(Knott 1882). There are usually two ICS with a diameter of up to 8 mm (Lang and Weigel 1983), connecting the CS anteriorly and posteriorly to the hypophysis and forming a venous circle (elliptic sinus, circular sinus, clinoid sinus). The anterior ICS is larger; the posterior can be completely absent. Knott (1882) found its absence in 26/44 cases (anatomical studies). In two cases he found the posterior ICS was larger and in one case it was the only one present. He found 3 transverse ICS in 15/44 cases. Small irregular venous sinuses coursing beneath the pituitary gland and draining into the ICS were already described by Winslow as socalled inferior circular sinus. Knott (1882) found this arrangement in only 6 cases, whereas in 12 others he found a single intercavernous vein beneath the pituitary body. In addition to the two ICS, Doyon et al. (1974) and others (Lasjaunias et al. 2001) have described a posterior communication via an occipital transverse sinus which probably corresponds to the basilar plexus.

Yasuda et al. (2004, 2005) in his recent study of the medial wall of the CS also described an inferior intercavernous sinus that connects the paired CS. These ICS extend across the midline between the meningeal dural layer covering the inferior aspect of the pituitary gland and the endosteal layer covering the floor of the sella.

Meningeal Veins

The meningeal veins (MV) are often developed as pairs, enclosed by dura sheets. Commonly, they accompany meningeal arteries and provide venous drainage for the cranial dura mater and are considered meningeal sinuses by some authors (Oka et al. 1985). They course between the arteries and the overlying bone which lead to a compression of their lumen and the typical radiographic appearance of parallel channels (Oka et al. 1985). They communicate with the superior sagittal sinus and unify to form two trunci anteriorly and posteriorly, which accompany the branches of the meningeal arteries. At the base of the skull they pass through the foramen spinosum and ovale to connect with the pterygoid plexus. The veins accompanying the anterior branch of the MMA drain into the SPPS, the CS or emissary veins (Oka et al. 1985).

Veins of the Foramen Rotundum, Emissary Vein

Knott (1882) observed in 2 cases (2/44) an additional small tributary in the form of an emissary vein passing through the canalis rotundus accompany-

ing the maxillary division of the trigeminal nerve. It is connected with the extracranial pterygoid plexus and is considered by Browder and Kaplan (1976) a draining vein (rarely several small veins). Knott (1882) found in 23/44 subjects an inconstant vein lying in the dura mater on the inner surface of the greater sphenoid wing.

On rare occasions, the basal vein or the inferior ventricular vein was found to anastomose with the lateral wall of the CS (Lang 1983).

3.3.2.3

Drainage of the Cavernous Sinus (E erent Veins)

Superior Petrosal Sinus, Sinus petrobasilaris (Langer), Sinus tentorii lateralis (Weber), Sinus petrosus superficialis

The superior petrosal sinus (SPS) courses between the layers of the tentorium cerebelli over the superior aspect of the petrous bone and connects the posterior superior angle of the CS with the proximal sigmoid sinus or the transverse sinus (Oka et al. 1985). It receives blood from the cerebellar veins, the lateral mesencephalic vein, the petrosal vein, the paracavernous sinus, the basal vein and the arcuate vein. Although often considered a channel draining the CS, the SPS may actually not drain blood from the CS. The blood flow is frequently directed from posterior to anterior, towards a confluence at the petrous apex, which is formed by the posterior part of the CS, a (transverse) communicating sinus, connecting the posterior CS and the IPS, draining the blood towards the IJV (Krayenbühl and Yasargil 1997). In contrast to Theron (1972), in our experience the posterior CS is relatively often visible in vertebral angiograms, which is explained by this direction of the flowing blood within the SPS. Knott (1882) found a complete absence of this sinus in 3/45 cases.

Inferior Petrosal Sinus, Sinus petrosus profundus, Sinus petro-occipitalis superior (Trolard)

The inferior petrosal sinus (IPS), shorter but larger than the SPS, represents the main posterior drainage of the CS towards the bulb of the internal jugular vein. It courses along the lower edge of the petrous bone and the petro-occipital suture, lying between the petrous pyramid and the clivus in the petroclival fissure. It reaches the jugular foramen and passes together with the glossopharyngeal, vagus and accessory nerve to enter the internal jugular vein (IJV). It receives blood from veins of the labyrinth, the

cochlear aqueduct, medulla, pons and cerebellum. The IPS has a close relationship with the abducens nerve, which is imbedded in the wall of its sagittal segment.

According to Boskovic et al. (1963) this sinus has a diameter of 7–10 mm in 93% of cases and has horizontal and sagittal parts. According to Lang (1983) it enters the posterior fossa 2–6 mm medial to the trigeminal pore (Dorello’s canal) with a diameter of 7–10 mm in its longitudinal and 0.5–5 mm in its transverse part. The actual entry is usually situated below the superior sphenopetrosal ligament and also contains the sixth CN in its lateral angle.

Shiu et al. (1968) studied 346 patients undergoing petrosal sinus sampling and described 4 different types of junction between the IPS and the IJV (Fig. 3.11):

Miller et al. (1993) modified this classification and added an incomplete type IV which is characterized by a small vein between the IJV and the point at which the IPS anastomoses with the vertebral venous plexus. It needs to be remembered, however, that these descriptions are exclusively based on retrograde venous angiograms and therefore may, due to variable venous hemodynamics, thrombosis of the IPS and suboptimal image quality, not fully reflect the true anatomy in this area. Thus, some vascular details like the IPCV was not recognized as such, although opacified in some of the figures (Fig. 2C therein).

Knott (1882) and others (Labalette 1891; Sappey 1888; Theile 1843) described in the nineteenth century a connection between the IPS and the IJV below the skull base (3/8 of an inch). Lang and Weigel (1983) found such a variant in 10% of their specimens. The authors describe in addition an inferior petrosal sinus canal in 2%–3% of the cases. Surprisingly, this variant is not included in the classical description of Shiu et al. (1968). Recent studies have shown that the IPS, as an anatomical variant, may also enter the IJV far below the skull base (up 4–5 cm) (Gailloud et al. 1997). Gailloud et al. (1997) have named this deep termination of the IPS as the accessory jugular vein. We have observed this situation in three patients with a vascular lesion of the CS (see Fig. 7.37) and were able to successfully catheterize

Fig. 3.11. Four main types of IPS-IJV connections as described by Shiu (from Shiu et al., 1968). This widely used classification is based on 2D angiograms from the pre-DSA era. It does not display all anatomic details in this area, which is in part due to the technique used for the cavernous sinus venography. Retrograde opacification of the IPS and CS can be limited by local hemodynamics and thrombotic processes. Furthermore, modern bi-plane DSA imaging provides significanly better spatial resolution (see Chap. 7). The inferior petroclival vein and the internal carotid artery venous plexus are not illustrated, although visualized in modern high-quality phlebograms. In AP view the vein lies next to the IPS and follows an almost parallel course. Neither the fact that the IPS may be thrombosed and thus only partially filled, nor the possibility of an abberant inferior petrosal sinus (deep termination below the skull base) was known or taken into account at that time

this aberrant inferior petrosal sinus (Benndorf and Campi 2001). According to Katsuta et al. (1997) the IPS, in its course along the petroclival fissure, also has connections with the CVP through the intrapetrosal veins and with the venous channel, called the inferior petroclival vein (IPCV, see below) which courses along the extracranial surface of the petroclival fissure [probably identical to the petro-occipi- tal sinus described by Aubin et al. (1974) and others (Clay and Vignaud 1974)]. The IPS forms within the jugular foramen a venous confluens of 2–3 mm diameter, receiving blood from the venous plexus of the hypoglossus canal, the inferior petroclival vein

and tributaries from the vertebral venous plexus and the posterior condylar vein. Sometimes two main channels connect the petrosal confluens with the jugular bulb (Katsuta et al. 1997).

Venous Plexus of the Hypoglossal Canal, Anterior Condylar Vein

This plexus is also referred to as the anterior condylar vein (ACV), which connects the marginal sinus with the JB. It is crossed by trabeculae and may empty into the lower end of the IPS or directly into the IJV. Occasionally it drains into the lower end of the sigmoid sinus and may communicate with the posterior condylar vein and the vertebral plexus (Katsuta et al. 1997).

Posterior Condylar Vein

The posterior condylar vein (PCV) passes through the posterior condylar foramen and courses in the posterior condylar canal to connect the vertebral venous plexus with the junction of the sigmoid sinus and the JB. The posterior condylar foramen opens into the posteromedial junction of the sigmoid sinus and the JB. It has communications with the confluens at the lower end of the IPS and via a bony channel with the anterior condylar vein (venous plexus of the hypoglossal canal) (Katsuta et al. 1997).

Lateral Condylar Vein

The lateral condylar vein (LCV) runs between the inferior aspect of the external orifice of the hypoglossal canal (anterior condylar canal) and the suboccipital cavernous sinus as described by San Millan Ruiz et al. (2002) and Arnautovic et al. (1997).

Inferior Petroclival Vein

The inferior petroclival vein (IPCV) (Fig. 3.12, see also Figs. 7.85–7.90) is, according to Katsuta et al. (1997), a “mirror image” to the IPS coursing along the extracranial surface of the petroclival fissure and empties into the confluens at the lower end of the IPS (Rhoton 2000). This vein is in some studies, probably erroneously called inferior petro-occiptal vein (San Millan Ruiz et al. 2002).

Petro-occipital Sinus, Sinus petro-occipitalis inferior, petro-occipital vein (Padget)

This very small sinus, described by Trolard (1890), also originates from the CS, courses parallel to the IPS, but outside the skull, along the petro-occipital suture, enters the IPS just before

its connection with the IJV and can be seen in phlebograms of the jugular vein (Aubin et al. 1974) or in orbital phlebograms (Clay and Vignaud 1974). Because of its small caliber and its proximity to the IPS it may be easily superimposed and is best visualized in the projection after Aubin et al. (1974) and can be missed on standard angiographic projections (see Chap. 7). The projection is called after Hirtz and was suggested by Aubin. Both, the petro-occipital sinus and the inferior petroclival vein are possibly referring to the same vascular structure.

Transverse Occipital Sinus (Doyen)

Doyen et al. (1974), while reviewing the ophthalmic veins, described a sinus as transverse occipital sinus that connects both posterior CSs horizontally along the upper surface of the clivus. In another figure (Fig. 7.64 therein) this sinus travels caudally to reach the foramen magnum. I have not be able to find this sinus mentioned elsewhere other than in Aubin et al. (1974) who describe it as the communication of the two posterior CS with the anterior segments of the IPS and the SPS.

Basilar Plexus (Virchow)

The basilar plexus (BP) consists of several communicating venous channels between the dural sheets above the dorsum sellae of the sphenoid bone and the bony skull base. The largest and most constant connection across the midline between the CS (Renn and Rhoton 1975) extends from the dorsum sellae and the pars basilaris of the sphenoid bone downwards, to become the marginal sinus. Its upper part, lying over the rostral end of the clivus, is connected with the posterior CS and both SPS. Its lower part is connected with both IPS and with the anterior internal vertebral venous plexus. According to Lang (1979a) it regularly contains clival rami of the MHT.

Marginal Sinus

This sinus lies between the layers of the dura mater adjacent to the anterior inner surface of the foramen magnum. It receives blood from the basilar plexus and communicates with the occipital sinus and the anterior internal venous plexus. Tubbs et al. (2006) recently studied further the anatomy and found significant communication between the marginal sinus and the veins of the hypoglossal canal, and that the vertebral artery was noted to pierce this sinus in the majority of the cases.

Internal Carotid Artery Venous Plexus,

Sinus Venous Caroticus (Haike), Carotid Sinus, Pericarotid Plexus

The internal carotid artery venous plexus (ICAVP) was first described by Rektorzik in 1858 as pars intracanalem sinus caroticus, and descends from the inferior cavernous sinus enclosing the carotid artery more or less completely at the lower part of the carotid canal (Knott 1882) (Fig. 3.12). This plexus converges to form one or more trunks which open fi- nally into the IJV and was found by Knott (1882) in each case of his series. Haike (1902) found that the sinus resembles the architecture of the CS, consisting of numerous small vascular spaces in children that confluent to larger lacunes with age. To him it appeared in some cases more like a plexus, in others more like a sinus. Knosp et al. (1987a) considers the plexus in many cases a very important drainage system and found it more prominent in the fetus when compared to the adult condition. Lang (1983) found this plexus running together with the artery, sympathetic nerves and tissue of the arterial sheath to form two longitudinal veins at the external aperture of the carotid canal which originate near the bend of the ICA. The initial part of the plexus communicates with the IPS via a medial intrapetrosal vein in about 46% and via a lateral intrapetrosal vein in 50% of the cases. The former terminates about 11 mm above the jugular foramen, the latter terminates in the vicinity of the foramen (Lang and Weigel 1983). Paullus et al. (1977) found this (periarterial venous) plexus in 76% of the cases, in 24% it was poorly developed or absent. It extended an average of 7.6 mm into the canal and was located between the floor of the middle cranial fossa and the artery in only 4%. A fistula between the ICA and this plexus could mimic a true CCF. This sinus is difficult to identify on angiograms or phlebograms and may be superimposed by the IPS or IPCV, and thus is not shown by several authors investigating the angiographic anatomy in the CS–IPS region (Hanafee et al. 1968; Shiu et al. 1968; Miller DL et al. 1993). Aubin (1974) demonstrated its visibility in axial projections, although it may be superimposed by the petro-occipital sinus (IPCV). Modern high-resolution DSA and threedimensional DSA proves valuable in visualizing Rektorzik plexus (Chap. 7).

Foramen Ovale Plexus (Trigeminal Sinus), Sphenoid Emissary, “Rete” of the Foramen Ovale

Nuhn (in Knott 1882) described a pair of small veins, which pass through the foramen ovale to

reach the pterygoid plexus. Knott found this plexus quite variable: in 18/45 cases he found a pair of veins on both sides, in 10 cases he found a pair on one side and a single vein on the other, in 11 cases there was a single vein on each side and in 5/45 cases a total absence on one side. Henderson (1966) called this sinus trigeminal sinus. Browder and Kaplan (1976) found a number of veins draining the pachymeninx of the inferior part of the cerebral hemispheres converging to the middle cranial fossa and coursing to the foramen ovale, where channels from the ventrolateral aspect of the CS join them. Padget (1956b) assumed that this plexus may not necessarily be concerned with the venous drainage, especially in the young. In adults, however, we see this plexus often drains the SMCV directly, bypasses the CS and may reach a diameter of up 6 mm (roughly the diameter of the foramen).

Vein of the Sphenoid Foramen (Foramen Venosum, Foramen of Vesalius)

A very small, inconstant vein supplementing the foramen ovale plexus when passing through the foramen venosum the margins of which are usually not ossified at birth (Padget 1956b).

Foramen Lacerum Plexus

Knott (1882) constantly found a varying number of small veins passing through the foramen lacerum and always being connected with the cavernous sinus. The emissary veins connect with the pterygoid plexus but also with the inferior petroclival vein.

Pterygoid Plexus

The pterygoid plexus (PP) is an extensive network of small venous channels in the neighborhood of the IMA and lies lateral and medial to the lateral pterygoid muscle and is connected with the CS via sphenobasal emissary channels passing through the foramina in the middle cranial fossa. It receives blood from the transbasal veins (middle meningeal veins, foramen ovale plexus, carotid venous plexus, vein of the foramen rotundum, vein of the foramen venosum). The pterygoid plexus also communicates with the ophthalmic veins through the inferior orbital fissure, with the anterior facial vein via a deep facial branch and receives tributaries corresponding to branches of the pterygopalatine maxillary artery segment (Osborn 1981). It usually drains posteriorly into the maxillary vein, which forms together with the superficial temporal vein the retromandibular vein. Although the latter vessel is usually