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A2
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A2
A1
P1
AcomA
DS
ThPA
A1
scciICA
AchA
PcomA
P1
P2
OA
MCA
scciICA
PcomA
P2
SCA
Fig. 31.21 Posterosuperior view of the circle of Willis in a
dry skull. A1, fi rst segment of the anterior cerebral artery; A2,
second (postcommunicating) segment of the anterior cerebral
artery; AchA, anterior choroidal artery; AcomA, anterior
communicating artery; DS, dorsum sellae; MCA, middle
cerebral artery; OA, ophthalmic artery; P1, fi rst segment of the
posterior cerebral artery; P2, second segment of the posterior
cerebral artery; PcomA, posterior communicating artery; SCA,
superior cerebellar artery; scciICA, supracavernous cisternal
internal carotid artery; ThPA, thalamoperforating arteries.
Intracranial Vascular Anatomy
OA
scciICA
ACA
MCA
scciICA
PcomA
P2
SCA
Fig. 31.22 Posterosuperior view of the supracavernous
cisternal segment of the internal carotid artery highlighting
the division into the anterior and middle cerebral arteries in
a dry skull. ACA, anterior cerebral artery; DS, dorsum sellae;
MCA, middle cerebral artery; OA, ophthalmic artery; P1,
fi rst segment of the posterior cerebral artery; P2, second
segment of the posterior cerebral artery; PcomA, posterior
communicating artery; SCA, superior cerebellar artery; scciICA,
supracavernous cisternal internal carotid artery.
ACA
DS
P1
(M1 segment). The upper and lower trunk arteries thus
formed turn upwards and run in the Sylvian fissure
lateral to the insular cortex (M2 segment), before they
turn laterally in the horizontal portion of the fissure
above the temporal and below the frontal lobe opercular surfaces (M3 segment). They emerge from the fissure in a series of branches (M4 segment). These turn
inferiorly or superiorly to, respectively, supply the cortex of the temporal and frontal lobes.
31.2 Superior and Inferior
Intercavernous Sinuses
(and Cavernous Sinuses)
31.2.1 Intercavernous Sinuses via
the Transsphenoidal Corridor
Once a complete anterior sphenoidotomy is performed,
the gate to the middle cranial fossa is exposed. The sellar
floor with the pituitary prominence is usually evident in
the posterior part of the sphenoidal roof with the planum sphenoidalis anterior to it and the clival recess, a
space bordered laterally by the paraclival prominences
of the ICA and superiorly by the pituitary prominence,
below it (Fig. 31.23). Once the overlying bone is removed,
the periosteal and dural layers become evident. Endoscopically, a superior and an inferior intercavernous sinus can be easily seen (Fig. 31.24). This complex vascular
network lies within the dural layers. Occasionally, these
sinuses can be united, producing a complete covering of
the gland. Small arteriolar branches can be found on the
dural surface. By removing the dural layer, the pituitary
gland is exposed.
31.2.2 Cavernous Sinuses via the
Transpterygoidal Corridor
Once an anteroposterior ethmoido-sphenoidotomy and
a wide middle-meatus antrostomy are performed, the
posterior wall of the maxillary sinus, the sphenoid sinus, the orbital apex area, and the base of the pterygoid are exposed. The sphenopalatine foramen is then
identified and the pterygomaxillary fossa is opened,
widening its foramen with a Citelli forceps. The medial
portion of the posterior wall of the maxillary sinus is
then removed. By lateralizing the contents of the pter-
men rotundum can be seen. Once the vidian artery is
transected, the base of the pterygoid and the base of
the sphenoid are drilled, especially at the bony margin
that links both foramina (vidian and rotundum). In this
manner, it is possible to have a complete view of the
pterygoid recess of the sphenoid, even if it is greatly
pneumatized, and a wide access to the floor of the middle cranial fossa. Access to the CS is accomplished by
removal of the posterolateral bony wall of the sphenoid
sinus and exposing the periosteal dura, starting from
the bony prominence of the parasellar ICA, upward to
the optic nerve (Fig. 31.6). Once the periosteal dura is
removed, the ICA is exposed throughout its intracavernous course. Laterally to the posterior vertical segment (paraclival) of the ICA, the abducens nerve can
be identified as entering the CS, while medially to it
the sympathetic plexus of the ICA comes into view
(Fig. 31.25). The cranial nerves III and IV contained in
the dural duplication of the lateral wall of the CS as well
as the V1 and V2 branches can be observed (Fig. 31.26).
From an endoscopic point of view, the oculomotor
nerve superiorly and the abducent inferiorly define
a triangular area with a posterior base formed by the
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pl
s
mt
SS
cpsICA
cpsICA
ON
ACP
CS
iics
it
CS
cpsICA
VIcn
cpsICA
Fig. 31.23 Schematic representation of the transsphenoidal corridor to the intercavernous sinuses after wide sphenoidotomy. BA,
basilar artery; cl, clivus; cpsICA, cavernous parasellar internal carotid artery; ds, dorsum sellae; ics, intercavernous sinus; it, inferior
turbinate; mt, middle turbinate; pl, planum sphenoidalis; s, sella; ss, sphenoid sinus.
sics
iics
ICS
DS
BA
cl
C
Fig. 31.24 Endoscopic view of the circular sinus. The superior
and inferior intercavernous sinuses connect the two cavernous
sinuses between them. ACP, anterior clinoid process; C, clivus;
cpcICA, cavernous paraclival internal carotid artery; cpsICA,
cavernous parasellar internal carotid artery; CS, cavernous
sinus; iics, inferior intercavernous sinus; ON, optic nerve; sics,
superior intercavernous sinus; splICA, supralacerum internal
carotid artery; VA, vidian artery; VC, vidian (pterygoid) canal;
V2, maxillary branch of trigeminal nerve.
336
cpcICA
splICA
VA
VC
V2
C
cpcICA
Fig. 31.25 The medial dural wall of the cavernous sinus
has been opened and the abducens nerve running inside
the cavernous sinus lateral to the carotid is visible. C, clivus;
cpcICA, cavernous paraclival internal carotid artery; cpsICA,
cavernous parasellar internal carotid artery; CS, cavernous
sinus; iics, inferior intercavernous sinus; V2, maxillary branch of
trigeminal nerve; VIcn, abducens nerve.
V2
IIIcn
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ILT
IVcn
CS
VA
VIcn
V1
V2
VC
cpsICA
PG
cpcICA
C
splICA
Fig. 31.26 Endoscopic view of the nerves on the lateral
dural wall of the cavernous sinus. C, clivus; cpcICA, cavernous
paraclival internal carotid artery; cpsICA, cavernous parasellar
internal carotid artery; CS, cavernous sinus; ILT, inferolateral
trunk; PG, pituitary gland; splICA, supralacerum internal
carotid artery; VA, vidian artery; VC, vidian (pterygoid) canal;
V1, ophthalmic branch of trigeminal nerve; V2, maxillary
branch of trigeminal nerve; IIIcn, oculomotor nerve; IVcn,
trochlear nerve; VIcn, abducens nerve.
anterior bend of the parasellar ICA. The outer surface
of this area contains the cranial nerve IV and a portion
of the V1 branch of the trigeminal nerve. The abducens
nerve superiorly and V2 inferiorly enclose a quadrangular area, demarcated anteriorly by the bony lateral
wall of the sphenoid sinus extending from the SOF to
the foramen rotundum, and posteriorly by the paraclival
segment of ICA between VI and V2. The V1 branch and
the branches of the ILT pass through this area.
Finally, the so-called “quadrangular space” (also
known as the “front door” to Meckel’s cave) can be
identified by exposing the area delimited superiorly
by the VI cranial nerve and V2, and inferiorly by the
vidian nerve, while its anterior and posterior margins
are accordingly defined by the bony lateral wall of the
sphenoid sinus extending from the foramen rotundum
to the pterygoid canal and by the short paraclival-located supralacerum portion, delimited superiorly by V2
and inferiorly by the petrolingual ligament. The dura is
then opened within the “quadrangular space,” which
is bounded by the paraclival ICA medially, V2 laterally,
and the horizontal petrous ICA inferiorly. Its superior
boundary is formed by the abducens nerve, which runs
obliquely and superiorly. The abducens runs near V1
within the CS along its inferior margin as they extend
into the SOF. The best way to avoid injury to the abducent nerves is not to cross the superior margin of V2.
Once the “quadrangular space” is entered, the Gasserian ganglion is exposed laterally and can be retracted
laterally, allowing for visualization of the petrous apex.
To reach the lateral aspect of Meckel’s cave, the anteromedial (between V1 and V2) and the anterolateral
Intracranial Vascular Anatomy
(between V2 and V3) middle fossa triangles are used.
The meningeal dural layer can be opened, using these
landmarks, along these corridors to expose the subarachnoid space of the anterior temporal fossa.
31
Anatomy
The CSs are connected to each other via the superior and
inferior inter-CSs (Fig. 31.20). Together, they are called the
circular sinus.
on the lateral surface of the pituitary gland. Its dural walls
surround a venous-filled space through which a segment
of the ICA (cavernous paraclival and parasellar) with its
branches, the abducens nerve, and the sympathetic plexus
run (Fig. 31.25). The sinus extends from the SOF anteriorly
to the area lateral to the dorsum sellae posteriorly.
lateral and medial walls join anteriorly along the SOF and
underneath along the upper border of V2. The roof of the
CS is located below the level of the ACP and is closely related to the dural sheath of the optic nerve.
The lateral wall of the CS blends into the dura covering
Meckel’s cave, which is a dural recess within the middle
cranial fossa, where the two layers of the dura mater split
and envelop the Gasserian ganglion.
of the CS, two easily recognizable layers are evident: the
meningeal and the endosteal. The meningeal layer faces the
brain and superiorly forms the diaphragma sellae, while
the endosteal (outer layer) invests superiorly the nerves in
the lateral wall and inferiorly adheres with the bone.
Endoscopically, the most identifiable nervous structure
visible within the CS is the abducens nerve, with its typical inferior-to-superior and medial-to-lateral course. A
rich neural network can be seen between the sympathetic fibers of the ICA and the abducens nerve. The VI nerve
pierces the dura of the PCF, on the clivus (Fig. 31.18).
Sheaths of dura and arachnoid follow the nerve within the
basilar plexus and then through the inferior petrosal sinus
28
(IPS).
enters the CS through the lower part of its posterior wall,
below the superior petrosphenoidal ligament, also known
as the Gruber’s ligament, that forms the Dorello’s canal, at
the apex of the petrous bone (Fig. 31.5).
anterior and rostrally lateral to the paraclival intracavernous ICA and then along the inferior border of the horizontal portion of the ICAc. It courses inside the lateral part of
the CS, medial and almost parallel to V1.
abducens nerve plane, the oculomotor nerve, the trochlear nerve, V1, and inferiorly V2 can be seen (Fig. 31.26).
The abducens nerve enters the orbit through the SOF and
it courses laterally to the oculomotor nerve to innervate
the lateral rectus muscle of the eye (Fig. 31.27).
The oculomotor nerve enters the posterior part of the
roof of the CS included in its own cistern (oculomotor cistern) and runs within the lateral wall of the CS, lateral to
the intracavernous ICA. This nerve forms the inferior border of the optic struct triangle and runs anteriorly to the
upper part of the anterior vertical portion of the parasellar ICA to reach the SOF.
divides into two branches: a superior one for the superior
rectus and superior levator palpebrae and an inferior one
for the medial rectus, inferior rectus, and inferior oblique
muscles (Fig. 31.27).
The trochlear nerve after crossing the superior petrosal
sinus (SPS) can pierce the roof of the CS and runs through
its lateral wall to reach the SOF and enter the orbit to
innervate the superior oblique muscle. The IV nerve,
32
The CS with its neurovascular content lies
24
The
33
24
In the lateral wall
24,33
The nerve presents a short course superiorly and
34
The nerve runs
24
Laterally to the
22
As the nerve enters the orbit, it
8
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Intracranial Vascular Anatomy
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IIIcn
ON
OA
cpsICA
CS
VIcn
V1
V2
36
ON
OA
ON
sbIIIcn
IIIcn
ibIIIcn
OrAp
MRM
ab
Fig. 31.27 (a,b) Endoscopic views after complete exposure of the cavernous sinus, optic canal, and superior orbital fi ssure. cpsICA,
cavernous parasellar internal carotid artery; CS, cavernous sinus; ibIIIcn, inferior branch of oculomotor nerve; MRM, medial rectus
muscle; OA, ophthalmic artery; ON, optic nerve; OrAp, orbital apex; sbIIIcn, superior branch of oculomotor nerve; IIIcn, oculomotor
nerve; V1, ophthalmic branch of trigeminal nerve; V2, maxillary branch of trigeminal nerve; VIcn, abducens nerve.
within the CS, passes above the oculomotor nerve (more
or less at the level of the optic strut) and becomes the
most superior structure of the CS.
an arachnoidal sheath surrounds the trochlear nerve that
is always superior to V1.
28
Typically, the homonymous ar-
35
Within the CS course,
tery accompanies the trochlear nerve, which is similar in
size to the nerve.
The ophthalmic (V1) nerve courses within the lateral
wall of the CS below the trochlear nerve. It lies laterally to
CS
VIcn
cpsICA
V1
ibIIIcn
V2
and is usually larger posteriorly and becomes smaller
near its apex. This sinus travels within the attachment
of the tentorium cerebelli and receives drainage from
the cerebellum, the inferior cerebral veins, and the
veins of the tympanic cavity. Most commonly, the SPS
is superior to the porus trigeminus, through which the
trigeminal nerve enters the Meckel’s cave (Fig. 31.18),
but sometimes the sinus lies inferiorly or divides, creating a venous ring around it.
ON
sbIIIcn
OrAp
the abducens nerve and runs anterosuperiorly to gain the
SOF. A small branch for the tentorium cerebelli has been
described.
nasociliary, frontal, and lacrimal nerves.
limit of the CS, where the lateral wall joins the medial
wall of the CS. The nerve crosses the lateral aspect of the
paraclival ICA. This nerve also can be identified anteriorly, passing through the foramen rotundum, which is
superior and lateral to the vidian canal. The nerve enters
the inferior orbital fissure and runs on the floor of the
orbit in the infraorbital groove/canal to reach the infraorbital foramen where it splits into palpebral, lateral nasal,
and labial branches.
28
Close to the SOF, it splits into three nerves:
The maxillary nerve (V2) corresponds to the inferior
31.3.2 Inferior Petrosal Sinus
The IPS is a paired venous structure at the base of the skull
that drains venous blood from the CS to the junction of the
sigmoid sinus and the superior jugular bulb on each side
(Fig. 31.28). Along its course, the IPS receives venous drain-
age from the internal auditory vein, medulla oblongata,
pons, and inferior cerebellar surface. It originates from the
posteroinferior aspect of the CS and usually passes beneath
the superior petrosphenoidal ligament (Gruber’s ligament)
with the abducens nerve (Fig. 31.4 and Fig. 31.5). Subsequently, it runs into the inferior petrosal sulcus (also known
as the petro-occipital fissure), a groove within the PCF at the
junction of the petrous portion of the temporal bone and
the basilar part of the occipital bone, until entering the jug-
31.3 Superior and Inferior
Petrosal Sinuses
31.3.1 Superior Petrosal Sinus
The SPS is a small, narrow venous channel located at
the junction between the middle and PCF. It originates
in the CS (Fig. 31.4 and Fig. 31.5) and passes dorsolaterally to drain into the transverse–sigmoid junction.
The sinus runs in the superior petrosal sulcus and begins from the posterolateral base of the petrous ridge
and travels at an anteromedial angle toward its apex,
ular foramen. The IPS traverses the anteromedial portion of
the jugular foramen along with the meningeal branch of the
ascending pharyngeal artery. Variations of the relationship
of the IPS and the cranial nerves exiting through the jugular
foramen have been reported. In the proximal portion of the
jugular foramen, the IPS most commonly courses inferior to
the horizontal portion of the glossopharyngeal nerve. Further distal in the foramen, the IPS courses in an inferolateral
direction between the descending portion of the glossopharyngeal nerve anteriorly and the vagus and accessory
nerves posteriorly. Rarely (10%), the IPS drains in the internal jugular vein below the cranial base and not in the jugular bulb where it lies lateral to the glossopharyngeal, vagus,
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Intracranial Vascular Anatomy
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CS
IPS
PS
FM
BP
TS
ACP
DS
SPS
PA
C
POF
JT
HC
37,38
LWS
ACP
SPS
IAM
SS
Fig. 31.28 Endocranial view of a dry skull showing the course
of the inferior petrosal sinus into the petro-occipital fi ssure.
ACP, anterior clinoid process; BP, basilar plexus; C, clivus; CS,
cavernous sinus; DS, Dorsum sellae; FM, foramen magnum;
HC, hypoglossal canal; IAM, internal acoustic meatus; IPS,
inferior petrosal sinus; JF, jugular foramen; JT, jugular tubercle;
LWS, lesser wing of the sphenoid; PA, petrous apex; POF,
petro-occipital fi ssure; PS, planum sphenoidalis; SPS, superior
petrosal sinus; SS, sigmoid sinus; TS, tuberculum sellae.
and spinal accessory nerves. In these cases, the sinus passes
usually through a foramen present in the inferior petrosal
sulcus. Sometimes, a poorly developed IPS has connections
with the plexus of the hypoglossal canal (also named the
anterior condylar vein), the petroclival vein, or the posterior
condylar emissary vein. This last vein puts the vertebral venous plexus into communication with the jugular bulb.
The IPSs are connected to one another through the basilar
plexus, located between periosteum and dura at the level
of the clivus.
PA
JF
JT
HC
31.4 Basilar Artery (and
Vertebrobasilar System
including Vertebral Arteries)
The BA arises from the confluence of the two vertebral
arteries (VA) at the pontomedullary junction between
the cranial nerves VI forming the vertebrobasilar system
(Fig. 31.29). Four VA segments are identified. In the present chapter, only the intradural segment (VA4) will be
described in detail, while the extradural segments (VA1,
VA2, VA3) will be only mentioned.
31.4.1 (VA1) Extraosseous
segment, (VA2) Foraminal Segment,
and (VA3) Extraspinal Segment
The VAs arise from the subclavian arteries and course
posterosuperiorly to enter the C6 transverse foramen.
SCA
AICA
VA
ThPA
BA
VA
ro
ca
Fig. 31.29 Posterior view of the vertebrobasilar system in a
dry skull. AICA, anterior inferior cerebellar artery; BA, basilar
artery; C, clivus; ca, caudal branch of superior cerebellar
artery; cm, caudomedial branch of anterior inferior cerebellar
artery; PCA, posterior cerebral artery; rl, rostrolateral branch of
anterior inferior cerebellar artery; ro, rostral branch of superior
cerebellar artery; SCA, superior cerebellar artery; ThPA,
thalamoperforating arteries; VA, vertebral artery.
Unnamed segmental branches arise from VA1 to supply
the cervical musculature and lower cervical spinal cord.
The VA2 segment courses superiorly through the C6–C3
transverse foramina until it reaches the atlas (C2), where
it first turns superolaterally and then turns upwards to
pass through the axis (C1) transverse foramen. An ante-
rior meningeal artery and additional unnamed segmental
branches arise from VA2. The VA3 segment begins after
the VA exits the C1 transverse foramen. It lies on top of
the C1 ring, curving posteromedially around the atlanto-occipital joint before making a sharp anterosuperior
turn to pierce the dura at the foramen magnum. The only
major VA3 branch is the posterior meningeal artery.
C
AICA
cm
SCA
ro
ca
rl
31.4.2 Intradural Segment (VA4)
Intradural Segment of VA via the
Transclival Corridor
Exposure of the clivus, craniovertebral junction, and anterior portion of the foramen magnum is achieved by
creating an inferior hinged mucofascial flap from the
nasopharyngeal mucosa and nasopharyngeal fascia. The
longus capitis, rectus capitis anterior, and anterior atlanto-occipital membrane are exposed and resected to
expose the anterior ring of C1, the capsule of the atlanto-occipital joint, and the apical ligament. During this approach, a wide bilateral sphenoidotomy with removal of
the floor of the sphenoid sinus and drilling of the base of
the pterygoid is fundamental to expose the vidian canal
with the vidian nerve and artery running through it, to
join the ICA at its anterior genu at the level of the FL which
is the superolateral limit of the inferior clival fenestration.
The paraclival tracts of the ICA are the lateral limits of the
superior clival fenestration, while the occipital condyles
are the inferolateral limit of the inferior clival fenestration.
For exposure of the VA at its dural entry point in the posterior fossa, condyle resection may be required to achieve
a more inferolateral exposure, prior identification of the
PCA
27
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ds
s
mt
it
pphICA
cl
a
Fig. 31.30 (a,b) Schematic representations of the transclival corridor to the vertebral and basilar arteries after wide sphenoidotomy
and drilling of the superior, middle, and inferior clivus. BA, basilar artery; cl, clivus; ds, dorsum sellae; it, inferior turbinate; mt, middle
turbinate; pphICA, parapharyngeal internal carotid artery; s, sella.
hypoglossal canal situated posterior and lateral to the supracondylar groove (a distinctive groove at the external
surface of the condyle of the occipital bone), which is an
excellent landmark for identifying the superior limit of the
hypoglossal canal. Once the clivus is drilled, the underlying dura mater and its basilar venous plexus are exposed
(Fig. 31.30).
25
After opening the dura, the premedullary
and inferior portions of the prepontine and lateral cerebellomedullary cisterns come into direct view. The VAs
are identified along their cisternal course up to the vertebrobasilar junction (VBJ) at the pontomedullary junction,
together with their branches which are the anterior spinal
artery, the posterior spinal arteries, the posterior inferior
cerebellar artery (PICA), and medullary perforating branches. The abducens nerve can be identified immediately ros-
tral to the VBJ bilaterally, while the lower cranial nerves
are visualized just before their entrance into the jugular
foramen.
39
Finally, the rootlets that form the hypoglossal
nerve are identified arising from the medulla in the preolivary sulcus and passing forward and laterally through the
subarachnoid space, passing behind the vertebral artery to
reach the hypoglossal canal (Fig. 31.31).
BA
pphICA
cl
b
segment of the VA passes just superior to the dorsal
and ventral roots of the first cervical nerve and just
anterior to the posterior spinal artery, the dentate ligament, and the spinal accessory nerve. In its ascending course, the anterior and lateral surfaces of the
lateral medullary segments face the occipital condyles,
the hypoglossal canals, and the jugular tubercles.
2. The anterior medullary segment begins after the preolivary sulcus and runs in between the hypoglossal
rootlets or anterior to them and crosses the pyramid
to reach the pontomedullary sulcus, where it unites
with the opposite VA to form the BA. The anterior
medullary segment rests on the clivus.
The VA in its intradural segment gives off the anterior
spinal artery, the posterior spinal arteries, the PICA, and
medullary perforating branches. The anterior spinal artery
is formed by the union of the paired anterior ventral spinal arteries which originate from the anterior medullary
segment of the VAs near the origin of the BA and join to
form a single trunk (in front of the spinal cord). In most
40
cases, the junction of the anteroventral spinal arteries is
above the level of the foramen magnum. The anterior spi-
BA
cl
nal artery descends through the foramen magnum on the
Anatomy
Once the VA becomes intradural, it courses superomedially behind the clivus and in front of the medulla. Moreover, this fourth segment can be divided into (1) lateral
and (2) anterior medullary parts, before it joins the opposite VA to form the BA. Usually the left VA is the dominant
one as it is wider than the right one.
1. The lateral medullary segment of the VA from the
entrance into the intradural compartment courses
anteromedially and superiorly through the lower cranial nerve rootlets and lateral to the medulla to reach
the preolivary sulcus. This segment begins at the dural foramen just inferior to the lateral edge of the foramen magnum. The first cervical nerve exits and the
posterior spinal artery enters the spinal canal through
this dural foramen with the VA. The initial intradural
anterior surface of the medulla and the spinal cord in or
near the anteromedian fissure.
27
The PICA arises from the distal VA at the anterolateral aspect of the brainstem near the inferior olive, passes
posteriorly around the medulla to course the glossopharyngeal, vagus, and accessory nerves, and then curves
around the tonsil to enter the cerebellomedullary fissure
(Fig. 31.32). On exiting the cerebellomedullary fissure, its
branches (the medial and lateral trunks) are distributed to
the vermis and hemisphere of the suboccipital surface. The
PICA gives off perforating, choroidal, and cortical (vermian,
tonsillar, and hemispheric groups) arteries. It is the largest
branch of the VA and has the most complex relationship
with the cranial nerves of any other cerebellar artery.
In more detail:
• The PICA is separated into five sections: anteri-
or medullary, lateral medullary, tonsillomedullary,
340
Intracranial Vascular Anatomy
X
X
IcIcnn
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VA
BA
IX-X-XIcn
XIIcn
VII-VIIIcn
XIIcn
XIIcn
BA
VIcn
VA
VA
VA
ASA
XIIcn
a
Fig. 31.31 (a,b) Endoscopic intradural views of the clivus and craniovertebral junction. The vertebral arteries, vertebrobasilar
junction, and spinomedullary junction are exposed. The anterior spinal artery as well as the acousticofacial bundle and the lower
cranial nerves are visible. The rootlets of the hypoglossal nerve that converge on the dural orifi ce of the hypoglossal canal and then
exit medially to the parapharyngeal internal carotid artery can be seen. ASA, anterior spinal artery; BA, basilar artery; VA, vertebral
artery; VIcn, abducens nerve; VIIcn, facial nerve; VIIIcn, vestibulocochlear nerve; IXcn, glossopharyngeal nerve; Xcn, vagus nerve; XIcn,
accessory nerve; XIIcn, hypoglossal nerve.
b
ASA
telovelotonsillar, and cortical. The anterior medullary
section is intimately related to the hypoglossal nerve
(XII), which lies anterior and between PICA and its parent vertebral artery (Fig. 31.32). The lateral medullary
section runs around the inferior surface of the olive until reaching the cranial nerve XI. The tonsillomedullary
section carries the artery between the spinal and cranial
roots of the cranial nerve XI and then behind the cranial
nerves X and IX (forming a caudally convex loop) to the
telovelotonsillar section where it runs medial to the cerebellar tonsil (forming a cranially convex loop). It enters
the telovelotonsillar section at the midpoint of the tonsil.
Then it runs medial to the tonsil and lateral to the vermis
to reach the cortical surface of the cerebellar hemisphere
and its last (cortical) section. Here it bifurcates into two
terminal branches: a smaller medial trunk that supplies
the vermis and adjacent part of the hemisphere and a
larger lateral trunk that supplies most of the hemispheric and tonsillar part of the suboccipital surface.
27
31.4.3 Basilar artery
(a) Basilar Artery via Transplanum
Supra-Retrosellar Corridor
By passing infrachiasmatically or by transposing the pituitary gland (Fig. 31.10), the basilar apex network and its
close relationship with the oculomotor nerves is evident
(Fig. 31.11 and Fig. 31.19). These nerves pass below and
slightly lateral to the PcomA in cases of normal configuration (Fig. 31.33 and Fig. 31.34), while in cases of fetal configuration the nerve runs beneath or medial to PcomA.
The oculomotor nerves arise in the interpeduncular cistern, on the medial side of the cerebral peduncle and run
anterolaterally to reach the roof of the CS. The Liliequist’s
cmAICA
PICA
VA
VA
Fig. 31.32 Endoscopic view of the medulla oblongata and
upper spine regions. The relationship between the lower
cranial nerves and the posterior inferior cerebellar artery
is visible. cmAICA, caudomedial branch of anterior inferior
cerebellar artery; PICA, posterior inferior cerebellar artery; VA,
vertebral artery. IXcn, glossopharyngeal nerve; Xcn, vagus
nerve; XIcn, accessory nerve; XIIcn, hypoglossal nerve.
PICA
IXcn
Xcn
XIcn
XIIcn
membrane, an arachnoidal structure, is attached superiorly to the oculomotor nerves and anteriorly to the dorsum sellae. More anteriorly, the oculomotor nerves pierce
the roof of the CS. In the anterior incisural space below
the floor of the third ventricle, the posterior part of the
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PcomA
P2
PcomA
P2
IIIcn
Fig. 31.33 Endoscopic view of the basilar apex region. Two
superior cerebellar arteries arising from the right side of the
basilar artery are evident. BA, basilar artery; P1, fi rst segment
of the posterior cerebral artery; P2, second segment of the
posterior cerebral artery; PcomA, posterior communicating
artery; SCA, superior cerebellar artery; IIIcn, oculomotor nerve.
P1
SCA
SCA
IIIcn
P1
BA
Willis circle is exposed. At the level of the interpeduncular cistern, below the mammillary bodies, the peduncular
veins are visible. More superiorly and laterally, it is possible to see the basal vein running on the lateral surface of
the mesencephalon in the ambient cistern.
39
(b) The Transclival Corridor
Through a clival window (Fig. 31.30), the PCF and the
upper spine can be exposed. After opening the dura
mater, all the neurovascular structures become visible
(Fig. 31.31).
their exit from the vertebral canal. The PICA and the anterior spinal artery are also visible. There is a close relationship between the lower cranial nerves and PICA at this
level (Fig. 31.32). The hypoglossal nerve can be identified
above the first tract of the intradural vertebral artery; usually, this nerve arises (in the preolivary sulcus) as a series
of rootlets that converge on the dural orifices of the hypoglossal canal where a significant venous network lies.
In the superior portion, the basilar tip region in front
of the midbrain is visible behind the pituitary gland
(Fig. 31.35). The PCAs arise directly from the BA as terminal branches, usually with a perpendicular direction. The
first segment of the PCA is termed P1 before joining the
PcomA, and thereafter is known as P2. Below the PCAs, the
SCAs originate from the lateral aspect of the BA, usually
just one vessel from each side; more than one SCAs coming from one side is rarely seen. Normally, the SCA crosses
below the oculomotor nerve and, going posteriorly, it runs
below the trochlear nerve and above the trigeminal one.
Occasionally, the lateral loop reaches the root entry zone
of the trigeminal nerve. The arterial network from PCA
is usually well visible in front of the cerebral peduncles;
it is possible to see the TPAs that arise from P1 and enter the brain by passing through the posterior perforated
substance and the medial part of the cerebral peduncles
in the area behind the mammillary bodies. Endoscopically,
they are undistinguishable from the premammillary arteries, which are branches from the PcomA entering the same
8
In the inferior portion, the VA can be seen at
SCA
P1
IIIcn
P1
PcomA
SCA
IIIcn
Fig. 31.34 Endoscopic view of the basilar tip in the
interpeduncular fossa. BA, basilar artery; P1, fi rst segment of
the posterior cerebral artery; PcomA, posterior communicating
artery; SCA, superior cerebellar artery; IIIcn, oculomotor nerve.
BA
area. With respect to the TPAs, the thalamogeniculate
arteries arise directly from the P2 beneath the lateral thalamus and penetrate part of the roof of the ambient cistern.
At this level, it is usually possible to see the peduncle vein
running posteriorly around the cerebral peduncle, above
the PCA and below the thalamus. The pontotrigeminal vein
arises on the superior and cerebellar peduncles and passes rostrally to the trigeminal nerve. The anterior, medial,
and lateral pontomesencephalic veins can also be seen at
this level. The transverse pontine veins are a group of veins
that transverse the pons at various levels. They run laterally above or below the trigeminal nerve to drain into the
superior petrosal vein, the pontotrigeminal vein, or that of
the cerebellopontine fissure. At the mesencephalic level,
the lateral anterior pontomesencephalic vein may anastomose with the basal and peduncular vein and that of the
pontomesencephalic sulcus. Caudally, at pons level, it joins
the transverse pontine vein (usually in the midpons) and
the pontomedullary sulcus vein in close proximity to the
abducens nerve. A close relationship between the anterior
inferior cerebellar artery (AICA) and veins is present. The
AICA arises at the junction of the lower one-third and upper two-thirds of the BA and runs laterally on the anterolateral surface of the pons in contact with the abducens
nerve, which may cross superiorly or inferiorly (Fig. 31.36).
More laterally, in the cerebellopontine angle it is possible to see a complex neurovascular network, given principally by the superior petrous veins (SPVs), the AICA, and its
branches, and the cranial nerves V to VIII. The superior petrosal veins are divided into medial, intermediate, and lateral groups depending on whether they enter the middle,
intermediate, or lateral third of the SPS, respectively.
27
The
SPV is formed approximately at the trigeminal nerve level
by the transverse pontine vein, the pontotrigeminal veins,
the cerebellopontine fissure veins, and the vein of the middle cerebellar peduncle. A close view of the area around
the internal acoustic meatus, by means of a 45-degree
lens, allows clear identification of the labyrinth artery
that nearly always arises from the AICA (Fig. 31.37). In
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PcomA
P1
IIIcn
Fig. 31.35 Endoscopic view of the basilar artery through a
clival corridor using a 45-degree endoscope. AICA, anterior
inferior cerebellar artery; BA, basilar artery; P, pons; PBs,
pontine branches; PcomA, posterior communicating artery;
P1, fi rst segment of the posterior cerebral artery; SCA,
superior cerebellar artery; IIIcn, oculomotor nerve; Vcn, fi fth
(trigeminal) cranial nerve.
SCA
BA
PBs
PBs
P
AICA
PcomA
P1
SCA
BA
IIIcn
Vcn
AICA
two-thirds of the cases, there are two or three branches.27
Recurrent perforating arteries, arising from the AICA, run
along the facial and vestibulocochlear nerves to reach the
brain stem, surrounding the entry zones of these nerves.
Anatomy
The two VAs unite at or just below the pontomedullary
junction to form the BA between the cranial nerves VI.
The BA courses superiorly in the prepontine cistern, lying
between the clivus in front and the pons behind. It terminates in the interpeduncular fossa by dividing into two
PCAs (Fig. 31.35). Numerous small but critical basilar per-
forating arteries arise from the entire dorsal surface of the
BA to supply the pons and midbrain. The first major named
BA branch is the AICA. The AICA arises from the proximal
BA and courses ventromedially to the cranial nerves VII
and VIII, frequently looping into the internal auditory meatus. It supplies both nerves as well as a relatively thin strip
of the cerebellar hemisphere that lies directly behind the
petrous temporal bone. The superior cerebellar artery (SCA)
originates from each side of the distal BA, courses laterally
below the III cranial nerve, and then curves posterolaterally around the midbrain just below the tentorium and IV
cranial nerve and above the V cranial nerve. SCA branches
ramify over the surface of the superior cerebellum and upper vermis, curving into the great horizontal fissure.
In more detail:
• The AICA arises at the junction of the lower one-third
and upper t
wo-thirds of the BA and runs laterally on
PBs
SCA
BA
BA
AICA
Vcn
VIcn
VIIIcn
SCA
IIIcn
PBs
P
VIcn
Fig. 31.36 Endoscopic view of the posterior cranial fossa
at the pontomesencephalic region. AICA, anterior inferior
cerebellar artery; BA, basilar artery; P, pons; PBs, pontine
branches; SCA, superior cerebellar artery; IIIcn, oculomotor
nerve; VIcn, abducens nerve.
AICA
rlAICA
cmAICA
cmAICA
Fig. 31.37 Endoscopic view of the anterior inferior cerebellar
artery and its branches. AICA, anterior inferior cerebellar artery;
cmAICA, caudomedial branch of anterior inferior cerebellar
artery; IAM, internal acoustic meatus; rlAICA, rostrolateral
branch of anterior inferior cerebellar artery; Vcn, trigeminal
nerve; VIcn, abducens nerve; VIIcn, facial nerve; VIIIcn,
vestibulocochlear nerve; IXcn, glossopharyngeal nerve; Xcn,
vagus nerve; XIcn, accessory nerve; XIIcn, hypoglossal nerve.
IXcn
Xcn
XIcn
XIIcn
IIIcn
PBs
AICA
VIcn
IAM
VIIcn
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Vcn
IAM
P1
P1
PcomA
VIIcn
VIIIcn
LA
rlAICA
JF
IX-X-XIcn
VIcn
Fig. 31.38 Endoscopic view of the internal acoustic meatus
and jugular foramen using a 45-degree endoscope. cmAICA,
caudomedial branch of anterior inferior cerebellar artery; IAM,
internal acoustic meatus; JF, jugular foramen; LA, labyrinthine
artery; rlAICA, rostrolateral branch of anterior inferior cerebellar
artery; Vcn, trigeminal nerve; VIcn, abducens nerve; VIIcn, facial
nerve; VIIIcn, vestibulocochlear nerve; IXcn, glossopharyngeal
nerve; Xcn, vagus nerve; XIcn, accessory nerve.
LA
cmAICA
the anterolateral surface of the pons in contact with the
abducens nerve (Fig. 31.36). It may cross superiorly or
inferi orly the abducens nerve. It anastomoses on the
surface of the cerebellar hemispheres with branches of
the PICA and the SCAs. On reaching the acousticofacial
bundle, before or after crossing the rootlets, the AICA
bifurcates into its two major branches: the caudomedi-
al and rostrolateral (Fig. 31.37). The caudomedial artery
runs inferomedially toward the anterior and medial border of the cerebellum and supplies the middle cerebellar
peduncle and biventral lobule. The rostrolateral trunk
runs laterally and curls around the flocculus, and then it
courses within the petrosal fissure from where the hemispheric branches to the superior and inferior semilunar
lobule are given off. The internal auditory artery (labyrin-
thine artery) arises usually from the AICA, rarely directly
from the BA, and runs laterally to the internal auditory
canal where it gives cochlear and vestibular branches to
feed the inner ear (Fig. 31.38). A small branch, the subarcuate artery supplies the meninges of the subarcuate
fossa and the bone of the semicircular canals.
27
• One or more (usually two to four) SCAs arise near the
apex of the BA (Fig. 31.33) just proximal to the origin
of the PCA from which is separated by the oculomotor
nerve medially and the trochlear nerve laterally. The
SCA parallels the course of the PCA around the cerebral peduncle in the ambient cistern (Fig. 31.39). Lying
below the free edge of the tentorium, it bifurcates on
the lateral side of the brainstem into two major trunks:
a rostral one for the vermis and a caudal one for the
hemisphere (Fig. 31.40).
27
P1
SCA
IIIcn
IIIcn
ca
ro
Vcn
VIcn
SCA
IIIcn
BA
Fig. 31.39 Endoscopic view of the ambient cistern. BA,
basilar artery; IIIcn, oculomotor nerve; P1, fi rst segment of the
posterior cerebral artery; PcomA, posterior communicating
artery; SCA, superior cerebellar artery.
SCA
BA
Fig. 31.40 Endoscopic view of the superior cerebellar artery
and its branches. BA, basilar artery; ca, caudal branch of
superior cerebellar artery; IIIcn, oculomotor nerve; P1, fi rst
segment of the posterior cerebral artery; ro, rostral branch of
superior cerebellar artery; SCA, superior cerebellar artery; Vcn,
trigeminal nerve; VIcn, abducens nerve.
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