**7. Endoscopy for skull base lesions**

The pioneering work of neuroendoscopy for skull base tumors was done by Carrau and colleagues [62], who reported their original experience of endonasal transsphenoidal hypophysectomy at the University of Pittsburgh. The endoscopic approach was expanded by de Divitiis and colleagues [4] to include other lesions of the sellar and parasellar regions. The bilateral endonasal endoscopic approach now allows for visualization of tumors at the anterior skull base up to the crista galli and down to the level of C2 [7]. **Figure 8** shows how endoscope can access and treat the pathologies in


The application of endoscopic endonasal surgery (EES) for excision of pituitary adenoma and craniopharyngioma has been reported with encouraging results and low morbidity [7]. The surgical corridor for the endoscopic approach for sellar or suprasellar tumors should be tailored on the basis of the extent of lesion.

#### **Figure 8.**

*Angles of endoscope by which the broad area of skull base (A. Frontal base, B. Sella, C. upper clivus, D. lower clivus) can be approached.*

Supradiaphragmatic lesions can be removed via the endonasal route. On the other hand the suprasellar prechiasmatic preinfundibular lesions can be removed with the transtuberculum-transplanum sphenoidale approach [2].

Tumors of the tuberculum sellae region present different type of difficulties for endoscopic endonasal surgery. It is a compact anatomic region where fine and critical microvasculature can be seen with the potential involvement of the Circle of Willis. The endoscopic procedure may leave high flow CSF leaks which may require the use of a vascularized flap, the most common example of which is the nasal septal flap [63, 64]. A natural extension for the endoscopic endonasal approach is anteriorly along the skull base to the planum and cribriform for resection of planum/cribriform [65].

EES has become a powerful tool for treating pathology of the clivus, the petroclival region, along with the intradural posterior fossa lesions immediately adjacent to the clivus. Traditionally, the pathologies in the clival and paraclival regions have been difficult to approach, especially for those, which have significant extension in the sagittal plain and/or had significant bilateral extension. For these tumors, often a combination of open approaches was required, as evidenced by dividing the clivus into thirds, each requiring a separate approach [66].

Expanded Endonasal Approaches (EEA) is versatile in the sagittal plain so much so that the ventral surgical corridor provides access to lesions that extend bilaterally across the midline. A single endonasal corridor can be sufficient to access large tumors that span the entire clival region. An endoscopic approach to the upper clivus provides midline access to the interpeduncular cistern, basilar artery, mammillary bodies and the floor of the third ventricle. The upper clivus or "sellar clivus" is formed by the posterior clinoid processes and the dorsum sella, which needs to be accessed and resected during this approach. A middle transclival approach provides access to the ventral pons and prepontine cistern, the basilar trunk and anterior inferior cerebellar artery, as well as the cisternal segment of the abducens nerve. The paraclival ICAs and the petroclival fissure set the lateral limit of the sphenoidal clivus. Laterally, the middle transclival exposure is limited by the interdural segment of cranial nerve VI [67].

#### *Frontiers in Clinical Neurosurgery*

Meckel's cave lesions can be approached by expanding the exposures laterally and inferiorly to the cavernous sinus [68–70]. The lower transclival approach through the lower segment of the clivus, which lies below the roof of the choana, allows access to the premedullary cistern and ventral medullary surface, the vertebral arteries, vertebrobasilar junction and posterior inferior cerebellar arteries and the cranial nerves IX–XII [9]. **Figures 9**–**11** are showing an endoscopic view of the cerebellopontine angle [71].

#### **Figure 9.**

*Endoscopic view of interpeduncular fossa and posterior part of CW. 1-basilar trunk, 2- superior cerebellar artery (SCA), 3- oculomotor nerve, 4-P1 artery, 5-posterior communicating artery, 6-basilar pons, 7 mamillary bodies & 8-Thalamoperforators.*

#### **Figure 10.**

*Endoscopic view through a cerebellopontine angle approach showing part of interpeduncular fossa and right sided medial temporal lobe. (a) Ps-pituitary stalk; po-pons; ds - dorsum sellae; and cl-clivus. (b) Ps-pituitary stalk; mtl - medial temporal lobe; on - oculomotor nerve; mb-midbrain; an-abducent nerve; po-pons. (c) On-oculomotor nerve; mtl-medial temporal lobe; and sca-superior cerebellar artery. (d) Ps-pituitary stalk; ma-right mamillary body; ba- basillar artery; pca-posterior cerebral artery; and sca-superior cerebellar artery.*

#### **Figure 11.**

*Endoscopic view through a cerebellopontine angle approach showing upper pons. Fv-facial and vestibulo cochlear nerve complex; p - petrous; r-retractor; pv - petrosal vein, Tr -trigeminal nerve; Po - pons; Cl - clivus; and t – tentorium cerebelli.*

**Figure 12.** *Route of endoscopic transposition of vascularized TPTMF flap for CSF leak repair on a 3D reconstructed skull.*

CSF rhinorrhoea, which commonly occurs as the result of trauma and iatrogenic disruption of the skull base as in EES & EEA and secondary to inflammation, neoplasm and pseudotumour syndromes, can be treated with endoscopic treatment. Skull base defects can be repaired with endoscopic remodeling of tissue planes and complete separation of the sinonasal cavities from the cranial space to carry out a multilayered reconstruction. Single layer of autologous fat or fascia, followed by tissue sealant may well be sufficient for small bony defects. Larger skull base defects with a high-volume intraoperative CSF leaks require robust closure with multiple layers. This multilayered closure can be achieved with an autologous fat graft in the bony defect followed by fascia lata, bony buttress and tissue sealant. These larger defects of skull base can be supplemented with a gasket seal closure [2, 63]. Currently, there are options of using multilayer closure with artificial dural substitutes and tissue sealants which work nicely in many cases. Sometimes, lack of vascularity may affect the integrity of the closure. Fortes et al. described endoscopic repair of CSF leak by transpterygoid transposition of a temporoparietal fascia flap [72] and Kawsar et al. showed good long-term outcome ranging from 6 months to 4 years in their small series. The pathway of the flap through the endoscopic route has been shown in **Figure 12** [73].
