Spontaneous Disappearance of Intracranial Unruptured Saccular Aneurysms: Two Case Reports.

Spontaneous disappearance of small unruptured intracranial aneurysms is exceedingly rare, as most reported cases describe resolution in the context of rupture, giant aneurysms, or dissecting lesions. We report two cases of small saccular aneurysms that vanished during long-term magnetic resonance angiography follow-up. In Case 1, a 3.6 × 3.4-mm right internal carotid artery aneurysm demonstrated gradual reduction followed by complete disappearance, coinciding with proximal hemodynamic alteration. In Case 2, a 3.6 × 2.9 mm aneurysm of the right middle cerebral artery remained stable for 7 years before disappearing following asymptomatic M2 branch occlusion. These observations suggest that spontaneous thrombosis may occur even after prolonged morphologic stability, particularly when hemodynamic conditions shift toward reduced flow or altered shear stress. Recognition of this phenomenon is important for follow-up strategies, and additional imaging details are provided in the supplementary material.