Similar Safety in Centers with Low and High Volumes of Endovascular Treatments for Unruptured Intracranial Aneurysms: Evaluation of the Analysis of Treatment by Endovascular Approach of Nonruptured Aneurysms Study

Discussion

The ATENA study was conducted in 27 neurointerventional centers in France and Canada from June 2005 to October 2006. It showed that the endovascular treatment of unruptured intracranial aneurysms was feasible in a high percentage of cases with low morbidity and mortality rates. However, an important unanswered question was whether the results were similar among centers that treated high and low volumes of patients—that is, it was unknown whether the treatment in a center with a high volume of cases was associated with a better outcome.

Previously, patients with ruptured or unruptured aneurysms with surgery or endovascular treatments have been studied to evaluate the relationships between physician experience, volumes of patients treated, and clinical and anatomic outcomes.^3–9 Most series were based on a retrospective analysis of a data base of hospital discharges and were fraught with the limitations of this type of evaluation.

Previous analyses were conducted in the patient population that had been treated surgically. Solomon et al³ reviewed all discharges in the state of New York from 1987 to 1993 that had been recorded in the Statewide Planning and Research Cooperative System of the New York Department of Health. They analyzed the outcomes of all patients who underwent craniotomy for cerebral aneurysms (n = 4034). They found that hospitals that performed >30 craniotomies per year had a 43% lower mortality rate than hospitals that performed fewer craniotomies (8.8% versus 15.5% mortality rates, respectively). They found similar results for patients who underwent craniotomy for unruptured cerebral aneurysms. Hospitals that performed >30 craniotomies per year had a 43% lower mortality rate than hospitals that performed fewer craniotomies (4.6% versus 8.1% mortality rates, respectively). Similar results were reported for older patients with ruptured aneurysms treated by the surgical approach.⁴ The 30-day mortality rates for patients with subarachnoid hemorrhage who were treated surgically were 20.5%, 18.4%, and 14.3% in hospitals averaging <1, between 1 and 5, and ≥5 interventions per year, respectively. Similar results were observed in a more recent series that studied mortality and morbidity after surgical treatment of unruptured intracranial aneurysms in the United States (1996–2000). The hospitals that treated ≥20 patients/year had fewer adverse outcomes (15.6%) and lower mortality rates (1.6%) compared with hospitals with <4 cases/year (23.8% adverse outcomes and a 2.2% mortality rate).⁸

One study analyzed the effects of endovascular services and volume of cases/hospital on cerebral aneurysm treatment outcomes. They showed that patients were less likely to die in the hospital when treated at institutions that more frequently used coil embolization and angioplasty for vasospasm.⁵ However, the volume of cases/hospital was not independently associated with in-hospital deaths. Johnston et al⁶ analyzed the data base from the Office of Statewide Health Planning and Development of California for the period of 1990–1998. They showed that compared with surgery, endovascular therapy of unruptured aneurysms was associated with a lower risk of adverse outcomes and in-hospital deaths. The volume of cases/hospital was also associated with outcome.

A subsequent analysis of patients treated for ruptured or unruptured aneurysms was conducted in New York State. Again, that study showed that the volume of cases/hospital and the propensity of a hospital to use endovascular therapy were both independently associated with better outcomes.⁷ In cases that involved clipping of either ruptured and unruptured aneurysms, a higher volume of cases/hospital was associated with a better outcome. In cases that involved embolization, the volume of cases/hospital was either not significantly associated with outcome or demonstrated a statistically weaker association than clipping. In addition, Hoh et al⁹ conducted a retrospective cohort study with the Nationwide Inpatient Sample of 1996–2000. They demonstrated that hospitals with >23 admissions per year had fewer adverse outcomes (5.2%) than hospitals with <4 admissions per year (17.6%). The mortality rate was lower at hospitals with high volumes of cases (1.0% versus 3.7%), but the difference was insignificant. Finally, the length of stay was shorter at hospitals with high volumes of cases.

We used a completely different approach in our analysis of the relationship between the volume of cases/hospital and the outcome of endovascular treatment for UIA. We did not use a discharge data base but the results from a prospective consecutive multicenter controlled study. This methodology can avoid much of the bias that might be encountered in a retrospective analysis of a discharge data base, but it is also associated with some limitations (see below). The population of the ATENA study was divided into 2 groups according to the volume of cases/hospital. The groups were defined to ensure approximately the same number of centers in each group. Group A included 13 centers that treated ≤20 patients, and group B included 14 centers that treated >20 patients during the inclusion period. With other cut-points (see “Materials and Methods”), similar results were obtained (not shown). Although the patient and aneurysm characteristics were similar in both groups, the endovascular treatment was performed differently in groups A and B. The standard coiling technique was more frequently used in group A (62.4% versus 50.4% in group B, P = .001), and stent placement was more frequently used in group B (9.6% versus 2.5% in group A, P = .0016). These results suggested that centers with high volumes of cases may facilitate the use of complex techniques like stent placement.

The most important factor in evaluating any treatment is certainly clinical outcome, as assessed by morbidity and mortality. Morbidity was not significantly different in groups A and B (1.8% and 1.7%, respectively). The mortality rate was higher in group A (2.3% versus 1%), but the difference was not significant. Hoh et al⁹ reported somewhat different outcomes. They found that discharge to a facility other than home occurred at a rate of 5.2% from hospitals with high volumes of cases compared with 17.6% from hospitals with low volumes of cases. However, with findings similar to ours, they reported a lower mortality rate at hospitals with high volumes of cases (1.0% versus 3.7%), but the difference was not significant.

Several factors may explain the discrepancies between studies. Our series was conducted more recently (2005–2006) and during a shorter period than the analysis of Hoh et al (1996–2000).⁹ Endovascular treatment started in France in 1993–1994; thus, most teams participating in the ATENA study had long experience with the endovascular treatment of unruptured aneurysms. This was probably not the case in the series of Hoh et al. Moreover, the development of more appropriate devices (coils, microcatheters, microguidewires, etc) has facilitated the performance of endovascular treatments. Also, the series of Hoh et al included a very large (81) number of centers compared with the number included in our series (27); thus, the levels of clinical experience were presumably more heterogeneous in the series of Hoh et al. Finally, we defined high- and low-volume hospitals differently than Hoh et al did. They defined high-volume hospitals as those with >23 admissions per year and low-volume hospitals as those with <4 admissions per year; in contrast, we defined high- and low-volume hospitals as those with greater or less than 20 admissions during 17 months, respectively.

We found that the anatomic outcomes were similar in both groups. Despite the difference in treatment techniques (see above), the relative percentages of complete occlusion, neck remnant, and aneurysm remnants were not different in groups A and B. However, this result was based on the occlusion rates determined by the practitioners that performed the procedures. A core lab analysis of anatomic results is necessary to confirm that the results were similar in both groups.

Our study had several limitations. First, it was not possible to evaluate the results according to the physician volume. Because 1, 2, or more physicians can be involved in the treatment of a single patient, it was not possible to calculate the physician volume by simply dividing the number of patients treated by the number of physicians in each center. Second, it was not possible to evaluate the effect of physician experience because we had no information regarding the years of experience or the number of unruptured aneurysms treated before the study for each physician. It was previously shown that the risk of complications with coil embolization of unruptured aneurysms was dramatically reduced with physician experience.¹¹ Third, we were unable to take into account the global experience within each center for the treatment of intracranial aneurysms, including ruptured aneurysms, and of neurovascular procedures in general. Finally, our definitions of high- and low-volume centers for groups A and B were arbitrary; in fact, there is a continuum from group A to group B, and this may partially explain our results. However, results were similar whatever the cut-point used to define the low and high-volume centers.