MRI Vessel Wall Imaging and Treatment of an Aneurysm at the Atlanto-Axial Segment of an Aberrant Vertebral Artery.

We report a case of unique location of an aneurysm at the atlanto-axial extradural segment of a unilateral aberrant vertebral artery. The MRI vessel wall imaging findings and possible mechanism of aneurysm formation were discussed. A 5 mm extracranial vertebral artery aneurysm located at the interlaminar space between C1 and C2 was diagnosed in a woman presenting with occipital headache. The index vertebral artery ran an aberrant course at the V3 segment, where it entered the dura between C1 and C2 instead of the usual atlanto-occipital space. MR vessel wall imaging showed homogenous wall enhancement of the aneurysm sac. We surmise the anomalous course of the vertebral artery subjected the V3 segment to repeated shearing force secondary to the atlanto-axial rotational neck movement. This led to vessel wall trauma and inflammation, and subsequent aneurysm formation. The aneurysm was successfully treated with endovascular coiling with resolution of symptoms.