Persistent trigeminal artery: A rare vascular carotid-basilar anastomosis

A 32-year-old female patient admitted to our hospital complaining of persistent headache of approximately four months duration. General physical examination revealed no pathology or other abnormalities. The patient initially underwent a brain magnetic resonance (MR) examination to investigate the cause of the headache.

The MR examination revealed an abnormal flow void arising from the proximal carvenous internal carotid artery (ICA) extending toward the basilar artery (Fig. 1). The following MR angiography (MRA) confirmed the presence of an anomalous vessel coming off the posterior aspect of the carvenous ICA and running posteriorly to the basilar artery (Fig. 2a, b). It also showed the existence of one single vertebral artery (Fig. 2b). No posterior communicating arteries where identified during the examination (Fig. 2c). The above findings are suggestive of Saltzman type 1 persistent trigeminal artery (PTA). Sagittal MR angiogram revealed the characteristic "tau sign" very well (Fig. 3).

The primitive trigeminal artery (PTA) together with otic (acoustic), hypoglossal and proatlantal intersegmental arteries are the four known primitive carotid-vertebrobasilar anastomoses. These four pairs of arteries first appear at the 4- to 5-mm embryonic stage and they retrograde with the development of the posterior communicating and vertebral arteries [1]. Even though these communictions normally regress in the fetus, we occasionally find them in adult life [1, 2].

Trigeminal artery is the most commonly seen of these arteries at a percentage that has been stated to reach 85% [3]. The angiographic incidence of PTA is thought to be between 0.06% and 0.6% reaching the 1% considering the undiagnosed or unreported cases [4, 5, 6]. The PTA forms a connection between the intracarvenous portion of the internal carotid artery and the basilar artery and it usually coexists with an hypoplastic vertebral artery and PCoA ipsilaterally [2]. Occasionally, the PTA communicates directly with a cerebellar artery only, without supplying the basilar artery. This artery is called the PTA variant [7, 8]. The PTA is located adjacent to many cranial nerves. Oculomotor, trochlear and abducens nerves are found inferiorly and the ophthalmic branch of the trigeminal nerve is found laterally [9].

Two types of PTA are recognised. In Saltzman type 1, the PTA supplies the distal basilar artery (between the superior cerebral arteries and the anterior inferior cerebellar arteries), the proximal basilar artery may be hypoplastic and the PCoAs may be absent. In Saltzman type 2, the PTA supplies the superior cerebellar arteries, while the PCoAs are present and supply the posterior cerebral arteries [10, 11].

PTA can be revealed at conventional angiography, magnetic resonance imaging (MRI) or angiography(MRA), or contrast-enhanced computed tomography angiography (CTA). In patients with a PTA, T1W sagittal or parasagittal MR images may show the anomalous vessel as an abnormal flow void arising from carvenous left carotid artery exteding toward basilar artery. The combination of the two branches of ICA (as it turns from a vertical to a horizontal course) and the proximal portion of the trigeminal artery creates a configuration resembling the Greek letter 'T' (tau). The sign is suggestive of a PTA [12, 13, 14].