Complex aneurysms are defined as those with a dome to neck ratio of 1.5:1, those larger than 2 cm without a defined neck, those that have a major artery originating from the neck or sac, those that have considerable atherosclerotic changes in the neck, those with a significant thrombus in the lumen, fusiform aneurysms, and blister aneurysms (Table 1). Such aneurysms cannot be occluded by simple microsurgical clipping or standard endovascular coiling techniques without occluding the parent vessel or its major branches. Available strategies for preserving or reconstructing the parent vessel or major branches are: microsurgical (clip reconstruction or bypass) or endovascular (balloon-assisted coiling or stent-assisted coiling, endovascular parent vessel occlusion). The strategy of choice for a particular case has to be individualized and depends on multiple patient and aneurysm characteristics. A simplified protocol we use in making treatment decision is shown in Fig. 1.
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