***TASK*** the task is to summarize an input biomedical literature in six sentences 
    ***INPUT*** the input is a biomedical literature 
    ***OUTPUT*** the output is the summary of an input biomedical literature in six sentences 
    ***DOCUMENTATION*** 
    ***EXAMPLES*** 
    Input: Background and Purpose — The annual risk of ischemic stroke distal to ≥50 % asymptomatic carotid stenoses was ≈2 % to 3 % in early cohort studies and subsequent r and omized trials of endarterectomy . This risk might have fallen in recent years owing to improvements in medical treatment , but there are no published prognostic data from studies initiated within the last 10 years . Methods — In a population -based study of all patients with transient ischemic attack ( TIA ) or stroke in the Oxford Vascular Study , we studied the risk of TIA and stroke in patients with ≥50 % contralateral asymptomatic carotid stenoses recruited consecutively from 2002 to 2009 and given intensive contemporary medical treatment .  Results — Of 1153 consecutively imaged patients presenting with stroke or TIA , 101 ( 8.8 % ) had ≥50 % asymptomatic carotid stenoses ( mean age , 75 years ; 39 % women ; 40 % age ≥80 years ) . During 301 patient-years of follow-up ( mean , 3 years ) , there were 6 ischemic events in the territory of an asymptomatic stenosis , 1 minor stroke ( initially 50 % to 69 % stenosis ) , and 5 TIAs ( 2 initially 50 % to 69 % stenosis ; 3 to 70 % to 99 % stenosis ) , 3 of which led to subsequent endarterectomy . The average annual event rates on medical treatment were 0.34 % ( 95 % CI , 0.01 to 1.87 ) for any ipsilateral ischemic stroke , 0 % ( 95 % CI , 0.00 to 0.99 ) for disabling ipsilateral stroke , and 1.78 % ( 95 % CI , 0.58 to 4.16 ) for ipsilateral TIA . Conclusions — In the first study of the prognosis of ≥50 % asymptomatic carotid stenosis to be initiated in the last 10 years , the risk of stroke on intensive contemporary medical treatment was low . Larger studies are required to determine whether this apparent improvement in prognosis is generalizable BACKGROUND AND PURPOSE The estimated prevalence of intracranial atherosclerotic disease ( IAD ) in patients with stenosis of the extracranial internal carotid artery ( ICA ) varies between 20 % and 50 % . The benefits of carotid endarterectomy ( CE ) in patients with both IAD and symptomatic extracranial ICA stenosis are uncertain . METHODS The association between IAD and other vascular risk factors and with the risk of stroke at 3 years were studied in patients with symptomatic extracranial ICA stenosis who participated in the North American Symptomatic Carotid Endarterectomy Trial ( NASCET ) . Since the NASCET protocol excluded severe IAD , only a modest number of patients in this category could be studied . RESULTS IAD was observed in one third of the patients . In medically treated patients , the relative risk of stroke associated with IAD varied from 1.3 ( 95 % CI , 0.9 to 1.9 ) with extracranial ICA stenosis of < 50 % to 1.8 ( 95 % CI , 1.1 to 3.2 ) with 85 % to 99 % ICA stenosis . In contrast , IAD did not affect the risk of stroke among surgically treated patients . To prevent 1 stroke ipsilateral to the symptomatic ICA stenosis over 3 years in patients who have also IAD , 12 patients with 50 % to 69 % , 5 patients with 70 % to 84 % , and 3 patients with 85 % to 99 % ICA stenosis have to undergo CE . In patients without IAD these numbers are 26 , 7 , and 6 , respectively . CONCLUSIONS IAD is an independent risk factor for subsequent stroke in medically treated patients with symptomatic ICA stenosis . CE reduces this risk . The additional risk imposed by IAD in medically treated patients enhances the value of CE in patients with moderate symptomatic extracranial ICA stenosis . Detection of IAD , requiring angiography , is an important prelude to planning CE in symptomatic patients with moderate extracranial ICA stenosis A prospect i ve study of acute cerebrovascular disease in a community of about 105,000 people is reported . The study protocol combined rapid clinical assessment of patients with accurate diagnosis of the pathological type of stroke by CT or necropsy , whether or not they were admitted to hospital . The study population was defined as those people who were registered with one of 50 collaborating general practitioners ( GPs ) . Referrals to the study were primarily from the GPs though , to ensure complete case ascertainment , hospital casualty and admission registers , death certificates and special data from the Oxford Record Linkage Study were also scrutinized . Six hundred and seventy five cases of clinical ly definite first-ever in a lifetime stroke were registered in four years yielding a crude annual incidence of 1.60/1,000 or 2.00/1,000 when adjusted to the 1981 population of Engl and and Wales . The age and sex specific incidence rates for first stroke showed a steep rise with age for both sexes . The odds of a male sustaining a first stroke were 26 % greater than those of a female . Ninety one per cent of patients were examined in a median time of four days after the event by a study neurologist and 88 % had cerebral CT or necropsy The development of fibrinolytic agents such as streptokinase and recombinant tissue type plasminogen activator ( r-TPA ) and other modalities of treatment in acute ischemic stroke , has raised the need for a more precise knowledge of the pathophysiology of the acute phases of ischemic stroke as it pertains to prediction of clinical outcome . In a prospect i ve analysis , 80 patients were studied within less than 6 h from the onset of symptoms by means of a detailed protocol including  clinical evaluation  ,  cerebral computed tomography , digital angiography and ultrasound transcranial Doppler sonography . Early angiography revealed a complete arterial occlusion in 76 % of cases , the majority of which were intracranial ( 66 % ) . Seventy percent of the occlusions that were retested were removed within 1 week . Potential embolic sources were found in more than 80 % of cases . Patients with documented intracranial occlusion and scarce or absent collateral filling at early angiography , had the worst clinical outcome ( P less than 0.05 ) , based on mortality data and the Canadian Neurological Scale . The 30-day mortality rate was 25 % . Survival was significantly better ( P less than 0.01 ) in patients with a Canadian Neurological Score on entry of greater than or equal to 6.5 than in patients with a less than 6.5 value . Our data indicate that early pathophysiological studies augment the clinical information and should be taken into account in the design and analysis of therapeutic trials of acute ischemic stroke BACKGROUND Transient monocular blindness associated with internal-carotid-artery stenosis is a risk factor for stroke . The effect of carotid endarterectomy in patients who present with transient monocular blindness has not been determined . METHODS We compared the risk of stroke among patients presenting with transient monocular blindness with the risk among patients presenting with hemispheric transient ischemic attack . The effect of endarterectomy was assessed in patients with transient monocular blindness . The analyses were based on data from the North American Symptomatic Carotid Endarterectomy Trial . RESULTS A total of 198 medically treated patients with transient monocular blindness had a three-year risk of ipsilateral stroke that was approximately half of that among 417 medically treated patients with hemispheric transient ischemic attack ( adjusted hazard ratio , 0.53 ; 95 percent confidence interval , 0.30 to 0.94 ) . Six factors were associated with a higher risk of stroke in patients with monocular blindness -- an age of 75 years or more , male sex , a history of hemispheric transient ischemic attack or stroke , a history of intermittent claudication , stenosis of 80 to 94 percent of the luminal diameter , and the absence of collateral circulation . The three-year risk of stroke with medical treatment for patients with zero or one risk factor was 1.8 percent , with two risk factors 12.3 percent , and with three or more risk factors 24.2 percent ( P=0.003 ) . The three-year absolute reduction in the risk of stroke associated with endarterectomy was -2.2 percent ( i.e. , a 2.2 percent increase in risk ) among patients with zero or one risk factor , 4.9 percent among those with two risk factors , and 14.3 percent among those with three or more risk factors ( P=0.23 by a test for interaction ) . CONCLUSIONS Among patients with internal-carotidartery stenosis , the prognosis was better for those presenting with transient monocular blindness than for those presenting with hemispheric transient ischemic attack . Among patients with transient monocular blindness , carotid endarterectomy may be beneficial when other risk factors for stroke are also present BACKGROUND AND PURPOSE Blood supply through collateral pathways improves regional cerebral blood flow and may protect against ischemic events . The effect of collaterals on the risk of stroke and transient ischemic attack ( TIA ) , in the presence of angiographic severe internal carotid artery ( ICA ) stenosis , was assessed . METHODS Angiographic collateral filling through anterior communicating and posterior communicating arteries and retro grade filling through ophthalmic arteries were determined in all patients at entry into the North American Symptomatic Carotid Endarterectomy Trial .  Kaplan-Meier event-free survival analyses were performed on 339 medically treated and 342 surgically treated patients . RESULTS The presence of collaterals supplying the symptomatic ICA increased with severity of stenosis . Two-year risk of hemispheric stroke in medically treated patients with severe ICA stenosis was reduced in the presence of collaterals : 27.8 % to 11.3 % ( P=0.005 ) . Similar reductions were observed for hemispheric TIA ( 36.1 % versus 19.1 % ; P=0.008 ) and disabling or fatal strokes ( 13.3 % versus 6.3 % ; P=0.11 ) . For surgically treated patients , the perioperative risk of hemispheric stroke was 1.1 % in the presence of collaterals versus 4 . 9 % when absent . The 2-year stroke risks for surgical patients with and without collaterals were 5.9 % versus 8.4 % , respectively . Neither comparison in the surgical group was statistically significant . The observed reductions were independent of the degree of ICA stenosis and other vascular risk factors . CONCLUSIONS Collaterals are associated with a lower risk of hemispheric stroke and TIA , both long term and perioperatively . Angiographic identification of collaterals assists in identifying patients with severe ICA stenosis at lower risk of stroke and TIA 
    Output:
    
On re- analysis , there were no statistically significant differences between the trials in the risks of any of the main outcomes in either of the treatment groups or in the effects of surgery .
Benefit from surgery was greatest in men , patients aged 75 years or over , and patients r and omised within two weeks after their last ischaemic event and fell rapidly with increasing delay .
Endarterectomy is of some benefit for 50 % to 69 % symptomatic stenosis and highly beneficial for 70 % to 99 % stenosis without near-occlusion .
Benefit in patients with carotid near-occlusion is marginal in the short-term and uncertain in the long-term .
These results are generalisable only to surgically-fit patients operated on by surgeons with low complication rates ( less than 7 % risk of stroke and death ) .