Neurosurgical Anatomy Flashcards
Surface anatomy of the pterion?
2.5cm above the zygomatic arch and 1.5cm behind the frontal process of the zygomatic bone.
Which bones contribute to the pterion?
Frontal
Parietal
Temporal
Greater wing of the sphenoid
Def: Asterion
Junction of the lamboid, occipitomastoid and parietomastoid sutures
What does the asterion overlie?
The junction of the transverse and sigmoid sinuses
Surface anatomy of the Sylvian fissure?
Marked by a line drawn from the lateral canthus to a point 75% of the distance from the nasion to the external occipital protuberance
Surface anatomy of the central sulcus
4-5cm posterior to the coronal suture
This is also at a point approximately 2cm posterior to the mid-position of the arc joining the nasion and the external occipital protuberance
Surface anatomy of the SSS?
Runs posteriorly from the nasion to the external occipital protuberance in the midline
Surface markings of the transverse sinus?
From the level of the occipital protuberance towards the mastoid at the same level as a line projected posteriorly from the zygomatic arch.
Subarachnoid cisterns:
1
Olfactory cistern
Subarachnoid cisterns:
2a
2b
Callosal cistern
Lamina terminalis cistern
Subarachnoid cisterns:
3
4
Chiasmatic cistern
Carotid cistern
Subarachnoid cisterns:
5
6
7
Sylvian cistern
Crural cistern
Interpeduncular cistern
Subarachnoid cisterns:
8
9
10
Ambient cistern
Superior CP cistern
Pre-pontine cistern
Subarachnoid cisterns:
11
12
13
Inferior CP cistern
Anterior spinal
Posterior spinal
Cistern contents:
Lamina terminalis
AComm and branches
Cistern contents:
Chiasmatic
Precommunicating ACA
Optic nerves
Cistern contents:
Carotid
ICA
Pcomm origin
Anterior choroidal
Cistern contents:
Sylvian
MCA
Cistern contents:
Crural
Anterior choroidal
Medial posterior choridal
Cistern contents:
Interpeduncular
Basilar bifurcation
PCAs
III
Cistern contents:
Ambient
PCA
SCA
Basal veins
IV
Cistern contents:
Quadrigeminal
Vein of Galen
Distal pericallosal arteries
Distal PCA and SCA
IV
Cistern contents:
Prepontine
Basilar
AICA
SCA
VI
Cistern contents:
Premedullary
Vertebral
PICA
XII
Cistern contents:
CP
AICA
V, VII, VIII
Cistern contents:
Cerebellomedullary
Vertebral
PICA
IX, X, XI, XII
Cistern contents:
Cisterna magna
Distal PICA
Craniospinal junction
Lateral view of the frontal lobe
Anterior to central sulcus
Precentral
Superior
Middle
Inferior frontal gyri
Where are the pyramidal cells of Betz found?
In layer 5 of the primary motor cortex
Which components of the inferior frontal gyrus comprise the motor speech cortex in the dominant hemisphere?
Triangular
Frontal opercular
Medial view of the frontal lobe
Predominantly superior frontal gyrus and cingulate gyrus
Basal view of frontal lobe
Gyrus rectus medially and orbital gyri laterally
Which rami characterise the Sylvian fissure?
Short anterior and ascending rami that demarcate the apex of the triangular portion of the inferior frontal gyrus
Medial surface of temporal lobe?
Parahippocampal gyrus
Dentate gyrus and fimbria lying more superiorly
The posterior surface of the parahippocampal gyrus is intersected by the calcarine sulcus with the upper part merging with the cingulate gyrus and the lower part with the lingula.
What structures are defined by the collateral sulcus?
The lateral margin of the lingual and the parahippocampal gyrus.
What defines the posterior limit of the parietal lobe?
A line drawn from the parieto-occipital sulcus to the inferiorly located preoccipital notch
Components of the parietal lobe
Superior and inferior lobules (latter made up of the supramarginal and angular gyrus)
What lies above the calcarine sulcus?
And below?
The cuneus above
Lingula below
Components of the insula?
Central sulcus demarcates a large anterior component containing several short gyri and a posterior part comprising two long gyri
Surrounded by the circular sulcus
Which artery crosses the insula?
M2
What are vulnerable to injury during resection of insula tumours?
Lenticulostriate perforators coming off the M1
Their damage can result in a motor deficit.
The most lateral lenticulostriate perforator acts as the medial margin of such a resection.
Approach to cerebellar hemispheric lesions
Transcortical dissection
Approach to quadrigeminal or undersurface tentorial lesions?
Supracerebellar, infratentorial route
How to access CPA?
Retraction of the petrous part of the cerebellar hemisphere
What can be used to assist in identifying CN VII and VIII?
Choroid plexus protruding from the foramen of Luschke
Where do the hypoglossal rootlets arise?
The preolivary sulcus
Types of WM fibres?
Association
Commissural
Projection
Types of association fibres
Short U arcuate fibres which interconnect adjacent gyri
Long fibres interconnect more distant gyri
Examples of long association fibres
Uncinate fasciculus
Cingulum
SLF
ILF
What structures are connected by the uncinate fasciculus?
Temporal and frontal lobes
What structures are connected by the cingulum?
Cingulate and parahippocampal gyri and the septal region
Structures connected by the SLF?
Frontal
Temporal
Parietal
Occipital lobes
Location of the SLF?
Superior to insula
Deep to the extreme and external capsules but superficial to the optic radiation and internal capsule
Structures connected by the ILF
Temporal and occipital lobes
What constitutes the inferior occipitofrontal fasciculus?
Group of fibres that traverse from the prefrontal region dorsal to the frontal fibres of the uncinate fasciculus
Continue posteriorly to the middle and posterior temporal region.
What are the principle commissural fibres
Corpus callosum
Anterior commissure
Parts of the corpus callosum
Rostrum
Genu
Body
Splenium
Tapetum
Tapetum etymology
Carpet
What is the tapetum of the corpus callosum?
Formed primarily by decussating fibres in the splenium of the corpus callosum that arch over the atrium of the lateral ventricle and course inferiorly in the lateral wall of the posterior and somewhat into the temporal horns of the lateral ventricle
What structures connect the frontal lobes?
Forceps minor via the genu
Structures connecting the occipital lobe?
Fibres of forceps major which project to the tapetum and interconnect via the splenium
Location of the fibres of the tapetum?
Lateral wall of the trigone and temporal horn
Separate these parts of the ventricle from the more laterally placed optic radiation.
Location of the anterior commissure?
The anterior wall of the third
Connect the temporal lobes
What happens to fibres from the upper part of the LGN?
Course directly to the visual cortex superior to the calcarine sulcus
What happens to fibres from the lower part of the LGB?
Loop forwards into the temporal lobe as Meyer’s loop, before turning posteriorly towards the inferior part of the primary visual cortex
How to approach cerebral hemispheric lesions?
Trans-gyral or trans-sulcal
How to approach lesions in the posterior part of the body or atrium of the lateral ventricle?
Transcortical approach through the superior parietal lobule.
What route can be taken to access the frontal horn and body of the lateral ventricle or lesions at the foramen of Monro?
Transcallosal approach
Can be extended to access the third ventricle.
General poinst in transcallosal approach
Interhemispheric, avoiding inadvertent injury to the callosomarginal and pericallosal arteries and to any bridging veins.
2cm incision in the corpus callosum.
What can help determine the laterality of the ventricle in the transcallosal approach?
Thalamostriate vein is lateral to the choroid plexus
Transchoroidal approach
Allows access to the third ventricle
Gentle retraction of the choroid plexus and opening the relatively avascular taenia fornicis from the foramen of Monro posteriorly for around 1cm.
Most common approaches to brainstem lesions
Midline suboccipital telovelar approach
Retrosigmoid approach
Lateral supracerebellar infratentorial approach
Extended pterional
Use of the suboccipital approach
Tumours within the fourth ventricle or in the floor.
Midline division of the vermis can expose this region but may be associated with neurological deficits including truncal ataxia and temporary speech disturbance
Use of telovelar approach
Access to fourth with less risk of complications.
Cerebellar tonsils retracted laterally and superiorly with care taken not to occlude PICA
Roof of the fourth exposed which consists of the tela choroidea inferiorly and the inferior medullary velum superiorly
Telovelar approach overview
Suboccipital craniotomy
Dissection proceeds to the uvulotonsillar space and cerebellomedullary fissure.
The uvulotonsillar space is exposed by retracting the uvula superomedially and the tonsil laterally.
A paramedian incision through the tela, just lateral to the uvula of the vermis enables good visualisation of the floor of the fourth.
The incision can be extended into the inferior medullary velum.
What structures should be protected when exposing the lateral recess of the fourth?
Superior and inferior cerebellar peduncles
What structures are at risk during telovelar approach?
The PICA and its branches, particularly as it courses medially into the cerebellomedullary fissure
What are the preferred zones for posterior entry into the brainstem?
Supra and infracollicular approaches either side of the facial colliculus
Midline longitudinal sulcus should be avoided due to its proximity to the MLF
What is the taenia fornicis?
Attachment of the choroid plexus to the fornix
What is the benefit of opening the taenia fornicis?
Facilitates opening of the choroidal fissure
Hazards in the velum interpositum
Internal cerebral veins
Terminal branches of the medial posterior choroidal artery
Hazads in telovelar
PICA
Vermis
Inferior cerebellar peduncles
Significant structures in floor of fourth
You are exposing the floor of the fourth ventricle by the telovelar approach. Which one of the following vessels do you expect to find as you open the tela choroidea and the inferior medullary velum?
Anterior inferior cerebellar artery
Vertebral artery
Superior cerebellar artery
Posterior inferior cerebellar artery
Superior vermian vein
PICA
A 60-year-old patient presented with a history of getting muddled and being vague. On examination she had a mental test score of 25/30. There was no evidence of papilloedema, cranial nerve deficit or limb deficit. An MRI scan showed a large intraventricular tumour with calcification
What is the most likely diagnosis?
Calcification commonly occurs in central neurocytomas. Intraventricular meningiomas can calcify; however, they are usually located in the trigone rather than in the anterior body. Colloid cysts are usually located in the third ventricle at the foramen of Monro. Choroid plexus tumours are very rare in this age group.
A 60-year-old patient presented with a history of getting muddled and being vague. On examination, she had a mental test score of 25/30. There was no evidence of papilloedema, cranial nerve deficit or limb deficit. An MRI scan showed a large intraventricular tumour with calcification
MRI shows a central neurocytoma
What surgical approach would you adopt?
Transcallosal approach
Transcortical is also possible though there would be a higher post-operative risk of seizures
A 45-year-old male presented with an abrupt onset of moderate headache, dysarthria, impaired balance and diplopia. Examination revealed brainstem signs, including facial numbness, facial weakness, nystagmus, a sixth cranial nerve palsy, weak cough and cerebellar signs, including ataxia, past-pointing and dysdiadochokinesia. A similar constellation of symptoms occurred 2 years previously; the patient made a good recovery after this earlier episode. An MRI scan showed a lesion in the pons
What is the most likely Dx?
Cavernous haemangioma
A 45-year-old male presented with an abrupt onset of moderate headache, dysarthria, impaired balance and diplopia. Examination revealed brainstem signs, including facial numbness, facial weakness, nystagmus, a sixth cranial nerve palsy, weak cough and cerebellar signs, including ataxia, past-pointing and dysdiadochokinesia. A similar constellation of symptoms occurred 2 years previously; the patient made a good recovery after this earlier episode. An MRI scan showed a lesion in the pons
Cavernous haemangioma shown on MR.
What surgical approach
Midline suboccipital craniotomy with telovelar approach.
Indication for endoscopic transplanum transtuberculum approach?
Pituitary tumours with suprasellar extension
Tuberculum sellae or planum meningiomas
Craniopharyngiomas
Patient positioning for endoscopic endonasal transplanum approach
Supine
Mayfield
Neutral or slight rotation towards surgeon
Location of the medial optico-carotid recess?
Lateral aspect of the tuberculum
What does the lateral optico-carotid recess represent?
Base of the optic strut
What is the chiasmatic sulcus?
Region from the tuberculum to the limbus sphenoidale that extends between the optic canals
Midline structures of the sphenoid from rostrocaudal
Planum
Limbus
Chiasmatic sulcus
Tuberculum
Sella
What forms the roof of the optic canal?
Anterior root of the lesser wing of the sphenoid
What forms the floor of the optic canal?
Posterior root or the optic strut
What is the significance of the blue arrow?
Represents the direction of drilling of the anterior clinoid process to disconnect it from the anterior root
What bone must be removed in endoscopic endonasal approach?
Wide bilateral sphenoidotomy. posterior ethmoidectomy and posterior septectomy
CR=
Car=
LOCR=
Clival recess
Carotid recesse
Lateral optico-carotid recess.
Bony drilling of sphenoid during endonasal approach
Intraphenoidal septations thinned using a drill
Bone overlying the sela and chaismatic sulcus egg shelled with diamond burr.
Thinned bone can be dissected off whilst maintaining dural integrity.
What is the limbus dura
Thickened dural fold that overlies the limbus sphenoidale
Where is the MOCR and to what does it correspond
At the lateral end of the tuberculum strut.
Corresponds to the transition between the paraclinoidal and supraclinoidal segments of the ICA.
What constitutes the para sellar ICA?
Combination of cavernous and paraclinoidal segment
Boundaries of the true osseous canal
Medially- the body of the sphenoid
Inferiorly the optic strut
Superiorly the roof is formed by the anterior root of the lesser wing of the sphenoid
What covers the preforaminal segment of the optic canal?
Falciform ligament
Approach to opening the endonasal dura?
Incision starting at the upper part of sell and continuing across the superior intercavernous sinus, tuberculum dura and chiasmatic sulcus before turning laterally across the limbus and planum dura.
Dural flap then retracted laterally with two further diaphragmatic cuts made. (one towards the infundibulum and the second towards the distal dural ring.
What structures are seen in the suprasellar infrachiasmatic space?
Superior hypophysial arteries
Should be preserved
What structures are seen in the suprasella suprachiasmatic space?
Gyrus rectus
ACA
AComm
What can be used to cover defect after endonasal transplanar surgery?
Vascularised nasoseptal flap
What are the anatomical hazards with endonasal approach
ICA segemnts
Careful drilling with constant irrigation to reduce risk of optic nerve damage during canal decompression
A medially looping ophthalmic artery could be injured
SHA should be preserved
Fronto-orbital branches may loop inferiorly
What structures can be accessed by a midline sub-frontal transcranial approach?
Anterior cranial fossa meningiomas or to access the region around the lamina terminalis, the retrosellar space and the interpeduncular cistern.
What is the beneift of the subfrontal transbasal approach?
Obviates the need for performing a separate orbital osteotomy and allows adequate rostrocaudal visualisation from the top of the third ventricle superiorly to the interpeduncular cistern inferiorly.
Patient positioning for subfrontal transbasal approach?
Three-pin
Elevated 15-20 degrees to allow additional venous return.
Neck extended to allow the frontal lobe to fall away from the anterior cranial fossa.
A lumbar drain may be used.
Incision in the sub frontal transbasal approach?
Bicoronal incision made from one zygoma to the other (no more than 1cm anterior to the tragus).
By staying below the galea during the dissection, a separate vascularised pericranial flap is raised.
What is the purpose interfascial dissection during subfrontal transbasal approach?
The superficial layer of the deep temporal fascia is raised with the fat pad on either side.
In order to protect the frontal branch of the facial nerve.
How far forwad should the pericranial flap be raised during sub frontal transcranial approach?
As the scalp is mobilised anteriorly, the nasofrontal suture and the orbital rims should be visualised.
Which structure should be preserved whilst raising scalp flap during subfrontal approach?
The supraorbital nerve and artery which should be visualised as the supra-orbital rim is reached.
Features of the craniotomy in the subfrontal approach
Incorporates the anterior wall of the frontal sinus.
Start low with the inferior osteotomy, starting at the nasofrontal suture and extending laterally over both orbital limbs.
The craniotomy thus follows the contour of the anterior skull base in the coronal plane and allows the bone flap to be as low as possible eliminating the need for a separate supraorbital osteotomy.
What happens to the frontal sinus during subfrontal approach?
The anterior wall is opened.
The mucosa and the posterior table are then removed, cranialising the sinus.
What happens to the nasofrontal ducts?
Packed prior to the extradural exposure of the anterior cranial fossa or opening of the dura.
Where is the posterior ethmoidal artery found?
Lies at the junction of the cribriform plate and the planum sphenoidale and can be used to differentiate between the two?
Extradural technique in subfrontal approach
Both olfactory sulci can be transected and the crista Galli be removed, accessing the planum sphenoidale, limbus and chiasmatic sulcus.
Opening the dura during subfrontal approach
Based on the SSS which is ligated with 2x 2’0 silk sutures followed by division of the falx and subsequent dissection of the olfactory tract from the basal frontal lobe.
Closure of the dura after subtotal approach?
Pericranial flap laid across the anterior fossa and the cranialised frontal sinus.
Pericranium can also be stitched to the frontobasal dura with multiple sutures.
Fat grafts often required to fill up dead space.
What is the purpose of exenterating the sinonasal mucosa during subfrontal transbasal craniotomy?
Prevents future infection or mucocoele.
Which structures should be appreciated during the intradural portion of subfrontal approach
Midline perforators from AComm.
Indications for anterior clinoidectomy?
Clipping of paraclinoidal or paraophthalmic aneurysms
Resection of medial sphenoid wing or clinoidal meningiomas
Optic canal decompression
Location of the anterior clinoid process
A triangular shaped bone that overlies the proximal part of the intradural supraclinoidal ICA.
Projects from the posteromedial border of the lesser wing of the sphenoid and has three attachments
Bony Attachments of the clinoid
Continuous with the lesser wing of sphenoid laterally
Medially connected to the sphenoid bone by an anterior root running between the limbus sphenoidale and the ACP
Posterior root/optic strut.
Ligaments of the anterior clinoid process
Anterior petronclinoidal
Interclinoidal ligaments
Falciform road
Which portion of the ICA is close to the ACP?
The posterior surface of the optic strut is intimately related to the clinoidal segment of the ICA
What is a consideration when drilling the ACP?
Forms the lateral border of the OC and so any drilling in this area should be undertaken with copious irrigation.
Borders of the Oculomotor triangle
Anterior petroclinoid dural fold (APCF)
Posterior petroclinoid dura fold (PCDF)
Interclinoid dural fold (ICF)
Contents of the oculomotor triangle?
Oculomotor nerve
Horizontal segment of ICA
What separates the SOF from the optic canal?
The optic strut
What struts are found either side of the SOF?
Optic struct
Maxillary strut
What marks the lateral end of the SOF?
Meingio-orbital band.
What is the aim in dividing the MOB?
To establish a plane between the dura propria and the membrane overlying the cavernous sinus.
What manoeuvres are required for full clinodecomty?
Lesser wing of the sphenoid drilled down to unroof the SOF
Base of the ACP is drilled from a lateral to a medial direction to disconnect it from the anterior root.
Connection with the optic strut is drilled out by advancing from the lateral to the inferior aspect of the optic canal.
Circumferential plane established around the hollowed-out ACP.
The remaining part of the ACP attached to the ligaments (anterior petroclinoidal and interclinoidal) delivered using rongeurs.
What is exposed by the clinoidecomty?
Exposes the clinoidal ICA and intradurally the proximal part of the supraclinoidal ICA.
The dura is opened in a curvilinear fashion that is extended onto the falciform ligament and distal ring if required.
What is the Dolenc technique?
Division of the meningo-orbital band and subsequent sharp dissection to find the right plane between the dura propria and the membrane covering the lateral wall of the cavernous sinus.
What is the purpose of pre-op fine cut CTH ahead of anterior clinodiectomy
To define the osseous anatomy and check for the prsence of an osseous carotico-clinoidal ring around the ICA.
The clinoidectomy may need to be completed extra and intra-durally if present.
Breach of the sphenoid or ethmoid sinus during ACP removal?
Covered with bone wax to prevent CSF leak
Which one of the following structures does the medial optico-carotid recess represent?
Lateral aspect of the tuberculum
Medial aspect of the tuberculum
Lateral aspect of the limbus sphenoidal
Lateral aspect of the tuberculum
Which one of the following anatomical spaces is visualized after the opening of the dura over the chiasmatic sulcus?
Suprasellar infrachiasmatic space
Interpeduncular cistern
Pre-pontine cistern
Suprasellar infrachiasmatic space
Other than the optic nerve, which one of the following cranial nerves is susceptible to injury during an anterior clinoidectomy?
Maxillary division of the trigeminal nerve
Mandibular division of the trigeminal nerve
Oculomotor nerve
CN3
Line defining the transverse sinus?
Root of zygoma to the inion
Line defining the junction between the sigmoid and transverse sinuses?
A line drawn from the mastoid groove to the line drawn from inion to root of zygoma.
What is the purpose of identifying this point?
Burrhole placed posteroinferior to this point should safely expose the sinus junction
Standard position for CPA access?
Lateral decubitus position with 3 point fixation.
Neck flexed and rotated approximately 10 degrees away from the affected side
Vertex angle for acess to trigeminal nerve during CPA acess?
Vertex parallel to floor
Vertex angle during access to the facial nerve?
Vertex tilted 10-15 eegrees
Incision during access to CPA
Curved incision with the concave side facing the ear is made, one third above and two thirds below the junction of the transverse and sigmoid sinuses.
Options for bone removal in retrosigmoid approach
Some perform craniotomy rather than craniectomy due to perceived difference in rate of post-op CSF leak and associated morbidity though there may be an increased risk of sinus damage.
Size of craniotomy for retrosigmoid approach
2cm
Mastoid emmissary vein
Connects the posterior auricular vein with the sigmoid sinus.
May be a source of bleeding during retrosigmoid approach.
Approach to dural opening in retrosigmoid approach
Dura adjacent to the junction of the sigmoid and transverse sinuses must be clearly identified.
CSF drained to reduce posterior fossa pressure and relax the cerebellum to minimise retraction injury.
What angle must be visualised during retrosigmoid approach?
The angle between the tentorium and dura overlying the petrous temporal bone.
Approach to retraction when accessing the CPA
Retractor is first placed horizontally over the cortical surface to identify the junction between the tentorium and petrous dura before being gradually positioned more vertically to achieve deeper exposure.
Retraction at the angle and moving slowly laterally rather than directly retracting reduces the risk of injury to the facial and vestibulocochlear nerve complex.
Superior petrosal venous complex
Usually encountered during retro sigmoid approach tot the CPA.
Care should be taken not to place vein under tension as can be avulsed from the superior petrosal sinus.
Mat be necessary to sacrifice this vein to gain adequate exposure. Venous infarction is a rare occurrence
Relationship of nerves to vessels in the superior neurovascular complex
3+4 superior to SCA
V inferior to SCA as it passes around the midbrain to the cerebellomesencephalic fissure where it runs on the superior cerebellar peduncle to supply the tentorial surface of the cerebellum.
What is the trigeminal root entry zone?
First 1cm of the nerve after it has emerged from the lateral pons and is the area most commonly compressed by the SCA.
Relationship of the motor component of trigeminal to its sensory root?
Medial to the sensory root, crosses under the Gasserian ganglion and exits through the foramen ovale
Gasserian ganglion
Trigeminal ganglion found in Meckel’s cave
Relationship of the SCA to the trigeminal nerve
The main trunk of the SCA passes above the trigeminal, frequently dividing into rostral and caudal branches.
These branches loop caudally and encounter the trigeminal
(AICA may rarely also compress if it loops superiorly)
Venous compression of the trigeminal
Transverse pontine veins are the most frequent (usually converge to form a single trunk)
Superior petrosal veins
What landmarks guide the surgeon in identification of CN VII
Pontomedullary sulcus (arises at the lateral margin)
Choroid plexus of the foramen of Luschka
Flocculus which lies just below.
Junction of GP, CN X and spinal accessory nerves with medulla, CN VII will enter the brainstem 2-3mm above, at the interseciton with the lateral end of the pontomedullary sulcus
What are the five nerves at the IAC
Cochlear
Superior and inferior vestibular nerves
Facial nerve (nervus intermedius)
Which arterial branches are at risk when accessing the IAC?
Which of these can be sacrificed?
The subarcuate, labyrinthine, recurrent perforating and internal auditory arteries (arise from AICA)
The subarcuate artery
Origin of glossopharyngeal and vagus nerves
Rootlets in the postolivary sulcus along the posterior edge of the superior third of the olive
Origin of the spinal accessory nerve rootlets
Along the posterior edge of the inferior two thirds of the olive
Origin of hypoglossal nerve
Series of rootlets in the pre-olivary sulcus, exiting along the anterior two-thirds of the anterior olive
Which one of the following is the most reliable landmark in a surgical approach to the trigeminal nerve for locating the junction of the transverse and sigmoid sinuses?
A line that is a vertical extension of the mastoid groove meeting a line between the root of the zygoma and the inion
A point halfway between the inion and the root of the zygoma
A point one-third of the distance between the tip of the mastoid process and the inion
The asterion
The mastoid emissary vein
There are several variations in anatomy, particularly in adults, as regards the location of the asterion and the mastoid emissary vein. The asterion is sometimes difficult to identify; however, it can overlie the transverse–sigmoid sinus junction and can be a useful confirmation if it corresponds to the surface landmark located using the root of the zygoma, the inion and the mastoid groov
Which one of the following is the most accurate anatomical description of the location of the facial nerve on the brainstem?
It arises at the medial margin of the pontomedullary sulcus
It lies below the choroid plexus of the foramen of Luschka
It lies just above the flocculus
It lies lateral to the vestibulocochlear nerve
It sits on a line formed by the glossopharyngeal, vagus and hypoglossal nerves
The facial nerve arises at the lateral margin of the pontomedullary sulcus, above the choroid plexus at the foramen of Luschka. It is on a line formed by the glossopharyngeal, vagus and spinal accessory nerves rather than the hypoglossal nerve, which lies more anteriorly. It lies medial to the vestibulocochlear nerve. In vestibular schwannoma surgery, when anatomy in this region is inevitably distorted, visualization of the choroid plexus can help identify the facial and vestibulocochlear nerve origin; when a facial nerve monitor or stimulator is being used, the nerve can be identified anterior and medial to the tumour and vestibulocochlear nerve.
What is a reliable landmark for C1?
C1 transverse process which can be palpated adjacent to the mastoid process
Helps identify the vertebral artery during muscular dissection
Possible approaches to foramen magnum and inferior clivus
From both ventral and dorsal directions
Simple midline posterior suboccipital approach
Far lateral craniotomy and transcondylar extension
Ventral endoscopic endonasal approach
Components of far lateral approach
Suboccipital craniotomy and C1-hemi or complete laminectomy
With or without removal of parts of the occipital condyle
Purpose of transcondylar exposure
Removal of the posterior part of the occipital condyle improves access to the lower clivus and the area anterior to hte medulla
Purpose of the supracondylar, transtubercular exposure
Involves drilling of the hypoglossal canal followed by the jugular tubercle.
Improves access to the anterior aspect of the brainstem and vbisualisation ofn the origin of PICa from the vetebral artery
Purpose of paracondylar exposure
Avoids drilling the occipital condyle.
Provides access to the posterior part of the jugular foramena nd can be combined with a transmastoid approach
Positioning for far lateral
Modified park bench or three quarter prone position can be used
Skin incision for far lateral
Inverted U-shape or horsehoe incision.
Both the medial and lateral limb can be extended inferiorly to perform the cervical laminectomy and to visualised the C1 transverse process.
*
What structures should gbe identified and marked during the far lateral approach
Inion- marks the midline
Transverse sigmoid junction- marked using the mastoid tip, auricle and zygomatic arch.
The superior extent of the incision should extend above the inion.
What portion of the occipital bone is drilled during the paracondylar approach?
Jugular process of the occipital bone- elliptical area (site of the attachment of rectus capitis lateralis) to access the jugular bulb and jugular foramen.
What does the dashed line represent
The portion of the occipital condyle drilled in a transcondylar approach without risking occipitocervical instability or breaching the hypoglossal canal
What is the jugular tubercle?
Bony prominence above the hypoglossal canal which constitutes a shelf on which the lower cranial nerves sit away on their way from the brainstem to the jugular foramen
Layers of extracranial muscles during far lateral approach
Superficial:
Trapezius, splenius capitis, SCM]
Middle:
Semispinalis capitis, longissimus capitis
Deep:
Superior oblique
Inferior oblique
Rectus capitiits posterior major
Relationship of the occipital artery to longissimusn capitis
May lie superficial or deep
When will the suboccipital triangle be demostrated during far lateral approach?
After reflection of the second layer of muscle
What are the borders of the suboccipital triangle?
Floor formed by the atlanto-occipital membrane
Contents of the suboccipital triangle
V3 segment of vertebral with muscular branch
C1 dorsal nerve root inferior to the artery
Vertebral venous plexus.
What structures are particularly vulnerable to injury in the suboccipital triangle
Muscular branch of the vetrebral artery
Vertebral venous plexus
How is control of the vertebral venous plexus achieved?
Superior oblique is reflected laterally whilst preserving the attachment of rectus capitis lateralis
Dissection of the rectus capitis posterior minor which is usually seen deep to recuts capitis posterior major allows full exposure.
Origin of the posterior meningeal artery
Typically arises extradurally from the suboccipital part of the vertebral artery and enters the cranium through the foramen magnum
Extent of craniotomy in far lateral
Extends from the asterion laterally to the edge of the sigmoid sinus.
Posterior condylar vein (PCV) is the landmark for completion
SHould also include either partial or total removal of the arch of C1.
What is an important landmark for the paracondylar expiosure?
Rectus capitis lateralis, attached to the jugular process of the occipital bone.
Why should drilling of the jugular tubercle be undertaken with caution
To avoid thermal damage to the lower cranial nerves
Opening of dura in far lateral
J-shape from transverse-sigmoid junction, extending inferomedially towards the FM, posterior to the dural entry point of the vertebral artery.
Cervical dura opened linearly in a paramedian fashion.
Relationship of the vertebral artery to levator scapula
Lies medial to its upper attachment
What is the important initial step in the ventral endonasal endoscopic approach to the FM
Flattening of the midline maxillary crest overlying the hard palate
Removal of fascial and muscle layers to expose the inferior clivus
This provides a bony exposure inferior to the floor of the sphenoid sinus and superior to the anterior arch of C1
Extent of superolateral exposure during the ventral endoscopic endonasal approach to the foramen magnum
Extends to the lower aspect of the foramen lacerum whilst the inferolateral exposure includes the occipital condyles and the atlantooccipital joints bilaterally.
How can the endonasal corridor be widened?
By perfomring a medial condylectomy
Dural opening at the level of the anterior arch of C1 will expose what structures?
Premedullary cistern
V4 segment of vert
ASA
PICA which travels posteromedially through the rootlets of the hypoglossal nerve towards the cerebellomedullary fissure.
Relationship of the hypoglossal to the vertebral artery
Originates from the preolivary sulcus
Lies posterior to the vertebral artery whilst the C1 rootlets are anterior.
What is a possible complication of extensive condylar resection
If it breaches the hypoglossal canal it may lead to occipitocervical instability necessitating occipitocervical fusion.
Which of the following are the names of the critical vessel contained within the suboccipital triangle and the muscles forming this triangle?
Vertebral artery/Superior oblique, inferior oblique and rectus capitis lateralis major
Occipital artery/Superior oblique, inferior oblique and rectus capitis lateralis minor
Occipital artery/Sternocleidomastoid, splenius capitis and trapezius
Vertebral artery is the key vascular structure in the suboccipital triangle, which is prone to injury during approaches like far-lateral directed to this region.
The suboccipital triangle also contains the vertebral venous plexus and the muscular branch coming off the vertebral artery. Rarely the posterior inferior cerebellar artery can arise extradurally from the vertebral artery.
Which one of the following describes the relationship of the dural entry point of the vertebral artery in relation to the occipital condyle–C1 joint?
Medial
Lateral
Superior
Inferior
Medial
Indications for occipital approach
Occipital convexity meningiomas
Small posterior tentorial meningiomas
Occipital parenchymal lesions
Patient positioning for occipital approach
Prone, Mayfield
Bed in reverse Trendelenberg with knees flexed and shoulders tucked against body.
Head flexed with 2 finger breadths from hin to sternum in neutral or slightly turned.
In neutral position, the inion should be the highest point of the surgical field
Skin incision for occipital craniotomy
Horsehoe or Linear
Horsehoe: starting on midline at the inion, runs upwards, turning laterally and ending just posterior to the mastoid.
Linear can be alone the midline or lateral but parallel to the midline
Critical structures in occipital incision
Occipital artery
GON
What are these key structures
OB- occipital bone
OPS- occipitoparietal suture
PB- parietal bone
LS- lamboidal suture
IN- inion
Options for craniotomy in occipital craniotomy
Median occipital
Paramedian occipital
Median occipital craniotomy planning
Burrhole 1: lateral and superior to TS
Burrhole 2: Inferior aspect of SSS
Burrhole 3: Superior aspect of SSS
Craniotomy from burrhole 1-> 2 and 1->3
Last cut performed on midline just over the SSS
What are the key landmarks in designing the occipital craniotyom
Medial- SSS
Inferiorly- Just above the TS
Superiorly- Lamboidal suture
Laterally- Occipitoparietal suture
Critical structures during occipital craniotomy
SSS
TS
Structures exposed during occipital craniotomy
OPS- occipitopareital suture
IOG- Inferior occpoital gyrus
DV- Draining vein
LOS- lateral occipital sulcus
SOG- Superior occipital gyrus
SSS- Superior sagittal sinus
What is happening here?
What structures are seen?
What should be avoided?
Medial retraction of the occipital lobe.
T- tentorium, F- falx
Care should be taken to avoid retraction of the calacarine sulcus
IOG- inferior occipital gyrus
SCG- Superior calcarine gyrus
ICG- Inferior calcarine gyrus
SOG- Superior occipital gyrus
LOS- Lateral occipital sulcus
Intradural expsoure during occipital craniotomy
Parenchymal strucutres: Inferior, superior, lateral OG
Arachnoidal layer: posterior aspect of the interhemispheric cisterns
Veins: occiptal cortical veins, vein of Labbe, Trolard, SSS, TS
Dura mater: tentorium + falx
Indications for midline suboccipital approach
Parenchymal lesions of lower cerebellum or upper cerebellum (supracerebellar infratentorial approach)
Fourth ventricular lesions
Dorsal brainstem lesions
Meningiomas of medial posterior fossa
PICA aneursysm
Dorsal meningiomas of the FM
Chiar malformation
Patient position for midline suboccipital craniotomy
Prone, flexed in Mayfield
Body in a slight concorde position with 10-20 degrees of reverse trendelenberg
Head maintained straight and flexed leaving a space of about 2cm between chin and chest
Shoulders pulled down for safe position of arms on armrests
Highest point of body is the T3 spinous process
Highest point of surgical field in midline suboccipital craniotomy
Inion
Skin incision for midline suboccipital craniotomy
3cm above inion on midline ending at the spinous process of the axis (C2)
Layers of soft tissue dissection during suboccipital midline craniotomy
Myofascial level
Muscles:
Trapezius
Nichal ligament incised down midline
Semispinalis capitis and rectus capitis posterior major detached and reflected laterally.
Trapezius muscle is attached along which line?
External occpital crest/superior nuchal line
Bony exposure during midline suboccipital craniotomy
Subperiosteal dissection of occipital squama laterally from midline to both sides is performed
Subperisoteal dissection of the posterior arch of C1 vertebra is performed
Identification of vertebral arteries during subperiosteal dissection for midline suboccipital craniotomy
Dissect the muscle layer laterally from the posterior atlantooccipital membrane (blunt dissection)
Critical structures during subperisoteal dissection in midline subocciptal craniotomy
Vertrebral arteries
GON (found close to external occiital protruberance)
Burrhole location during midline suboccipital craniotomy
1 and 2 placed 1cm paramedial and 1cm below superior nuchal line
3 and 4 about 3 cm below the previous two burrholes at the extreme lateral aspect of surgical field
Craniotomy landmark for midline suboccopital approach
Lateral extend 3.5cm paramedian at the level of the inferior nuchal line
Superiorly runs 1cm below superior nuchal line
Inferiorly inferior margin is the foramen magnum
Use of C1 laminectomy
Used for surgical decompression in Chiari amlformation
Emphases placed on protecting the vertebral arteries
Burrhole location for superacerebelar infratentorial approach
Two far laterally over TS
+/- 1 over the SSS
Two paramedian burrholes below either side of the occipital sinus
Critical structures during craniotomy in midline suboccipital appraoch
Vertebral arteries particularly during C1 laminectomy
TS and SSS (during supracerebellar, infatenetorial apporach
Bone flap location for transcallosal approach
1/3 behind, 2/3 in front of anterior coronal suture due to relative paucity of veins draining into SSS
Use of supine cranial positioning
Used for procedures in frontal, temporal, anterior parietal areas, cranial base approaches
Use of three-quarter prone cranial positionin
Posteiror parietal occipital, suboccipital areas
Room set up for right frontotemporal craniotomy
Anaesthetist to patient’s left
Scrub nurse to right of patient
Suction, cautery at patient’s feet
Surgeon at patient’s head
Facial nerve zygoma
Incisions reaching zygoma more than 1.5cm anterior to ear commonly interrupt facial nerve
Frontozygomatic point
Located on orbital rim, 2.5cm above the level at which the upper edge of the zygomatic arch joins the orbital rim
Location of pterion
3cm behind frontozygomatic point on the sylvian fissure line (i.e. from frontozygomatic point to 3/4 along line from nasion to inion)
Upper rolandic point
2cm behind midpoint of line from nasion to inion
Lower rolandic point
Line extending from midpoint of upper margin of zygomatic arch to the upper rolandic point crosses the sylvian fissure line
Keyhole
3cm anterior to pterion, above lateral end of superior orbital rim
Positioning of pins
Avoid thin bone over frontal sinus mastoid air cells and temporalis muscle
Avoid previous burrhole/craniectomy sites
Avoid VP shunt tubing/valves
French correlation with diameter
Three French units= 1mm
Triangles of the cavernous sinus
1 Anterior
2 Medial
3 Superior
4 Lateral
5 Posterolateral
6 Posteromedial
7 Posteroinferior
8 Anterolateral triangle
9 Far Lateral triangle
10 Anterior tip of cavernous sinus route
11 Extended trans-sphenoidal route
12 Premeatal triangle
13 Post meatal triangle
Anterior triangle of the cavernous sinus
Triangle 1
Epidural space exposed by removal of ACP
Extradural optic nerve
Fibrous dural ring
Medial wall of SOF
Medial triangle of the cavernous sinus
Triangle 2
Intradural caroitd
PCP
Porus oculomotorius and siphon angle of the carotid artery
Used to approach intracavernous aneurysms/tumours
Superior triangle of the cavernous sinus
Triangle 3
Bordered by CN III and IV respectively
Posterior margin is edge of tumour along petrous ridge
Entry corridor to locate the meningohypophyseal trunk
Lateral triangle of cavernous sinus
Triangle 4
AKA Parkinson’s triangle
Trochlear nerve medially
V1 laterally
Dura of petrous ridge posteriorly
Can be opened to expose CN VI as it crosses carotid.
Parkinson’s triangle
PGK
L PL PM
Lateral triangle
IV + V1
Posterolateral triangle
Glassocks’
Triangle 5
Describes location of horizontal intrapetrous ICA
Foramen ovale, spinosum, posterior border of V3 and cochlear apex define this space
Glassock’s triangle
PGK
L PL PM
Posterolateral
Foramen ovale, spinosum, posterior border of V3 and cochlear apex define this space
Posteromedial triangle
Triangle 6
Kawase’s
Anterior petrous projection of the volume of bone that can be removed to make a window in petrous apex to posterior fossa
Delimited by cochlea, porus trigeminus and posterior border of V3 at the posterior apex of posterolateral (Glassock’s triangle)
Allows access to anterior brainstem and root of trigeminal without encountering neurovascular structures in the bone
Kawase’s triangle
PGK
L PL PM
Posteromedial
Cochlea, porus trigeminus, posterior border of V3 at posterior apex of PL triangle (Glassock’s)
Posteroinferior triangle
Triangle 7
Porus trigeminus, posterior clinoid and entrance to Dorello’s canal define this traignle
Incision here exposes petrosphenoidal ligament which forms roof of Dorello’s canal
VI can be seen here
