The occipital artery (OA) is typically a branch of the external carotid artery (ECA), arising from its posterior aspect and coursing toward the occipital region to supply the posterior scalp and neck muscles [1]. However, numerous atypical origins of the OA have been reported, including origins from the internal carotid artery (ICA), thyrocervical trunk, inferior thyroid artery, vertebral artery (VA), and the ascending cervical branch of the inferior thyroid artery [2]. Uchino et al. [3] retrospectively reviewed magnetic resonance angiography (MRA) images of 2,866 patients that included the carotid bifurcation. They observed six cases (seven arteries) with variant origins of the OA, representing an incidence of 0.21%. Specifically, the OA arose from the ICA in four patients (five arteries), from the carotid bifurcation in one patient, and from the VA in one patient. Notably, five of the seven variant arteries occurred on the right side, suggesting a possible lateral preference in these anomalies.

The posterior inferior cerebellar artery (PICA) is the largest branch of the VA and typically originates intracranially before the junction of the VAs to form the basilar artery [1]. Variations in the origin of the PICA are clinically important due to their implications in cerebellar circulation and potential involvement in cerebrovascular diseases [4]. The PICA may arise from the basilar artery or may be absent unilaterally, while an extradural origin of the PICA has been documented in 5%–20% of cases [5]. Additionally, origins of the PICA from the ICA, posterior meningeal artery, hypoglossal artery, and proatlantal artery have been reported [5]. A systematic review reported that an extradural origin of the PICA from the V3 segment of the VA was found in 1.6% (41/2,511) of the European population and 0.6% (6/975) of the American population, indicating a slightly higher prevalence in Europeans [6].

The coexistence of multiple rare vascular anomalies in a single patient is uncommon and requires additional caution in clinical practice. Such anomalies can increase the risk of inadvertent vascular injury during surgical or endovascular procedures and may affect hemodynamics, potentially leading to neurological deficits [7]. Understanding these variations is significant for neurosurgeons, interventional radiologists, and clinicians involved in head and neck interventions to prevent complications.

In this case report, we present a 38-year-old female patient with a unique combination of two rare anatomical variants: an OA originating from the VA and an extradural PICA arising from the V3 segment of the VA. Additionally, the presence of the suboccipital artery of Salmon (SAS) originating from the same segment enhances the complexity of the vascular anatomy. To our knowledge, this is the first documented case of such a coexistence. We discuss the clinical significance of these findings and emphasize the importance of detailed vascular imaging in preoperative planning to ensure patient safety and optimal outcomes.

During a retrospective analysis of archived conventional catheter digital subtraction angiograms (DSAs), we identified a unique unilateral coexistence of two rare anatomical variants in a 38-year-old female patient. The case was documented using Horos 3.3.6 software for MacOS (Horos Project). The study was approved by the responsible authorities (affiliation 3, approval no. 2093/1 March 2022), and written informed consent was obtained from the patient. The patient underwent a DSA examination to investigate potential arteriovenous malformations. Bilateral analysis revealed that the anatomical variants were present on the left side (Fig. 1), while the right side exhibited normal arterial anatomy (Fig. 2).

The DSA of the left VA revealed an anomalous origin of the OA from the horizontal portion of the V3 segment of the VA. The OA exhibited a tortuous course toward the occipital region, giving rise to several branches: an auricular branch that coursed superolaterally, a superficial descending branch supplying the upper posterior neck muscles and cutaneous tissues, a superficial collateral branch, and terminal branches supplying the posterior half of the scalp (Fig. 1).

Immediately inferior to the origin of the OA from the VA, we observed the SAS, also originating from the horizontal part of the V3 segment. The SAS had a short trunk that bifurcated into two main branches.

Additionally, from the same portion of the V3 segment, an extradural PICA was seen to originate. It followed its characteristic tortuous course, entered the skull through the foramen magnum, formed an upward loop, and branched into its medial and lateral terminal trunks (Fig. 1).

Variant origins of the OA are rare but have been documented (Table 1) [3, 5, 8-10]. Uchino et al. [3] reported an incidence of 0.21% for the OA originating from the ICA and the VA in a study of 2,866 patients, reporting only one case of VA-origin left OA in a 51-year-old female. Such variations are clinically significant as the OA occasionally provides the main inflow to the basilar artery [8]. In certain animals like the ox, horse, cow, and elephant, the OA serves as the primary feeding vessel to the vertebrobasilar system, analogous to the human occipital-basilar collateral pathway [8]. In humans, two collateral systems facilitate communication between the ECA and intracranial vessels [11]: (1) the facial-angular-ophthalmic arteries to the carotid siphon and (2) the OA and its muscular branches to the vertebral and basilar arteries. When the OA originates from the VA, the second collateral system is absent, leaving the facial-angular-ophthalmic pathway as the sole route. This has significant implications in surgery involving the face, an area of interest primarily for maxillofacial and plastic surgery; if the latter pathway is compromised, the lack of an alternative route may result in inadequate blood supply, increasing the risk of ischemic complications.

Traditionally, the OA’s course is divided into three segments [9]: digastric, suboccipital (horizontal), and occipital (subgaleal). The first segment extends from its origin to the point where it emerges from the occipital groove of the mastoid process and is closely associated with the posterior belly of the digastric muscle. The second, or horizontal segment, runs from the occipital groove to the superior nuchal line, passing between the splenius capitis and semispinalis capitis muscles. The third segment extends beneath the galea aponeurotica above the occipital muscle before dividing into terminal branches. In our case, due to the OA’s origin from the VA, the digastric segment is absent, resulting in only the suboccipital and occipital segments. Notably, the superficial descending branch arises directly from the main trunk rather than from a common descending trunk, which was observed in approximately 25% of cases [10].

The extradural origin of the PICA is exceedingly rare, with a prevalence of less than 1% [12]. An extradural PICA has significant clinical implications because any aneurysms originating from it would also be extradural, potentially altering surgical management [13]. Extradural exposure of the V3 segment of the VA carries a risk of injuring an extradural PICA if this variant is not identified preoperatively. This risk is heightened during lateral transcondylar approaches to lesions of the foramen magnum or spinomedullary area and during transtemporal approaches to glomus jugular tumors [13]. Therefore, it is important to perform bilateral selective vertebral angiography before such procedures to delineate the anatomical variations of the VA and its branches, assess the dominance of the VAs, and evaluate collateral circulation. Preoperative identification of these variations can prevent serious surgical complications [13].

The SAS is another important vessel in the occipital region, with two described course patterns [14]: type I, featuring a single main trunk with collateral branches, and type II, involving a short common trunk that divides into two branches. In our case, the SAS exhibited a type II pattern. Macchi et al. [14] also report a case of the origin of the PICA from the V3 segment of the VA. There is an 80% chance of communication between the SAS and branches of the OA, which can have implications for regional blood flow and surgical planning [14]. In our angiographic evaluation, the limitations of DSA prevented us from confirming any anastomotic branches between the SAS and OA. While some branches appeared superimposed on imaging, the lack of planar or sectional views inherent to DSA means we cannot definitively state that anastomoses exist between these vessels. Despite its status as the gold standard for vascular imaging [15], DSA may benefit from complementary imaging modalities. We recommend the use of computed tomography angiography or MRA to provide detailed planar images that can confirm arterial communications, refining the understanding of complex vascular anatomies.

The coexistence of an OA originating from the VA, an extradural PICA from the V3 segment of the VA, and a type II SAS in a single patient is unprecedented. Awareness of such variants is important for clinicians to prevent inadvertent vascular injuries, particularly in procedures involving the craniovertebral junction and posterior cranial fossa.