Case volume does not correlate with outcome after cerebral aneurysm clipping: a nationwide study in Japan

The present study analyzed the impact of case volume on outcome after cerebral aneurysm clipping at all 382 core neurosurgical training centers certified by the Japan Neurosurgical Society. A survey requested information on all clipping surgeries for cerebral aneurysms performed during 2003. Among these centers, 369 (96.6%) responded to our request and data satisfactory for analysis were obtained for 11,974 patients. Clinical condition was graded on admission according to the classification of the World Federation of Neurosurgical Societies. Outcomes were evaluated at discharge using the modified Rankin scale. Case volume at centers was divided into three groups based on the number of clippings (<30, 30-50, >or=50) performed in 2003. Totals of 7,578 (63.3%) and 4,396 (36.7%) patients underwent clipping for ruptured and unruptured aneurysms, respectively. The mortality rate was 9.6% for patients with ruptured aneurysms, and 0.2% for patients with unruptured aneurysms. No significant correlation was detected between case volume and outcome for either ruptured (Spearman's correlation coefficient = 0.034, p = 0.483) or unruptured aneurysms (Spearman's correlation coefficient = 0.029, p = 0.562). Furthermore, no relationships between case volume and outcome were identified for ruptured aneurysms in each neurological grade or unruptured aneurysms (Kruskal-Wallis test).