Introduction: Giant fusiform M1 segment aneurysms involving lenticulostriates and the MCA bifurcation present a significant treatment challenge. The current work reports the first histopathologic analysis of a giant fusiform MCA aneurysm initially treated with a endovascular flow diverter (EFD), followed by surgical resection for subsequent EFD thrombosis and regional mass effect.

Methods: The specimen was fixed in formalin and tangentially sectioned to the EFD. The EFD was pen-striped leaving the thrombus internal to the EFD lumen intact. The sectioned specimens were then processed in a prostate tissue cassette, sectioned, and stained.

Results: Cross-sections demonstrate two concentric and distinct clot formations. The EFD lumen thrombus demonstrates, intact red blood cells intermixed with fibrin and disintegrating leukocytes while the thrombus external to the EFD displays brightly eosinophilic, laminated, fibrin with erythrocytes and scattered foamy macrophages (Fig 2 A- D). The aneurysmal wall is composed of a thin tunica adventitia distorted by fibrocalcific atherosclerosis. The lack of neutrophils, presence of macrophages, and more robust fibrin laminations indicate an older clot surrounding the EFD. [1] CD61 immunohistochemical stains, demonstrate intense brown staining consistent with platelet aggregation in both thrombi; however, the overall platelet density is lower in the older clot as compared to the clot that formed within the EFD (Fig 2 E-F). Additionally, there is a distinctive rim of CD61 immunoreactivity in the portion of older clot immediately external to the pipeline. [2]

Conclusions: Our case presentation offers new insights into the mechanisms for parent vessel thrombosis in the treatment of cerebral aneurysms and raises new questions about the role of acute aneurysm thrombosis in the maintenance of parent vessel patency. While this new technology provides additional options for the treatment of large aneurysms, caution must be exercised when applying a new technology to indications outside of formal indications.

Patient Care: Offers mechanistic insight for parent vessel thrombosis of giant aneurysms

Learning Objectives: To illustrate the histopathology of parent vessel thrombosis

References: 1. Fineschi, Vittorio, et al. "Histological age determination of venous thrombosis: a neglected forensic task in fatal pulmonary thrombo-embolism." Forensic science international 186.1 (2009): 22-28. 2. Galindo, María, et al. "Immunohistochemical detection of intravascular platelet microthrombi in patients with lupus nephritis and anti-phospholipid antibodies." Rheumatology 48.8 (2009): 1003-1007.