REZ 360°

REZzzzzzzzzz360°
31-1-2016
11.50 pm
Root exit zone & Root entry zone

Great teachers – All this is their work .
I am just the reader of their books .
Prof. Paolo castelnuovo
Prof. Aldo Stamm Prof. Mario Sanna
Prof. Magnan

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3rd nerve in interpeduncular fossa

Roof - two triangles:
1. clinoid (anterior)
2. oculomotor (posterior)
ACP anterior clinoid process, APCF anterior petroclinoid fold, DS dorsum sellae, ICF interclinoid
fold, PF pituitary fossa, PLL petrolingual ligament (inferior sphenopetrosal ligament),
PPCF posterior petroclinoid fold, PS planum sphenoidale, SSPL superior sphenopetrosal
ligament (Gruber’s ligament), TS tuberculum sellae, black asterisk middle clinoid process , CSR
cavernous sinus roof , white asterisk oculomotor nerve
If the Gruber’s ligament is ossificated it is called Wegener’s
bridge.

Oculomotor cistern
Cranial nerve III enters the roof included in its own cistern
(oculomotor cistern).
Oculomotor cistern goes upto
anterior clinoid tip

Endoscopic lateral skull base – 4th
coming from posteriorly over the
superior cerebellar artery [ in this
picture has 2 branches

The superior cerebellar artery (SCA) and the trochlear nerve (IV)
are well observed superior to the trigeminal nerve (V) – in
accoustic neroma surgery by translabyrinthine approach

4th nerve under tentorium in subtemporal approach after cutting the
tentorium & lifting it , you are seeing 4th nerve insertion [ yellow arrow = REZ
of 4th nerve ]

The trochlear nerve is divided into 5 segments: cisternal, tentorial,
cavernous, fissural ( in superior orbital fissure ) and orbital.
The cisternal segment exits the midbrain and courses through the
quadrigeminal and ambiens cisterns towards the TC. The tentorial segment
starts when the nerve pierces the TC, usually posterior to the postero-lateral
margin of the oculomotor triangle. This segment ends at the level of the
anterior petroclinoid fold. This portion is in close relationship with the
spheno-petro-clival venous gulf and the petrous apex (Iaconetta et al. 2012 ).

Cadaveric dissection image taken
with a 30-degree endoscope
following removal of the superior
third of the clivus, visualizing the
small trochlear nerve seen running
along the tentorial membrane edge.
Posterior view of the left CPA with a 30° angled
endoscope gives a view of CPA contents and
permitsobservation of the blind spots by “looking
around the corner.” V indicates trigeminal nerve; VI,
abducens nerve; IV, trochlear nerve; VII, facial nerve
anteriorly hidden by VIII; VIII, vestibulocochlear
nerve; IX, glossopharyngeal nerve; X, vagusnerve;
XI, spinal accessory nerve; XII, hypoglossal nerve;
aica, anterior-inferior cerebellar artery; DV, Dandy’s
vein or superior petrosal vein; SPS, superior
petrosal sinus; Tent, tentorium.

Trochlear nerve. Lateral view of the right parasellar area. A triangular piece of the tentorium has been
removed while preserving the tentorial edge to expose the site at which the trochlear nerve pierces the
lower margin of tentorium. The trochlear nerve courses medial to the tentorial edge. It is the longest and
thinnest cranial nerve. It ia the only nerve to arise from the dorsal aspect of the brain stem. The trochlear
nerve arises from the midbrain below the inferior colliculi and passes around the brain stem near the
junction of the midbrain and pons to reach the lower margin of the tentorial edge. The trochlear nerve
pierces the tentorial edge (arrow) just behind the anterior attachment of the tentorium. and passes forward
in the lateral wall of the cavernous sinus below the oculomotor nerve. The upper edge of the posterior root
of the trigeminal nerve is in the lower margin of the exposure.

With 30 degree scope - The major sensory root and minor motor root
within the Meckel cavity This close-up view allows visualisation of the
roof and the bifurcation at the bottom of the Meckel cavity.

6th nerve (the snake nerve)
6th nerve originates above the VBJ [
vertebro-basillar junction ] – Prof.
Amin Kassam

6th nerve origin is above or below AICA or has two
rootlets of origin

After removal of the anterior component of the tumor Trans-labyrinthine
approach , the basilar artery (BA) is clearly visible, as well as the abducent
nerve (VI) and the origin of the anteroinferior cerebellar artery (AICA).

Closer view of the inferior area of the left CPA, with
tip of the endoscope between the acousticofacial nerve bundle and lower cranial nerves. PICA
originating from the vertebral arterycan be seen forming a loop near the REZ of the facial nerve.
AICA arises from the more medial basilar artery and traverses under the acousticofacial nerve
bundle to supply the anterior surface of cerebellum. Abducens nerve (VI) is occasionally formed
by two different nerve bundles as seen here.

Cadaveric dissection image demonstrating structures seen
following dissection of the lower third of the clivus. Note how
the basilar arteries and vertebral arteries can be extremely
tortuous in their course.

Pontomedullary junction = Vertebro-basillar junction
= Junction of Mid clivus & Lower clivus
The pontomedullary junction. The vertebral artery junction is at the level of
the junction of the inferior and midclivus. The basilar artery runs in a straight
line on the surface of the pons. The exit zones of the hypoglossal and
abducent nerves are at the same level. The abducent nerve exits from the
pontomedullary junction, and ascends in a rostral and lateral direction toward
the clivus.
The abducent nerve (VI) is seen in the prepontine
cistern after accoustic neuroma removal in close proximity
with the basilar artery. BA, basilar artery; BS,b rainstem.

The pontomedullary junction.
1. The exit zones of the hypoglossal and abducent nerves are at
the same level [ same vertical line when view from Transclival
approah ( through lower clivus ) ]
2. The abducent nerve exits from the pontomedullary junction, and ascends
in a rostral and lateral direction toward the clivus.
6th nerve originates above the
VBJ [ vertebro-basillar junction ]
– Prof. Amin Kassam

6th nerve – enters the dorellos canal –
Intradural course

Gulfar segment of 6th nerve (GS in left picture ) ( gVIcn in right picture ) - The
gulfar segment can be identified at the intersection of the sellar floor and the
proximal parasellar internal carotid artery (ICA) (Barges-Coll et al. 2010 ).
6th nerve enters dorello’s canal between
the meningeal layer of dura and the
periosteal layer of dura (POD).

(Left ear) Difference between the conventional 180° translabyrinthine
approach and the 320° transapical extension in the surgical view. The
dashed line demonstrates the working area for extended bone removal. After
this bone work is completed, the important structures can be controlled. AICA,
anterior inferior cerebellar artery; BA, basilar artery; V, trigeminal nerve; VI, abducent nerve.

4 parts – 1.Brainstem 2. CPA 3. IAC
4. Fundus of IAC

REZ of 7th nerve is caudal than 8th
nerve REZ

Intraoperative endoscopic picture
in a patient with right hemifacial
spasm showing PICA having
perpendicular contact with VII at
REZ
Intraoperative endoscopic picture
in a patient with right hemifacial
spasm after “decompression” of
VII at REZ by mobilizing PICA and
interposition of Teflon insulation

Microscopic view of the foramen of Luschka on an injected specimen.
ChP, choroid plexus; VII, facial nerve; VIII, vestibulocochlear nerve; LCN,
lower cranial nerves; LF, foramen of Luschka.

a Microscopic view of the foramen of Luschka (arrow). b At higher
magnification, the bulbopontine junction is clearly appreciated. AICA, anterior
inferior cerebellar artery; PV, plexus of veins; VI, abducent nerve; VII, facial
nerve; VIII, vestibulocochlear nerve.

a Landmarks for identifying the
foramen of Luschka. The choroid
plexus (CP) exits the foramen. AICA,
anterior inferior cerebellar artery;
Fl, flocculus; VI, abducent nerve; VII,
facial nerve; VIII, vestibulocochlear
nerve; IX, glossopharyngeal nerve.
Anatomy of the fourth ventricle and
the lateral recess and the location of
the cochlear nucleus. This illustration
shows an ideal placement of an array
of electrodes. Cbl, cerebellum; CP,
choroid plexus; d, dorsal cochlear
nucleus; Fl, flocculus; SS, sigmoid
sinus; v, ventral cochlear nucleus;
VII, facial nerve; VIII, stump of the
vestibulocochlear nerve.

Left Ménière disease: In around 40% of cases,
the anterior inferior cerebellar artery (aica)
forms a vascular
loop running toward the porus acusticus,
usually inferior to the
vestibulocochlear nerve bundle. Within the
vestibulocochlear nerve,
the vestibular fibers (Ve) are more superior
(rostral) and close to the
trigeminal nerve, and the cochlear nerve (Co)
is inferior (caudal)
and close to the lower cranial nerves (LCN).
Left Ménière disease: A small
dissector is inserted
into the inter-vestibulocochlear
cleavage plane to divide the
vestibulocochlear nerve into its
two parts.

Mneumonic is Circle inspector of Police [ CI ] – Cochlear nerve is inferior
In cisternal AFB cochlear nerve is
inferior to vestibular nerve
In IAC cochlear nerve is anterio-
inferior quadrant
At the end of tumor
removal, the most
lateral fundus part
of the internal
auditory meatus is
checked with an
endoscope. Often
there is residual
tumor (T) in the
fundus. Fn indicates
facial nerve; Cn,
cochlear nerve; Vn,
residual vestibular
nerve.

Vestibular neurotomy is
progressively performed with
microsurgical scissors.
Left endoscopic vestibular
neurotomy is complete.
The facial nerve located
anteroinferior to the vestibular nerve
is now
visible.

Left microsurgical vestibular neurotomy with terminal
fibers being dissected by blunt probe. co indicates the cochlear
nerve; ve, sectioned vestibular nerve; aica, anterior inferior
cerebellar artery.

The anterior inferior cerebellar artery, lying between
the auditory and facial nerves, is found in 38% of cases. –
5 Trigeminal nerve , 7 Facial nerve , 8 Vestibulocochlear nerve

Artist’s renderings showing posterior view of
the left IAM. ( a ) Subarcuate artery penetrates the dura of
the subarcuate fossa near the IAM. The labyrinthine artery
enters the meatus with the vestibulocochlear and the facial
nerves. ( b ) Laterally convex loop of the AICA is embedded
in the dura covering the subarcuate fossa, where it
gives off the subarcuate artery. ( c ) AICA loop is embedded
in the dura and bone ( arrow ) surrounding the subarcuate

fossa. ( d ) Dura over the subarcuate fossa has been incised,
and the dura with the adherent loop is dissected free from
the subarcuate fossa in preparation for opening the IAM.
( e ) Dura over the subarcuate fossa has been incised and
remains attached to the artery. The bone surrounding the
embedded AICA loop is removed with a 2-mm diamond
drill to displace the artery medially for exposure of the
IAM (From Tanriover and Rhoton [ 50 ] )

The posterior wall of the internal
acoustic meatus has been
removed. The cleavage plane
between the superior and inferior
vestibular nerves is especially
prominent.
The dura lining the internal
acoustic meatus has been
opened. The transverse crest
separates the superior vestibular
and facial nerves above from the
inferior vestibular and cochlear
nerves below.

Enlarged view of the nerves
within the meatus. The cochlear
nerve is partially hidden anterior
to the inferior vestibular nerve.
The cleavage plane between the
superior and inferior vestibular
and cochlear nerves has been
started laterally and extended
medially to expose the individual
nerve bundles

In Left Trans-labyrinthine Accoustic neuroma surgery - Possible
locations of the facial nerve (FN) in relation to the tumor are
shown. C, cochlear nerve; ant., anterior; post., posterior; sup.,
superior.

Possible locations of the facial nerve
(FN) in relation to the tumor are shown. C, cochlear
nerve; ant., anterior; post., posterior; sup., superior.

a The effect of tumor size on the facial nerve (FN). Note that as the
tumor grows in size the facial nerve becomes thinner and more fragile
(splayed). b With tumor dissected away, the splayed nerve can be better
visualized.

The various locations of the facial nerve (FN). a Superiorly pushed
facial nerve (left ear). b Inferiorly pushed facial nerve (left ear). c Posteriorly
pushed facial nerve (left ear). T, tumor.

a ) Normal neural
relationships with the eighth nerve dividing into its
three parts in the lateral meatus. The facial and
superior
vestibular nerves are above the transverse crest and
the
cochlear and inferior vestibular nerves are below.
The facial
nerve occupies the anterosuperior quadrant of the
lateral meatus.
( b ) The facial nerve is displaced
directly anteriorly.
This is a frequent direction of
displacement with
acoustic neuroma.
In Retrosigmoid approach

( c ) Another frequent direction of
displacement
of the facial nerve is anterior and
superior.
( d )The facial nerve is displaced
anteriorly and inferiorly by
tumor, which erodes the superior
wall of the meatus above
the nerves and grows into the
area above the nerves, displacing
them inferiorly

7up- 7th is above
Coca cola – cochlear n. is cola[=lower]

FN & SVN converge as they pass toward the fundus , while the CN & IVN can
be seen diverging from each other as they pass laterally to the fundus - ---
Basal turn of cochlea pushing away IVN from CN
See the cochlea in below photo

Translabyrinthine
approach – at fundus
Detachment of the superior vestibular nerve
(SVN). The facial nerve (FN) is clearly seen.
Arrows point at the canal where the superior
ampullary nerve was running.

While the dissection of the vestibular nerves is carried on further medially, adhesions (AD) between the facial nerve (FN) and the
vestibular nerves begin to be encountered. At this point careful, delicate dissectionshould be performed in order not to injure the
facial nerve. CN, cochlear While the dissection of the vestibular nerves is carried onfurther medially, adhesions (AD) between the
facial nerve (FN) and the vestibularnerves begin to be encountered. At this point careful, delicate dissectionshould be performed
in order not to injure the facial nerve. CN, cochlear

In some cases, the dissection of the adhesion bands will result in
bleeding, obscuring the plane of dissection. CN, cochlear nerve;
FN, facial nerve.

Identification of the facial nerve (FN) at the fundus of the internal auditory
canal is easier. TC, transverse crest; IVN, inferior vestibular nerve; SVN,
superior vestibular nerve.

Wrong technique for identifying the internal
auditory canal. Note that the posterior wall of
the canal is completely removed, while the superior
and inferior walls have not been drilled. The correct
way is to leave a thin shell over the dura of the
internal auditory canal until the two troughs are
created.

Identify the superior ampullary nerve first and then find the facial nerve
(F) after eliminating this nerve and the superior vestibular nerve (Vs).

middle cranial fossa photos
Fig. 5.30 A simple middle cranial fossa
approach has been established, and the
internal auditory canal dura has been
opened. A Anterior, B Bill’s bar, FN Facial
nerve, P Posterior, SSC Superior
semicircular canal, SV Superior vestibular
nerve
The acousticofacial bundle components have been
separated. Both the facial nerve (FN) cochlear nerve (CN)
can now be seen. AICA Anterior inferior cerebellar artery

The dura of the internal auditory canal
has been further removed. At
the level of the fundus, Bill’s bar (BB)
can be seen. AE, arcuate eminence; C,
cochlea; FN, facial nerve within the
internal auditory canal; GPN, greater
petrosal nerve; L, labyrinthine segment
of the facial nerve; SVN, superior
vestibular nerve.
At higher magnification, the
relationship at the fundus can be better
appreciated. AE, arcuate eminence; BB,
Bill’s bar; C, cochlea; FN(iac), internal
auditory canal segment of the facial
nerve; GG, geniculate ganglion; GPN,
greater petrosal nerve; L, labyrinthine
segment of the facial nerve; SVN,
superior vestibular nerve.

Exocranial & Endocranial views of Jugular Foramen : Within the JF area 2
venous compartement can be identified: a large postero-lateral_SIGMOID_venous channel and
a small antero-medial_PETROSAL_venous channel which can receive the drainage of the
inferior petrosal sinus (IPS). An intermediary neural compartment is located
between the venous ones and houses lower cranial nerves (IX, X, XI).
CC carotid canal, CR carotid ridge, ESF endolymphatic sac fossa, FS foramen spinosum, IAM internal acoustic
meatus, JT jugular tubercle, OC occipital condyle, PCF petroclival fi ssure, SAF subarcuate fossa, SP styloid
process, SSG sigmoid sinus groove, TB tympanic bone, VPTB vaginal process of the tympanic bone, white
arrow intrajugular process of the temporal bone, red arrow external ori fi ce of the hypoglossal canal, violet
arrow petroclival fi ssure, blue-sky arrow tubal isthmus, black arrow endocranial orifice of the hypoglossal
canal, orange arrow trigeminal impression, green arrow pyramidal fossa, black asterisks intrajugular ridge,
black circle intrajugularprocess of the occipital bone

FCB & JT & LCNs are at same level from anterior
to posterior
FCB = Fibrocartilago basalis , JT = Jugular tubercle , LCNs
Lower cranial nerves ( = 9th , 10th, 11th )

In Far lateral approach -- The lower cranial nerves have an intimate relationship with the
jugular tubercle (three black arrows). When the occipital bone and jugular tubercle are being drilled,
careful attention should be paid to avoiding damage to the lower cranial nerves. , Cbl cerebellum , ICA
internal carotid artery , OC occipital condyle , TP transverse process of the C1 vertebra , VA vertebral artery
, VIII cochleovestibular nerve , IX glossopharyngeal nerve , XI spinal accessory nerve

The right side of the
bulbomedullary junction. It is the
lowermost and narrowest part of
the posterior fossa. This area
requires special dissection prior
to endoscopic investigation
between the pontomedullary
stem and the jugular foramen.

The root fibers of the spinal
accessory nerve and the fibers of C1
and C2. The entrance of the vertebral
artery is the boundary between the
foramen magnum and the spinal part
of the accessory nerve.
A 30° endoscope provides an
overview of the medullary canal,

11th nerve behind left vertebral artery at cervico-medullary junction – listen
lecture at 23.25 min in this Prof. Amin Kassam video
https://www.youtube.com/watch?v=QoMCqwJ6Ke0
Through anterior skull base
approach
Through endoscopic lateral skull
base approach – The entrance of
the vertebral artery is the
boundary between the foramen
magnum and the spinal part of
the accessory nerve.

The accessory nerve (XI) is closely related to the vertebral artery (VA) at the point of
dural entrance. Note the dura attached to the artery at this level.
Endoscopic lateral skull base
approach

The accessory nerve (XI) is closely related to the vertebral artery (VA) at the
point of dural entrance. Note the dura attached to the artery at this level.

Intracranial hypoglossal region. Anterior endoscopic transnasal-transclival vision is
compared with a posterior retrosigmoid endoscopic one
JF jugular foramen, JT jugular tubercle, IO inferior olive, PICA posteroinferior cerebellar artery, VA vertebral artery, IXcn
glossopharygeal nerve, Xcn vagus nerve, XIcnCR cervical roots of accessory nerve, XIcnSR spinal roots of accessory nerve,
XIIcn hypoglossal nerve
Cranial nerves IX and X present a close relationship with the fi rst portion of the PICA. They are protected by the arachnoid
membrane (Roche et al. 2008 ) . The roots of cranial nerve XIcn from the spine pass through the foramen magnum posterior
to the vertebral artery. Within the hypoglossal canal, XIIcn is surrounded by a venous plexus and dural and arachnoid
sheets. Branches of the ascending pharyngeal artery coursing through the hypoglossal canal are seen in about 50 % of cases
(Lang 1995 ) . Also branches from the posterior meningeal artery have been described (Janfaza and Nadol 2001 ). The
transcisternal vein to the area of the JF can be seen. Also, veins to the hypoglossal canal can be present. The hypoglossal
nerve do not exit with VA. It can have maximum 3 outlets. On the contrary, C1 roots exit with the VA.

Left side. The lower cranial
nerves, with the poste-rior
inferior cerebellar artery
arising from the vertebral
artery in the background.
Neurovascular relationships
between the exit zone of the root
fiber bundles of the eleventh and
twelfth nerves, the posterior
inferior cerebellar and vertebral
arteries. Fibrous tissue is seen
around the vertebral artery.

The facial nerve can be clearly seen
in the middle part of the approach
after retracting the posteriorly lying
cochlear nerve. Separation of the
glossopharyngeal nerve (IX) from the
vagus (X) and accessory (XI) nerves
at the medial aspect of the jugular
foramen.
Further inferiorly, the ninth (IX),
tenth (X), and eleventh (XI) cranial
nerves can be seen exiting the skull
through the jugular foramen

Right side. The root fibers of the hypoglossal nerve (12) collect in two
bundles, which pierce the dura in two dural pori. The hypoglossal nerve
is situated more anteriorly and medially than the root fibers of the
lower cranial nerves. The arterial relationship is the vertebral artery,
with perforating arteries to the brain stem. The curved vertebral artery
displaces and stretches the hypoglossal nerve fibers.

A closer view of the anterior border
of the pontomedullary
stem and the vertebral artery
junction and origin
of the basilar artery. Perforating
arteries arise from the vertebral
and basilar arteries.
The endoscope is focusing on the
hypoglossal nerve
area. The posterior inferior
cerebellar artery arises from the
vertebral artery in the
background, and runs between
the two
bundles of the hypoglossal nerve.

PICA passes between two bundles of 12th nerve & between
two roots of 11th nerve – retrosigmoid endoscopic approach
The endoscope is focusing on the
hypoglossal nerve area. The posterior
inferior cerebellar artery arises from the
vertebral artery in the background, and
runs between the two bundles of the
hypoglossal nerve.
The posterior inferior cerebellar
artery travels through the nerve
fiber roots of the accessory nerve

PICA passes between two bundles of 12th nerve & between
two roots of 11th nerve – anterior skull base endoscopic
approach
Endoscopic Far-medial
approach PICA passes through 12th
nerve – retrosigmoid
endoscopic approach
PICA passes through
11th nerve –
retrosigmoid
endoscopic approach

The posterior inferior cerebellar artery travels
through the nerve fiber roots of the accessory
nerve and
encircles the brain stem. The course of the
vertebral artery is
inferior and anterior to the lower cranial nerves
and the
hypoglossal nerve. Fibrous tissue surrounds the
entrance of
the vertebral artery into the CPA.
9 Glossopharyngeal nerve
10 Vagus nerve
11 Accessory nerve
12 Hypoglossal nerve
PICA Posterior inferior cerebellar
artery
Vert. A Vertebral artery

A closer view of the pars nervosa
of the jugular foramen. The
glossopharyngeal nerve has its
own dural porus, which is
situated 0-3 mm upwards from
the dural porus of the tenth
cranial nerve. The vagus and the
accessory nerve exit the
posterior fossa together in a
sleeve of dura through the
jugular foramen.
Left side. The 30° angled endoscope
provides an overview of the inferior
part of the CPA. On the right lies the
acousticofacial nerve bundle, with
the anterior inferior cerebellar
artery; the glossopharyngeal nerve
and the vagus nerve, as multiple
filaments, form three to five major
nerve bundles and the accessory
nerve.

Relationship between the cochlear aqueduct and the lower cranial nerves. After rerouting of the facial nerve
and drilling away of the fallopian canal of the left temporal bone, the cochlear aqueduct (CA) has been
opened. The proximity of the glossopharyngeal nerve (IX) can be well appreciated. Since the nerve lies just
inferior to the cochlear aqueduct, the latter is used as a landmark to the nerve in the translabyrinthine
approach [ after drilling cochlea inferiorly ] , indicating the lower limit of drilling to avoid injury to the
glossopharyngeal nerve. ICA internal carotid artery , JB jugular bulb , SMF stylomastoid foramen

The glossopharyngeal nerve has its
own dural porus, which is situated
0-3 mm upwards from the dural
porus of the tenth cranial nerve. The
vagus and the accessory nerve exit
the posterior fossa together in a
sleeve of dura through the jugular
foramen.

The glossopharyngeal and vagus nerves are well
identified in the cerebellomedullary cistern before
entering the jugular foramen.

Cadaveric dissection with image taken just above the skeletonized hypoglossal canal
(HC) at the cerebellopontine angle. The anterior inferior cerebellar artery (AICA) can
be seen intimately associated with the vestibulocochlear nerve (CN VIII), facial nerve
(CN VII), and the nervus intermedius (NI). The posterior inferior cerebellar artery
(PICA) can be seen running between the vagus (CN X) and spinal and cranial portions
of the accessory nerves (CN XI – S, CN XI – C).

PICA encircling lower CNs . Observe
SCA & AICA

1. The HC divides the condylar region into the tubercular compartment
(superior) and the condylar compartment (inferior).
Tubercular compartment contains LPT lateral pharyngeal tubercle, PT
pharyngeal tubercle,
2. The SCG [Supracondylar groove] represents a reliable landmark for hypoglossal canal (HC) identification
(red arrow) (Morera et al. 2010 ) .

The tubercular compartment corresponds to the Jugular tubercle ( JT )
Line along the lateral pharyngeal tubercle [ LPT ] passes through
Jugular tubercle [ JT ] – so when you are drilling LPT in anterior skull
base you will land up on JT .
LPT lateral pharyngeal tubercle, OC
occipital condyle, PT pharyngeal
tubercle, SCG supracondylar groove
Jugular tubercle ( JT )

Line along the lateral pharyngeal tubercle [ LPT ] passes through Jugular tubercle [ JT
] – so when you are drilling LPT in anterior skull base you will land up on JT .
Red rings = hypoglossal canals , yellow
ring = pharyngeal tubercle [ PT ] , blue
rings = lateral pharyngeal tubercle [
LPT]

The condylar emissary canal is visible superior
to the right occipital condyle within the supracondylar fossa (small arrow). The left
hypoglossal canal can be seen through this oblique view being located below the
jugular tubercle (large arrow).

Line along the lateral pharyngeal tubercle [ LPT ] passes through Jugular
tubercle [ JT ] – so when you are drilling LPT in anterior skull base you will
land up on JT .
yellow ring = pharyngeal tubercle [ PT ] , blue rings = lateral pharyngeal
tubercle [ LPT] , green ring = Jugular tubercle

JT = Jugular Tubercle – Below this
tubercle is hypoglossal canal & above
is Internal Jugular foramen

The pontomedullary junction.
1. The exit zones of the hypoglossal and abducent nerves are at
the same level [ same vertical line when view from Transclival
approah ( through lower clivus ) ]
2. The abducent nerve exits from the pontomedullary junction, and ascends
in a rostral and lateral direction toward the clivus.

Two cerebellar lobes and the
medullary stem. The
posterior inferior cerebellar artery
encircles the medullary
stem. The opposite vertebral artery
exits from the dural porus
and raises the hypoglossal nerve.
The pontomedullary junction. The vertebral
artery junction is at the level of the junction
of the inferior and midclivus. The basilar
artery runs in a straight line on the surface
of the pons. The exit zones of the hypoglossal
and abducent nerves are at the same level.
The abducent nerve exits from
the pontomedullary junction, and ascends in
a rostral and lateral direction toward the
clivus.

A closer view of the anterior
border of the pontomedullary
stem and the vertebral artery
junction and origin
of the basilar artery.
Perforating arteries arise from
the vertebral and basilar
arteries.
The endoscope is focusing on
the hypoglossal nerve area.
The posterior inferior
cerebellar artery arises from
the vertebral artery in the
background, and runs
between the two bundles of
the hypoglossal nerve.

The PICA runs between the two
bundles of the hypoglossal nerve.
At a higher magnification, the nerves IX−XI
are seen coursing toward the jugular
foramen. The two bundles of the
hypoglossal nerve(XII) are closely related
to the vertebral artery (VA) before they
unite to course in the hypoglossal canal in
the partially drilled occipital condyle (OC).
XIs, spinal accessory nerve.

The root fibers of the hypoglossal nerve (12) collect in two
bundles
Cadaveric dissection image showing the hypoglossal nerve
exiting the hypoglossal foramen with its corresponding vein that
communicates the internal jugular vein with the basilar plexus.
HC, hypoglossal canal; CN XII, hypoglossal nerve and rootlets;
FM, foramen magnum; VA, vertebral artery; PICA, posterior
inferior cerebellar artery; BA, basilar artery; CN X, vagus nerve.

Through endoscopic lateral skull
base - The curved vertebral
artery displaces and stretches
the hypoglossal nerve fibers.
Through anterior skull base

Through lateral skull base - The curved
vertebral artery displaces and stretches the
hypoglossal nerve fibers.
Through lateral skull base - The opposite
vertebral artery exits from the dural porus
and stretches /raises the hypoglossal nerve.

Cadaveric dissection image taken following dissection of the right lower third of the
clivus. As the posterior inferior cerebellar artery (PICA) courses from the vertebral
artery (VA) it frequently runs through the rootlets that make up the hypoglossal nerve
(CN XII). It may tent these rootlets as it courses to the cerebellomedullary fissure to
run intimately with the cranial nerves IX – XI. CN X, vagus nerve; HC, hypoglossal canal;
IPS, inferior petrosal sinus; BA, basilar artery; FM, foramen magnum; A. AOM, anterior
atlanto-occipital membrane.

Cadaveric dissection image showing the hypoglossal nerve
exiting the hypoglossal foramen with its corresponding vein that
communicates the internal jugular vein with the basilar plexus.
HC, hypoglossal canal; CN XII, hypoglossal nerve and rootlets;
FM, foramen magnum; VA, vertebral artery; PICA, posterior
inferior cerebellar artery; BA, basilar artery; CN X, vagus nerve.

The hypoglossal nerve do not exit with VA. It can have maximum
3 outlets. On the contrary, C1 roots exit with the VA.

HC = hypoglossal canal , JT= Jugular Tubercle

At the inferior part of the
approach the lower cranial nerves
can be appreciated.
The relation between the inferior
petrosal sinus (ips) and the lower
cranial nerves.

The origin of the hypoglossal nerve (XII).
.
The drilled occipital condyle (OC) and
the hypoglossal canal (HC).

Posterior cranial fossa (jugular and hypoglossal areas); vision obtained with a 45° endoscope
through a clival window
AICA anteroinferior cerebellar artery, BA basilar artery, IO inferior olive, LA labyrinthine artery, PCA posterior cerebral artery, PcomA
posterior communicating artery, PICA posteroinferior cerebellar artery, POV preolivary vein, RPA recurrent perforating artery, SCA
superior cerebellar artery, SPV superior petrosal vein, VA vertebral artery, IIIcn oculomotor nerve, Vcn trigeminal nerve, VIcn abducens
nerve, VIIcn facial nerve, VIIIcn vestiboloacoustic (statoacoustic) nerve, IXcn glossopharyngeal nerve, Xcn vagus nerve, XIIcn hypoglossal
nerve
The LA usually originates from the AICA, rarely directly from the BA. It feeds the inner ear. AICA and SCA course through the
cerebellopontine cistern. AICA enters the lower part of cerebellopontine cistern and it usually bifurcates into its rostral and caudal trunks
within the cistern. PICA origins from the VA, near the inferior olive, and passes posteriorly around the medulla. It could pass rostral, caudal
or even between the rootlets of the hypoglossal nerve. RPA(s) are arteries that present a recurrent course and reach the root entry zone of
the VII and VIII cns. They send branches to these nerves and to the brainsterm around the root entry zone.

Nerves and vessels of the posterior cranial fossa. (a) Basilar tip region, endoscopic view (b) Right cerebellopontine angle, endoscopic view from
anterior. (c) Right laterobulbar region, endoscopic intracranial view. (d) Three-dimensional reconstruction of the posterior cranial fossa. AICA,
anteroinferior cerebellar artery; BA, basilar artery; DV, Dandy’s vein; Fl, flocculus; IIIcn (CS), intracavernous portion of the oculomotor nerve; IIIcn,
oculomotor nerve; IO, inferior olive; IXcn, glossopharyngeal nerve; IX–X, glossopharyngeal and vagus nerves; LA, labyrinthic artery; LPMVN,
lateropontomesencephalic vein network; P1, posterior cerebral artery (first segment); P2, posterior cerebral artery (second segment); PcomA,
posterior communicating artery; PICA, posteroinferior cerebellar artery; POV, preolivary vein; PV, peduncular vein; RPA, recurrent perforating
artery; SCA, superior cerebellar artery; SPV, superior petrosal vein; TGAs, thalamogeniculate arteries; TPAs, thalamoperforating arteries; VA,
vertebral artery; Vcn, trigeminal nerve; VIcn, abducens nerve;
VII–VIIIcn, facial nerve and vestibuloacoustic nerve; VIIcn, facial nerve; VIIIcn, vestibuloacoustic nerve; X/XIcn, vagus and accessory nerves; XIcn,
accessory nerve; XIIcn, hypoglossal nerve.

Microscopic Far-lateral
approach
Endoscopic Far-medial
approach

Hypoglossal is just behind the upper end of
parapharyngel carotid – very easy way to
identify 12th nerve in paraphayrngeal space
– Dr.Satish jain

The hypoglossal nerve exits from the hypoglossal canal medial to the ICAp. It lies posteriorly to
the vagus nerve and passes laterally between the internal jugular vein and ICAp.
The hypoglossal nerve is usually accompained, within the hypoglossal canal, by an emissary vein and arterial
branches from ascending pharyngeal artery and occipital artery.
C1 atlas, Cl clivus, CS cavernous sinus, CV condylar vein, FCB fi brocartilago basalis, HC hypoglossal canal,
ICAc cavernous portion of the internal carotid artery, ICAp parapharyngeal portion of the internal carotid
artery, JT jugular tubercle, OC occipital condyle, XIIcn hypoglossal nerve, violet arrow atlanto-occipital
joint

Endoscopic endonasal view of a cadaveric dissection showing transection of the right eustachian tube (ET)
attachment to foramen lacerum (FL). The hypoglossal nerve (XII) enters the hypoglossal canal just deep to
the ET and separates the occipital condyle (OC) and the jugular tubercle (JT). (BA, basilar artery; ICA,
internal carotid artery [paraclival segment]; IPS, inferior petrosal sinus; VN, vidian nerve.) B. Endoscopic
endonasal view of cadaveric dissection showing the parapharyngeal internal carotid artery (ICA) and
jugular foramen (JF) following transection and removal of the eustachian tube. (BA, basilar artery; IPS,
inferior petrosal sinus; FL, foramen lacerum; JT, jugular tubercle; OC, occipital condyle; XII, hypoglossal
nerve.)

12 th nerve access from Laterally
[ mastoid approach ]

The jugular process and the portion of the occipital condyle have been drilled out. The left
occipital condyle is identified below the jugular bulb and posterior to the internal jugular vein.
* occipital condyle , ICA internal carotid artery , IJV internal jugular vein , JB jugular bulb , LSC
lateral semicircular canal , P promontory , SS sigmoid sinus

11th nerve bisects the upper end of IJC whereas vertical part of 7th nerve bisects the jugular bulb .
The lateral aspect of the jugular bulb, sigmoid sinus, and internal jugular vein has been removed. On the
medial wall of the jugular bulb the inferior petrosal sinus is identified. The opening of the posterior
condylar vein is seen. * occipital condyle , ICA internal carotid artery , JB jugular bulb , P promontory ,
SS sigmoid sinus

* occipital condyle , IJV internal jugular vein , IPS inferior petrosal sinus
, JB jugular vein , PCV posterior condylar vein , SS sigmoid sinus

Note the relationship among the sigmoid sinus, jugular bulb, posterior condylar vein, vertebral
artery, and lower cranial nerves. C1 atlas , C2N C2 nerve , JB jugular bulb , PCV posterior
condylar vein SS sigmoid sinus , TP transverse process of C1 , VA vertebral artery , X vagus nerve
, XI spinal accessory nerve

The posterior condylar vein crossing the occipital condyle is noted.
ICA internal carotid artery , JB jugular bulb , PCV posterior condylar vein
IX glossopharyngeal nerve , XI spinal accessory nerve

Other spinal ( cervical ) nerves origin
with rootlets like in 12th nerve origin

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