Introduction

Persistent trigeminal artery (PTA) is the most common remnant of embryonic communication between the carotid artery and the vertebrobasilar systems. The PTA originates from the cavernous segment of the internal carotid artery (ICA) and communicates with the basilar artery (BA). Numerous anatomical and embryologic studies of PTA and PTA variant have been reported with an incidence of about 0.2–0.76 %. Dozens of aneurysms of PTA and PTA variant have been reported previously [1–17]. However, aneurysms with medial type PTA trunk are extremely rare [4, 10, 17]. PTA is divided into two types by Uchino: lateral PTA and medial PTA. The medial PTA is through cavernous sinus and takes a posterior course medial to the first branch of trigeminal nerve. It shows an intrasellar course in close contact to the pituitary gland. We report a case of saccular aneurysm in the left medial-type PTA trunk. The BA under the union is hypoplastic. The bilateral posterior communicating arteries (PComAs) are absent.

Case report

A 77-year-old woman who had symptom suggestive of vertebrobasilar insufficiency underwent cerebral magnetic resonance (MR) imaging and head and neck MR angiography (MRA) using a 3 T MR scanner. The patient had syncope once with a history of hypertension and heart disease.

Cerebral MR conventional imaging showed multiple small cerebral infarcts of the bilateral corona radiata. Non-contrast 3D TOF MRA images of the head and neck using a 3 T scanner revealed a left PTA, which originated from the left ICA C5 portion and connected to the BA, with a saccular aneurysm in the mid portion of its trunk and the hypoplasia of the BA under the union (Fig. 1a, b). The bilateral PComAs were absent. The diameter of the bilateral vertebral artery is smaller than that of the BA. The distal portion of BA can be well observed, but the proximal portion to the union is very faint. We made maximum-intensity projection (MIP) images of the MRA and VR (volume rendering) to see it more clearly. The axial source images of MRA also showed the medial type PTA which originated from the left ICA, through cavernous sinus and connected to the BA (Fig. 1c, d). We followed the aneurysm closely without treatment.

Fig. 1
figure 1

a A 77-year old woman who had symptom suggestive of vertebrobasilar insufficiency. 3D TOF MRA maximum-intensity projection image shows a primitive trigeminal artery (PTA) (long arrowhead) separate from the left carotid artery C5 portion and connected to the basilar artery, and an aneurysm arising from the PTA trunk (short arrowhead). The basilar artery proximal is hypoplasia (white arrow). b 3D TOF MRA volume rendering image shows the PTA (long arrowhead) separate from the left carotid artery C5 portion and connected to the basilar artery, and an aneurysm arising from the PTA trunk (white arrow). c 3D TOF MRA source image shows the PTA (white arrow) separate from the left carotid artery C5 portion, through cavernous sinus and connected to the basilar artery. d 3D TOF MRA source image shows a saccular aneurysm in the mid portion of the medial type PTA trunk

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Discussion

Carotid-vertebrobasilar anastomoses include the persistent PTA, the persistent primitive otic artery, the persistent primitive hypoglossal artery, and the persistent primitive proatlantal artery. These anastomoses, which are named according to their associated anatomy, link the primitive ICA and BA systems, and exist in early fetal life (4- to 5-mm embryonic stage) [6]. As the vascular system evolves, they are quickly remodeled, with the trigeminal artery being the last to disappear (7 to 14-mm embryonic stage) [6]. Rarely, the anastomoses persist into adulthood. PTA may co-exist with proatlantal intersegmental artery [12].

PTA and its variants are usually incidental findings. The angiographic anatomy and classification of PTA were first described by Saltzman and Wollschlaeger [9]. In Saltzman type 1 PTA, the PTA supplies the BA, posterior cerebral arteries (PCAs), and upper cerebellar arteries (SCAs) segment of the BA proximal to the insertion of the PTA may be hypoplastic and the PComAs may be absent. In Saltzman type 2 PTA, the PTA joins the BA below the origin of the SCA, and the PCA receives its blood supply predominantly through the patent PComA. Our case is considered as Saltzman type 1 PTA in which the bilateral PComAs were absent. In our case, the BA proximal segment was hypoplastic and the bilateral PComAs were absent, thus, the main supply to the posterior circulation came from the left ICA via the PTA.

PTA is also divided into two types by Uchino: lateral PTA and medial PTA. The lateral PTA arises from the cavernous ICA near the posterior genu curves laterally and posteriorly around the dorsum sellae. It runs underneath the abducens nerve and caudally between the trigeminal and abducens nerves to join the BA. The medial PTA is through cavernous sinus and takes a posterior course medial to the first branch of trigeminal nerve showing an intrasellar course in close contact to the pituitary gland. Medial type PTA has been reported to occur more rarely than lateral PTA. The literatures reported that the PTA can coexist with the pituitary adenoma [5]. Recognizing medial type PTA is very important in clinical practice when performing transsphenoidal surgery on the pituitary, because failure to do so could result in a massive hemorrhage [5, 8]. This case was a medial type PTA and the saccular aneurysm was located in the sella turcica. Being aware of this is crucial in clinical, because they can influence surgical and interventional procedures.

Hypoplasia of the BA proximal to the location where it joins the PTA has been reported. In our case, although the distal portion of the BA is easily observed, the portion that is proximal to the site of BA/PTA convergence is very faint. Due to the BA proximal segment was hypoplastic and the bilateral PComAs were absent in our case, the main supply to the posterior circulation come from the left ICA via the PTA. This has cautioned about the potential risk of posterior circulation ischemic lesions of the occlusion of the ICA and PTA.

An association of PTA with intracranial vascular anomalies has been reported, such as aneurysms, arteriovenous malformations, and Sturge-Weber syndrome, etc. of which the cerebral aneurysms are the most common. The specific hemodynamics of the PTA, which directly branches and receives blood flow from the ICA, may reduce the formation of aneurysm. The pathogenesis of the aneurysm is the result of congenital defects of the medial layer of this primitive vessel added to a hemodynamic stress due to the anatomic location of the PTA [14]. The symptom of PTA aneurysm includes cranial nerve palsy, carotid artery and cavernous sinus fistula, and subarachnoid hemorrhage, etc. The most prevalent symptom of PTA trunk aneurysm was subarachnoid hemorrhage. Our patient was found incidental.

Aneurysms can occur on either the PTA or PTA variant and are almost always saccular in form [16]. The prevalence of intracranial aneurysms in PTA and PTA variant is reported as 3–4.2 %, which is similar to that of the general population [3].This indicated that the aneurysms seem to have no association with PTA and PTA variant. The reported aneurysms of PTA were either at their origin, at their mid-portion, at their junction with BA [1], PComA [2], or at the medial cerebral artery, PCA and ICA. Yang et al. [15] and Yamamoto et al. [16] reported fusiform aneurysms of PTA variant. Although PTA associated aneurysms have occurred on the intracranial arteries, the cavernous ICA, intracranial the PTA and BA junction, and the PTA trunk, few of them were located on the PTA or PTA variant itself [1–17]. Our patient had an aneurysm on the PTA trunk. Rupture of such an aneurysm on the PTA might result in the formation of a PTA-cavernous sinus fistula [17]. PTA aneurysms may result in abducens or oculomotor nerve palsy which is more frequent in individuals with lateral type PTA.

Due to the deep location of PTA trunk aneurysm and close to the cranial nerves, the direct surgery is very difficult. Takase et al. [10] reported a PTA trunk aneurysm which successfully eliminated by clipping without sequelae. Endovascular treatment has been reported successfully used in the PTA aneurysm [4, 7]. Flow preservation is highly important in patients undergoing treatment for PTA trunk aneurysms, because insufficient flow in the main trunk of the PTA compromises the posterior circulation, and may induce dangerous post-treatment sequelae [4]. Our elder patient rejected further intervention, so management has relied on follow up.

Conclusions

To our knowledge, this is the first case of aneurysm arising from the medial type PTA trunk with hypoplastic BA under the union and the absence of the bilateral PComAs. Detailed MRA knowledge of the presence of this aneurysm is of great importance clinically.