of this study was to assess regional cerebral blood flow (rCBV) in areas of CT hypoattenuation appearing in the postoperative period in patients treated for aneurysmal subarachnoid hemorrhage (SAH) using xenon-enhanced CT scanning (Xe-CT).

METHODS

We analyzed 15 patients (5 male and 10 female; mean age 49.7+/-12.1 years) with SAH on CT performed on admission to hospital and who showed a low-density area within a well-defined vascular territory on CT scans after clipping or coiling of a saccular aneurysm. All zones of hypoattenuation were larger than 1 cm(2) and showed signs of a mass effect suggesting a subacute phase of evolution. Two aneurysms were detected in two patients. Aneurysms were located in the middle cerebral artery (n=7), in the anterior communicating artery (n=6), in the internal carotid artery (n=3), and in the posterior communicating artery (n=1). Treatments were surgical (n=8), endovascular (n=2) or both (n=1). A total of 36 Xe-CT studies were performed and rCBF values were measured in two different regions of interest (ROI): the low-density area, and an area of normal-appearing brain tissue located symmetrically in the contralateral hemisphere.

RESULTS

rCBF levels were significantly lower in the low-density area than in the contralateral normal-appearing area (P<0.01). In the low-density areas, irreversible ischemia (CBF <10 ml/100 g per minute) was present in 11/36 lesions (30.6%), ischemic penumbra (CBF 10-20 ml/100 g per minute) and oligemia (CBF 20-34 ml/100 g per minute) in 8/36 lesions (22.2%), relative hyperemia (CBF 34-55 ml/100 g per minute) in 7/36 lesions (19.4%), and absolute hyperemia (CBF >55 ml/100 g per minute) in 2/36 lesions (5.6%).

CONCLUSION

Our study confirmed that rCBF is reduced in new low-density lesions related to specific vascular territories. However, only about one-third of the lesions showed rCBF levels consistent with irreversible ischemia and in a relatively high proportion of lesions, rCBF levels indicated penumbral, oligemic and hyperemic areas.

Xenon-CT and transcranial Doppler in poor-grade or complicated aneurysmatic subarachnoid hemorrhage patients undergoing aggressive management of intracranial hypertension

OBJECTIVE

To evaluate whether elevated flow velocimetry values are associated with critically reduced cerebral blood flow values in deeply sedated patients with acute aneurysmatic subarachnoid hemorrhage and in whom the detection of clinical vasospasm is not feasible.

DESIGN

Retrospective analysis of prospectively collected data.

SETTING

Neurosurgical and trauma patients in an intensive care unit in a regional hospital.

PATIENTS AND PARTICIPANTS

Twenty-nine patients in the acute phase following subarachnoid hemorrhage who were sedated and ventilated for elevated intracranial pressure, transcranial Doppler vasospasm, or respiratory failure and were studied with at least a coupled xenon-CT/transcranial Doppler study.

MEASUREMENTS AND RESULTS

Combined measurement and comparison of cerebral blood flow by means of xenon-CT and of mean velocity by means of transcranial Doppler in middle cerebral artery territories. The case mix studied was consistent with patients' predominantly poor grade and with a complicated course. The results suggest that in sedated patients flow velocity and measured cortical mixed cerebral blood flow are not correlated, and, more specifically, that flow velocities values above 120 or 160 cm/s and Lindegaard index above 3 are not associated with an ischemic regional cerebral blood flow. Conversely, as many as 55% of the xenon-CT studies were associated with hyperemia.

CONCLUSIONS

In patients with elevated intracranial pressure, mean middle cerebral artery flow velocity or Lindegaard Index does not help to detect critical cerebral blood flow nor elevated cerebral blood flow.

Threshold of regional cerebral blood flow for infarction in patients with acute cerebral ischemia

Threshold of regional cerebral blood flow (rCBF) for cerebral tissue survival in relation to time was studied in patients with acute cerebral ischemia with xenon-enhanced computed tomography (XeCT). Case 1: A 58-year-old man with right hemiparesis, total aphasia and a high intensity area of 1 cm 2 in the left insula on diffusion weighted image underwent XeCT CBF study before and after intra-arterial local thrombolytic therapy (IALT) on the occluded middle cerebral artery (MCA) 4 hours and 7 hours after stroke onset, respectively. Case 2: A 65-year-old woman with recurrent transient ischemic attacks (TIAs) caused by severe stenosis of the left MCA underwent XeCT CBF study 5 hours after onset of the last attack. XeCT was conducted by 5-min wash-in method. In Case 1 the rCBF in the pre-IALT MCA territory was 4 to 19 ml/100 g/min. The area where rCBF in the post-IALT increased to above 15 ml/100g/min were saved, but the other area where it remained in the 9 to 14 ml/100 g/min evolved into infarct on subsequent CT scan/MR (magnetic resonance) imaging. The patient was discharged with only mild motor dysphasia. In Case 2 the left corona radiata showed rCBF of 7 ml/100 g/min and this area evolved into infarct on MR imaging. The patient was discharged home with right hemiparesis. Our results showed validity of the rCBF threshold in acute cerebral ischemia reported by Jones et al. Residual rCBF in the acute stage of cerebral ischemic stroke can predict the fate of the lesion.

Transition of areas of eosinophilic neurons and reactive astrocytes to delayed cortical infarcts after transient unilateral forebrain ischemia in Mongolian gerbils

The fate of postischemic tissues containing eosinophilic neurons (ENs), whether they remain viable or evolve into infarction, is largely unknown. We analyzed the time profile and distribution of ENs, reactive astrocytes (RAs), and infarction after transient cerebral ischemia. Unilateral forebrain ischemia was induced in Mongolian gerbils by two 10-min unilateral common carotid artery occlusions with a 5-h interval, and the brains at 24 h, 4 days, and 2, 4, and 16 weeks were prepared for morphometric analysis. Intra-ischemic laser Doppler flowmetry revealed significant ischemia, deeper in the anterior cortex, during carotid occlusion. Here, ENs appeared in the middle and deep layers at 24 h postischemia, and EN areas had extended to all cortical layers by 4 days. Large areas of high EN density turned into infarcts between 4 days and 4 weeks. In the posterior cortex, middle and deep cortical layers evolved low EN density areas without subsequent transformation into infarcts. RAs were consistently observed in areas with ENs, and RA areas with high EN density were largely transformed into infarcts between 4 days and 4 weeks postischemia. Areas of high, but not low, EN density were slowly transformed into infarcts after transient cerebral ischemia. Delayed astrocytic death took place in the RA areas with high EN density. In conclusion, density of ENs is an important indicator of delayed astrocytic death and infarction in postischemic tissue.

Diagnostic impact of the spectrum of ischemic cerebral blood flow thresholds in sedated subarachnoid hemorrhage patients

OBJECTIVE

Ischemia is the main cause of secondary damage in subarachnoid hemorrhage (SAH). Cerebral blood flow (CBF) measurement is useful to detect critical values. We analyzed the diagnostic impact of CBF ischemic thresholds to predict a new low attenuation area on computed tomography (CT) due to failure of large vessel perfusion.

METHODS

We analyzed 48 xenon CT (Xe-CT) studies from 10 patients with SAH. CBF measurements were obtained by means of Xe-CT and cortical regions of interest (ROls). The ROIs which appeared in a hypoattenuation area were recorded. Cortical CBF was tested for specificity and sensitivity as a predictor of hypoattenuation by means of a receiver operating characteristic curve.

RESULTS

Mean age was 58 (SD +/- 12.4) years. The median Fisher score and Hunt and Hess scale were 2 and 3, respectively. The area under the receiver operating characteristic curve was 0.912 (CI 0.896 to 0.926). The cut-off value for best accuracy was 6 mL/ 100 g/min, with a likelihood ratio of 37.

CONCLUSION

The present study suggests a threshold of 6 mL/100 g/ min as a predictor of a new low attenuation area. However, each clinician should choose the most useful threshold according to pre-test probability and the cost/effectiveness ratio of the applied therapies.

Reperfusion of low attenuation areas complicating subarachnoid hemorrhage

Hypoattenuation areas shown on brain CT scans after subarachnoid hemorrhage (SAH) are believed to be associated with persistent ischemia. The aim of this study was to evaluate regional cerebral blood flow (rCBF) in hypoattenuation areas and its evolution over time by means of Xenon CT (Xe-CT). We enrolled 16 patients with SAH who developed a hypoattenuation area in the middle cerebral artery territory. Patients were studied at time zero (the first Xe-CT), within 24 to 96 hours, and 96 hours after the initial Xe-CT. We analyzed 19 hypoattenuation areas caused by vascular distortion, vasospasm, or post-surgical embolization in 48 Xe-CT studies. Areas of hypoattenuation were divided in 2 groups according to initial rCBF. In the first group (n = 15), rCBF was initially above 6 mL/100 gr/min but only 2 were still ischemic (rCBF < 18 mL/ 100 gr/min) 96 hours after the first Xe-CT, while 7 (58%) were hyperemic. Conversely, in the second group with severe ischemia (rCBF < 6 mL/100 gr/min; n = 4) mean rCBF increased (p = 0.08) but still remained below the ischemic threshold. In severely ischemic lesions, rCBF reperfusion occurs but is probably marginally relevant. Conversely, in lesions not initially severely ischemic, residual CBF gradually improved and frequently became hyperemic. The functional recovery of these zones remains to be evaluated.

Dynamics of cytochrome c oxidase activity in acute ischemic stroke

We have investigated the dynamics of cytochrome c oxidase (COX) activity in the cerebrospinal fluid (CSF) and the erythrocyte haemolysate (EH) in 85 patients suffering from brain infarction (BI), reversible (RIA), or transient (TIA) ischemic attack from the perspective of mitochondrial affection in ischemia. In all patients, the COX activity was decreased in the CSF, especially within the first two days, indicating an acute inactivation or modification of mitochondrial proteins, probably mediated by free radicals. The gradual elevation of COX activity until the seventh day suggested that these changes may be reversible. The increase in the COX activity was established in the EH, with the highest values found in the BI, somewhat lower in the RIA, and the lowest in the TIA group, respectively. This could indicate a systemic compensatory response to an acute ischemia. Thus, COX activity in the CSF and EH in acute ischemia could be an indicator of brain metabolic dysfunction.

Cerebral blood flow in mean cerebral artery low density areas is not always ischemic in patients with aneurysmal subarachnoid hemorrhage--relationship with neurological outcome

Aneurysmal subarachnoid hemorrhage (SAH) can be complicated by reduction of regional cerebral blood flow (rCBF) from large conductance vessels leading to focal edema appearing as an area of hypoattenuation on CT. In this study we included 29 patients with SAH due to aneurysmal rupture, having 36 CT low density areas within the middle cerebral artery territory in whom a total of 56 Xenon-CT (Xe-CT) studies were performed. Collectively, we evaluated 70 hypoattenuated areas. rCBF levels were measured in two different regions of interest drawn manually on the CT scan, one in the low density area and the other in a corresponding contralateral area of normal-appearing brain tissue. In the low density area (22.6 +/- 22.7 ml/100 gr/min) rCBF levels were significantly lower than in the contralateral area (32.8 +/- 17.1 7 ml/100 gr/min) (p = 0.0007). In the injured areas deep ischemia (CBF < 6 ml/ 100 g/min) was present in only 25.7% of Xe-CT studies, suggesting that hypodense areas are not always ischemic, whereas in 43.7% of the lesions/Xe-CT studies we found hyperemic values. Patients with a better outcome had hyperemic lesions, suggesting brain tissue recovery in injured areas.

Method for improving the accuracy of quantitative cerebral perfusion imaging

PURPOSE

To improve the accuracy of dynamic susceptibility contrast (DSC) measurements of cerebral blood flow (CBF) and volume (CBV).

MATERIALS AND METHODS

In eight volunteers, steady-state CBV (CBV(SS)) was measured using TrueFISP readout of inversion recovery (IR) before and after injection of a bolus of contrast. A standard DSC (STD) perfusion measurement was performed by echo-planar imaging (EPI) during passage of the bolus and subsequently used to calculate the CBF (CBF(DSC)) and CBV (CBV(DSC)). The ratio of CBV(SS) to CBV(DSC) was used to calibrate measurements of CBV and CBF on a subject-by-subject basis.

RESULTS

Agreement of values of CBV (1.77 +/- 0.27 mL/100 g in white matter (WM), 3.65 +/- 1.04 mL/100 g in gray matter (GM)), and CBF (23.6 +/- 2.4 mL/(100 g min) in WM, 57.3 +/- 18.2 mL/(100 g min) in GM) with published gold-standard values shows improvement after calibration. An F-test comparison of the coefficients of variation of the CBV and CBF showed a significant reduction, with calibration, of the variability of CBV in WM (P < 0.001) and GM (P < 0.03), and of CBF in WM (P < 0.0001).

CONCLUSION

The addition of a CBV(SS) measurement to an STD measurement of cerebral perfusion improves the accuracy of CBV and CBF measurements. The method may prove useful for assessing patients suffering from acute stroke.

Cerebral perfusion imaging in acute stroke

Herein, the author discusses four perfusion technologies, including the diffusible tracer methods-xenon-enhanced computed tomography (CT) and single photon emission CT with various radioisotopes-and the nondiffusible tracer techniques-CT perfusion and magnetic resonance (MR) perfusion and diffusion. The methods for and important issues in the performance of each technique are presented, along with the accuracy of the data acquired with each technique, as demonstrated with experimental studies. In addition, the use of each technique in the evaluation of patients with acute stroke and their relative advantages and disadvantages are presented.

Cerebral perfusion CT: technique and clinical applications

Perfusion computed tomography (CT) is a relatively new technique that allows rapid qualitative and quantitative evaluation of cerebral perfusion by generating maps of cerebral blood flow (CBF), cerebral blood volume (CBV), and mean transit time (MTT). The technique is based on the central volume principle (CBF = CBV/MTT) and requires the use of commercially available software employing complex deconvolution algorithms to produce the perfusion maps. Some controversies exist regarding this technique, including which artery to use as input vessel, the accuracy of quantitative results, and the reproducibility of results. Despite these controversies, perfusion CT has been found to be useful for noninvasive diagnosis of cerebral ischemia and infarction and for evaluation of vasospasm after subarachnoid hemorrhage. Perfusion CT has also been used for assessment of cerebrovascular reserve by using acetazolamide challenge in patients with intracranial vascular stenoses who are potential candidates for bypass surgery or neuroendovascular treatment, for the evaluation of patients undergoing temporary balloon occlusion to assess collateral flow and cerebrovascular reserve, and for the assessment of microvascular permeability in patients with intracranial neoplasms. This article is a review of the technique, clinical applications, and controversies surrounding perfusion CT.

Effects of reperfusion on ADC and CBF pixel-by-pixel dynamics in stroke: characterizing tissue fates using quantitative diffusion and perfusion imaging

The effects of reperfusion on the spatiotemporal dynamics of transient (60 minutes) focal ischemic brain injury in rats were evaluated on a pixel-by-pixel basis using quantitative cerebral blood flow (CBF) and apparent diffusion coefficient (ADC) measurements every 30 minutes for 3 hours and compared to post-mortem histology at 24 hours. Four biologically relevant clusters were classified based on ADC (0.53 +/- 0.02 x 10mm/s, SD) and CBF (0.30 +/- 0.09 ml/g/min) viability thresholds, namely: (1) the "normal" cluster with ADC and CBF > thresholds; (2) the "mismatch" cluster with ADC > threshold but CBF < threshold; (3) the "core" cluster with ADC and CBF < thresholds; and (4) "non-nourishing reperfusion zone" where ADC < threshold but CBF > threshold. The spatio-temporal progression of tissue volumes, ADC and CBF of each cluster were evaluated. Pixels of each cluster on the CBF-ADC space were mapped onto the image space. Following reperfusion, 28% of the "core" pixels and 90% of the "mismatch" (defined at 60 minutes) pixels were salvaged at 180 minutes, which correlated with histology. The ADC and CBF of subsequently salvaged tissues were significantly higher than those became infarcted. Salvaging "core" pixels indicated that reduced ADC was not synonymous with irreversible injury; duration of exposure and severity of reduced ADC and CBF were likely critical. Projection profiles showed a bimodal ADC, but uni-modal CBF, distributions. The ADC bimodal minima, obtained without histological correlation, were similar to the histology-derived ADC and CBF viability thresholds, and could have potential clinical applications. This study demonstrated a simple but powerful approach to evaluate, on a pixel-by-pixel basis, the spatio-temporal evolution of ischemic brain injury, and a potential for statistical prediction of tissue fate.

Comparison of caffeine-induced changes in cerebral blood flow and middle cerebral artery blood velocity shows that caffeine reduces middle cerebral artery diameter

Changes in cerebral blood flow (CBF) can be assessed directly with xenon clearance (XeC) or indirectly by measuring changes in middle cerebral artery blood velocity (Vmca) with transcranial Doppler (TCD). The aim of this study was to compare the changes in CBF and Vmca following caffeine ingestion. Nineteen patients (age 48-86, recovering from an acute stroke) and ten controls (age 52-85) were each studied twice. Bilateral measurements of CBF and Vmca were made before and after ingestion of 250 mg caffeine or matched placebo. The percentage change in CBF and Vmca after caffeine was calculated. Full results (CBF and Vmca) were obtained from 14 patients and 9 controls. There was no significant difference between patients and controls, so results were combined. Caffeine reduced CBF by 22% (95% confidence interval (CI) = 17% to 28%) and reduced Vmca by 13% (95% CI = 10% to 17%). The fall in Vmca was significantly less than that in CBF (p = 0.0016), showing that caffeine reduces mca diameter. Analysis based on Poiseuille flow in the arterioles suggests that caffeine reduced arteriole diameter by 5.9% (95% CI = 4.6% to 7.3%) and mca diameter by 4.3% (95% CI = 2.0% to 6.6%). TCD is being used as an alternative to XeC for assessing the effect of vasoconstrictors and vasodilators on CBF. This study has demonstrated that in mca diameter can be changed by the vasoactive agents, and that changes in Vmca do not necessarily reflect changes in CBF.

Lidocaine attenuates apoptosis in the ischemic penumbra and reduces infarct size after transient focal cerebral ischemia in rats

Lidocaine is a local anesthetic and antiarrhythmic agent. Although clinical and experimental studies have shown that an antiarrhythmic dose of lidocaine can protect the brain from ischemic damage, the underlying mechanisms are unknown. In the present study, we examined whether lidocaine inhibits neuronal apoptosis in the penumbra in a rat model of transient focal cerebral ischemia. Male Wistar rats underwent a 90-min temporary occlusion of middle cerebral artery. Lidocaine was given as an i.v. bolus (1.5 mg/kg) followed by an i.v. infusion (2 mg/kg/h) for 180 min, starting 30 min before ischemia. Rats were killed and brain samples were collected at 4 and 24 h after ischemia. Apoptotic changes were evaluated by immunohistochemistry for cytochrome c release and caspase-3 activation and terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) for DNA fragmentation. Cytochrome c release and caspase-3 activation were detected at 4 and 24 h after ischemia and DNA fragmentation was detected at 24 h. Double-labeling with NeuN, a neuronal marker, demonstrated that cytochrome c, caspase-3, and TUNEL were confined to neurons. Lidocaine reduced cytochrome c release and caspase-3 activation in the penumbra at 4 h and diminished DNA fragmentation in the penumbra at 24 h. Lidocaine treatment improved early electrophysiological recovery and reduced the size of the cortical infarct at 24 h, but had no significant effect on cerebral blood flow in either the penumbra or core during ischemia. These findings suggest that lidocaine attenuates apoptosis in the penumbra after transient focal cerebral ischemia. The infarct-reducing effects of lidocaine may be due, in part, to the inhibition of apoptotic cell death in the penumbra.

Perfusion-weighted magnetic resonance imaging of the brain: techniques and application in children

Perfusion-weighted magnetic resonance imaging (PWI) has been proposed as an attractive non-invasive tool for evaluating cerebral haemodynamics. Quantitative maps of cerebral blood flow (CBF), cerebral blood volume (CBV), mean transit time (MTT), time to peak (TTP) and various other haemodynamic parameters can be obtained. Recent advances in hard- and software made PWI available for clinical routine. Although PWI became common in adult neuroradiology, it remains challenging in pediatric neuroradiology. In this article, the different PWI techniques that render haemodynamic maps of the brain are presented and discussed. The normal developmental changes of the cerebral haemodynamics in children as measured by PWI are presented as well as the application of PWI in cerebral ischaemia, primary and secondary cerebral vasculopathies and in cerebral tumours.

Pixel-by-pixel spatiotemporal progression of focal ischemia derived using quantitative perfusion and diffusion imaging

Pixel-by-pixel spatiotemporal progression of focal ischemia (permanent occlusion) in rats was investigated using quantitative perfusion and diffusion magnetic resonance imaging every 30 minutes for 3 hours. The normal left-hemisphere apparent diffusion coefficient (ADC) was 0.76 +/- 0.03 x 10(-3) mm(2)/s and CBF was 0.7 +/- 0.3 mL x g(-1) x min(-1) (mean +/- SD, n=5). The ADC and CBF viability thresholds yielding the lesion volumes (LV) at 3 hours that best approximated the 2,3,5-triphenyltetrazolium chloride (TTC) infarct volumes (200 +/- 30 mm(3)) at 24 hours were 0.53 +/- 0.02 x 10(-3) mm(2)/s (30% +/- 2% reduction) and 0.30 +/- 0.09 mL x g(-1) x min(-1) (57% +/- 11% reduction), respectively. Temporal evolution of the ADC- and CBF-defined LV showed a significant "perfusion-diffusion mismatch" up to 2 hours (P < 0.05, n = 11), a potential therapeutic window. Based on the viability thresholds, three pixel clusters were identified on the CBF-ADC scatterplots: (1) a "normal" cluster with normal CBF and ADC, (2) an "ischemic core" cluster with markedly reduced CBF and ADC, and (3) a "mismatch" cluster with reduced CBF but slightly reduced ADC. These clusters were color-coded and mapped onto the image and CBF-ADC spaces. Lesions grew peripheral and medial to the initial ADC abnormality. In contrast to the CBF distribution, the ADC distribution in the ischemic hemisphere was bimodal; the relatively time-invariant bimodal-ADC minima were 0.57 +/- 0.02 x 10(-3) mm(2)/s (corresponding CBF 0.35 +/- 0.04 mL x g(-1) x min(-1)), surprisingly similar to the TTC-derived thresholds. Together, these results illustrate an analysis approach to systemically track the pixel-by-pixel spatiotemporal progression of acute ischemic brain injury.

Hypothermia for Acute Ischemic Stroke: Not Just Another Neuroprotectant

BACKGROUND

Based on empirical experience, hypothermia has long been known to be a potent putative neuroprotectant. Recent insights into the mechanisms of central ischemia and reperfusion suggest reasons why hypothermia may be an ideal modality for extending the time window for thrombolytic stroke therapy.

REVIEW SUMMARY

Hypothermia protects brain tissue from the effects of ischemia in multiple ways. It retards energy depletion, reduces intracellular acidosis, and lessens the ischemic overdose of excitatory neurotransmitters. This attenuates the influx of intracellular calcium, the herald of subsequent neuronal death. Additionally, hypothermia suppresses synthesis of oxygen free radicals involved in secondary damage associated with reperfusion. It also suppresses the mechanisms related to blood-brain barrier degeneration and post-ischemic remodeling. Animal and human data show that deep hypothermia is primarily protective and is used in several cardiothoracic and neurosurgical applications, and that mild hypothermia enhances recovery after focal and global ischemic brain injuries. Preliminary data on hypothermia in human stroke also show promising potential. Current methods of instituting hypothermia, including patient selection, temperature and timing, cooling methods, and complications are reviewed in detail.

CONCLUSIONS

Neuroprotection conferred by mild to moderate hypothermia is likely to undergo phase III clinical trials in various clinical settings. Novel technology promises a broad application even outside intensive care settings. Preliminary studies suggest that mild to moderate hypothermia is a useful adjunct to thrombolytic therapy for stroke. Timing, degree, and duration rules are being developed and methods of cooling further perfected to optimize the safety and efficacy of this promising approach.

Ischemia and stroke

Cell death following cerebral ischemia is mediated by a complex pathophysiologic interaction of different mechanisms. In this Chapter we will outline the basic principles as well as introduce in vitro and in vivo models of cerebral ischemia. Mechanistically, excitotoxicity, peri-infarct depolarization, inflammation and apoptosis seem to be the most relevant mediators of damage and are promising targets for neuroprotective strategies.

Anti-epileptic drugs as possible neuroprotectants in cerebral ischemia

Many similarities exist between cerebral ischemia and epilepsy regarding brain-damaging and auto-protective mechanisms that are activated following the injurious insult. Therefore, drugs that are effective in minimizing seizure-induced brain damage may also be useful in minimizing ischemic injury. Use of such drugs in stroke victims may have important clinical and financial advantages. Therefore, the authors conducted a Medline search of studies involving the use of anti-epileptic drugs (AEDs) as possible neuroprotectants and summarize the data. Most AEDs have been tested in animal models of focal or global ischemia and some were already tested in humans, for a possible neuroprotective effect. The existing data is rather scant and insufficient but it appears that only drugs that have multiple mechanisms of action seem to have some potential in conferring a degree of neuroprotection that could be clinically applicable to stroke patients. In conclusion, some of the newer AEDs show promise as possible neuroprotectants in the setup of acute ischemic stroke but more studies are needed before clinical trials in humans could be undertaken.

Bcl-2 overexpression protects against neuron loss within the ischemic margin following experimental stroke and inhibits cytochrome c translocation and caspase-3 activity

Bcl-2 protects against both apoptotic and necrotic death induced by several cerebral insults. We and others have previously demonstrated that defective herpes simplex virus vectors expressing Bcl-2 protect against various insults in vitro and in vivo, including cerebral ischemia. Because the infarct margin may be a region that is most amenable to treatment, we first determined whether gene transfer to the infarct margin is possible using a focal ischemia model. Since ischemic injury with and without reperfusion may occur by different mechanisms, we also determined whether Bcl-2 protects against focal cerebral ischemic injury either with or without reperfusion in rats. Bax expression, cytochrome c translocation and activated caspase-3 expression were also assessed. Viral vectors overexpressing Bcl-2 were delivered to the infarct margin. Reperfusion resulted in larger infarcts than permanent occlusion. Bcl-2 overexpression significantly improved neuron survival in both ischemia models. Bcl-2 overexpression did not alter overall Bax expression, but inhibited cytosolic accumulation of cytochrome c and caspase-3 activation. Thus, we provide the first evidence that gene transfer to the infarct margin is feasible, that overexpression of Bcl-2 protects against damage to the infarct margin induced by ischemia with and without reperfusion, and that Bcl-2 overexpression using gene therapy attenuates apoptosis-related proteins. This suggests a potential therapeutic strategy for stroke.

Pathogenesis of venous stroke: evaluation with diffusion- and perfusion-weighted MRI

Previous reports have demonstrated the diagnostic usefulness of diffusion- and perfusion-weighted magnetic resonance imaging (MRI) in the evaluation of cerebral venous thrombosis. However, the explanations ascribed for the pathophysiologic mechanisms of venous stroke in these reports were conflicting. Earlier reports supported prominent vasogenic edema associated with mild cytotoxic edema as the potential pathophysiologic mechanism. More recently, a few reports have found cytotoxic edema as the cause for venous stroke. The purpose of this report is to review the pathogenesis of cerebral venous thrombosis after taking into consideration the results of MRI findings. We report two cases of cerebral venous infarction, which had worsening symptoms and signs that resolved after intravenous heparin therapy. In both cases, findings on diffusion-weighted and perfusion-weighted MRI images were compatible with vasogenic edema and viable neuronal tissue. Both the patients recovered without any significant residual sequel. We support the hypothesis that in the pathogenesis of venous stroke vasogenic edema is the initial event, which may or may not be followed by cytotoxic edema eventually evolving to an infarction.

Delayed treatment with MLN519 reduces infarction and associated neurologic deficit caused by focal ischemic brain injury in rats via antiinflammatory mechanisms involving nuclear factor-kappaB activation, gliosis, and leukocyte infiltration

Secondary brain injury due to ischemia includes the infiltration of leukocytes into the brain parenchyma mediated by activation of nuclear factor-kappaB (NF-kappaB), which is activated by proteasome degradation. Neuroprotection with the proteasome inhibitor MLN519 has previously been reported to decrease ischemic brain injury in rats. The authors used higher doses of MLN519 to evaluate the neuroprotection therapeutic window after 24 hours of brain injury in rats as correlated to proteasome levels, activated NF-kappaB immunoreactivity, and leukocyte infiltration. Male Sprague-Dawley rats were subjected to 2-hour middle cerebral artery occlusion (MCAO) and recovery. MLN519 or vehicle was administered after injury with a single injection given in delayed increments of 2 hours (i.e., 4, 6, or 8 hours after MCAO). Treatment with MLN519 up to 6 hours after MCAO (4 hours after reperfusion) effectively reduced neuronal and astrocytic degeneration, decreased cortical infarct volume, and increased neurologic recovery. These effects were related to >80% reductions in blood proteasome levels, reduced neutrophil infiltration, and a decrease in activated NF-kappaB immunoreactivity. This improved neuroprotection profile and antiinflammatory effect of MLN519 provides an exciting avenue for potential treatment of focal ischemic brain injury in humans.

Imaging tutorial: differential diagnosis of bright lesions on diffusion-weighted MR images

High sensitivity (94%) and specificity (100%) have been reported in the diagnosis of acute cerebral infarction with diffusion-weighted magnetic resonance (MR) imaging. However, high signal intensity on diffusion-weighted MR images and low apparent diffusion coefficient values (similar to the findings in acute cerebral infarction) were reported in such diverse conditions as hemorrhage, abscess, lymphoma, and even Creutzfeldt-Jakob disease. The differential diagnosis of these conditions (eg, acute ischemic infarction and acute cerebral hemorrhage) is critical for the determination of appropriate treatment. The authors present a systematic review of bright lesions on diffusion-weighted MR images and their differential diagnosis, with emphasis on the practical and clinical approaches of differential diagnosis.

Functional neuroimaging in acute stroke

To the present day, the first and most widespread diagnostic approach in the assessment of acute stroke remains CT scan. Its sensitivity is very high (nearly 100%) in detecting intracerebral hemorrhage in the acute period, but its capability of revealing ischemic injury in the very first hours from symptom onset is relatively poor. Since the efficacy of thrombolytic treatment in acute stroke has been suggested by the ECASS and NINDS rt-PA trials, functional neuroimaging able to distinguish potentially salvageable tissue from irreversibly injured areas has acquired primary importance. The possibility to correctly identify the tissue of the ischemic penumbra within the first hours from symptom onset is essential for correct patient selection for thrombolitic treatment. Different imaging strategies are available for the definition of perfusion deficits within the acute time window; among these are positron emission tomography (PET), single photon emission computed tomography (SPECT), Xenon CT (XeCT), dynamic CT perfusion imaging (CTP), diffusion weighted magnetic resonance imaging (DW-MRI), and perfusion weighted magnetic resonance imaging (PW-MRI). Though each technique has its advantages and limitations to present day functional MRI remains the most widespread imaging technique in the assessment of acute stroke being more accessible than both SPECT and PET, and capable of giving information on both perfusion and tissue functional status in a single imaging session. In this paper we discuss the role of functional neuroimaging in acute stroke.

Inducible nitric oxide synthase expression in the ischemic core and penumbra after transient focal cerebral ischemia in mice

The present observations examined the hypothesis that the iNOS expression in the ischemic penumbra after a transient focal ischemic insult is involved in the recruitment of penumbra into infarction. The middle cerebral artery in mice was occluded for 2 h by an intraluminal filament and then recirculated. The measurement of iNOS activity, iNOS protein formation and NO concentration in the ischemic core and penumbra, and the determination of infarct volume were performed at 6, 12, 24 and 48 h after reperfusion. iNOS protein and iNOS enzymatic activity appeared at 6 h, peaked at 24 h, and declined at 48 h in the penumbra after reperfusion. iNOS protein was not detectable in contralateral area and in sham-operated brains. The time course of iNOS protein, enzymatic activity and NO concentration in the penumbra but not in the core matched the process of infarct maturation. Treatment with iNOS inhibitor aminoguanidine (100 mg.kg(-1), i.p.) at 6 and 12 h after reperfusion inhibited iNOS activity by 88.0 +/- 10.4% and reduced NO concentration by 48.5 +/- 8.3% in the penumbra, and lessened infarct size by 48.8 +/- 7.2%. The iNOS activity and NO level in the core were not affected by the administration of aminoguanidine. These results suggest that iNOS expression in the ischemic penumbra is involved in the recruitment of penumbra into infarction and thereby contributing to the enlargement of infarct.