Background M1 segment of middle cerebral artery (MCA) aneurysm is a relatively rare clinical condition. However, due to its complex geometry and deep location, microsurgical treatment is challenging. With the development of devices and techniques, the range of aneurysms that can be treated with endovascular treatment (EVT) continues to expand. We performed this study to define a specific form of M1 aneurysm that can be safely and effectively treated through EVT as M1 hilltop aneurysm, and to report our treatment results.

Methods Of 757 MCA aneurysm between December 2017 and October 2021, 54 M1 segment aneurysms were treated with EVT, and these aneurysms were designated M1 hilltop aneurysms. Clinical and radiographic data, including aneurysm characteristics, endovascular techniques, angiographic outcome, procedure-related complications and clinical outcomes at the time of the last follow-up, were collected and reviewed retrospectively.

Results Treatments were successful in all 54 cases, 21 cases were treated with coiling and 33 cases with stent-assist coiling (SAC). The mean height of the aneurysm was 4.35 ± 1.9 mm, the mean width was 4.59 ± 1.9 mm, and the mean neck size was 3.63 ± 1.4 mm. Of the 54 cases, 50 (92.6%) cases were identified as wide-neck aneurysms. The neck of aneurysm incorporating branch vessel was found in 49 (90.7%) cases. Immediate post-procedural angiogram showed favorable occlusion in 32 (59.3%), incomplete occlusion in 22 (40.7%). There were 4 (7.4%) procedures-related complications including thromboembolism and internal carotid artery dissection, but there were no cases of permanent neurological impairment. The mean follow-up duration was 18.2 months. During the follow-up period, there was no neurological deterioration or aneurysmal rupture in any of the patients. On 50 available follow-up angiographic studies, minor recurrence was found in 6 (12%) cases and major recurrence was found in 1 (2%) case. Recurrence was significantly related to aneurysm neck (OR 3.9, 95% CI 1.2 to 12.9, p = 0.025).

Conclusions EVT for M1 hilltop aneurysms appears to be safe and efficacious, with low mid-term recurrence rate. However, long-term and large cohort study will be needed.

Disclosures S. Park: None. W. Lee: None. J. Jung: None.