Basilar artery fenestration (or more simply, basilar fenestration) is the most common intracranial arterial fenestration and most common congenital anomaly of the basilar artery. This anatomic variant is characterised by duplication of a portion of the artery that are connected proximally and distally.

Epidemiology

Its reported prevalence is highly variable depending on the technique used:

• angiography: 0.6% ³ (presumably low due to preferential laminar flow in only one lumen at a time)
• CT angiography: 2.4% ²
• MR angiography: 1-2% ¹⁹
• autopsy: up to 5% ⁵

Pathology

An arterial fenestration is a segmental duplication wherein a single lumen divides into two lumens, both lined by endothelium, that rejoin to one distally. 

Fenestrations vary in size. At one extreme, basilar septation is a rare variant considered to be a miniature/aberrant basilar fenestration ⁸. At the other extreme, complete duplication can be considered extreme fenestration of the basilar artery ⁹.

Aetiology

Fenestration is thought to occur due to failed fusion of plexiform primitive longitudinal neural arteries ⁴.

Location

Basilar fenestration typically occurs at the lower end of the basilar artery just as the vertebral arteries join. However, it can also be seen in the mid-basilar and distal tip.

Treatment and prognosis

Basilar fenestration is generally considered an anatomic variant not requiring observation or treatment. However, adverse sequelae have been reported in the literature.

Complications

Fenestration may predispose to basilar artery aneurysm formation, presumably due to abnormal flow dynamics, but the magnitude of risk is inconsistent across studies ^1,6,12. Fenestration-associated aneurysms most often occur at the vertebrobasilar junction, followed by the basilar trunk, and are usually saccular in morphology ⁷. The reported prevalence of aneurysms in cases of basilar fenestration is 7% ³.

Thromboembolic posterior circulation infarcts have been reported with basilar artery fenestration ^10-18.

Differential diagnosis

The main differential consideration for a small fenestration is an acquired focal arterial dissection with a raised intimal flap dividing true and false lumens.