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(Stroke. 1996;27:260-265.)
© 1996 American Heart Association, Inc.

Articles

A Systematic Review of the Risks of Stroke and Death Due to Endarterectomy for Symptomatic Carotid Stenosis

P.M. Rothwell, MD; J. Slattery, MSc C.P. Warlow, FRCP

From the Department of Clinical Neurosciences, Western General Hospital, Edinburgh, UK.

	Abstract

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Background and Purpose Carotid endarterectomy reduces the risk of carotid territory ischemic stroke ipsilateral to a recently symptomatic severe carotid stenosis. However, the benefit is limited by the risks of stroke and death associated with the operation. Although reported surgical risks vary enormously, there has been no systematic review of the published literature.

Methods We performed a systematic review of mortality and the risk of stroke and/or death due to endarterectomy for symptomatic carotid stenosis in studies published since 1980.

Results Fifty-one studies fulfilled our criteria. Overall mortality was 1.62% (95% confidence interval [CI], 1.3 to 1.9), and the risk of stroke and/or death was 5.64% (95% CI, 4.4 to 6.9). However, there was significant heterogeneity of risk of stroke and/or death (P<.001). The risk varied systematically with the methods and the authorship of the study. The risk of stroke and/or death was highest in studies in which patients were assessed by a neurologist after surgery (7.7%; 95% CI, 5.0 to 10.2) and lowest in studies with a single author affiliated with a department of surgery (2.3%; 95% CI, 1.8 to 2.7). After correcting for study methodology, there was no temporal trend in the risk of stroke and/or death between 1980 and 1995.

Conclusions The reported risks of endarterectomy for symptomatic carotid stenosis show significantly greater variability than would be expected by chance. However, much of this variability can be accounted for by differences in methodology and authorship. The 5.6% overall risk of stroke and/or death is consistent with present guidelines.

Key Words: carotid endarterectomy • carotid stenosis • mortality • surgery

	Introduction

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Carotid endarterectomy reduces the risk of carotid territory ischemic stroke ipsilateral to a recently symptomatic severe (70% to 99%) carotid stenosis.^{1 2} This operation is one of the most frequently performed in western countries,³ and rates continue to rise.^{4 5} However, the benefit of endarterectomy is limited by the morbidity and mortality of the procedure, particularly the risks of stroke and death. These risks vary enormously in different reports,⁶ and reviews often fail to differentiate between the higher risks in symptomatic than asymptomatic patients.^{7 8} Recommended maximum complication rates for endarterectomy for symptomatic stenosis have been published,^{9 10} but these were based on nonsystematic review of a small number of selected reports. Nonsystematic reviews tend to be selective and may be biased by the opinions of the authors, whereas systematic reviews have clearly defined methods and should, in theory at least, be a more accurate reflection of the true state of the literature.¹¹

So what are the risks of stroke and death due to endarterectomy for symptomatic carotid stenosis? Is the apparent heterogeneity of reported risks greater than would be expected by chance alone? If so, can it be explained by differences in study methodology? Curiously, there has been no systematic review of the morbidity and mortality of carotid endarterectomy for symptomatic stenosis. We therefore performed a systematic review of all studies published since 1980 that reported the risks of stroke and death after carotid endarterectomy for symptomatic carotid stenosis. In combination with a previous overview by one of us,¹² we have looked at temporal trends in reported risk over the last 30 years.

	Methods

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The systematic review was confined to studies published since 1980 to reflect present-day surgical practice. We searched for all published studies reporting the morbidity and mortality of carotid endarterectomy. These were identified from CD-ROM (Cambridge MEDLINE, 1980 to 1994) using the search terms "carotid endarterectomy" and "carotid surgery." The Cochrane Collaboration Stroke Database¹³ and the reference lists of all articles identified electronically were also searched. Studies were included in the overview if they reported the risks of stroke and death within 30 days of carotid endarterectomy in patients with symptoms referable to the operated carotid artery (ie, amaurosis fugax, transient cerebral ischemic attack, retinal infarction, or completed stroke). Articles in which risks for patients undergoing surgery for symptomatic stenosis were not reported separately from risks for patients undergoing surgery for nonhemispheric symptoms or asymptomatic stenosis were excluded. Mortality and stroke risk were defined per operation. The 95% confidence intervals (CIs) of the overall risks of death and stroke and/or death were calculated allowing for extrabinomial variation.¹⁴ Standard methods of calculating CIs produce artificially narrow intervals when there is heterogeneity of risk between different studies.

Studies were divided into the following groups: those in which postoperative assessment was performed by a neurologist/general physician, those in which one or more authors were affiliated with a department of neurology/medicine but in which it was not explicitly stated who assessed outcome, those with multiple authors who were all affiliated with a department of surgery, and single-author studies with affiliation with a department of surgery. Studies were also stratified according to whether they were performed prospectively or retrospectively. These factors were analyzed, along with year of publication, in a multiple regression analysis of the operative risk of stroke and/or death.

The studies identified in this review were combined with those studies published before 1980 in a previous review.¹² Overall mortality and the risk of stroke and/or death were analyzed in 5-year periods.

	Results

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A total of 126 studies reporting the complications of carotid endarterectomy were identified. Only 51 studies^{2 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64} fulfilled our inclusion criteria. Sixty-nine studies were excluded because they did not report operative risks in patients with symptomatic stenosis alone, and a further six studies were excluded because they reported only percentage risks without giving the number of operations, strokes, or deaths on which these were based. The overall mortality estimate was based on 17 105 operations. One study reported only deaths, and so the overall estimate of stroke and/or death was based on 15 956 operations.

Overall mortality due to surgery for symptomatic stenosis (Fig 1) was 1.62% (95% CI, 1.3 to 1.9). Two studies did not give the cause of death.^{17 48} Among the remainder, the overall risk of fatal stroke was 0.86% (95% CI, 0.70 to 1.02), and the overall risk of nonstroke death was 0.70% (95% CI, 0.56 to 0.84). The risk of stroke and/or death due to endarterectomy for symptomatic stenosis (Fig 1) was 5.64% (95% CI, 4.4 to 6.9). There was, however, significant heterogeneity (²=203, df=49, P<.001) in the reported risk of stroke and/or death.

View larger version (44K): [in this window] [in a new window]   Figure 1. Mortality and the risks of stroke and/or death due to carotid endarterectomy for symptomatic carotid stenosis in 51 studies published since 1980. The risks in each study are represented by a square (observed risk) and a line (95% confidence interval of the observed risk) and are reported per operation rather than per patient. The variance of the observed risk is proportional to the size of the square. The overall risk is represented by a diamond.

Study Methodology and Authorship
Nine reports (2605 operations) stated that a neurologist or physician performed the postoperative assessment.^{2 22 24 27 28 30 51 52 64} Eleven reports (3217 operations) included a neurologist or physician among the authors but did not state who assessed outcome.^{20 31 32 34 35 37 42 49 59 60 63} There were five single-author reports (1849 operations) with affiliation with departments of surgery.^{16 18 29 44 50} The remaining 26 reports had multiple authors, all of whom were affiliated with departments of surgery (8375 operations). In 19 of the 51 reports (6591 operations), it was clear from the methods that the study had been performed prospectively.^{2 16 17 20 22 23 24 25 26 27 28 30 35 43 51 52 55 59 64}

The risk of stroke and/or death varied according to the category of report (Table 1). The risk was highest in the reports with assessment by a neurologist or physician and lowest in reports with single-surgeon authors. There was no significant difference in overall risk between prospective and retrospective studies, although there was a trend toward a higher risk in the former (Table 1).

View this table: [in this window] [in a new window]   Table 1. Mortality and Risk of Stroke and/or Death According to Study Methodology and Authorship

A multiple regression analysis revealed that after correction for year of publication much of the apparent heterogeneity of the operative risk of stroke and/or death was related to differences in study methodology (Table 2). Compared with studies with multiple-surgeon authors, studies with assessment by a neurologist or physician and those with a neurologist or physician as an author reported significantly higher risks. Studies with a single surgeon as the author reported significantly lower risks.

View this table: [in this window] [in a new window]   Table 2. Multiple Regression Analysis of Effect of Study Methodology on Reported Operative Risk of Stroke and/or Death

Time Trends in Operative Risk
Mortality and risk of stroke and/or death due to carotid endarterectomy for symptomatic stenosis were reported in 11 studies published before 1980 in our previous overview.¹² Three studies published in the 1960s reported high operative risks, but since 1970 reported operative mortality and risk of stroke and/or death have been significantly lower overall (Fig 2). However, although there has been no change in operative mortality over the 15-year period covered by the present overview, there has been a significant increase in the reported operative risk of stroke and/or death: 1980 to 1984, 4.34% (95% CI, 2.26 to 6.42); 1985 to 1989, 5.28% (4.40 to 6.16); 1990 to 1994, 6.08 (5.30 to 6.86). However, when corrected for differences in study methodology and authorship in a multiple regression analysis, year of publication was not independently associated with risk of stroke and death (Table 2).

View larger version (16K): [in this window] [in a new window]   Figure 2. Mortality and risk of stroke and/or death due to endarterectomy for symptomatic carotid stenosis during 5-year periods from 1965 to 1994. Pre-1980 data were derived from a previous review.12 The error bars represent the 95% confidence intervals (CI) of the risk estimates, and the risks are reported per operation rather than per patient.

	Discussion

Top Abstract Introduction Methods Results Discussion References

 
This study is the first systematic review of the mortality and stroke morbidity of carotid endarterectomy for symptomatic carotid stenosis. However, electronic searches invariably miss some published studies,⁶⁵ and so it is unlikely that the review is definitive. However, inclusion of studies is unlikely to have been consistently biased with respect to the reported mortality or morbidity. Indeed, bias between studies resulting from differences in study methodology is likely to have been far greater than any exclusion bias. Similarly, although 60% of the studies reporting the complications of carotid endarterectomy identified from the literature search did not fulfill our inclusion criteria, selection should not have introduced bias. The majority of studies were excluded simply because the risks of endarterectomy were reported for combined populations of symptomatic and asymptomatic studies. Since the risks of surgery for asymptomatic stenosis are consistently lower than for symptomatic stenosis,⁶⁶ combined risks would have been difficult to interpret.

The reported risks of stroke and death were very statistically heterogeneous. Of course, differences between studies may reflect differences in case mix or surgical experience, but our analysis suggests that differences in study methodology accounted for much of the heterogeneity. Studies in which postoperative assessment was performed by a neurologist reported risks of stroke and death that were on average over three times higher than those reported in studies by single surgeons. Possible explanations for the disparity are discussed in detail elsewhere.⁶⁷ Briefly, the first possibility is scientific fraud. A few surgeons might have dishonestly reported low morbidity. Second and more likely is publication bias. Surgeons with particularly high operative stroke risks might be less likely to publish their results. Third and perhaps most likely is diagnostic bias. Surgeons might simply be less able to diagnose minor or unusual strokes than neurologists. Finally, surgeons might be more likely to undertake a study in the first place if they thought that their record was good, whereas neurologists might be more interested if they thought that the operative risks were high. Which if any of these biases account for our results is unclear. However, our findings do support the recommendation that audit of the morbidity and mortality rates for carotid endarterectomy should be independently validated.¹⁰ Surprisingly, there was little difference between the risks in prospective as opposed to retrospective studies, although the distinction was somewhat blurred in many reports.

In view of the apparent biases introduced by different study methodologies and the marked heterogeneity of the reported risks of stroke and death, it is difficult to know how the 5.6% overall risk of stroke and/or death should be interpreted. It is comparable to the risks reported in the recent large trials of endarterectomy for symptomatic stenosis^{1 2} and to the American Heart Association guidelines.¹⁰ Moreover, despite the overall statistical heterogeneity, only two^{29 44} of the 51 studies reviewed reported statistically significantly lower risks of stroke and/or death. Both were reported by single surgeons, and in both the risks of nonfatal stroke were little more than double the risks of death. As a general rule, case fatality for first stroke is usually about 10% to 20%.⁶⁸ In keeping with this, the ratios of nonfatal operative stroke to operative mortality in the North American Symptomatic Carotid Endarterectomy Trial (NASCET)² and the European Carotid Surgery Trial (ECST)¹ were 6 and 8, respectively. Studies that reported much lower ratios may well have missed a proportion of nonfatal strokes.

How should individual surgeons compare their own complication rates with the overall rates reported here or the recommended maximum acceptable complication rates published elsewhere?^{9 10} The wide CIs around the complication rates in the relatively small numbers of cases that constitute the recent experience of most surgeons make meaningful comparison very difficult. For example, a surgeon or trainee with a true 2% operative risk of stroke and/or death must perform over 150 operations before the upper 95% CI of his risk falls below the 5.64% overall risk in this review. Similarly, a surgeon with a 10% risk also must perform at least 150 procedures before his risk can be said to be significantly greater than the norm. This results in a "catch-22" situation, whereby a surgeon is unlikely to be sure whether he is sufficiently competent to routinely perform carotid endarterectomy for symptomatic carotid stenosis unless or until he has already performed 150 procedures. There is, of course, no way around this problem, and surgeons, auditors, and patients simply have to cope with the uncertainty.

The reported risk of stroke and/or death due to surgery for symptomatic carotid stenosis has increased since 1980, but this appears to be accounted for by temporal trends in study methodology. When study methodology was taken into account in the multiple logistic regression analysis, year of publication was no longer independently associated with risk of stroke and/or death. Moreover, temporal trends are difficult to interpret anyway, given the changes in the frequency with which the operation has been performed over the past 20 years and likely changes in the characteristics of patients undergoing surgery after the results of recent clinical trials.^{1 2} However, some general conclusions can be drawn. For example, the increasing use in recent years of intensive perioperative monitoring does not seem to have had a marked effect on reported operative risk. The utility of this approach cannot, of course, be properly assessed without randomized clinical trials, but our results suggest that any benefit is unlikely to be large.

In conclusion, the reported mortality and risk of stroke and/or death due to endarterectomy for symptomatic carotid stenosis are heterogeneous. Much of the variability, however, is due to differences in study methodology. In particular, outcome assessment by a neurologist was associated with a higher risk of stroke and death than that by surgeons, and studies with single-surgeon authors were associated with a lower risk of stroke and death than those in which the authorship comprised two or more surgeons. The overall risks of death and stroke and/or death were in line with current recommendations for surgery for symptomatic carotid stenosis.

	Acknowledgments

 
This study was funded by the United Kingdom Medical Research Council (Dr Rothwell and Mr Slattery).

	Footnotes

 
Reprint requests to Dr P.M. Rothwell, Western General Hospital, Crewe Rd, Edinburgh EH4 2XU, UK. E-mail pmr@skull.dcn.ed.ac.uk.

Received October 3, 1995; revision received November 14, 1995; accepted November 14, 1995.

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