Outflow angle: a risk factor for thromboembolic complications in coil embolisation for treating unruptured middle cerebral artery bifurcation aneurysms

Thromboembolic complications are a relevant risk in coil embolisation of cerebral aneurysms. This retrospective study aimed to assess the treatment outcomes of unruptured middle cerebral artery (MCA) bifurcation aneurysms and the predictors for thromboembolic complications based on the aneurysm morphological characteristics. We examined the following three features: inflow angle, outflow angle (OA), and bifurcation angle, formed by the aneurysm and neighbouring blood vessels.A total of 32 MCA bifurcation aneurysms were retrospectively investigated in 32 patients treated consecutively at our institute between April 2008 and March 2019. The predictors for thromboembolic complications were analysed in two groups: patients with and without thromboembolic complications.Perioperative thromboembolic complications were detected in six patients (18.8%), including two and six intra- and post-procedural thromboembolic complications, respectively; all cases were resolved. Regarding the aneurysms' morphological characteristics, the group with thromboembolic complications showed a significantly smaller OA (55.58° ± 14.05° vs. 86.04° ± 28.58°, P = 0.01) than the group without complications. Multivariate analysis revealed that smoking habits and OA 70° were significant predictors of thromboembolic complications (smoking habits: P = 0.01, odds ratio [OR]: 6.89, 95% confidence interval [CI]: 3.78-12.62; OA 70°: P = 0.04, OR: 3.19, 95% CI: 1.52-6.56).Our findings indicate significant pre-procedural predictors of thromboembolic complications to consider for safe treatment; clipping should be preferred to coil embolisation in high-risk patients. The method of choice should be considered in each case to allow the safe treatment of unruptured MCA aneurysms.