Postcoiling Aneurysm Tilting: A Disturbing Finding?

Discussion

Surgical intervention for basilar tip aneurysm is difficult and is associated with high morbidity. The challenge arises from operating in a location deep in the brain, often with limited surgical accessibility and in association with delicate anatomic structures in close proximity to the lesion (1). In the early 1990s, controlled detachable coils were introduced as a less invasive alternative for treating aneurysms. The location of the aneurysm is less important than the morphology of the aneurysm, when one considers patients for endovascular therapy. In general, basilar tip aneurysms are favorable for microcatherization and therapy with coils (2, 3). Surgical complications from brain and nerve retraction are avoided with endovascular approaches; however, selective embolization can be associated with other immediate and long-term complications, primarily hemorrhagic or ischemic strokes or both (2–4).

After selective embolization, the two patients demonstrated a change in orientation of the aneurysm. Whether it has to be considered as an incidental finding without clinical implication might be answered by our first patient. In this case, we believe that tilting of the aneurysm led to compression of the left third nerve, resulting in a new, progressive unilateral third nerve palsy.

In the second patient, angiography performed over a 3-year time span demonstrated tilting of the aneurysm with marked posterior bending of the basilar trunk. The final MR image showed enlargement of the lateral and third ventricles, which were likely due to anterior compression of the aqueduct of sylvius by the aneurysm (Fig 2E). The patient, however, remained asymptomatic.

It is not known whether the dramatic change in orientation of aneurysms after endovascular therapy described in this report could occur in another location. The fact that the basilar tip is anatomically unrestricted, “floating in the interpedoncular cistern,” may make this structure susceptible to reorientation.

There are several hypothesis that could explain these morphologic changes. A change in the orientation of the hemodynamic inflow away from the fundus could lead to reorientation of the aneurysm. The “supportive” nature of inflowing pressurized blood is eliminated by the presence of the coils, thus allowing the aneurysm to “fall.” Because there is no vasovasorium in intracranial arteries, the walls of nongiant aneurysms depend on flow within the aneurysm for their viability. As a wall degenerates, there is reduced support for the coil mass, which could lead to reorientation. Another possibility is that the pulsatile flow into the anteriorly situated residual aneurysm accounts for the reorientation of the aneurysm. A final possibility is that the weight of the coils themselves leads to ptosis and reorientation based on added weight in the aneurysm fundus.

Conclusion

With the endovascular approach for the treatment of intracranial aneurysm, the endocranial environment adjacent to the aneurysm is theoretically unmodified. Coils within an aneurysm usually do not change the morphology of the aneurysm unless there is significant recanalization with subsequent aneurysmal growth. We have presented two patients whose aneurysms demonstrated fundal ptosis after endovascular embolization. This interesting finding, which seems to be rare, can have clinical repercussions, as demonstrated in the case of our first patient. It is not known whether such a change in orientation could be present in other locations. The anatomic environment of the basilar artery may make it more prone to fundal ptosis postembolization than other vessels.