Arterial communications between the carotid and vertebrobasilar systems in the fetus may occasionally persist in the adult.
A primitive trigeminal artery is the most cephalad and common of these persistent fetal anastomoses.
The persistent trigeminal artery usually arises from the presellar ICA as it exits the carotid canal and enters the cavernous sinus and it extends posteriorly to join the distal third of the basilar artery usually between the origins of the superior and anterior inferior cerebellar arteries.
The anomalous vessel may have a parasellar course or may run through the middle of the dorsum sella with nearly equal frequency.
Flow in the anastomosis is usually from the ICA to the basilar artery.
An increased occurrence of other coexisting intracranial vascular abnormalities has been reported in as many as 25% of patients with a persistent trigeminal artery (i.e. intracranial aneurysms, arteriovenous malformations, carotid-cavernous fistulae, and moyamoya).
The clinical relevance of a persistent trigeminal artery is debatable as most cases are discovered incidentally.
Knowledge of the presence of a trigeminal artery is vital before performing intracranial sellar-parasellar or vascular intervention.
Sagittal MR images may show the anomalous vessel as an abnormal flow void coming off the posterior aspect of the cavernous ICA and running a somewhat horizontal course posteriorly to the basilar artery.
The tau sign refers to the unusual configuration of signal voids in the presellar ICA and in a persistent trigeminal artery, as viewed on parasagittal T1-weighted MR images. The combination of the vertical and horizontal segments of the ICA and the proximal portion of the trigeminal artery creates the outline of the Greek letter tau.