Endoluminal cryotherapy to prevent recanalization after endovascular occlusion with platinum coils

Mechanisms of Healing in Coiled Intracranial Aneurysms: A Review of the Literature

Recanalization of intracranial aneurysms following endovascular coiling remains a frustratingly common occurrence. An understanding of the molecular and histopathologic mechanisms of aneurysm healing following coil embolization is essential to improving aneurysm occlusion rates. Histopathologic studies in coiled human and experimental aneurysms suggest that during the first month postcoiling, thrombus formation and active inflammation occur within the aneurysm dome. Several months following embolization, the aneurysm is excluded from the parent vessel by formation of a neointimal layer, which is often thin and discontinuous, across the aneurysm neck. Numerous coil modifications and systemic therapies have been tested in animals and humans in an attempt to improve the aneurysm-healing process; these modifications have met with variable levels of success. In this review, we summarize the histopathologic and molecular biology of aneurysm healing and discuss how these findings have been applied in an attempt to improve angiographic outcomes in patients with intracranial aneurysms.

Thrombosis heralding aneurysmal rupture: an exploration of potential mechanisms in a novel giant swine aneurysm model

BACKGROUND AND PURPOSE

The relationship between aneurysm dimensions, flow, thrombosis, and rupture remains poorly understood. We attempted to clarify this relationship by exploring various swine aneurysm models.

MATERIALS AND METHODS

Bilateral carotid aneurysms were constructed according to 3 protocols in 24 animals: small aneurysms with wide necks (group 1; n = 6 animals); small aneurysms with small necks (group 2; n = 4 animals), and giant aneurysms with large necks (group 3; n = 14 animals). Group 3 included 3 subgroups, related to testing the model in various experimental conditions: The neck was clipped in 3 animals; venous pouches lacked an endothelial lining in 4 animals; and 7 were control animals. Animals were followed until rupture, or for 1-4 weeks. Angiography was performed postoperatively and before euthanasia. We studied lesion pathology, paying attention to thrombosis, recanalization, wall composition, and perianeurysmal hemorrhage.

RESULTS

Groups differed significantly in aneurysm dimensions and aspect ratio (P = .002). Ruptures occurred more frequently in animals with untreated giant aneurysms (7/7) than in animals with small wide-neck (0/6) or small-neck (2/4) aneurysms (P = .002). Ruptures occurred only in animals with thrombosed aneurysms. Lesions lacking an endothelial lining and 5 of 6 clipped venous pouches thrombosed but did not rupture. One giant lesion ruptured despite complete clipping. The wall was deficient in α-actin and was infiltrated with inflammatory cells and erythrocytes in all thrombosed cases, ruptured or not. Ruptures were associated with recanalizing channels in 9 of 10 cases.

CONCLUSIONS

Thrombosis, inflammation, and recanalization may precipitate aneurysmal ruptures in a swine model.

Radiofrequency endothelial ablation prevents recanalization after endovascular coil occlusion: in vitro and in vivo assessment

PURPOSE

Coil embolization of intracranial aneurysms may be followed by recurrences. Radiofrequency (RF) ablation of the endothelium may prevent recanalization after coil embolization.

MATERIALS AND METHODS

The authors performed in vitro experiments in chicken meat and egg white models to investigate the thermal distribution and geometry of lesions created with RF applied through standard coils alone or by using a prototype RF electrode inserted in a coil or a mass of coils. A mathematic model was designed to predict perianeurysmal isotherm lesions by using the bio-heat equation. In an in vivo coil arterial occlusion model (six dogs), the authors compared angiographic and pathologic results of coil embolization (n = 8) with those of coil embolization preceded by RF ablation (n = 7) by using a cardiac electrode at 1 month.

RESULTS

Current coils offer high impedance (400 Omega) at high current frequencies and are damaged by RF transmission. A dedicated electrode generated reproducible lesions, but contact with coils interferes with lesion reproducibility. When the coil mass was used, a uniform RF lesion that conformed to the coil mass shape was produced. The mathematic model predicted a uniform heat distribution within 1 mm from the coil mass periphery. Arterial coil embolization led to occlusion followed by recanalization (n = 8), whereas RF ablation (20-30 W for 60 seconds) prevented recanalization in all coil-occluded arteries (P < .001, chi(2) test). Pathologic findings helped confirm complete arterial occlusion with RF ablation. One animal developed brachial plexus injury with excessive levels of RF ablation.

CONCLUSIONS

RF ablation can prevent recanalization after coil occlusion-at least in the arterial model. Modifications of coils, dedicated neurovascular electrodes, and technique optimization remain necessary before considering a clinical application.

Mechanisms of occlusion and recanalization in canine carotid bifurcation aneurysms embolized with platinum coils: an alternative concept

BACKGROUND AND PURPOSE

Endovascular treatment of aneurysms may result in complete or incomplete occlusions or may be followed by recurrences. The goal of the present study was to better define pathologic features associated with so-called healing or recurrences after coiling and to propose an alternative concept to the currently accepted view.

MATERIALS AND METHODS

Experimental canine venous pouch aneurysms were created by using a T-type (group A, N = 29) or a Y-type constructed bifurcation (group B, N = 37) between the carotid arteries. Coil embolization was performed 2 weeks later; and angiography, immediately after and at 12 weeks. Angiographic results, neointima formation at the neck, endothelialization, and organization of thrombus were compared between groups by using qualitative scores and immunohistochemistry.

RESULTS

Angiographic results at 3 months were significantly better in group A than in group B (P = .001). Macroscopic neointimal scores were also better (P = .012). Only 10/32 aneurysms with satisfactory results at angiography were completely sealed by neointima formation. Animals with residual or recurrent aneurysms had significantly worse neointimal scores than those with completely occluded ones (P = .0003). On histologic sections, the neointima was constantly present in "healed" and in recurrent aneurysms. This neointima was a multicellular layer of alpha-actin+ cells in a collagenous matrix, covered with a single layer of nitric oxide synthetase (NOS+) endothelial cells, whether it completely occluded the neck of the aneurysm or dived into the recurring or residual space between the aneurysm wall and the coil mass embedded in organizing thrombus.

CONCLUSION

Complete angiographic occlusions at 3 months can be associated with incomplete neointimal closure of the neck at pathology. Thrombus organization, endothelialization, and neointima formation can occur concurrently with recurrences.