Skull Base Neurosurgery in one page : Easy facts to develop insight

Every neurosurgical trainee should develop an insight about the nuances of skull base neurosurgery. If you are able to make a bur hole in skull, you have already taken first step in understanding and practicing skull base neurosurgery.
Aim of skull base neurosurgery is to reach deeper regions of the brain with minimal retraction of the brain tissue. Prolong retraction of brain parenchyma leads to edema, ischemia and brain damage and, in turn,  increases post operative morbidity.

One should learn to draw the lateral view of skull depicting zygoma, pterion and orbit.

Fronto-temporal craniotomy or pterional craniotomy is very useful for approaching middle cranial fossa lesions. But this often requires prolong temporal lobe retraction, so adding orbito-zygomatic craniotomy makes the job easy. Therefore, little more work on skull base helps to reduce morbidity.

To understand the skull base surgery, one should know that cranial cavity can be divided into anterior, middle and posterior cranial fossa ( learn to draw a line diagram showing ACF, MCF & PCF).

ACF floor can further be zoomed to see midline structures like crista galli, olfactory groove, planum sphenoidale and tuberculum sellae. Laterally roof of the orbit forms the floor of anterior cranial foosa (ACF). The anterior skull base tumors can be meningiomas (olfactory groove meningioma, planum sphenoidale meningioma or tuberculum sellae meningioma. For CSF rhinorrhea repair, excision & repair of intranasal encephalocele, exciosion of intracranial extension of ethenioneuroblastoma and excision of intracranial fungal granulomas extending into nasal cavity, and chordoma of the floor of the ACF, one should know bicoronal scalp inciosion,  bifrontal craniotomy just above the orbit and anterior to the coronal suture, exteriorization of frontal air sinus, clipping and cutting of the superior sagittal sinus close to crista galli and opening the dura and retracting base of the frontal lobe backwards.

Mid sagittal image of the MRI is very easy to understand the relation of intracranial structures to the nasal cavity and how Trans nasal transphenoidal and transoral odondoidectomy  is done. This image will also help you understand the supracerebellar infra tentorial approach for operating pineal tumor

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Like usefulness of fronto temporal craniotomy for anteriorly placed lesions, midline sub occipital craniotomy is basic and of equal importance for operating lesions of midline posterior fossa structures. Scalp incision is made from external occipital protuberance to the C2 spinous process. Scalp is retracted and a bur hole is made in the occipital squama and the suboccipital craniectomy is done. Dura is opened in 'Y' shaped manner to avoid injury to the occipital sinus. Midline suoccipital craniectomy is done for operating Medulloblastoma, Cerebellar ependymoma, 4th ventricular tumor, posterior fossa decompression for Chiari malformation.If lesion is situated laterally like cerebellar astrocytoma the paramedian suboccipital craniectomy is done. far lateral, retromastoid cranectomy are done for other posterior fossa lesions.

TNTS is abbreviation for for transnasal transsphenoidal surgery & 

TOO stands for transoral surgery

In Frontotemporal craniotomy, at the supraorbita ridge care should be taken to identify and preserve the supraorbital nerve and vessel passing through the supraorbital foramen or notch. These can be released with the aid of a chisel or drill. A bur hole is made at the McCarty keyhole. The sphenoid ridge is made as flat as possible down to the level of the meningo-orbital artery.

In orbital osteotomy , the peroorbita is separated from the roof of the orbit for a distance of 3 cm posteriorly from the supraorbital ridge, and it is also separated along the lateral wall of the orbit. A sagittal cut is made in the roof of the orbit from the cranial to the orbit side to a depth of at 3 cm posterior to the supraorbital ridge. A second cut is made above the zygomatic process of the frontal boneand extended posteriorly as deep as possible within the orbit. Third cut is a coronal cut made across the roof of the orbit to connect the previous cuts. In orbitozygomatic osteotomy, this coronal cut across the roof of the orbit extends laterally  from the sagittal cut to the inferior orbital fissure. A third cut is made parallel to, but several millimeters above, the zygomaticomaxillary suture.

In Transbasal approach,  after making the McCarty keyhole bilaterally, a bifrontal craniotomy is done as close to the floor of the frontal fossa as possible. The posterior wall of the frontal air sinus is removed and thereby cranializing the sinus.The medial floor of the frontal fossa, including the roof of the ethmoidal air cells, is removed . The lateral margins of the the bone removal are the medial walls of the orbit. In Extended Transbasal approaches,  a bone cut is made through the nasofrontal suture, sagittal cuts are made just medial to the supraorbital notch on each side. This allows en bloc removal of the central portion of the supraorbital bar.

Trans-sphenoidal approach is used for sellar lesions , especially pituitary adenoma, through nasal cavity.

In Anterolateral Extradural/ Intraduralapproach for cavernous sinus, after cranio-orbitozygomatic osteotomy, the optic canal is unroofed and its medial wall drilled. The superior orbital fissure should be u roofed and the anterior clinod process resected. The floor of the middle fossa between the SOF and the foramen rotundum is drilled and the foramen is enlarged. The middle meningeal artery is identified and followed to the foramen spinosum. It is coagulated and divided , thus allowing the dura to be elevated from the floor of the middle fossa. The dura is dissected in a lateral to medial fashion until the greater superficial petrosal nerve (GSPN) is encounterd.To avoid the risk of injury to the facial nerve, the GSPN is transected and the bone of the Glassock’s triangle is drilled away, exposing the posterior loop of the ICA and freeing it from its bony canal to obtain proximal control. The dura is then ready for opening.

In Preauricular Subtemporal-Infratemporal approach, a temporal craniotomy is performed, temporal dura is stripped from the floor of the middle fossa and arcuate eminence is identified. This is the posterior extent of the dissection. The dural separation is then continued anteriorly to reach the foramen spinosum, where the MMA can be seen as it enters the cranial cavity. Zygomatic osteotomy is done . Anteriorly a V-shaped cut is made at the level of the frontozygomatic and zygomaticomaxillary suture. The posterior cut is made to include the condylar fossa. This cut is also V-shaped, with the apex of V just short of the foramen spinosum and the limbs spanning the anterior and posterior extent of the zygomatic root. V3 (mandibular) and  V2 ( maxillary) divisions of the fifth nerve then are exposed at the foramen ovale and rotundum, respectively. The GSPN may be cut close to the facial hiatus.

Transpetrosal approach include translabyrinthine and transcochlear approaches require division of the external ear canal and mastoidectomy.

Retrosigmoid craniotomy or craniectomy is used for surgery of.cerebellopontine angle tumors. A retroauricular skin incision is made.

Midline or Paramedian suboccipital craniectomy is used for approaching posterior foassa lesion.

Extreme lateral approach is used for vertebral and vertebral- basilar junction aneurysm.

For approaching the posterior third ventricular lesions and Pineal tumors, supracerebellar infratentorial, occipital transtentrorial approach and interhemispheric approaches are useful.

Sources: Operative Neurosurgical anatomy by Damirez T. Fossett &Anthony J.Caputy (Thieme)