REZzzzzzzzzz360°19-7-2015

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

3rd nerve in interpeduncular fossa

Liliequist membrane

Roof - two triangles: 1. clinoid (anterior)

2. oculomotor (posterior)

ACP anterior clinoid process, APCF anterior petroclinoid fold, DS dorsum sellae, ICF interclinoidfold, 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

4th nerve

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

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° angledendoscope 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 infe- rior 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.

5th nerve

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, withtip 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 prepontinecistern 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 ) - Thegulfar 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° translabyrinthineapproach and the 320° transapical extension in the surgical view. The

dashed line demonstrates the working area for extended bone removal. Afterthis bone work is completed, the important structures can be controlled. AICA,

anterior inferior cerebellar artery; BA, basilar artery; V, trigeminal nerve; VI, abducent nerve.

7th& 8th nerve [ AFB ]

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

From Clinical anatomy book

Brainstem

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

Closer view of the inferior area of the left CPA, withtip 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.

Intraoperative endoscopic picture in a patient with right hemifacial spasm showing PICA havingperpendicular 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 highermagnification, the bulbopontine junction is clearly appreciated. AICA, anterior

inferior cerebellar artery; PV, plexus of veins; VI, abducent nerve; VII, facialnerve; VIII, vestibulocochlear nerve.

a Landmarks for identifying the foramen of Luschka. The choroidplexus (CP) exits the foramen. AICA, anterior inferior cerebellar artery;Fl, flocculus; VI, abducent nerve; VII, facial nerve; VIII, vestibulocochlearnerve; IX, glossopharyngeal nerve.

Anatomy of the fourth ventricle andthe lateral recess and the location of the cochlear nucleus. This illustrationshows 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.

CPA

Left Ménière disease: In around 40% of cases,the anterior inferior cerebellar artery (aica) forms a vascularloop running toward the porus acusticus, usually inferior to thevestibulocochlear nerve bundle. Within the vestibulocochlear nerve,the vestibular fibers (Ve) are more superior (rostral) and close to thetrigeminal nerve, and the cochlear nerve (Co) is inferior (caudal)and close to the lower cranial nerves (LCN).

Left Ménière disease: A small dissector is insertedinto the inter-vestibulocochlear cleavage plane to divide thevestibulocochlear 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 withmicrosurgical scissors.

Left endoscopic vestibular neurotomy is complete.The facial nerve located anteroinferior to the vestibular nerve is nowvisible.

Left microsurgical vestibular neurotomy with terminalfibers being dissected by blunt probe. co indicates the cochlear

nerve; ve, sectioned vestibular nerve; aica, anterior inferiorcerebellar artery.

The anterior inferior cerebellar artery, lying betweenthe 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 ofthe left IAM. ( a ) Subarcuate artery penetrates the dura ofthe subarcuate fossa near the IAM. The labyrinthine arteryenters the meatus with the vestibulocochlear and the facialnerves. ( b ) Laterally convex loop of the AICA is embedded

in the dura covering the subarcuate fossa, where itgives off the subarcuate artery. ( c ) AICA loop is embeddedin 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 fromthe subarcuate fossa in preparation for opening the IAM.( e ) Dura over the subarcuate fossa has been incised andremains attached to the artery. The bone surrounding theembedded AICA loop is removed with a 2-mm diamonddrill to displace the artery medially for exposure of the

IAM (From Tanriover and Rhoton [ 50 ] )

IAC

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 theinferior 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, cochlearnerve; ant., anterior; post., posterior; sup., superior.

a The effect of tumor size on the facial nerve (FN). Note that as thetumor grows in size the facial nerve becomes thinner and more fragile

(splayed). b With tumor dissected away, the splayed nerve can be bettervisualized.

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

pushed facial nerve (left ear). T, tumor.

a ) Normal neuralrelationships with the eighth nerve dividing into itsthree parts in the lateral meatus. The facial and superiorvestibular nerves are above the transverse crest and thecochlear and inferior vestibular nerves are below. The facialnerve occupies the anterosuperior quadrant of the lateral meatus.

( b ) The facial nerve is displaced directly anteriorly.This is a frequent direction of displacement withacoustic neuroma.

In Retrosigmoid approach

( c ) Another frequent direction of displacementof the facial nerve is anterior and superior.

( d )The facial nerve is displaced anteriorly and inferiorly bytumor, which erodes the superior wall of the meatus abovethe nerves and grows into the area above the nerves, displacingthem inferiorly

Fundus of IAC

7up- 7th is aboveCoca 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 inbleeding, 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 internalauditory canal. Note that the posterior wall of

the canal is completely removed, while the superiorand inferior walls have not been drilled. The correct

way is to leave a thin shell over the dura of theinternal 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. Atthe 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, greaterpetrosal nerve; L, labyrinthine segment of the facial nerve; SVN, superiorvestibular nerve.

At higher magnification, the relationship at the fundus can be betterappreciated. AE, arcuate eminence; BB, Bill’s bar; C, cochlea; FN(iac), internalauditory canal segment of the facial nerve; GG, geniculate ganglion; GPN,greater petrosal nerve; L, labyrinthine segment of the facial nerve; SVN,superior vestibular nerve.

9th, 10th & 11th 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 nerveCranial 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 pontomedullarystem and the vertebral artery junction and originof the basilar artery. Perforating arteries arise from the vertebraland basilar arteries.

The endoscope is focusing on the hypoglossal nervearea. The posterior inferior cerebellar artery arises from thevertebral artery in the background, and runs between the twobundles 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 thevertebral 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 travelsthrough the nerve fiber roots of the accessory nerve andencircles the brain stem. The course of the vertebral artery isinferior and anterior to the lower cranial nerves and thehypoglossal nerve. Fibrous tissue surrounds the entrance ofthe vertebral artery into the CPA.

9 Glossopharyngeal nerve10 Vagus nerve11 Accessory nerve12 Hypoglossal nervePICA Posterior inferior cerebellar arteryVert. 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 theacousticofacial 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

12th nerve

JT= Jugular Tubercle

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 superiorto 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. Theposterior inferior cerebellar artery encircles the medullarystem. The opposite vertebral artery exits from the dural porusand 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 surfaceof the pons. The exit zones of the hypoglossal and abducent nerves are at the same level. The abducent nerve exits fromthe 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 originof 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.

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.

The root fibers of the hypoglossal nerve (12) collect in two

bundlesCadaveric 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

Closer view of the inferior area of the left CPA, withtip 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.

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

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).

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The drilled occipital condyle (OC) and the hypoglossal canal (HC).

Posterior cranial fossa (jugular and hypoglossal areas); vision obtained with a 45° endoscopethrough 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

nerveThe 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.

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 nerveCranial 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.

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

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

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

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

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