Clinical History
A 58-year-old patient presented with clinical symptoms of acute ischaemic stroke and was referred to our department for MR imaging of the brain.
Imaging Findings
TOF MR Cerebral Arteriogram showed an anomalous artery arising from the cavernous segment of the left ICA, running posteriorly, lateral to the sella turcica, curving medially at the level of dorsum sella and passing through it to join the mid-basilar artery inferior to the origin of superior cerebellar arteries - suggestive of lateral (petrosal) variant of persistent trigeminal artery (PTA).
Sagittal MIP image of MRA TOF showing the "TAU" or "Trident" sign formed by the ICA and the proximal portion of the PTA.
V4 segments of both vertebral arteries and the proximal basilar artery were hypoplastic.
The right posterior cerebral artery (PCA) had a fetal configuration, whereas the left PCA was arising from the basilar artery with normal ipsilateral posterior communicating artery (PCOM).
Discussion
At 5 weeks of gestation during the embryological development, the internal carotid arteries (ICAs) extend from the paired dorsal aortic arches and anastomose with the paired longitudinal neural arteries that form a primitive vertebrobasilar system at four major sites [1, 2]. These fetal anastamoses are trigeminal, otic, hypoglossal and proatlantal intersegmental arteries, named according to their neighbouring structures [1, 2]. Failure of regression of these vessels during normal embryological development lead to persistent primitive carotid-vertebrobasilar anastamoses [1].
Persistent primitive trigeminal artery is the most cephalic and the most common type among them with a prevalence rate of 0.1-0.6% [1]. It arises from the ICA immediately after its exits from the carotid canal, runs posteriorly and joins the mid or distal third of the basilar artery usually between the origins of the superior cerebellar and anterior inferior cerebellar artery (AICA) [1, 3]. It may take a parasellar or intrasellar (transhypophyseal) course and is divided accordingly as lateral (petrosal) and medial (sphenoidal) variants respectively [2-4]. Ipsilateral posterior communicating artery (PCOM), vertebral artery and the basilar artery caudal to the anastamosis are usually hypoplastic [3].
In Saltzman type 1, PTA terminates in the basilar artery supplying both posterior cerebral arteries (PCAs) and superior cerebellar arteries (SCAs) with the absence of PCOMs [1, 5]. In Saltzman type 2, PTA terminates in the basilar artery supplying only SCAs on both sides whereas the PCAs are supplied by the intact PCOMs [1, 5]. In some cases, PTA arising from the precavernous segment of ICA may end directly as cerebellar artery without joining the basilar artery known as variants of PTA (Saltzman type 3) [1, 2, 5]. AICA is the most common of these, but SCA and posterior inferior cerebellar artery (PICA) may also arise from the internal carotid artery [1].
A classic finding indicative of PTA on sagittal MR images is the “Tau” sign due to its resemblance with the Greek letter “Tau” [3, 5]. The vertical limb and anterior horizontal limbs of “Tau” are formed by the ICA and the posterior limb is formed by the proximal portion of PTA [5]. Persistent otic artery may mimic PTA but its origin (petrous segment of ICA) is lower and the vertical limb of the ICA is seen above its origin [1, 5].
There are few case reports on PTA causing oculomotor palsy, abducent nerve palsy, trigeminal neuralgia and hyperprolactinaemia. Identifying PTA is vital before performing intracranial sellar-parasellar or vascular surgery [3, 4].
Differential Diagnosis List
Saltzman Type 2 persistent trigeminal artery (lateral / petrosal variant).
Variants of persistent trigeminal artery
Persistent primitive otic artery
Final Diagnosis
Saltzman Type 2 persistent trigeminal artery (lateral / petrosal variant).
