Introduction

Giant aneurysm at vertebrobasilar junction is still hard to manage even with the modern microsurgical and interventional technique. When a young patient with this lesion present with tolerable symptoms, neurosurgeons often do a proximal coil occlusion (Hunterian occlusion) rather than open surgery. Over the two months after this procedure for our patient, the aneurysm was neither shrunken nor thrombosed and finally bled fatally. After the experience of this patient, the authors postulate and discuss the possible mechanisms for this aneurysmal bleeding after Hunterian occlusion.

Case Report

A thirty two-year-old woman was admitted to our affiliated hospital with complaining of dysphagia and left hemiparesis which developed one week before. She had no specific past medical history. At admission, she was alert and showed mild left hemiparesis (Grade IV). Brain MRI (Fig. 1A) and CT angiography (Fig. 1B) revealed a giant

aneurysm occupying the right cerebellopontine angle and compressing the brainstem. The aneurysm was at the right vertebrobasilar junction and was measured 32.7 x 19.0 mm in size (Fig. 2A). Proximal coil embolization to the right vertebral artery was performed (Fig. 2B). After the embolization, the aneurysm was faintly visualized from the left vertebral angiogram (Fig. 2C). But CT angiogram taken 2 days later still showed an unthrombosed aneurysm. She was discharged without further management. Two months later she was readmitted to our hospital complaining of more aggravated dysphagia. Brain CT (Fig. 1C) and CT angiogram (Fig. 1D) showed a slightly-enlarged aneurysm. Shortly after the admission she became comatose and brain CT (Fig. 1E) and CT angiogram (Fig. 1F) showed thick SAH, hydrocephalus and enlarged aneurysm. Far lateral suboccipital approach was done from the left side and the aneurysm was trapped by clipping at vertebrobasilar junction just distal to the aneurysm. After the operation, she

didn’'t regain consciousness and finally succumbed.

Discussion

Proximal parent artery occlusion began in the 1950’'s for the giant aneurysms of the carotid system. Later in late 1970’'s, detachable balloon and in 1990’'s, Guglielmi detachable techniques were introduced for the same purpose of treating unclippable giant cerebral aneurysms. Giant vertebrobasilar aneurysm is a formidable vascular lesion to treat because of its relation with surrounding brain stem and cranial nerves. Untreated giant vertebrobasilar aneurysm ruptures or compresses brain stem and cerebellum to result in ischemia or hemorrhagic infarction to death.8)11) Surgical treatment such as clipping or wrapping under extracorporeal circulation could be an option.10) But due to its difficulty of clipping the neck, endovascular proximal occlusions of the vertebral artery or arteries were employed.1)3)4)7) If test

occlusion of basilar artery is tolerable, bilateral vertebral occlusion can be a viable alternative for managing such a lesion. The patient with large size (>1 mm in diameter) of the posterior communicating arteries usually tolerate test basilar occlusion well and the result of bilateral vertebral occlusion is acceptable. If the patients do not tolerate the test occlusion due to poorly developed posterior communicating artery, the external carotid to posterior cerebral artery bypass can be preceded before the vertebral artery occlusion.1)11)

Stent graft for a very complicated giant vertebrobasilar aneurysm can also be considered for treating such an aneurysm.5) There are reports that aneurysm progressively enlarged after bilateral vertebral occlusion or trapping by occluding proximal and distal to the aneurysm and thereby completely eliminated from the circulation or thrombosed.2)6)9) One suggested mechanism which explains enlargement of the thrombosed aneurysm was that well developed vasa vasorum around the proximal parent artery

made a transmural vascular connection with intra aneurysmal thrombus.6)

The angiogram of our patient showed the aneurysm was not thrombosed after proximal occlusion. A very short segment of vertebral artery distal to the giant aneurysm suggested direct pressure from the flow in the left vertebral and basilar arteries. Pascal’'s law states that if pressure is applied to fluid in a

confined system, the fluid then transmits that same pressure in all directions at the same rate. As a result, the power transmitted to the inner wall of the system is proportional to the ratio between the area of applying pressure and the area of the inner wall of the system. The author speculated that in

this patient, occlusion of the proximal parent artery produced the giant aneurysm a closed fluid filled system with one inlet. Continuous pressure into the aneurysmal lumen through a small inlet built up a progressively high power against the aneurysmal wall. Because of the ratio between the area of the inlet and the inner aneurysmal wall, much higher pressure was transmitted to the aneurysmal wall and

finally ruptured. Of course giant aneurysms often thrombose after proximal parent arterial occlusion, but some do not thrombose completely. Our patient’'s aneurysm didn’'t thrombose after proximal occlusion and presumably the wall was under progressively added stress to result in fatal bleeding.

Conclusion

For treating unclippable giant vertebrobasilar aneurysms proximal vertebral occlusion can be an alternative. If the aneurysm is not thrombosed after occlusion, we have to be concerned about rupture because of the aforementioned mechanism. We have to consider other management options such as bilateral vertebral occlusion with or without bypass surgery or stent graft with or without coil mbolization.

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