In tertiary syphilis, inflammation of the adventitia and the vasa vasorum of the aorta lead to weakening of the media and gradual aneurysm formation. These aneurysms typically affect the arch of the aorta, and may involve the root of the aorta, leading to incompetence of the aortic valve. They may be multiple. The pathogenesis is uncertain, but a congenital defect of the media is postulated. These aneurysms are not identified at birth, but seem to develop over years.
Rupture results in subarachnoid hemorrhage.