Histopathological analysis of in vivo specimens of recurrent aneurysms after coil embolization

BACKGROUND: Aneurysm recurrence after coil embolization remains a challenging problem. OBJECTIVE: To determine the histopathological features of recurrent aneurysm specimens and explore the mechanism of aneurysm recurrence. METHODS: Nine aneurysm specimens were collected from eight patients who underwent clipping for aneurysm recurrence within 2 years after embolization. All specimens were sectioned and embedded in resin, stained with hematoxylin-eosin (H&E), Masson stain, and immunohistochemical staining for smooth muscle actin (SMA) and CD68+ antibodies, and were examined under light microscopy. RESULTS: Five aneurysms were surgically clipped owing to post-embolic subarachnoid hemorrhage, while the other four aneurysms had dangerous recanalization detected on follow-up imaging. Five aneurysms had self-growth and four aneurysms had coil compactions. Gross observation showed that each recurrent aneurysm was wrapped by a thrombus and the aneurysm wall; some coils protruded from the pseudocapsule in some ruptured aneurysms. Microscopically, H&E staining showed that three types of thrombi (fresh thrombus, granulation tissue, and scar tissue) coexisted in one section. In addition, characteristic unstable and unorganized thrombi with empty spaces were found in the neck cavity. Immunohistochemical staining showed that the SMA stain was discontinued and incomplete, and CD68+ antibody and H&E staining revealed inflammatory infiltrate in the aneurysm wall. CONCLUSION: The coexistence of three types of thrombi is the main characteristic of recurrent aneurysms. The formation of stable thrombus may be one of the key points of aneurysm recurrence. Smooth muscle cell damage and infiltration of inflammatory cells in the aneurysm wall probably contribute to the recanalization.