Microsurgical Anatomy of the Cavernous Sinus

Yoshihiro MIYAZAKI, Isao YAMAMOTO*, Shogo SHINOZUKA,

and Osamu SATO

Department of Neurosurgery, Tokai University School of Medicine, Isehara, Kanagawa;*Department of Neurosurgery

, Yokohama City University School of Medicine, Yokohama

Abstract

The cavernous sinuses of 50 adult cadavers were examined to investigate the relationships of the blood vessels and cranial nerves, important structures during surgery in this sinus. The first and second divisions of the fifth cranial nerve were embedded in the deep dural layer of the cavernous sinus and were supplied by the two main branches of the intracavernous carotid artery. The meningohypophyseal artery supplied the sixth cranial nerve in Dorello's canal and the third and fourth cranial nerves where they entered the dura. The inferolateral trunk supplied the third, fourth, fifth, and sixth cranial nerves. The size of the meningohypophyseal artery was usually inversely proportional to the size of the inferolateral trunk. The capsular artery did not supply the cranial nerves. The cavernous sinus can be

approached through various routes: a) superior, through the anteromedial or medial triangle; b) lateral, through the paramedial, Parkinson's, anterolateral, and lateral triangles; c) inferior, through the posterolateral and posteromedial triangles; and d) from the inferomedial walls. The choice of surgical approach depends mainly on the location of the lesion to be treated.

Key words: carotid artery, cavernous sinus, cranial nerve, microsurgery,

microsurgical anatomy, surgical approach

Received May 12, 1993; Accepted August 18, 1993

Author's present address: Y. Miyazaki, M.D., Yokohama Miyazaki Hospital of Neurosurgery, Yokohama, Japan.

Introduction

The anatomy of the cavernous sinus has been the sub

ject of many studies since the similarity of the fibrous trabeculae to the corpus cavernosum of the penis was first described in 1732,5,9,18,21,24,26,29,32,33,37,40,41,42,46,47,49) Parkinson") pioneered the direct surgical approach to the cavernous sinus through a triangle bordered by the third and fourth cranial nerves superiorly and the first division of the fifth cranial nerve inferiorly, but this procedure was not widely adopted because of the potential risk of profound hypothermia and circulatory arrest.6) Recently, the increased interest in direct surgical approaches to the cavernous sinus has required a more detailed anatomical knowledge of this inaccessible area.

This study attempts to define the important relationships of the microsurgical anatomy for the various surgical approaches to the cavernous sinus.

Anatomical Study

The cadaveric heads of 10 adults and 20 tissue blocks

containing 50 cavernous sinuses preserved in for

malin were dissected under the operating microscope.

I. General relationships The cavernous sinus is an irregular cuboid located

just lateral to the sella turcica and pituitary gland (Fig. 1).

Roof: The roof of the cavernous sinus is trapezoid. The anterior border of this trapezoid corresponds to the anterior rim of the origin of the C2

portion of the internal carotid artery (ICA) medially and the lateral end of the superior orbital fissure laterally. The posteromedial point is the posterior clinoid process; the posterolateral point is the dural entrance of the fourth cranial nerve. The roof is

formed by two components: the anterior bony structure, which is the medial half of the lesser wing; and

the posterior dural membranous layer. The anterior

clinoid process is situated above the anterior part of

the roof.

Fig. 1 Lateral posterosuperior view of the cavernous

sinus. A: anterior rim of the origin of the C2

portion of the ICA, ACA: anterior cerebral

artery, B: posterior clinoid process, C: dural en

trance of the fourth cranial nerve, D: lateral

end of the superior orbital fissure, E-H: lower

margin of the foramen rotundum, F:

posteromedial border of the carotid sulcus, G:

medial border of the foramen ovale, Gr.Pet.N:

greater superficial petrosal nerve, Gen.Gang:

geniculate ganglion, MCA: middle cerebral

artery, MMA: middle meningeal artery, II

VII: second to seventh cranial nerves.

Floor: The floor is formed by the medial portion

of the greater wing of the sphenoid bone, including

the lower margin of the foramen rotundum anterior

ly and foramen ovale posterolaterally.

Medial wall: The medial wall of the cavernous sinus is formed at the top third by the lateral wall of

the pituitary gland and at the lower two-thirds by the

most medial greater wing including the carotid

sulcus. The middle clinoid process is a small

eminence of the lateral part of the sella turcica,

which forms the superior margin of the carotid

sulcus, and the ICA courses between the anterior and

middle clinoid processes. Kobayashi et al.") called

the dural pouch lying between the superior margin of the carotid sulcus and the carotid artery the carotid

cave. Lateral wall: The lateral wall of the sinus consists

of two membranous folds compressing an outer dural layer and an inner layer of loose connective tissue, which envelopes the third, fourth, and fifth cranial nerves. The inner layer around the third cranial nerve is rather thick and relatively easily dissected. However, the inner layer around the fourth cranial nerve and the first division of the fifth cranial nerve is thin and often firmly attached to the outer layer, resulting in an uniformly thick wall. The third cranial nerve penetrates the roof slightly anteromedial to the penetration site of the fourth cranial nerve, which forms the posterosuperior angle of the lateral wall of the cavernous sinus. At the anterior lateral wall, the third and fourth cranial nerves and the first division of the fifth cranial nerve meet just before entering the superior orbital fissure, where the two layers unite to form a thick single wall. As the third, fourth, and fifth cranial nerves ap

proach the superior orbital fissure, the fourth cranial nerve courses inferolaterally to the third cranial nerve, which is situated immediately below the anterior clinoid process. The sixth cranial nerve

pierces the dura mater lateral to the dorsum sellae, runs through Dorello's canal, and enters the posterior wall of the cavernous sinus, and then courses between the first division of the fifth cranial nerve and ICA within the cavernous sinus. Nathan et al.,") Harris and Rhoton,'8 and Miyazaki32) noted that the sixth cranial nerve split into two or more trunks in the cavernous sinus and finally fused into a single trunk, which then continued to the lateral rectus muscle. We found that the sixth cranial nerve split into two branches in only one (2%) of the 50 sinuses. Anterior wall: The anterior wall is a relatively

shallow rectangle formed by the anteromedial portion of the greater wing, which includes the superior orbital fissure above and the foramen rotundum below. The third and fourth cranial nerves and the first division of the fifth cranial nerve pass through the superior orbital fissure and the second division of the fifth cranial nerve passes through the foramen rotundum. Posterior wall: The posterior wall is bounded by

the ligament petroclinoidea posterior above, which forms the tentorial edge situated between the

posterior clinoid process and the superior border of the petrous temporal bone, and the petrous apex below. The superolateral border of the clivus is situated at a point two-thirds along the medial border of the posterior wall. The trigeminal ganglion occupies the trigeminal (Meckel's) cave in the dura

mater, covering the trigeminal impression near the

petrous apex. It is medial to the petrous ICA and the

posterior part of the cavernous sinus, and inferior to the greater petrosal nerve, the petrous apex, and the

foramen lacerum.

The first and second divisions of the fifth cranial

nerve then pass forward into the cavernous sinus

close to the lateral part of the posterior wall. The

sixth cranial nerve runs through Dorello's canal and

pierces the dura mater lateral to the clivus in the cavernous sinus. Within the cavernous sinus, this

sixth cranial nerve courses along the carotid artery,

medial and parallel to the first division of the fifth

cranial nerve.

Fig. 2 Origins of the 56 meningohypophyseal arteries

(upper) and the 38 inferolateral trunks (lower) in the 50 cavernous sinuses. Superior (Sup.), medial (Med.), inferior (Inf.), and lateral

(Lat.) surfaces of the ICA are shown in cross section at the C4, C4_5, and C5 portions.

II. Arterial relationships

After entering the cavernous sinus, the ICA first

ascends toward the posteromedial border of the

carotid sulcus, then curves anteriorly along the

carotid sulcus, and again courses upward to emerge

from the cavernous sinus inferomedial to the anterior clinoid process. A variable number (0-5) of arterial branches arise from the cavernous ICA. The most consistent branches are the meningohypo

physeal artery (MHA) and the inferolateral trunk (ILT). The MHA, which Tran-Dinh47) described as the

posterior trunk, was present in 48 (96%) and was a single trunk in 40 (80%) of the 50 specimens examined. Among the total of 56 MHAs found, 71 % originated at the C4_5 junction, 25% at the C5, and 4% at the C4 of the cavernous ICA; 72% arose from the superior aspect of the ICA (Fig. 2 upper). Although many investigators4" 8'37) reported that the MHA typically gave rise to three branches: the tentorial artery (artery of Bernasconi-Cassinari), the inferior hypophyseal artery, and the dorsal meningeal artery, we divided the MHA into four branches. The dorsal meningeal artery, which occurred in 46 (96%) of the 48 specimens examined, courses superior or lateral to the sixth cranial nerve and always runs into Dorello's canal, finally supplying the dura over the clivus (Fig. 3). Brassier et al.5) called this artery the lateral artery of the clivus. The tentorial artery, which occurred in 96% of specimens, runs posteriorly or posterolaterally to the roof of the cavernous

sinus, as well as the dural porus of the fifth cranial nerve, and then supplies the free edge of the tentorium (Fig. 3 upper). The inferior hypophyseal artery, present in 75% of our dissections, courses medially and supplies the medial wall of the cavernous sinus and the posterior part of the pituitary

gland (Fig. 3). The lateral artery occurred in only 25% of specimens, passing laterally and supplying the trigeminal ganglion and the adjacent dura mater

(Fig. 3). Brassier et al.5) described six possible collateral

branches arising from the C5 portion of the cavernous ICA: 1) the posteroinferior hypophyseal artery, 2) the lateral artery of the clivus, 3) the lateral artery

of the trigeminal ganglion; corresponding to the inferior hypophyseal, dorsal meningeal, and lateral arteries, respectively; 4) the artery of the free margin

of the tentorium cerebelli, 5) the basal tentorial artery, which was called the tentorial artery, and 6) the recurrent artery of the foramen lacerum. The recurrent artery of the foramen lacerum was small and occurred in 30% of cases. Our study could not distinguish this artery from the lateral artery.

Parkinson 31) and Harris and Rhoton18) called the ILT the artery of the inferior cavernous sinus, while Tran-Dinh47) called it the lateral trunk. This artery occurred as a single branch in 38 (76%) of our 50 specimens. Of these, 68% originated at the C4i 26%

Fig. 3 Superior view of the right cavernous sinus. upper: The dura in the roof of the cavernous sinus has been removed to expose the intracavernous portion of the ICA. The meningohypophyseal

artery (Men.Hyp.A) arises from the lateral surface at the C4_5 junction of the ICA and gives rise to the lateral (Lat.A), dorsal meningeal (Dor.Men.A), tentorial (Tent.A), and inferior hypophyseal

arteries (Inf.Hyp.A). The dorsal meningeal artery runs into Dorello's canal. Infund: infun dibulum, Opht.A: ophthalmic artery. lower: The anterior clinoid process, the roof of the optic

canal, and the dura mater in the wall of the cavernous sinus have been removed. The menin

gohypophyseal artery arises from the superior surface at the C5 portion of the ICA. The tentorial artery runs posterolaterally to the dural porus of the fifth cranial nerve. The inferolateral trunk

(Inf.Lat.Tr) arises from the lateral surface of the C4 portion of the ICA and gives rise to two branches, the anteromedial (Ant.Med.Br) and the posterior branches (Post.Br). The capsular

artery (Cap.A) arises from the medial surface at the C4 portion of the ICA. PCA: posterior cerebral artery.

Fig. 4 Lateral view of the right cavernous sinus. The lateral wall of the cavernous sinus has been removed to expose the third and fourth cranial nerves and the intracavernous portion of the ICA. upper:

The first and second divisions of the fifth cranial nerve has been reflected anteriorly. The in ferolateral trunk (Inf.Lat.Tr) arises from the lateral surface of the C4 portion of the ICA and gives

rise to four branches: the anteromedial (Ant.Med.Br), anterolateral (Ant.Lat.Br), superior

(Sup.Br), and posterior branches (Post.Br). The anterolateral branch anastomoses with the branch of the internal maxillary artery (Int.Max.A). The caliber of the inferolateral trunk is larger than

that of the meningohypophyseal artery (Men.Hyp.A). lower: The second division of the fifth cranial nerve has been reflected. The intracavernous portion of the ICA contains the meningo

hypophyseal artery, but no inferolateral trunk. The accessory meningeal artery (Acc.Men.A) supplies the fifth cranial nerve.

Fig. 5 Posterosuperior view of the left cavernous sinus after removal of the roof, showing the relation ship between the size of the meningohypophyseal artery (Men.Hyp.A) and the inferolateral trunk

(Inf.Lat.Tr). upper: The caliber of the meningohypophyseal artery (arrow), which arises from the superior surface at the C4_5 junction of the ICA, is larger than that of the inferolateral trunk (ar

rowhead). The third cranial nerve passes just below the anterior clinoid process (Ant.Clin). Post.Clin: posterior clinoid process. lower: The proximal C4 portion of ICA has been retracted

medially to expose the origin of the inferolateral trunk (arrowhead). The caliber of the in ferolateral trunk is larger than that of the meningohypophyseal artery (arrow).

at the C4_5 junction, and 5% at the C5 of the cavernous ICA; 74% arose from the inferior aspect and 16% from the lateral aspect of the ICA (Fig. 2 lower). After a short common stem, the ILT divided into four branches, as described by Knosp et al.24) and Brassier et al.') The anteromedial branch, which oc

curred in 89% of our specimens, courses along the lateral aspect of the cavernous ICA to the superior orbital fissure and finally supplies the third, fourth, and sixth cranial nerves, and the first division of the fifth cranial nerve (Figs. 3 lower and 4 upper). The anterolateral branch, which occurred in 84% of specimens, passes lateral to the fifth cranial nerve and at times courses through the foramen rotundum and forms anastomoses with branches of the internal maxillary artery (Fig. 4 upper). The posterior branch, present in 74% of our specimens, courses

posteriorly along the lateral or superior aspect of the cavernous ICA and supplies the fifth cranial nerve and its ganglion, as well as the sixth cranial nerve near Dorello's canal (Fig. 3 lower). The superior branch, present in 53% of specimens, runs transversely parallel to the third and fourth cranial nerves at their dural entrance. In contrast to the almost constant presence of the MHA, many investigators5,18,24,27,2s,37,41) found the ILT in about 80-90% of their cases. The accessory meningeal artery, which

passes through the foramen ovale, supplies the ILT territory when this artery is not present (Fig. 4 lower). The branches of the MHA, especially the lateral artery, also supply the ILT area in these circumstances. Sixty-five percent (20/31) of our specimens demonstrated an inversely proportional relationship between the sizes of the MHA and the ILT (Figs. 4 upper and 5).

The capsular artery (McConnell) occurred in 6

(15%) of our 40 specimens examined and originated from the medial surface of the C4 portion of the cavernous ICA to supply the medial wall of the cavernous sinus (Fig. 3 lower). Harris and Rhoton18) and McConnell31) found the capsular artery in 28 and 50% of specimens, respectively. Tran-Dinh47) called the capsular artery the medial group of the cavernous branches of the ICA.

The ophthalmic artery usually arises from the C2

portion of the ICA, but in 5% (2/40) of our cases this artery arose from the C3 portion. Other investigators '8'20,47) found that the ophthalmic artery arose from the C3 portion in 7-15.6% of their specimens.

Fig. 6 Blood supply to the third, fourth, sixth (upper), and fifth cranial nerves (lower) in the 50 caver

nous sinuses. ®: meningohypophyseal ar tery, E] : inferolateral trunk.

III. Blood supply to the cranial nerves The blood supply to the proximal and distal parts

of the third and fourth cranial nerves in the cavernous sinus was almost equally distributed (44

60%). The major supplier was the MHA in 79% (26/ 33) of specimens for the third cranial nerve, and 74% (31/42) for the fourth cranial nerve. The sixth cranial nerve was supplied mainly by the branches of the MHA (95%, 40/42), especially the dorsal meningeal artery, and partly by the ILT (5%, 2/42) at the posterior part of the cavernous sinus. The more distal part of the sixth cranial nerve in the cavernous sinus was supplied only by the tentorial artery of the MHA (Fig. 6 upper).

The blood supply to the fifth cranial nerve was located near the trigeminal ganglion in 76% of 50 specimens: in the first division in 48%, in the second division in 68%, and in the third division in 68% of specimens. The trigeminal ganglion was supplied by the MHA in 45% and the ILT in 55% of 38 specimens. The first division of the fifth cranial nerve was supplied by the ILT in 92% (22/24) of specimens, but the second and third divisions

were supplied mainly by the ILT (Fig. 6 lower). In 6% (3/50) of our specimens, the accessory menin

geal artery rather than the ILT supplied the second division (Fig. 4 lower). Knosp et al. 14) also reported that the ILT was replaced by the accessory menin

geal artery. Asbury et al.3) reported a recurrent arterial supply

from the intradural ophthalmic artery to the intracavernous portion of the third cranial nerve, but we found that the intracavernous ophthalmic artery and the capsular artery did not supply blood to the cranial nerves.

Discussion

Surgical approaches to the cavernous sinus can be

divided into superior, lateral, inferior, and in

feromedial routes.

I. Superior approaches The superior intradural or extradural approach is

directed through a frontotemporal craniotomy. Dolenc''8) usually removes the anterior clinoid

process and opens the optic canal before opening the dura mater. During removal of the lateral orbital wall between the superior orbital fissure and the foramen rotundum, care should be taken not to damage the orbital periosteum.19) In contrast, Hakuba et al. 13,11) first open the dura mater and then expose the anterior clinoid process and the medial sphenoid ridge, which are resected with extensive unroofing of the optic canal. The roof of the cavernous sinus provides a triangular area between the third cranial nerve, the anterior and the posterior clinoid processes of 5-17 mm2. Drilling the anterior clinoid process, the roof of the optic canal, and occasionally the posterior clinoid process, will enlarge this triangular area to a 16-69 mm2 rectangle.") Sekhar et al.") described the triangular area of the roof after removal of the anterior clinoid process together with the thin underlying dura mater as the "clinoid space ." This space varies in dimensions according to the size of the anterior clinoid process which occupied the space lateral to the emergence of the ICA from the cavernous sinus.

Anteromedial (Dolenc) or medial (Hakuba) triangle: The triangular space formed by the line between the C2 origin of the ICA and posterior clinoid

process as the medial border, the medial aspect of the third cranial nerve as the lateral border, and the dura extending between the dural entry point of the third cranial nerve and the posterior clinoid process as the posterior border was called the medial triangle by Hakuba et al.13,'4,17,30) and the anteromedial

triangle by Dolenc9,48) (Fig. 7A). This triangle is facilitated by the complete removal of the anterior clinoid process and allows good access to the superior space of the cavernous sinus, particularly the C3 and C4 portions of the ICA, with less risk of sacrificing arterial branches.') The origin of the ophthalmic artery is identified by medial displacement of the second cranial nerve. The ophthalmic segment of the ICA gives rise to 3.6 branches (2-5

branches) to the second cranial nerve, the pituitary

gland, and the floor of the third ventricle.") Care must be taken to preserve these branches. The ILT originated from the C4 portion of the ICA in 68% of our specimens, so the origin of the ILT can be identified easily. However, the origin of the MHA is difficult to expose, because the MHA originated from the C4 portion in only 4% of our specimens. One of the important anatomical relationships in the anterior clinoid process is the third cranial nerve passing just below, 21) which cleaves the ICA and the third cranial nerve (Fig. 5 upper). Therefore, the dissection should be continued along the ICA carefully to prevent injury to the third cranial nerve when removing the lower edge of the anterior

clinoid process. Although the third cranial nerve is enveloped by a membranous dural sheath, the fourth cranial nerve is not usually covered, so care must be taken to avoid damage when exposing the fourth cranial nerve. '7 The fifth cranial nerve is not so well exposed by this anteromedial or medial ap

proach as by the lateral approach, and the sixth cranial nerve not at all.

II. Lateral approaches The lateral intradural approach requires either a

frontotemporal or a temporal craniotomy. The lower part of the squamous temporal bone and the zygomatic arch should be removed to minimize retraction of the temporal lobe. As the third, fourth, and fifth cranial nerves are usually not clearly visible through the outer dural layer of the lateral wall of the cavernous sinus, this layer should be peeled off to identify the location of these nerves.

The triangular area between the third, fourth, and fifth cranial nerves varies from 5 to 39 mm2 in the lateral wall of the cavernous sinus.33) Five patterns have been identified for the arrangement of the cranial nerves. In pattern 1, occurring in 60% of our 40 specimens compared to 31.4% reported by Lang and Reiter,26) the fourth cranial nerve runs close to the third cranial nerve and the triangular space occupies the space between the fourth cranial nerve and the first division of the fifth cranial nerve corresponding to Parkinson's triangle. In pattern 2, oc

Fig. 7 Various surgical approaches to the cavernous sinus. A: The anteromedial or medial triangle

(asterisks) seen through a right frontotemporal craniotomy. The anterior clinoid process and the dural roof of the cavernous sinus have been removed to expose the superior space of the cavernous

sinus. Inf.Lat.Tr: inferolateral trunk, Men.Hyp.A: meningohypophyseal artery. B: The

paramedial triangle (asterisks) seen through a right frontotemporal craniotomy. Bone removal in cludes the lateral part of the anterior clinoid process (Ant.Clin) and the greater and lesser sphenoid

wings to expose the C4 portion of the ICA and the inferolateral trunk. AchA: anterior choroidal artery, PcoA: posterior communicating artery, Sup.Orb.Fiss: superior orbital fissure. C: Parkin

son's triangle (asterisks) seen through a large right frontotemporal craniotomy. The lateral wall of the cavernous sinus has been opened to expose the proximal C3 and C4 portions of the ICA. The

sixth cranial nerve is also identified. D: The anterolateral triangle (asterisks) seen through a large right frontotemporal craniotomy with temporal lobe retraction. The second division of the fifth cranial nerve has been displaced inferiorly to expose the inferolateral and posterior spaces of the cavernous sinus. The origins of the meningohypophyseal artery and the inferolateral trunk are easily identified. F.Rotundum: foramen rotundum.

curring in 15% of our cases and 53.5% according to Lang and Reiter,26' the fourth cranial nerve runs close to the first division of the fifth cranial nerve and the triangular space is found between the third and fourth cranial nerves, corresponding to the

paramedial triangle. In pattern 3, found in 15% of our specimens, the fourth cranial nerve runs between the third cranial nerve and the first division of the fifth cranial nerve so there are triangular spaces both above and below the fourth cranial nerve. In pattern

4, in 5% of our specimens, the posterior portion of the fourth cranial nerve runs close to the third cranial nerve and the anterior portion runs close to the fifth cranial nerve. In pattern 5, in 5% of our specimens, the third, fourth, and fifth cranial nerves

run closely together and the triangular spaces are very small (Fig. 8).

Fig. 7 Various surgical approaches to the cavernous sinus. E: The lateral triangle (asterisks) seen through a right extradural subtemporal approach. The lateral wall of the cavernous sinus has been opened to expose the lateral aspect of the C4 portion of the ICA. The middle meningeal artery

(MMA), foramen rotundum, and foramen ovale (F.Ovale) are exposed. F: The posterolateral triangle (asterisks) seen through an extradural subtemporal approach following a large right

frontotemporal craniotomy. The petrous ICA (Pet.ICA), greater (Gr.Pet.N) and lesser petrosal nerves (Les.Pet.N), geniculate ganglion (Gen.Gang), and cochlea have been exposed by remov ing the bone in the middle cranial fossa. F.Spinosum: foramen spinosum, Sym.Tr: sympathetic

trunk, Ten.Tym.M: tensor tympani muscle, Vid.A: Vidian artery. G: The posteromedial tri angle (asterisks) seen through a right extradural infratemporal approach with temporal lobe re traction. The anterolateral brainstem is exposed by drilling the anterior pyramidal bone. PCA:

posterior cerebral artery, Post.Clin: posterior clinoid process, SCA: superior cerebellar artery, Sup.Pet.Sinus: superior petrous sinus, Temp.Lobe: temporal lobe, Trig.Gang: trigeminal

ganglion. H: The inferomedial transnasal-transsphenoidal approach to the cavernous sinus. The carotid prominence (Car.Prom), optic canal, and trigeminal prominence (Trig.Prom) are iden

tified in the anterior view of the sphenoid sinus. The bone and dura along the right lateral wall of the sphenoid sinus have been removed to expose the C4 portion of the ICA and the origin of the in ferolateral trunk.

When the lateral wall is opened, preservation of the main arterial supply to the cranial nerves should be attempted. Injury to the accessory meningeal

artery may cause ophthalmoplegia when the ILT is

absent.")

Fig. 8 Arrangements of the cranial nerves in the lateral wall of the 40 cavernous sinuses. A:

pattern 1, Parkinson's triangle (60%); B: pat tern 2, paramedial triangle (15%); C: pattern

3, triangular areas above and below the fourth cranial nerve (15%); D: pattern 4, anterolateral triangle (5%); E: pattern 5, no triangular areas in the lateral wall (5%).

Paramedial triangle: The paramedial triangle is formed in the lateral wall of the cavernous sinus by the lateral aspect of the third cranial nerve (the medial border), the medial aspect of the fourth cranial nerve (the lateral border), and the dura extending between the dural entry points of the third and fourth cranial nerves (the posterior border)')

(Fig. 7B). Retraction of the third cranial nerve superiorly and the fourth cranial nerve inferiorly provides a narrow access to the superolateral space of the cavernous sinus. The C4 portion of the ICA, and the origin of the ILT and its branches can be visualized in this space. However, the C3 and C5 portions of the ICA are poorly exposed and there is a

potential risk of damaging the third and fourth cranial nerves by displacement. The sixth cranial nerve is partially seen with this approach, but not

quite as well as with other lateral approaches. Parkinson's triangle: Parkinson's triangle 3') be

tween the fourth cranial nerve and first division of the fifth cranial nerve is an isosceles triangle, and can

be consistently identified (Fig. 7C). The dimensions of this triangle averaged 4 x 12 x 13 mm in our specimens. Harris and Rhoton18) and Sekhar et a1.42) reported average measurements of 6 X 13 X 14 and 5 x 17 x 17 mm, respectively. This approach is best for exposing the lateral space of the cavernous sinus,

particularly at the proximal C3 and C4 portions of the ICA, except for the medial surface, because the inner

membranous layer of the lateral wall of this area is thinner than that of the paramedial triangle 21) and the venous plexus is generally sparse lateral to the ICA.42) However, the anteroinferior space of the cavernous sinus is poorly exposed by this approach. The origin of the ILT and its anteromedial, anterolateral, and superior branches are relatively easily identified, but the distal parts of the C3 and C5

portions of the ICA, the origins of the MHA, and the ophthalmic artery are not visualized within this triangle. As the sixth cranial nerve passes lateral to the C4 portion of the ICA towards the superior orbital fissure and usually below the branches of the ILT and the MHA at this level, identifying the main trunk of the MHA before exposing the sixth cranial nerve is important to prevent injury to this nerve.

Anterolateral triangle (Mullan): The anterolateral triangle is defined by the lateral aspect of the first division of the fifth cranial nerve as the medial border, the medial aspect of the second division of the fifth cranial nerve as the lateral border, and the anterolateral wall of the bony middle cranial fossa as the posterior border', 14) (Fig. 7D). Exposure of the cavernous sinus through this approach is restricted to the distal C4 and C5 portions of the ICA, and the origins of the MHA and the ILT. The sixth cranial nerve is inferomedial to the first division of the fifth cranial nerve as it courses anteroinferiorly towards the superior orbital fissure. Therefore, there is a risk of damaging the first and second divisions of the fifth cranial nerve and the sixth cranial nerve through retraction of the first and second divisions of the fifth cranial nerve to expose the sixth cranial nerve. The ophthalmic artery is not exposed through this approach.

Lateral triangle: The lateral triangle is another triangular area on the lateral wall of the cavernous sinus, formed by the lateral aspect of the second division of the fifth cranial nerve (the medial border), the medial aspect of the third division (the lateral border), and the line between the foramen rotundum and the foramen ovale (the anterior border)') (Fig. 7E). This triangle allows good access to the anterior and lateral aspects of the C4 portion of the ICA as well as the origin of the ILT. The anterior half of the sixth cranial nerve is also well exposed.42) As this ap

proach is usually an extradural route with minimal retraction of the temporal lobe, the landmarks of the anterior bony middle cranial fossa, such as the middle meningeal artery, the foramen rotundum, and the foramen ovale, are easily identified. However, the superomedial space of the cavernous sinus is

poorly exposed. The ophthalmic artery and the distal part of the C3 portion of the ICA are also not visual

ized with this approach.

III. Inferior approaches The floor of the middle cranial fossa is exposed

through an extradural subtemporal or infratemporal approach to minimize temporal lobe retraction.") The temporal dura mater is separated from the bony middle cranial fossa to expose the middle meningeal artery, the greater superficial petrosal nerve, facial hiatus, the arcuate eminence, and tympanic tegmen. The branches of the middle meningeal artery are easily identified and the dura mater is separated until the foramen spinosum is exposed. The petrosal artery, a branch of the middle meningeal artery running alongside the greater superficial petrosal nerve, supplies the geniculate ganglion and the seventh cranial nerve.") The petrosal artery arose proximal to the foramen spinosum in 58% and distal in 42% of cases reported by Paullus et a1.38) The greater superficial petrosal nerve, which usually adheres to the surrounding dura, is encountered posterior to the foramen spinosum and provides a reliable landmark for orientation during the inferior approach. As this nerve runs above and parallel to the horizontal segment of the petrous ICA, the petrous bone is drilled off to expose this artery. During bone removal, it is important to remember that the junction of this nerve with the geniculate ganglion is usually covered by bone. However, there is no bony covering in 30% of cases, and in 15% of these the bone is also absent over the genu of the seventh cranial nerve and the

geniculate ganglion .31) The facial hiatus, a narrow groove between the trigeminal impression medially and the arcuate eminence laterally, transmits the

greater superficial petrosal nerve and the petrosal artery.36) Paullus et al.") reported two branches arising from the horizontal segment of the petrous ICA: the Vadian or pterygoid artery (in 30% of cases) and the periosteal artery (in 8%). These branches arose

predominantly from the inferior (55%) or anteroinferior (35%) surfaces.

Posterolateral (Glasscock) triangle: The

posterolateral triangle is the space formed by the greater superficial petrosal nerve as the medial border, the line between the foramen spinosum and the arcuate eminence as the lateral border, and the

posterior aspect of the third division of the fifth cranial nerve as the anterior border9,12) (Fig. 7F). This approach best exposes the petrous portion of the ICA. Using an epidural route, three landmarks in the floor of the middle cranial fossa are indispensable: the middle meningeal artery, the arcuate eminence, and the greater superficial petrosal nerve.2) The middle meningeal artery is interrupted at its emergence

from the foramen spinosum. During removal of the bone over the petrous ICA, the bone posterosuperior to the genu of the petrous ICA should not be removed to avoid injuring the geniculate ganglion and the cochlea. 15,43,45) The greater superficial petrosal nerve must sometimes be divided in the area

posterior to the foramen spinosum to gain more access and prevent traction of the geniculate gan

glion. 12,39,44) When drilling off the arcuate eminence, an effort must be made to preserve the thin membranous bone covering the anterior semicircular canal 36)

The inferior approach to the cavernous sinus is only possible when the third division of the fifth cranial nerve and the trigeminal ganglion are tem

porarily displaced upward after the foramen ovale is unroofed, but it is usually difficult to reach the inferolateral part of the cavernous sinus without dividing the third division of the fifth cranial nerve. The sphenoid sinus may be exposed by removing the bone anterior to the third division of the fifth cranial nerve and superior to the pterygoid processes.")

Posteromedial (Kawase) triangle: The posteromedial triangle is formed by the lines connecting the hiatus of the greater superficial petrosal nerve and the posterior aspect of the fifth cranial nerve, the sphenopetrosal groove with the greater superficial

petrosal nerve, and the posterior border of the fifth cranial nerve9,23) (Fig. 7G). Exposure of the anterolateral brainstem is feasible through this triangle by drilling away the anterior pyramidal bone,') in an area surrounded by the trigeminal ganglion anteriorly, the cochlear organ posteriorly, and the carotid canal and internal auditory canal inferiorly.22,23) This drilling may open the eustachian tube and mastoid air cells. As the internal auditory canal runs along the line extending from the external auditory meatus to the tympanic tegmen, care must be taken to preserve the dura mater of the internal auditory meatus during opening to avoid damaging the seventh and eighth cranial nerves.23)

This triangle only exposes the posteroinferior space of the cavernous sinus, especially the C5 portion of the ICA, after drilling the foramen lacerum.42) Other advantages of this approach include minimizing temporal lobe retraction and the risk of

damage to the temporal bridging veins such as the vein of Labbe.23)

IV. Inferomedial approaches

The best exposure of the medial wall of the caver

nous sinus is obtained by the inferomedial approach

through the transnasal-transsphenoidal route 21,29)

Removal of the bone in the lateral wall of the

sphenoid sinus exposes the dura mater forming the medial wall of the cavernous sinus. Three bony landmarks on the lateral wall of the sphenoid sinus are valuable: the carotid prominence, trigeminal prominence, and optic canal.",") The carotid sulcus of the medial part of the greater wing of the sphenoid bone causes the carotid prominence within the sphenoid sinus, and is more conspicuous at the C4 and C5 portions of the ICA. The trigeminal prominence is a prominent bulge into the lateral wall of the sphenoid sinus corresponding to the second division of the fifth cranial nerve just proximal to the foramen rotundum. However, the trigeminal

ganglion, the first and second divisions of the fifth cranial nerve, and sixth cranial nerve are located far from the medial wall of the cavernous sinus, so these nerves are difficult to expose through the inferomedial route. The optic canal protruding into the superolateral part of the sphenoid sinus corresponds to the superomedial part of the cavernous sinus (Fig. 7H).

This approach can provide good visualization of the C4 and C5 portions of the ICA and the second division of the fifth cranial nerve through an entirely extracranial route.29) Medial retraction of the C4 portion of the ICA allows access to the origin of the ILT. The disadvantages include difficulty in obtaining adequate control of bleeding from the ICA, a

potential risk of cerebrospinal fluid leakage, and the narrow viewing angle. 19,42)

Clearly, no single approach can provide exposure

to all parts of the cavernous sinus. Therefore, the

selection of the most appropriate surgical approach

depends mainly upon the location of the lesion and

partly on the surgeon's experience.

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Address reprint requests to: Y. Miyazaki, M.D.,

Yokohama Miyazaki Hospital of Neurosurgery, 218

9 Kariba-cho, Hodogaya-ku, Yokohama 240,

Japan.