Case report A 51-year-old male presented with left-sided hemiparesis and aphasia upon awakening to an outside facility with a NIHSS of 9. CT displayed a basilar artery (BA) occlusion. Upon presentation to our institution his NIHSS had progressed to 11.

Angiography displayed a BA thrombus and mechanical thrombectomy was performed with a TICI score of 2B. The large basilar aneurysm was then visualized, measuring 1.1 x 1.2 cm, at which no intervention was performed.

Angiography at 4 and 11 months yielded no change and partial luminal thrombosis, respectively. No intervention was performed due to vertebral stenosis and the patients stable neurologic examination. A two-year follow-up angiogram showed the right vertebral artery to have a normal caliber and course with abrupt termination distal to the PICA and no aneurysm was visualized. The left vertebral was catheterized and found to have a normal course and caliber with normal filling of the left PICA and AICA. The basilar was noted to be occluded for two-thirds of its superior course.

Discussion: Many giant posterior circulation aneurysms are known to thrombose spontaneously. Cases of concurrent thrombosis of the parent vessel have also been described.^5–7 Multiple mechanisms for this have been suggested in the literature. Maeda, Usui, Tsutsumi, and Iijima specifically described proximal spread of a giant basilar aneurysm dissection with thrombus formation from an intramural hemorrhage within the aneurysm. Giant aneurysms are known to have a vascularized wall where hemorrhages may occur and contribute to their growth. This has been discovered to occur either by angiogenesis in the arterial wall or the vasa vasorum.⁶

We hypothesize this mechanism occurred in our case, where the large aneurysm was initially partially thrombosed with concurrent complete BA occlusion. The initial thrombectomy was performed due to the progressing NIHSS. During follow-up angiography, the patient’s neurologic exam remained stable therefore a conservative approach was chosen. If the large aneurysm microhemorrhaged and dissected, the dissection most likely spread superiorly into the remaining portion of the BA creating a thrombus which instigated the patient’s presenting symptoms. This was confirmed by MRI, which showed acute/subacute infarction within the pons and left cerebellar vermis on DWI.

Fakhouri and McCullough described a case with aggressive intra-arterial thrombolysis although studies have argued that outcomes are variable in these cases.^1,3–5,10 Indications for intervention of these aneurysms include mass effect of the surrounding eloquent parenchyma or strokes. Treatment options include open bypass, stent-assisted coiling, hunterian ligation or flow-diversion. Treatment options have significant risks with flow diversion having the lowest mortality of twenty-five percent. Treatment patterns have evolved as endovascular techniques have become advanced. ^2,8,9

Conclusion There are few reported cases of spontaneous large basilar aneurysm thrombosis without intervention. Small retrospective studies have shown a variety of outcomes.^1,3,4 Further research is required in order to standardize treatment and improve outcomes. Because posterior circulation aneurysms are variable in size, character, behavior and vascular territory, individualized treatment plans remain the best option for patients.

Disclosures: T. Dawley: None.