The Superior Cerebellar Artery

The superior cerebellar artery (SCA) is often described as the most consistent artery of the infratentorial group. Some variations were noted especially in the proximal part of the artery, such as duplication, triplication, and unusual origin from the basilar artery. It is consistently described as bifurcating into rostral and caudal branches when originating solely from the distal basilar artery. Duplication is mostly encountered as two trunks originating from the basilar artery, with the superior trunk bearing rostral and the inferior trunk caudal branches, respectively. In 14 brains with 28 superior cerebellar arteries, which form the basis of this article, the majority of the duplications were related to the basilar-artery origin of a marginal branch that is also one of the cortical branches of the SCA. Moreover, this kind of proximal origin of the marginal branch resulted in its being richer in perforators than one of distal origin. Other variations that were noted in our study were also those related to marginal branches. The majority of duplications of the SCA were related to the basilar-artery origin of the marginal branches. A proximal origin seems to confer on the marginal artery the property of being an important supplier of perforators. This perforator dominancy, as a result of proximal origin, of the marginal artery might be important clinically in cerebellopontine angle and clivus surgery.

T he superior cerebellar artery (SCA) arises near the apex of the basilar artery, extends to near the pontomesencephalic junction, encircles the brainstem through the cerebellomesencephalic fissure, and supplies the tentorial and the superior petrosal surfaces of the cerebellum. 1 Direct or indirect perforating branches and precerebellar branches also emerge from the SCA, supplying pons, cerebral and the cerebellar peduncles, and deep cerebellar nuclei. 1,2 Even though the SCA is considered to be the most consistent artery of the infratentorial branches, inconsistencies are noted in its anatomy. To elucidate the anatomy of posterior circulation in the brain and to revise previous reports, we present the SCA territories in the 14 human cadaver brains studied.

MATERIALS AND METHODS
Fourteen human cadaver brains were included in this study. Thirteen of the brains were obtained from fresh autopsy cadavers and 3 of these were injected with a quick-acting silicone mixture, described earlier for injection into autopsy brains. 3 One other brain was obtained from our anatomy section and was injected with silicone in the same manner. The injected brains were studied after formalin fixation for 1 month. Ten brains from autopsy had not been injected with silicone ( Fig. 1), and 9 were studied on autopsy day. One underwent formalin fixation for 3 months. The territories of the SCAs, along with their branches, were studied under magnification by surgical loupes (X3) (Fig. 2).

Origin of SCA
Different variations at origin of SCA including duplication, and triplication, and origin from posterior cerebral artery were reported previously. 2,4-6 Bilateral occurrence of those variations was, however, encountered more infrequently. Duplications of the SCA were found to be 28, 4 14, 2 20, 5 and 5.9% in some studies. 6 Triplications were also noted as being 2 2 and 8% 4 in some of these studies. Bilateral duplication is another variation that is rare and has been reported in 2%. 2,6 Origin of the SCA from the posterior cerebral artery was found in 4% 2 of cases; bilateral occurrence of this variation is extremely rare. 6,7 In 14 brains in our study, 28 SCAs were examined. Seven of those 28 arose as 2 trunks, and duplication was noted as being present in 25% (Fig. 3). Bilateral duplication was present in one of the brains (7.1%).
CAs originated from the basilar artery on both sides in 13 of the brains, whereas in one of the brains, both the SCAs arose bilaterally from the posterior cerebral artery (Fig. 4). Origin from the posterior cerebral artery origin was seen in 2 out of 28 SCAs (7.1%).

Main Trunk, Main Branches, and the Marginal Artery
In classic descriptions, the main trunk of the SCA divides into 2 main branches during its proximal course at the level of the anterior or lateral brainstem, and these 2 branches are named rostral and caudal, or medial and lateral branches, respectively. 1,2,5,8 Some characteristics can be summarized for main trunks and the main branches of SCA: The bifurcation of the SCA was found to occur 0.6 to 34 mm (average, 18.5 mm) from its origin and was mostly lateral to brainstem 2 ; in 70% of the cases, it was lateral, and in 30%, anterior to the brainstem, 5 mostly at the level of the ambient cistern. 8  Before summarizing our results describing the characteristics of the main trunks and branches of the SCAs in our study, some discussions of the marginal branches are needed. The lateral or the caudal branches of the SCA are called marginal branches, 8,9 and they are thought to supply the superolateral cerebellar hemisphere and deep nuclei. The marginal branch of the SCA has been described by Hardy et al 2 and Rhoton 1 as a distinctive one that ''is the first cortical branch when present, usually arises from the lateral pontomesencephalic   segment of SCA, does not enter the cerebellomesencephalic fissure as do the other cortical branches, may arise from caudal or main trunk or from basilar artery as a variant of a duplicate origin of the SCA and supplies superior petrosal surface.'' It was noted as being present in 53% of hemispheres, originating from the caudal trunk (40%), the main trunk (10%), and the basilar artery (2%) in decreasing order of frequency. 1,2 In our study, 21 of the SCAs arose as single trunks, and 20 of them exhibited bifurcation into rostral and caudal trunks. One of these SCAs did not exhibit bifurcation and, instead, the marginal artery proximally was the first cortical branch, and the superior vermian artery at the distal part of the cerebellomesencephalic fissure was a second. This pattern was accepted as constituting a solitary SCA without bifurcation. The bifurcation sites of the SCAs, which arose as single trunks in our study, were mostly lateral to the brainstem (67.8%) and were not always more distal than the level of the ambient cistern.
In 7 duplicated SCAs in our study, only 2 exhibited the distribution pattern of a solitary SCA, in that superiorly located trunks behaved like a rostral branch of a solitary SCA, whereas the inferiorly located vessels were like a caudal branch (Fig. 3). In 5 other duplications, the inferiorly located trunk was a marginal artery, whereas the superiorly located branches supplied the rest of the SCA territory (Figs. 5, 6). In those 5 duplications, the superior trunks bifurcated into rostral and caudal trunks in 3, but were not seen to bifurcate in 2; instead, they gave off hemispheric branches during their courses in the cerebellomesencephalic fissures. In a unique finding, in one of the solitary SCAs, the rostral branch was anastomosing with the distal part of the caudal branch inside the cerebellomesencephalic fissure (Fig. 7).

Perforators
The perforating branches of SCA are of 2 types: direct and circumflex perforating branches. 2,5 Although the direct type has a straight course toward the brainstem, the circumflex type winds around it, and sends branches into it along this course. The circumflex arteries are of 2 forms. The long branch travels more than 90 degrees around the circumference of the brainstem; the short circumflex travels shorter distances. 1,2 Previously, 69% of SCAs arising as a single trunk were found to give perforating branches before their bifurcation into rostral and caudal trunks, and only 7 of the caudal and 2 of the rostral trunks failed to give perforators. 2 The average numbers of perforating branches arising from the main, rostral, and caudal trunks of SCAs were 2, 5, and 2, respectively. 2 The main trunk mostly gave rise to a circumflex artery (61%) and then a direct perforating artery (25%), and a short circumflex artery (14%). 2 The perforating branches from the main trunks terminated in the tegmentum in the region of the junction between the superior and middle cerebellar peduncles, interpeduncular fossa, cerebellar peduncles, and collicular regions in decreasing order of  frequency. 2 The perforating branches from the rostral and the caudal trunks were mostly circumflex in type and terminated in the junction between the superior and middle cerebellar peduncles and thereafter in the inferior colliculus, the cerebral peduncle, and the interpeduncular fossa. 2 The marginal artery was also found to give rise to perforators in 7 hemispheres out of 26 (27%) and to an area of middle-cerebellar peduncle. 2 In 21 of the SCAs in our study that were observed to have solitary trunks, 17 main trunks had 37 perforating branches. Direct perforating branches were the most commonly encountered type, and a total number of 18 were verified (Fig. 9). The number of direct perforators from the main trunk was between 0 and 4. Again, 11 long circumflex with 8 short circumflex branches were observed to diverge from the main trunks and their numbers were between 0 and 2. The main vascular territories of these perforating branches that arose from the main trunks were interpeduncular fossa, cerebral peduncle, pons, cerebellar peduncle, and the tectal plate.
In 20 of the solitary SCAs bifurcating into rostral and caudal trunks, 17 of the rostral and the 18 of the caudal trunks gave off perforators. Of the 48 perforating branches arising from the rostral trunks, 20 were long circumflex, 16 were short circumflex, and 12 were direct perforating branches, whereas in 34 perforating branches arising from caudal trunks, 14 were long circumflex, 10 were short circumflex, and 10 were direct perforating branches. The territories of the perforating branches from the rostral and the caudal trunks were the cerebral cerebellar peduncles and the tectal plate.
In the 7 duplicated SCAs observed in our study, 22 perforating branches were identified. The superior trunks gave off 6 direct perforating, 2 long circumflex, and 1 short circumflex branches, whereas the inferior trunks provided 9 direct perforating, 1 long circumflex, and 3 short circumflex branches.
The marginal artery also gave off perforating branches. As mentioned before, in 5 of the duplicated SCAs, the inferior trunk was a marginal artery, and only 1 of the marginal arteries in this subgroup did not have perforating branches. In all, 22 perforating branches arose from the duplicated SCAs. Six of the direct perforating and the 3 of the short circumflex branches arose from the marginal arteries originating directly from the basilar arteries.
One of the 2 marginal arteries originating from main trunk also provided 1 direct perforating and 2 short circumflex arteries. Interestingly, any of the marginal arteries originating from caudal branches or inferior trunks of duplicated SCAs could give off perforating branches.

Precerebellar Arteries
The precerebellar arteries are described as small arteries arising from the trunks and the cortical branches of SCAs within the cerebellomesencephalic fissures. 1,2 These arteries tether the distal part of the SCA, and their dissection is difficult; as many as 8 are found to arise within the fissure. 1,2 Their territories are described as including the deep cerebellar nuclei, the inferior colliculi, and the superior medullary velum, according to whether they originate from the cortical branches lateral to the vermis or from the cortical branches of the vermis. 1,2 We could observe precerebellar arteries in our specimens injected with silicone (Fig. 10), but could not discern a general pattern of these arteries in noninjected autopsy brains.

Cortical Branches
The main cortical territories of the SCAs were the tentorial surface, upper part of the suboccipital surface, superior part of the vermis, and the superior part of the petrosal surface (Fig. 11). A frequent pattern was supply to the vermis by a vermian artery arising from the rostral trunk and bifurcating at the area that meets the vermis; supply to the medial tentorial and medial superior suboccipital surfaces was mostly through hemispheric branches arising from the rostral trunk (1 to 3 hemispheric branches). Supply to the lateral tentorial and lateral suboccipital surfaces was through hemispheric branches arising from caudal trunk (1 to 3 branches), and to the superior petrosal surface of the cerebellum through the marginal branch of the SCA. This general distribution in our study is concordant with previous findings 1,2 ; 3 of the SCAs in our study (1 in solitary group and 2 in duplicated group) were, however, not bifurcated, instead giving off cortical arteries throughout their courses. The cortical branches in 2 of these 3 SCAs were the most lateral hemispheric branch, which arose first, and the vermian artery, which was the last. The superior vermian artery was the first cortical branch originating from the other SCA.

DISCUSSION
Almost every previous anatomic description of the brain and its vessels has been revised with new gross anatomic original contributions in the last few decades, for example, the excellent studies pioneered by Rhoton. The studies on SCA are also examples of these. 1,2 The SCA has been described as the most consistent of the infratentorial cerebellar arteries in its extent and area of supply, 1,10 with some variations, seen as duplication, in 5.9% to 28% of cases, as triplication in 2% to 8%, and as having a unilateral or bilateral posteriorcerebral-artery origin in 4% and 1%, respectively. 2,[4][5][6] Our results corroborated these earlier findings of SCA anatomy, with a duplication rate of 25%, and a bilateral origin from the posterior cerebral artery in one of the specimens. We describe significant differences from other studies peripherally.
First, the origins of the marginal branches in our study were significantly different from those in previous 1,2 studies, in that 25% of the marginal branches in our study had a basilar-artery origin. This finding changes the classic description of the duplication of SCAs as ''superior trunk of duplication behaving as a rostral and inferior trunk behaving as a caudal branches respectively'' to ''superior trunk replacing a single SCA and an inferior trunk providing the marginal artery (5 out of the 7 duplicated SCAs).''  Second, solitary SCAs were found to bifurcate into rostral and the caudal trunks previously, 1,2 but we also observed one solitary trunk, without bifurcation, giving off the cortical branches during its course. Reanastomosis between the rostral and caudal branches of the SCA in the cerebellomesencephalic fissure in one specimen seems to be an original finding (Fig. 7).
The perforating branches were not different from those in previous descriptions 1,2 in their courses, but their number was smaller. An interesting finding was regarding the perforating branches arising from the marginal artery if the latter originated from the basilar artery (25%) or from the main trunk (10%) of the SCA; these marginal arteries were rich in perforating branches, but those more distal in origin did not yield frequent perforators in our specimens. This finding indicates that the marginal branch with a proximal origin is likely to yield perforating branches.
Descriptions of cortical arteries were concordant with previous findings. 1,2 The superior part of the vermis and the cerebellar hemisphere at the tentorial and the superior suboccipital surfaces, with the petrosal surface, constituted the main territory of the SCA. In short, rostral branches tend to supply to the vermis and the paravermian area, whereas the caudal branches tend to supply more laterally.
In conclusion, marginal branches can frequently originate from the basilar artery to form a pattern of duplicated SCAs, and such proximal branches are apparently rich in perforating branches. These findings emphasize that the marginal perforators are clinically more important than their cortical supply, especially when they originate more proximally. This can be an important surgical point with upper clival and tentorial tumors.