Any creature with a trigeminal nerve has a trigeminal artery, which has as much right to life, liberty, and pursuit of happiness as facial or hypoglossal, for example. Which is why the adage “Persistent” is a misnomer. Yes, we all call it that, but it is important to recognize that this term is a product of an earlier, incomplete understanding of vascular embryology. Although the trigeminal is usually below angiographic resolution, it nevertheless exists in everyone. It is seldom the vessel selected by development to serve as an intracranial anastomosis between the anterior and posterior circulations — even the more usual PCOM is far from constant, and subject to the important embryologic variation of the fetal disposition. The great Lasjaunias considered the PCOM to be an integral part of the anterior circulation (caudal ramus, see dedicated Neurovascular Evolution page). However, some have challenged that notion on the basis of certain internal carotid artery variations that do not seem to conform to Lasjaunias’ model, and proposed that the PCOM is in fact an anastomotic vessel, like the trigeminal. This, to me, seems to be at odds with the nature of the trigeminal artery as the vessel of the first branchial arch, which seems to have no correlate to the PCOM; I have no proof for either idea, but love to digress, as happened just now…
Getting back to the point, the trigeminal artery is an important potential conduit between the carotid and basilar arteries. It does follow the course of the trigeminal nerve. The classic anastomosis is between the basilar trunk, between the AICA and SCA, and the MHT of the ICA. In the full spectrum of possibilities, the trigeminal can arise from either the MHT or the ILT, and anywhere between the SCA and the AICA , including rare variants where it is the exclusive source of either complete or partial AICA and SCA territory supply (see below). Presence of the trigeminal artery need not imply hypoplasia of the PCOM; in fact, in most instances of angiographically resolvable trigeminal, the PCOM is present as well.
Here is an example, also found on the Neurovascular Evolution page; the “classic” trigeminal (green arrows) extends from the basilar trunk between the AICA and SCA, to the MHT, and is associated in this case with hypoplasia of the PCOM. Essentially all vascular variants are associated with a slightly increased incidence of brain aneurysms, as evidenced by the coil mass (brown arrows)
In the case below, the trigeminal artery itself is associated with a large aneurysm. Is it intradural or extradural?
Trigeminal supply to the AICA
In this variant, the trigeminal artery (white arrow), supplies the lateral cerebellar territory classically belonging to the AICA
AP view of carotid injections showing the AICA (black arrow), with corresponding wedge-shaped void of its territory seen on the right vert injection. Notice fetal disposition of both PCOMs.
This variant can also be appreciated by an astute neuroradiologist on a CT angiogram
Another example of a trigeminal artery (red) ending in AICA (yellow)