Better Outcomes with Treatment by Coiling Relative to Clipping of Unruptured Intracranial Aneurysms in the United States, 2001–2008

Discussion

In this sample of patients treated in the United States from 2001 to 2008, we have found that endovascular coiling of unruptured intracranial aneurysms was associated with significantly less morbidity and mortality than surgical clipping. While we cannot know for any individual case in our study the relative merits of endovascular therapy versus open surgery versus observation, the results from the NIS data base suggest that patients treated in the United States with surgery generally face a significantly higher risk of adverse outcome than patients treated with endovascular therapy. This trend does not necessarily imply that all the patients treated with surgery should have been offered endovascular therapy, because for many of these patients, endovascular therapy may have been an inadequate option. While some have recommended that in treating unruptured cerebral aneurysms, “microsurgical clipping rather than endovascular coiling should be the first treatment choice in low-risk cases,”⁴ our study offers compelling data that surgical treatment is being performed at a higher periprocedural risk of morbidity and mortality than coiling for a great number of patients in the United States.

According to Cowan et al,³ the percentage of unruptured aneurysms treated by endovascular therapy increased from 11% in 1998% to 43% in 2003. Our study shows that the percentage increased to a high of 63.8% in 2006 and then fell to 55.8% by 2007, for unknown reasons, rising again in 2008 to 63.2%. Since 2004, a greater percentage of patients with unruptured aneurysms has been treated with coiling than with clipping. As coiling has become more available and refined, it is reasonable to expect that treating centers are becoming more adept at recognizing patients who can be expected to have a better outcome with coiling. Surgical outcomes might also improve if higher risk surgical patients, such as those with basilar aneurysms, are increasingly directed toward endovascular therapy. However, the NIS data do not show any trend of improving outcomes with either clipping or coiling individually. Rather, they show an improvement in outcomes correlated with a higher fraction of patients being treated with coiling, with the lowest morbidity for all patients occurring in 2006 when the fraction of coiled patients was highest. Of course, the patients in the NIS data base represent the entire spectrum of cerebral aneurysms, and some patients in the sample were undoubtedly better candidates for clipping than for coiling. However, the data from the NIS sample clearly show that the increasing adoption of endovascular coiling is strongly associated with a decreasing risk of morbidity and mortality. This finding would support an argument for coiling as the first-line therapy for unruptured aneurysms in patients who are considered good candidates for both forms of treatment.

It might be true that many patients with a low risk for clipping are increasingly being treated with coiling, which would lead to a gradual shift toward a higher risk population among clipping patients. If this trend was the explanation for our findings, it would follow that greater attention should be paid to patient selection for clipping. The ISUIA⁵ showed that risk factors for adverse surgical outcomes included increasing age, increasing size of aneurysm, location of aneurysm in the posterior circulation, history of ischemic cerebrovascular disease, and the presence of aneurysmal symptoms other than rupture. Such risk factors should be adequately integrated into practice.

Because risk of rupture is related to aneurysm size, the acceptable risk for therapy is also related to aneurysm size. The American Heart Association recommendations⁶ note that treatment of small unruptured cerebral aneurysms cannot generally be advocated. ISUIA⁵showed that the size of an intracranial aneurysm is a key determinant in assessing the risk of future rupture. Based on ISUIA, the risk of rupture of small (<7 mm) anterior circulation aneurysms is quite low and would not justify the risks of treatment with clipping or coiling observed in our study. Information about the size and location of aneurysms treated in the NIS is not available, so we cannot ascertain the relative risk of rupture in this population according to the natural history data from ISUIA. However, in the general population, most aneurysms are <7 mm and are located in the anterior circulation,⁷ so it is reasonable to suspect that a significant fraction of patients treated in the NIS population had small anterior circulation aneurysms.

The NIS results in our study show higher morbidity and mortality for clipping than is typically reported in single-center series, which was reported to be 7.9% on average.⁸ The difference could be explained by publication bias (ie, a greater tendency among centers with the best results to publish their findings). It is also possible that the single-center series reported by surgeons might be less accurate in reporting their outcomes. With carotid endarterectomy, adverse event rates tend to be significantly lower when assessed by the treating surgeon than when they are evaluated by a neurologist following surgery.^9,10 The NIS data used in our study are somewhat free from such biases because the coding process is rather objective.

The number of unruptured aneurysms treated in the United States increased steadily from 2001 to 2008 (Fig 1). This change is probably due to expanded use of CT and MR imaging leading to the discovery of increasing numbers of incidental unruptured aneurysms. All increases in the number of treated aneurysms are accounted for by an increase in the coiling procedures, because from 2001 to 2008, the rate of coiling increased by a factor of 10, whereas surgical clipping rates remained rather constant (Fig 1). The marked increase in the number of treated unruptured aneurysms occurred despite the report of fairly benign natural history data in USUIA. Thus, ISUIA apparently has had little impact on decisions regarding treatment of unruptured aneurysms in the United States, perhaps due to criticisms raised about the trial methods.¹¹ A randomized trial may be indicated to definitively assess the efficacy of treatment of unruptured cerebral aneurysms.¹²

A recent study of the NIS from 2000 to 2006 showed less morbidity and mortality with coiling of unruptured aneurysms than with surgical clipping.¹³ Our study includes data from 2007 and 2008 and further confirms the decreased morbidity with coiling by showing that the morbidity and mortality increased in 2007 in association with an increase in the rate of surgical clipping and then fell again in 2008 in association with a decrease in the rate of surgical clipping (Table 2).

Our study updates previous evaluations of the NIS that took place before the use of endovascular therapy was widespread. Treatment with coiling was uncommon from 1996 to 2000 compared with 2001–2008 (421 cases in the NIS for 1996–2000¹ versus 34,125 cases for 2001–2008); therefore, the sample of coiling patients in our study is much larger than that in previous reports. Mortality for coiling decreased from 1.7% in 1996–2000¹ to 0.6% in 2001–2008. Mortality for clipping decreased from 2.1% in 1996–2000² to 1.2% in 2001–2008. The morbidity associated with coiling decreased from 7.6% in 1996–2000¹ to 4.9% in 2001–2008, and the morbidity associated with clipping decreased from 16.1% in 1996–2000² to 14.0% in 2001–2008. The decrease in morbidity and mortality associated with coiling between these time periods is likely due to advances in endovascular techniques. Because the technique of surgical clipping has not changed recently, this argument is less likely to be applicable to open surgical cases, which saw a relatively smaller decrease in morbidity and mortality compared with coiling.