With an increased knowledge of the surgical anatomy and multidisciplinary approach, most of the cranial basal lesions can be dealt with safely. Newer techniques in reconstruction of blood vessels and  nerves, sophisticated neuro anesthesia and intensive post operative nursing are of great help. It is possible to keep the brain retraction to the minimum and damage to the blood vessels and nerves are avoided with newer skull base techniques.

 The aim is to get the extra room under the brain.

 SURGICAL ANATOMY:

  A) Osteology:

 1) The body of the sphenoid occupies the central portion. The tuberculum sellae is a transverse ridge , that separates the chiasmal sulcus anteriorly from the sella turcica posteriorly. The sella is a rounded hollow that cradles the pituitary gland.

 2) The sides of the body slop down and laterally, grooved by the sigmoid curve of the ICA, to the floor of the middle fossa.The anterior and posterior clinoid processes are important landmarks and areas of dural attachment .Occasionally there is a middle clinoid process, that may be bridged to the anterior clinoid, so forming a caroticoclinoid foramen through which passes the ICA.

 3) The lateral recesses are the middle fossae proper and triangle shaped, limited anteriorly by the spenoid ridge and posteriorly by the petrous ridge.

      -The  anterior wall  is formed by the greater wing of the sphenoid.

      -The floor is by the greater wing anteriorly, and the petrous ridge posteriorly. Laterally, between  the two  is the  squamous temporal bone.

      -Thelateral wall is made up of the greater wing of the spenoid anteriorly and the squamous temporal bone posteriorly.

      -The posterior wall is by the petrous ridge.

 The floor and lateral walls are grooved by the middle meningeal artery. The superior surface of the petrous ridge has several important markings.

      -Medially, near the apex is an impression for the trigeminal ganglion as it lies in the Meckel's cave. The ICA runs directly under this and the bony canal may be dehiscent.

      -Laterally is the thin tegmen tympani, roofing the middle ear and mastoid.

      - Anteromedially, lies the arcuate eminence overlying the superior semicircular canal. Further antero-medially, lie the canals for the greater and lesser superficial petrosal nerves. GSPN may   be traced to the geniculate ganglion and facial nerve in the IAC.The bone may be dehiscent over the geniculate ganglion.

      -The petrous ridge is longitudinally grooved by the superior petrosal sinus where the tentorium cerebelli attaches.

4) The foraminae:

      -Anteriorly lies the superior orbital fissure, which leads to the orbital apex.

      -Foramen rotundum lies behind and inferior to superior orbital fissure and transmits the maxillary division of the trigeminal nerve.

      -Foramen ovale lies posterolateral to the foramen rotundum and transmits the mandibular division of the trigeminal nerve, the accessory meningeal artery, the lesser superficial petrosal nerve and emissary veins to the pterygoid plexus.

      -The foramen spinosum lies posterolateral to the foramen ovale and transmits the middle meningeal artery.

      -The petrous apex articulate with the sphenoid and occipital bone medially and so forms a rounded opening to the carotid canal (cranial counterpart of the foramen lacerum) on the under surface of the skull base.

1-optic canal 2-superior orbital fissure 3-For.rotundam 4-Venous For. 5-For. ovale 6-For.spinosum 7-For. lacerum 8-Groove for GSPN 9-Groove for mid.men.art

5) The temporal bone itself contains several important structures.       -The sigmoid sinus ends in the jugular bulb.       -The 7th and 8th nerves enter the porus-acusticus and IAC. The 7th nerve traverses the middle ear and mastoid. The 8th nerve ends at the inner ear.       -The eustachian tube arises at the protympanum and runs anteromedially and inferiorly .The  tube is one third bony and two thirds cartilaginous.        -Directly medial to the origin of the bony eustachian tube lies the ICA.

  B) Intracranial contents:

 1 ) The dural arrangement is complex and densely adherent in the regions of clinoid processes, petrous  and sphenoid ridges and around the basal foraminae. In the midline it forms a transverse dural plate, the diapraghma selle, roofing the pituitary fossa. Laterally the dural plate forms the roof of a basin beside the body of the sphenoid,the cavernous sinus.

 2) The cavernous sinus is a plexus of veins that lies within the layers of the dura beside the sphenoid sinus. The lateral border of the roof is the anterior petroclinoid fold and the posterior border is the posterior petroclinoid fold.

The ICA is the main structure within the sinus. The 6th nerve is the only nerve within and lies in close opposition to the lateral wall of the ICA. The cranial nerves the 3rd, 4th, and 5th are variably related to each other in the lateral wall.

Parkinson has outlined triangles between these nerves that can be used to gain access safely to the cavernous sinus.

Anterior venous connections are the superior ophthalmic vein and spheno-parietal sinus. Superiorly, the cavernous sinus drains the superficial middle cerebral and inferior cerebral veins. Medially is the intercavernous plexus to form the circular sinus. Inferiorly,  the emissary veins pass to the pterygoid plexus. Posteriorly it drains into the superior and inferior petrosal sinuses and into the basilar plexus between the dural layers over the clivus.

 3) The motor and sensory roots of the 5th nerve pass underneath the free edge of the tentorium cerebelli and into the Meckel's cave, which contains the motor root and trigeminal (Gasserian) ganglion, which  overlies the petrous apex and ICA. The ganglion is variably enclosed by the subarchnoid space and  CSF. The cranial nerves V1, V2 and V3 pass from the ganglion into the lateral wall of the cavernous sinuses.  The motor root passes with V3 through the foramen ovale.

 4) The temporal lobe fills most of the rest of the fossa.

 5) The inferior anastomatic vein (of LABBE) connects the superficial middle cerebral vein to the transverse sinus just before it becomes the sigmoid sinus. Injury to this vein may result in infarction of the motor cortex. The superior anastomatic vein (of TROLARD) connects the middle cerebral vein to the superior cerebral veins.

 6) The greater petrosal nerve (GSPN) and the lesser petrosal nerve (LSPN) run parallely beneath the dura along the anterior edge of the petrous bone as it runs to the foramen lacerum .It is also a landmark for the ICA which lies just deep and parallel to it.

 7) The internal carotid artery is the most important structure at risk during surgery. 

It is divided into four parts:

      -The cervical portion arises at the 3rd and 4th cervical vertebrae, runs superiorly to the external  carotid artery and deep to the digastric muscle and styloid apparatus. The glenoid fossa is a bony landmark for the higher parts of the ICA at the eustachian tube level.

        This portion has no branches.

      -The intra temporal ICA has a vertical and a horizontal segment: -The vertical segment (C1) begins at the canal where it is anchored very firmly by a fibrinous ring. It ascends for 5mm, turns anteromedially into the horizontal segment(C2)  which runs forward in the petrous bone directly related antero-laterally to the eustachian tube in this portion.

     -The cavernous portion of the ICA  ( C3 ) is very thin walled.

     -The supracavernous ( C4 ) portion begins as the artery pierces the dura in the roof of the cavernous sinus medial to the anterior clinoid process, passes backward below the optic nerve to the anterior perforated substance where it in the circle of Willis.

Drilling medially through the glenoid fossa leads straight into the bony eustachian tube and superiorly, into the middle cranial fossa. Anteromedially is the foramen spinosum , and then the foramen ovale.

Further medially, is the eustachian tube (cartilaginous) and more medially, the carotid artery.

SURGERY:

 Preoperative work-up:

 1) MRI with and without gadolinium is valuable .Fine sections reveals encasement of the blood and suggest the nature of the lesion.

 2) CT with bone windows shows the bony changes.

 3) Cerebral  angiography reveals the vasculature of the lesion and cross circulation. Pre operative embolization may be required in selected cases.

 4) Discussion with the ENT surgeon, the Anesthetist and scrub nurse regarding the objective of the procedure, whether radical excision or otherwise, and positioning,  CSF drainage during the procedure etc, is a must.

 5) Neurophysiological monitoring, if available, may be useful.

 Approaches:

 The sphenoid-wing meningiomas, sphenocavernous and cavernous lesions, tumors of and around The petrous bone and some complex and giant aneurysms are better dealt with skull base approach. Pituitary adenomas and Craniopharyngiomas with para-sellar extension may require this route for radical excision. Tumors from the infratemporal fossa may also extend into the intracranial cavity. With added modifications, the middle fossa approaches may be employed for lesions around the lower clivus as well.

 The extent of the lesion, objective of the surgery, availability of the facilities and experience of the surgeon in skull-base surgery decides the approach, as in any surgery of any kind.

 1)Anterolateral approach:

 This approach is recommended for lesions around and above the level of the upper clivus, above the level of the crossing of the 5th and 6th nerves from posterior to middle fossa.

 --Under general anesthesia, the patient is positioned with the head turned to the opposite side.  

 --The common and internal carotids are exposed at the neck for future temporary occlusion.

 --Through a bicoronal (if a frontobasal approach is also planned) or a frontotemporal scalp incision, a frontotemporal craniotomy is made .I prefer a free bone flap.

-The anterior zygomatic osteotomy is made at or lateral to the zygomaticomaxillary suture.

      -The posterior zygomatic osteotomy is then made through the condylar fossa or just anterior.

      -The entire orbital rim, the zygomatic arch and condylar fossa may then be removed as a single piece.

 --Next, the drilling of the sphenoid wing is done untill the base of the clinoid.Ideally the ant.clinoid is removed extradurally, some prefer to  remove the anterior clinoid intradurally to prevent injury to the surrounding structures.

 --CSF drainage at this stage helps.

 --V2 and V3 branches are exposed extradurally in the subtemporal area, and the superior orbital  fissure is decompressed.

 --The dura-pericranial hitch stiches are applied. The dura is opened and turned anteroinferiorly  as a flap.

 --Another slit in the dura along the sylvian fissure provides a protective cover to the frontal and temporal   cortex

 --The sylvian fissure is opened laterally and the branches of the middle cerebral artery are followed proximally

 --The optic canal is decompressed. The optic nerve sheath is opened to mobilize the nerve. The dural rings around the ICA are opened.

 --The tumor is removed in piecemeal using microsurgical techniques. It is prudent to leave  behind the tumor bits adherent to vital structures.

 --If necessary, the cavernous sinus is opened at a point where the lesion presents as a bulge or through one of the parkinson's triangle and the dural layer is peeled away. The 3^rd, 4th and 5th nerves are at risk and must be protected at this stage. In the presence of a lesion, the venous plexus is collapsed, and bleeding is not a problem. The dissection of the 6th nerve can be difficult and must be done carefully. Some bits of the lesion may have to be left attached to  the ICA. Small tears in the artery may need to be sutured after temporary occlusion of the  carotid at the neck. When the artery is completely encased, excision of the involved, the ICA may be contemplated with a vein graft bypass. Some prefer ICA or ECA to MCA bypass.  Many leave the adherent tumor behind.

When direct surgery is planned, intracavernous giant aneurysms are dealt with, after temporary occlusion of the carotid at the neck.

   Induced hypertension, mild hypothermia and barbiturate coma are used during vascular occlusion. 

 --The clival and sphenoidal bone may need to be drilled on occasions for complete tumour removal.

 --The tentorium overlying the Meckel's cave may be opened, exposing the prepontine and interpeduncular area, to access into the posterior fossa, if required.

 --Following excision, the cavernous sinus is repaired with fascia lata. If the sphenoid sinus is  entered, it is packed with fat and the dura is closed watertight.

 --The orbito-zygomatic arch and then the bone flap are replaced, followed by the scalp closure.

The petrous ICA is often partially exposed without any bony covering just posterior and medial to V3 and middle meningeal artery and inferior to the GSPN.

 The bone between the middle cranial fossa and mandibular fossa may be removed to expose the genu of ICA.

Care must be taken not to injure the cochlea or the geniculate ganglion and the facial nerve which lie immediately posterior   and superior to the genu.

 The ICA is identified in one area, the entire ICA is progressively exposed and unroofed.

The bone medial to V3 and lateral to ICA may have to be drilled just medial to V3.The lumen of  the eustachian tube is  cauterized and packed with muscle and fat and closed.

The jugular bulb and cranial nerves 9,10 and 11 lie immediately posterior to the vertical segment of the petrous ICA.

The petrous apex medial to ICA can be progressively removed and  the midclival and petrous apex dura can be exposed.

Medial to the vertical ICA, progressive removal of the bone will allow unroofing of the 12th nerve.

Now, the entire petrous and upper cervical ICA is exposed and mobilized.

 --The sphenoid sinus is approached anteriorly between V2 and V3.

 --The tumor is removed in piecemeal using microsurgical techniques.

 --The cavernous sinus may be entered extradurally or intradurally to complete the tumor removal with an appropriate    dural     incision.

 --The V3 may be divided to access the lower clivus, sphenoid and opposite petrous apex.

 --The defects are closed with an autologous fascia lata graft.

   The dead space is filled by a vascularized temporalis muscle flap or a distant microvascular   free flap.

 --A post-operative CSF drainage is often employed to prevent a CSF leak.

   Many patients require some type of rehabilitation for ocular, facial, swallowing and speech disorders  postoperatively.