Basilar artery fenestration: an incidental finding in a patient with headache

A previously healthy 59-year-old female patient was admitted to the emergency department with a “thunderclap” headache, localised in the occipital lobe. At neurological examination no pathologic findings were identified. An unenhanced CT scan excluded subarachnoid haemorrhage.

The patient was visited by a neurologist and due to clinical suspicion of an arteriovenous malformation (AVM) a CT angiography was performed.
CT angiography showed a focal fusiform ectasia (4.5 mm) of the proximal tract of the basilar artery, with a bulging of the right wall with an eccentric hypodensity adherent to it, highly suggestive for acute dissection or for a partially thrombosed aneurysm (Fig. 1). For further definition, a cerebral angiography was performed.
Angiography excluded dissection or thrombosis and revealed the presence of a basilar artery fenestration of an extension of 7-8 mm. This appeared to be slightly asymmetric, characterised by the right vessel being thinner than the left one. Both lumina appeared patent (Fig. 2).
The cause of the headache was unknown and the patient was discharged from the hospital.

Fenestration of a cerebral vessel is a rare congenital anomaly, defined as a division of the arterial lumen into distinctly separate channels, each with its own endothelial and muscularis layers, while the adventitia may be shared. Fenestration should be differentiated from duplication, which consists of two distinct arteries with separate origins and no distal arterial convergence [1-2].
Intracranial arterial fenestration is more common in the vertebrobasilar arteries, as in our case, than in the arteries of the anterior circulation. Basilar artery fenestration has been found in 0.6% of angiographic examinations and approximately 5% of autopsies [3-4].
The basilar artery is formed by the fusion of paired longitudinal neural arteries during the 5th gestational week; as this fusion progresses, bridging arteries that temporarily connect the longitudinal neural arteries regress. If the bridging arteries fail to regress, the result is fenestration of the basilar artery [2].
The fenestration most commonly involves the proximal basilar trunk, close to the vertebrobasilar junction. Based upon the morphology and size, basilar artery fenestration can be classified in two types: slit-like shape and convex-lens-like shape [5].
An association has been observed between fenestration and intracranial aneurysms. It has been postulated that turbulent flow created by defects in the tunica media at the proximal and distal ends of a fenestrated segment leads to aneurysm formation. The reported frequency of aneurysm formation in cases of basilar artery fenestration is 7% [6].

Written informed patient consent for publication has been obtained.