Transitional aneurysms, also referred to as transitional paraclinoid aneurysms, are saccular cerebral aneurysms arising from the clinoid-ophthalmic segment of the internal carotid artery (ICA) and near the distal dural ring with potential risk for subarachnoid hemorrhage.
After an anterior turn of segment C5 (clinoid segment), the ICA leaves the cavernous sinus, passing through a dural ring called the “proximal dural ring”. The ICA then goes through a very small region where, although already out of the cavernous sinus, it is not yet intradural in location. After this short "transitional" segment, the ICA goes through another dural ring, the distal dural ring, and then becomes intradural.
The region between the two dural rings is "transitional" in location, that is, a point of transition between the two dural rings and transition from extradural to intradural. Aneurysms in this location have received several names, including juxtadural, transitional, and paraclinoid, among others 1. It is important because aneurysms distal to the distal dural ring are located in the subarachnoid space, and their rupture leads to subarachnoid hemorrhage.
Although the ophthalmic artery is usually located just distal to the distal dural ring and is therefore used as a reference for when the ICA becomes intradural, the ophthalmic artery can vary in origin and arise in an extradural (proximal) location in a minority of cases.