3.3.1.2

External Carotid Artery

The external carotid artery (ECA) gives off four major arteries, the branches of which contribute to the supply of the CS and the CNs.

3.3.1.2.1

Ascending Pharyngeal Artery

The ascending pharyngeal artery (APA), the smallest branch of the ECA, is a rather gracil, long vessel. It arises close to the origin of the ECA at its dorsal circumference and ascends between ICA and the pharyngeal wall to reach the base of the skull. Its meningeal branches are very small vessels, supplying the dura mater and have been described in detail by Lasjaunias and Moret (1976). One branch enters the skull through the foramen lacerum, another through the jugular foramen, and sometimes a third one through the hypoglossal canal.

The first, the carotid ramus (ramus caroticus), accompanies the ICA in its canal. It anastomoses at the level of the foramen lacerum with a small branch of the C5-portion (recurrent artery of the foramen lacerum).

The second, the jugular ramus (ramus jugularis), passes with CNs 9–11 through the jugular foramen and reaches the dura mater. It anastomoses within the sigmoid sinus with the dorsal branch of the occipital artery and within the inferior petrous sinus with the medial branch of the lateral clival artery.

The third, the hypoglossal ramus (ramus hypoglossus), accompanies and supplies the hypoglossal nerve. It branches further within the dura of the foramen magnum and anastomoses with the medial clival artery arising from the MHT (Lasjaunias and Moret 1976). This ramus does sometimes not exist, in which case a connection between the extradural course of the vertebral artery and a corresponding branch can be found.

3.3.1.2.2

Internal Maxillary Artery

The internal maxillary artery (IMA), or maxillary artery according to current terminology, is the largest terminal branch of the ECA consisting of a mandibular, a pterygoid and a pterygopalatine segment giving rise to at least 16 terminal branches. Because of their anastomoses with branches of the C4 segment of the ICA, the middle meningeal artery and

the accessory meningeal artery (both arising from the second, the pterygoid segment), are of greater importance in the context of this monograph. The branches of the third portion leave the pterygopalatine fossa through corresponding foramina and fissures (Allen et al. 1973) which account for a relative constant angiographic pattern. Among the anteriorly directed branches are the posterior superior alveolar artery, the infraorbital artery and the greater (descending) palatine artery. The sphenopalatine artery is considered the terminal branch of the maxillary artery and leaves the pterygopalatine fossa through the sphenopalatine foramen (Allen et al. 1973). The posteriorly directed branches are from medial to lateral the pterygovaginal artery, the artery of the pterygoid canal (Vidian) and the artery of the foramen rotundum (see also Figs. 7.77–7.79).

The artery of the foramen rotundum (AFR), because of its connection with the ILT, has particular importance (Djindjian and Merland 1973), lying within the pterygoid fossa and leaving it through the foramen rotundum as the most laterally coursing branch (Allen et al. 1973). It anastomoses with the anterolateral branch of the ILT (in older literature, artery of the inferior cavernous sinus). This very small artery has a diameter of about 150 microns (Lang 1979a) and is usually not visualized on standard angiograms, unless its diameter is increased (Ribeiro et al. 1984; Allen et al. 1974). Its course is usually oblique, posterior and cranial through the canal. It represents one of the most prominent branches in the supply of DCSFs, and may also play a role as collateral supply to the circle of Willis in case of ICA occlusions or as tumor feeding vessel for meningiomas of the sphenoid wing (Ribeiro et al. 1984).

The vidian artery (Arteria canalis pterygoidei Vidii, artery of the pterygoid canal) arising within the pterygopalatine fossa as a branch of the distal maxillary artery, courses within the vidian canal towards the foramen lacerum and may anastomose within the oropharyngeal roof with branches of the AMA and APA (Lasjaunias 1984). It can arise from the greater palatine artery and may continue and anastomose with the petrous segment of the ICA. The vidian artery is a remnant of the first aortic arch and has been demonstrated to arise from the inferior (55%) or anteroinferior (35%) aspect of the internal carotid artery within the petrous bone in 30% of anatomic specimens (Paullus et al. 1977; Quisling and Rhoton 1979). It runs here for an average of 7 mm along the anterior wall of the carotid canal before it emerges from the skull through the cartilage

of the foramen lacerum and enters the pterygoid canal and has an average diameter of less than 0.5 mm (Lang 1983). The vidian artery supplies the lateral pharyngeal recess, giving rise to branches supplying the auditory tube and the tympanic region (Allen et al. 1973). This artery can be involved in the arterial supply of DCSFs (Osborn 1980b).

In contrast the pterygovaginal artery arises from the distal part of the maxillary artery and passes posteriorly through the pterygovaginal canal along the roof of the nasopharynx and anastomoses with the inferomedial eustachian branch of the accessory meningeal artery, the Eustachian branch of the APA and the mandibular branch of the ICA (Lasjaunias et al. 2001). This most medial coursing branch is also called the pharyngeal artery and supplies the choanes, the pharynx and eustachian tube.

As mentioned by Allen et al. (1974), other collateral branches may traverse the lateral aspect of the superior orbital fissure and are called Aa. anastomoticae after Lie (1968). The latter erroneously described these branches as all passing through the supraorbital fissure and did not recognize the artery of the foramen rotundum.

Although neglected in several textbooks, modern high-resolution DSA and, in particular, 3D-DSA reveal that in some DCSFs these “Aa. anastomoticae”, passing through the SOF, are indeed identifiable. They course more cranially above the artery of the foramen rotundum and often contribute to the supply of DCSFs (see Chap. 7).

3.3.1.2.3

Middle Meningeal Artery

The middle meningeal artery (MMA) is the second ascending branch of the IMA and the largest of the dural arteries. Its main branch courses through the foramen spinosum to enter the skull. Of the four groups of branches (extracranial, basal, anterior and posterior) the extracranial and basal arteries are of special interest. The extracranial branches supply local structures, in particular the AMA with a branch to the foramen ovale and the above lying meninges. The basal group supplies the cranial fossa (Lasjaunias and Theron 1976). Of the latter, the inferior meningeal branches supplying the middle cranial fossa, as well as their anastomotic branches to the extracranial arteries, are of importance: to the APA via the carotid canal and the foramen lacerum, to the vidian artery via the foramen lacerum, to the AMA via the foramen ovale and to the IMA via the

foramen rotundum. Other anastomotic vessel are medial branches to the Gasserian ganglion and to the cavernous segment and their anastomoses to the ICA and other branches supplying the CS; finally anterior branches supplying the superior orbital fissure and their anastomoses. Sometimes one larger branch of the temporal segment of the MMA curves posteriorly and medially and contributes to the supply of the CS or even the tentorium. The MMA usually has anastomoses with the posterolateral branch of the MMA and may also give rise to the medial (marginal) tentorial artery (Benndorf 2008).

3.3.1.2.4

Accessory Meningeal Artery

The accessory meningeal artery (AMA) supplies the pharynx and the eustachian tube, sometimes the meninges. According to Baumel and Beard (1961) it arises almost as frequently from the IMA as from the MMA, depending on if the latter is of the deep or superficial variant. It never originates before the MMA. In the superficial variation (IMA lateral to the lateral pterygoid muscle), the AMA originates from the MMA, in the deep variation (IMA medial to the lateral pterygoid muscle), it stems directly from the IMA (Lasjaunias and Theron 1976; Vitek 1989) in 60% of cases, immediately following the origin of the MMA, and in the remaining cases from the middle portion of the pterygoid segment. In 24% of the cases it consists of more than one vessel. The AMA courses parallel to the superior boundary of the medial pterygoid muscle in a fascial plane between the medial and lateral pterygoid muscles (Vitek 1989). The artery ascends through the interaponeurotic space and divides below the skull base into its anterior and posterior rami. The anterior ramus courses along the eustachian tube, the posterior meningeal ramus courses in circa 10% through the foramen ovale (Lang 1979a) or the foramen of Vesalius to supply the lateral wall of the CS, the Gasserian ganglion and the antero-superior surface of the petrous bone. It may also contribute to the supply of the sphenoid sinus. In case of a dominant supply of the CS region by transcranial branches of the IMA, the AMA may supply the entire area, giving rise to four branches usually belonging to the ILT in 20% of the cases (Lasjaunias et al. 2001). Lang (1979b) describes the AMA as a branch of the MMA, contributing to the rich vascular network on the surface of the dura mater that is provided mainly by the latter (Djindjian and Merland 1973). Other small