Vertebrobasilar artery junction aneurysm associated with fenestration.

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Abstract
Fenestration of an intracranial artery is a rare occurrence. After the vertebral artery, the basilar artery is the second most frequent site of fenestration of intracranial arteries 1,2 . Alike arterial bifurcations, fenestrations resulting from developmental anomalies have a tendency to develop aneurysms 3 . Fenestration of the proximal basilar artery aneurysms located at the vertebrobasilar junction may be associated with aneurysms 1,4,5 . Aneurysms arising from the posterior circulation are estimated to be less than 15% of all intracranial aneurysms. The morbidity and mortality of open surgery for posterior circulation aneurysms are higher than those of the anterior circulation 6 . Endovascular treatment of vertebrobasilar junction aneurysms with Guglielmi Detachable Coils (GDC) can provide an alternative method of treatment 1,2 . We present a fenestration of vertebral artery associated with intracranial aneurysm and discuss its pathogenesis and management. CASE DISCUSSION The basilar artery is formed as a result of the regression of the trigeminal arteries. When the embryo is about 4mm long, the bilateral longitudinal neural arteries are connected laterally at multiple locations with the primitive hindbrain plexus. At this stage, two fusion phenomena take place, one at the pontomesencephalic sulcus that is related to the caudal divisions of the internal carotid arteries (ICAs), while the other fusion process involves the longitudinal neural artery system and vertebrobasilar maturation. Caudal basilar fusion occurs with late trigeminal involution and cranial basilar fusion with early involution, because the trigeminal artery is responsible for the flow changes in the vertebral system during arterial development. The fusion process is typically completed when the embryo is 9 mm long (i.e. around the 5 th fetal week). Failure of fusion of the neural arteries with regression of the bridging vessels between longitudinal arteries is an explanation for the phenomenon of basilar artery fenestration. A fenestration is defined as a single artery with two luminal channels. This is typically encountered in the vertebral artery or the ICAs in the neck. A lack of fusion of embryologically paired vessels, leads to segmentally unfused arteries. This condition can only exist where two embryological arteries fuse during development. Therefore, the basilar artery or the anterior spinal artery can harbor unfused segments. Duplication can occur where the ''double lumen'' is due to two embryologically different vessels that fuse during development; an additional vessel persists, whereas in fenestrations the two lumina correspond to a single artery. Non-fusion of the basilar artery usually involves the lower half of the vessel because of incomplete fusion of the longitudinal arteries in the craniocaudal direction, although distal non-fusions have been reported and are very rare 2,7 . Since the basilar artery develops by the midline fusion of the paired ventral...

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