Bifurcation of basilar artery

The bifurcation of the basilar artery (Basilar apex) is the terminal division point of the basilar artery where it splits into the two posterior cerebral arteries (PCAs), located at the level of the proximal midbrain, just distal to the oculomotor nerves (cranial nerve III).

Anatomical Context

The basilar artery is an unpaired midline vessel formed by the confluence of the two vertebral arteries at the base of the pons. It courses anterosuperiorly within the basilar sulcus of the pons, giving off several branches along its path including the anterior inferior cerebellar arteries (AICAs), pontine perforating arteries, and the superior cerebellar arteries (SCAs). At its most rostral end, the basilar artery terminates by dividing into the bilateral PCAs, and this division point constitutes the bifurcation.

Morphometric Features

• The overall length of the basilar artery averages approximately 30.7 mm (range 23.538.2 mm), with the segment just proximal to the bifurcation (the superior or distal segment) averaging about 3.6 mm in length and 4.2 mm in diameter.

• The bifurcation normally lies at or near the level of the suprasellar cistern; a bifurcation positioned above this plane may suggest elongation (dolichoectasia).

• The bifurcation angle (the angle between the two PCAs) averages approximately 103° ± 20.6° in normal individuals and tends to widen with age, particularly in females.

Morphological Configurations

The basilar bifurcation can be classified as either T-shaped or Y-shaped, depending on the angle at which the PCAs diverge from the basilar trunk. This distinction has been studied in the context of stroke outcomes, though current evidence does not show a significant difference in recanalization success or clinical outcome between the two configurations.

Clinical Significance

The basilar bifurcation is a site of major clinical importance:

• It is the most common location for posterior circulation aneurysms (basilar apex aneurysms). A wider bifurcation angle is significantly associated with aneurysm formation, even for small aneurysms (< 5 mm). Hemodynamic studies show that asymmetry in the vertebrobasilar junction can increase wall shear stress at the bifurcation, predisposing to aneurysm development.

• Penetrating arteries arising at the basilar apex supply the median and paramedian portions of the midbrain and thalamus, making occlusion at this level particularly devastating.

• The PCAs connect to the posterior communicating arteries, forming the posterior limb of the circle of Willis and providing an important route for collateral circulation between the anterior and posterior cerebral circulations.