Endoscopic skull base surgery

by 4rn0

Skull base surgery has gone through significant changes with the development of extended endoscopic endonasal approaches (EEA) over the last decade. Initially used for the transphenoidal removal of pituitary adenomas, the endoscopic transnasal approach gradually evolved into a way of accessing the whole ventral skull base. Improved visualisation, avoidance of brain retraction, the ability to access directly tumors with minimal damage to critical neural and vascular structures as well lack of external scars are among its obvious benefits.

Why favouring endoscopic skull base surgery over traditional approaches?

1. Pathway to the tumour:

Choosing the most direct approach to the tumour is vital in skull base surgery. Avoidance of frontal lobe retraction with its associated temporary (and occasionally permanent) neurological deficits is obviously a reason to favour the transnasal approach to traditional transcranial approaches. However, even more importantly, from an oncological resection point of view, unimpaired visualisation and access are important: Critical neurovascular structures (the carotid, the optic nerves, the oculomotor nerve etc) may block such direct access. For example in a patient who has a retrochiasmatic craniopharyngioma which is growing postero inferior to the optic nerve as clearly identifiable on the pre-operative MRI scans, an extended endonasal approach provides the most direct pathway to the tumour without the optic nerve obstructing access, as would be the case in a transcranial approach. This has been articulated by the Pittsburgh group, as the concept of “not crossing the nerves” – in other words, always chose the approach that does not include dissecting “behind” a critical neurovascular structure.

2. True team surgery

Traditionally “team” surgery was defined as surgeons working sequentially. However, both technically as well as conceptually, this approach is not valid in EEA: the anatomical knowledge, dissection principles and manual dexterity of all members of the team (otolaryngologists and neurosurgeons) are required throughout the whole procedure. In practical terms, and in most of our cases, this takes the form of “ 2 nostrils – four hands technique”   as a way of optimising visualisation and tissue handling.  How does this work? This so-called “3-4 hands technique”requires a good collaboration between two surgeons that should be perfectly tuned, one holding the endoscope as well as providing traction and another handling two surgical instruments inside the surgical field. The surgeon could proceed performing a bimanual dissection while the colleague holds the endoscope moving it dynamically and is able to insert other surgical instruments. The bimanual dissection also proves to be time efficient.  We found that our growth would not have been possible, without the mutual support between the otolaryngologist and the neurosurgeon, reflected in the operating room but also transferred to the multidisciplinary skull base clinic and a joint learning curve.

3. Direct view under high magnification:

Anyone working with a bright modern endoscope using high quality optics and a high definition digital setup can testify to the excellent image projected (see figure 1. for an intra-operative snap-shot of the software used to combine navigation scans and live video and figure 2). This image is shared by everyone in the operating room, facilitating communication between the team members and planning of surgery and anaesthesia.

intra-operative snap-shot of the software used to combine navigation scans and the live video images in a x year old patient with a tuberculum sella meningioma, after drillout of the tuberculum sella and planum sphenoidale. The Optic Chiasma as well as the Anterior cerebral arteries (A1 and A2) as well as the anterior communicating artery (ACA) and optic chiasm (OC) can be clearly visualised as the tumour (Tu) is being removed.

intra-operative photo after resecting a tuberculum sellae meningioma. The optic chiasm (A), anterior communicating artery (B), anterior cerebral artery (C) and cortex (D) are clearly visible.

4. Visualisation under angle:

holding a mirror, 30 and even 45 degree endoscopes can provide access to areas that would have been impossible to assess with the direct, straight view afforded by a microscope. This has proven especially helpful in assessing completeness of dissection and searching for tumour remnants in hidden angles, after the completion of tumour removal. Having said this however, the extended view should be coupled with adequate access in order to manipulate instruments using a 3 or 4 hand technique- and visualisation provided by the 30 degree endoscope should never be used as a substitute for adequate wide access.

5. Minimally invasive

This point is important, but we feel we should not take priority over radicality of excision. Indeed, avoiding a craniotomy means that, in many cases, we can avoid the morbidity (scarring and blood loss) associated with the approach as well as the neurological sequalae associated with brain retraction.  This results in many cases in faster recovery and reduced hospitalisation.