There are two possible definitions of SCA. One defines SCA as the artery with the most distal origin among the infratentorial cerebellar arteries. Another defines SCA as the artery that supplies the tentorial cerebellar surface. Because the former definition is more specific, it has been adopted as the definition of SCA.

SCA originates from the basilar artery (BA) near its apex. It dips caudally and encircles the brainstem near the pontomesencephalic sulcus. After passing CN V, SCA enters the cerebellomesencephalic fissure, where its branches supply the superior cerebellar peduncle and are finally distributed to the tentorial cerebellar surface (Fig. 3.2a–e). SCA usually arises as a single trunk, but may occasionally arise as a double trunk in 15–30 % of cases.

Hardy DG et al. [1] divided SCA into four segments: (1) Anterior pontomesencephalic, (2) Lateral pontomesencephalic, (3) Cerebellomesencephalic, and (4) Cortical (Table 3.1).

The anterior pontomesencephalic (pontine) segment begins at the origin of SCA, extending laterally below CNs III to the anterolateral margin of the brainstem (Fig. 3.2a, b).

The lateral pontomesencephalic (ambient) segment begins at the anterolateral margin of the brainstem and travels along the lateral part of the pons, parallel to CN IV (Fig. 3.2c, d). SCA often bifurcates into rostral and caudal trunks in this segment (Fig. 3.2d). The rostral trunk supplies the cerebellar vermis and the medial part of the cerebellar hemisphere, whereas the caudal trunk supplies the lateral part of the cerebellar hemisphere.

The cerebellomesencephalic (quadrigeminal) segment courses within the cerebellomesencephalic fissure (Fig. 3.2c). Bilateral SCAs become proximate in this segment. This segment gives rise to the precerebellar artery, which supplies the superior cerebellar peduncle.

The cortical segment gives rise to branches distal to the cerebellomesencephalic fissure, which pass below the tentorial edge and are distributed to the tentorial cerebellar surface. Branches of the cortical segment are divided into the vermian and hemispheric groups (Fig. 3.2a). The vermian branches ascend the culmen after dipping caudally at the level of the declive and anastomose with the vermian branches of PICA on the tentorial cerebellar surface. The marginal artery is the most proximal of the cortical branches. It usually arises from the pontomesencephalic segment and courses along the anterolateral margin of the cerebellum; therefore, it is also called the anterolateral marginal artery (Fig. 3.2d).

SCA passes near CNs III, IV, and V and frequently makes contact with them (Fig. 3.2a–c). Because the proximal part of SCA passes below CN III, the nerve is sandwiched by SCA inferiorly and the posterior cerebral artery (PCA) superiorly. CN IV arises below the inferior colliculus and traverses forward in the cerebellomesencephalic fissure. On reaching the lateral side of the brainstem, it courses below the lower surface of the tentorium, penetrating the dura mater at the posterior edge of the oculomotor trigone to enter the cavernous sinus. CN IV passes superomedial to the lateral pontomesencephalic segment of SCA as it traverses forward. CN V arising from the lateral part of the upper pons exists in the upper cerebellopontine angle. It traverses forward, beneath the tentorial attachment to enter the Meckel’s cave. The lateral pontomesencephalic segment of SCA encircles the brainstem and makes contact with CN V from the superomedial side while forming a caudal loop near or on the nerve (Fig. 3.2d). Compression of CN V by SCA may cause trigeminal neuralgia. SCA has a few potential points of contact with CN V because it bifurcates into rostral and caudal trunks in addition to forming a caudally projected loop (Fig. 3.2d, e) [2, 5].

The perforating branches of SCA are divided into direct and circumflex types [1]. The direct type pursues a straight course to enter the brainstem in the interpeduncular fossa. The circumflex type winds around the brainstem and supplies the tegmentum, cerebellar peduncle, and inferior colliculus. SCA gives rise to the precerebellar arteries in the cerebellomesencephalic fissure, which course along the superior cerebellar peduncle in the fissure and supply the inferior colliculus, superior medullary velum, and dentate nucleus. The superior colliculus is mainly supplied by PCA.

The four segments of SCA—a downward loop formation of the anterior and lateral pontomesencephalic segments and the rostral and caudal trunks—can be visualized using magnetic resonance imaging (MRI), magnetic resonance angiography (MRA), and conventional angiography. Before the advent of MRI and MRA, it was considered that the shape of SCA in the anterior–posterior view using angiography shows that of the brainstem, and the downward loop formation of the lateral pontomesencephalic segment of SCA may be related to the contact between the artery and CN V. Nowadays, the relationships between SCA and CNs can clearly be observed using MRI and MRA, including preconstructed MRA images. In cases of trigeminal neuralgia, careful attention should be paid to the relationship between CN V and SCA because SCA is often involved in the disease.

(Regarding imaging of SCA, please refer to Chap. 10: “Anatomy for Microvascular Decompression Procedures: Relationships between Cranial Nerves and Vessels, Preoperative Images, and Anatomy for the Stitched Sling Retraction Technique”)