Aneurysms of the dorsal (anterior) wall of the internal carotid artery (IC d-ANs) are rare. Although their clinical features are well described, their mechanism of pathogenesis is unknown. We present 6 cases, including an autopsy case, of IC d-AN and the pathogenesis of IC d-ANs on the basis of clinical records, anatomic findings and history. Most IC d-ANs have fragile walls and necks and can be described as “blister like.” In our series, 3 aneurysms were of the blister type and 3 were of the saccular type. Although the operative findings and clinical course differ a little between blister-type and saccular-type IC d-ANs, we could not conclude that these 2 types of aneurysm have different pathogenesis. Moreover, the possibility still remains that some blister-type aneurysms may grow to saccular aneurysms. Some authors report that the blister-type, or fragile wall, aneurysm may be a type of dissecting aneurysm. Intraoperative and pathologic findings suggest that these aneurysms may be pseudoaneurysms caused by a partial- or full-thickness tear through the wall of the artery,but they cannot be defined as dissecting aneurysms because there is no dissecting lumen. Moreover, if there are findings of dissecting aneurysm intraoperatively we should treat it as dissecting aneurysm. IC d-ANs should be defined more clearly on the basis of findings of the aneurysm and the parent artery because IC d-ANs are classified on the basis of intraoperative features, not pathologic findings.
