Small basilar artery fenestration mimicking an aneurysm.

Fenestration of the basilar artery is a well-known anatomical variant which can be misdiagnosed as arterial dissection. Misdiagnosis as an aneurysm is also possible and has been reported. A 41-year-old woman was referred to our hospital for evaluation of a suspected aneurysm of the proximal basilar artery trunk found on 1.5 T magnetic resonance angiography. More detailed 3 T magnetic resonance angiography revealed a small fenestration, not an aneurysm. The diameters of the vessels in the two limbs forming the fenestration were remarkably different: one limb had almost the same diameter as the main basilar artery, while the other had a very small diameter and coursed in an arc on the right outer side. The smaller limb of the fenestrated segment appeared to mimic an aneurysm on the initial magnetic resonance angiography. Careful imaging assessment is important to identify basilar artery fenestrations, especially small ones and those with a large difference in diameter between the two limbs.