1. Introduction
Transnasal endoscopic techniques have brought significant advancements to skull base
surgery. In the last 30 years, our understanding of skull base anatomy from an
endoscopic perspective has significantly improved, thus enhancing the value of
endoscopic transnasal surgery for treating skull base pathologies [1]
[2]
[3].
As the surgical corridors to the skull base are the nasal cavity and the pneumatized
sinuses, otolaryngologists (ENTs) drove much of the development of endoscopic skull
base surgery in the very early stage. However, the new technique allowed
neurosurgeons access to the skull base form below in a less traumatizing procedure
and in many centers all over the world a very close collaboration of ENT surgeons
and neurosurgeons started with interdisciplinary surgery addressing pathologies of
both sides of the skull base. This interdisciplinary teamwork fostered the further
development of special instruments and refinement of surgical approaches and
improved patients’ outcome.
The endoscopic surgical revolution began in different centers in the world with
pituitary surgery and has expanded to various approaches, reaching areas like the
parasellar region, the petrous apex, the jugular foramen, the infratemporal fossa,
and the upper parapharyngeal space [4]
[5]
[6]
[7].
The new techniques clearly demonstrated comparable or superior results in terms of
adequacy of resection and reconstruction, while reducing the typical high morbidity
of open approaches [8]. The choice of the
surgical approach is guided by the type and location of the disease, its
relationship with critical neurovascular structures, the biology of the lesion
treated, and the characteristics of the expected defect. The surgical exposure must
be balanced based on the need of a clear view of the area of interest, the
possibility of a complete ablative surgical phase, proper reconstruction, and
avoidance of complications. Several endoscopic techniques can be conceived as
modular approaches with incremental grades of surgical invasiveness, thus allowing
the surgeon to expand the approach in case the lesion extends unexpectedly, in order
to identify and protect vital neurovascular structures [3]
[9].
Considering the complexity of the anatomy involved, simplifying the intricate
geometry into a reliable schematization is necessary for creating a mental map of
skull base anatomy. Surgical modules categorized based on their relation with the
internal carotid artery in sagittal and coronal planes, provide access to the entire
ventral skull base. Sagittal plane approaches give exposure of median structures
extending from the posterior plate of frontal sinus to C2, between the internal
carotid arteries and orbits [9]
[10]
[11].
These modules include the following approaches: transfrontal, transcribriform,
transplanum–transtuberculum, transsellar, transclival, and transodontoid
(
[Table 1]) [5]
[6]
[9]. Paramedian and lateral skull
base approaches on the coronal plane include various depths of dissection to the
lamina papyracea, orbital roof, and orbital cavity, parasellar area, cavernous
sinus, superior petrous apex, Meckel’s cave, and pterygopalatine and infratemporal
fossa. The most posterior coronal approaches provide exposure of the inferior
petrous apex, lateral craniocervical junction, and upper parapharyngeal space. The
abovementioned approaches are linked together and can be combined based on the need
to control a wide variety of pathologies [10].
Moreover, in extended cases, the trans-nasal route can be combined with other
approaches (i. e. transoral, transorbital, transcervical, and transpetrosal),
integrated in a multiportal surgical strategy or staged procedures may be indicated
in particular cases [12]
[13]
Table 1 Classification of skull base endoscopic
approaches.
|
Midline approaches
|
Transfrontal, Transcribriform, Transplanum–transtuberculum,
Transsellar, Transclival, Transodontoid
|
|
Coronal approaches
|
Approaches to the orbit, Optic decompression, Transcavernous
Approach to the medial petrous, apex Transpterygomaxillary and
Infratemporal fossa, Upper and lower transpterygoid Suprapetrous
(Meckel’s cave) approach, Infrapetrous approach, Transcondylar –
Transjugular tuberculum Approaches to the upper parapharyngeal
space
|
Given the intrinsic rarity of skull base lesions, the learning curve of the team is
slow and needs continuous efforts to develop. For these reasons, patients with skull
base pathologies should be referred to dedicated centers with a high surgical
expertise, a high caseload and all the availability of all specialties
e. g.(neuro)radiologists, medical and radiation oncologists, ophthalmologists,
maxillofacial surgeons, and dedicated pathologists.
As multidisciplinary collaborative management of skull base pathologies by ENT
surgeons and neurosurgeons is crucial, the aim of this contribution is i) to give an
overview of various surgical approaches with emphasis of surgical anatomy and tips
and pearls of complication avoidance; ii) to provide evidence of the superiority of
patient’s outcome if treated by a interdisciplinary endoscopic skull base team [14].
2. Methods
The authors used their surgical experience to describe typical surgical endoscopic
skull base approaches and demonstrate representative cases. The scientific
literature is analyzed to provide a broad understanding of skull bas anatomy,
surgical anatomy, and adequacy of approaches with limits and potential
complications.
2.1 General surgical considerations of interdisciplinarx endoscopic skull
base surgery
Adequate preoperative surgical imaging must delineate the anatomical location,
the size and the extension of the pathology. Therefore, thin sliced CT scans and
MRI with dedicated sequences (eg. fat saturation, thin sliced T2 sequences etc.)
are necessary for surgical planning. Usually, image guided surgery is applied
for better intraoperative orientation and identification of critical
neurovascular structures. In case with optical image guidance, a rigid head
fixation is required while electromagnetic navigation may allow head positioning
in a head rest and intraoperative movement of the head.
The patient has general anesthesia with orotracheal intubation. A standard
anesthesiological preparation of the patient is done including placement of an
arterial line for continuous blood pressure monitoring, urinary catheter for
fluid balance monitoring, and placement of intravenous devices for sufficient
fluid replacement. Intermittent pneumatic compression (IPC) devices are used to
help prevent deep venous thrombosis.
A pharyngeal tamponade is placed to prevent aspiration of blood or flushing
solution. The anesthesiology team and the monitoring equipment is placed at the
patients’ feet or head during the operation.
Depending on the pathology endoscopic skull base surgery is performed either by
ENT surgeons if no dura invasion is assumed, according to the preoperative MRI.
If intracranial extension of the pathology is expected or proven by preoperative
imaging, a dedicated interdisciplinary team of ENT surgeons and neurosurgeons
should perform surgery interdisciplinary.
The patient is placed in a supine position with the upper part of the body
slightly elevated (max. 20°). The head is either placed on a head rest or
sharply fixed in a Mayfield clamp in neutral position with a minimal rotation
(10°) towards the main surgeon ([Fig.
1]).
Fig. 1 The patient is placed in a supine position with the upper
part of the body slightly elevated (max. 20°). The head is sharply fixed
in a Mayfield clamp in neutral position with a minimal rotation (10°)
towards the main surgeon.
CT and/ or MRI data are applied for co-referencing and image guidance during
surgery. After the preparation of the surgical field xylometazoline soaked
paddies are placed in both nostrils for mucosal decongestion. Meanwhile, a
sterile draping and a standardized time out are performed. Intravenous
antibiotic medication (e. g. 1.5g cefuroxime) is given before surgery
starts.
Intraoperative set up may vary according to the surgeon’s preference. In our
setup, the operating surgeon is standing on the patient’s right side while the
assisting surgeon is standing on the patient’s left side. The scrub nurse is
standing next to the main surgeon either on his left or right side. Other teams
propagate that both surgeons stand close next to each other on the patient’s
right side. The mutual positions of surgeons depend on the habits of the team
and are all valid when an effective 4 hands surgical technique is
guaranteed.
As visualization is mandatory monitors must be placed for continuous unobstructed
view without disturbing the ergonomic workflow during surgery for both surgeons
and the scrub nurse.
2.2 Midline approaches
2.2.1 Transfrontal approach
The frontal sinus can be used as a corridor to access the posterior plate of
the frontal bone [15]
[16]
[17]
[18]. The lesions that
mainly arise from this subsite are osteomas, inverted papillomas,
glomangiopericytomas, mucoceles and meningoencephaloceles, followed by
dermoid cysts and nasal gliomas [16]
[19]
[20]
[21]. The exposure phase of the endoscopic transfrontal approach
consists of harvesting a corridor corresponding to a Draf Type III (or IIb
in selected monolateral cases) sinusotomy. The anatomical boundaries of the
approach are: the nasal bones and anterior plate of the frontal bone,
anteriorly; the cribriform plate and nasal septum, posteriorly; the medial
and cranial portions of the orbits, laterally. Lesion with far lateral
frontal extension and involvement of the anterior frontal plate can be
rarely managed with a pure endoscopic transfrontal approach, since an
exposure allowing a satisfactory surgical maneuverability cannot be reached.
In these cases, pure open or combined strategies can be performed.
Furthermore, dysembriogenic lesions show typical extensions toward the
external nose, so a combined endoscopic or open rhinoplasty technique might
also be recommended for specific cases [16]. The lateral exposure over the meridian of the orbit,
traditionally considered the lateral limit of the endoscopic transfrontal
approach, can be expanded through an orbital transposition. This consist in
the sectioning of the anterior ethmoidal artery, which allows the inferior
and lateral dislocation of the orbit and lateral gain of precious
millimeters [22]. This surgical path
is particularly challenging, thus limiting its use to selected cases. The
area of exposure is limited and constant, especially laterally. The anatomy
of the patient really influences the surgical maneuverability: a narrow
corridor in the anteroposterior direction limits the movements of the
surgeons, and 4 hands dissection may not be applied. The damage of the
anterior frontal tissues, orbit and cribriform plate may lead to esthetic
and severe functional complications. Transdural dissection is particularly
complex due to the geometry of the defect and angle of approach, as well as
the presence of delicate vascular structures (i. e. superior sagittal sinus,
bridging veins, and orbitofrontal and frontopolar arteries). Finally, but
not less important, the reconstructive phase is hampered by different
factors. The anterior positions make the anterior and upper border of the
frontal endoscopic craniectomy difficult to be reached by posterior pedicled
mucosal flaps (nasoseptal flap). Since the orientation of the defect is
vertical, the intracranial pressure, combined with gravity, does not push
perpendicularly on the duroplasty, thus reducing its stability. Despite the
rarity of indications and the intrinsic complexity, it is crucial to master
this approach to manage lesions located in the far-anterior region of the
midline skull base. Furthermore, a secondary involvement for contiguity of
this area is quite common for lesions originating from nearby skull base
areas, such as the cribriform plate and ethmoidal roof [23] ([Fig. 2]).
Fig. 2 Case of anterior skull base osteoma, centered on the
posterior plate of the frontal bone, with intracranial extension and
valve mechanism and pneumoencephalon (white stars in figure a
and b). Image c shows the dural defect, obtained with
a transfrontal endoscopic approach (white arrow). Reconstruction was
performed in multilayer fashion, with intradural fat, intracranial
extradural fascial lata, and nasospetal flap (NSF in D). NS – Nasal
Septum.
2.2.2 Transcribriform approach
The transcribriform approach is conceived to endoscopically remove anterior
skull base lesions involving the cribriform plate and the ethmoidal roof
(also named fovea ethmoidalis) [5]
[24]. Over the last 20
years, this surgical route has been the primary technique for treating
various diseases, such as sinonasal malignancies, meningoencephaloceles,
meningiomas [25]
[26]
[27]
[28]
[29], schwannomas [30]
[31], and dysembryogenic lesions [32]. Moreover, selected spontaneous or post-traumatic
cerebrospinal fluid leaks may be treated with a tailored transcribriform
route. The main malignant histologies addressed with this approach are
squamous cell carcinomas, adenocarcinomas (mainly intestinal type or
non-intestinal type), and olfactory neuroblastomas [1]
[10]
[33]
[34]
[35]
[36]
[37]
[38]
[39]
[40]
[41]
[42], with good
oncologic results and minimal morbidity when compared with traditional open
approaches. Among the factors influencing the choice of the approach,
besides the biological behavior of the lesion, local extension, and
reconstructive options, the pre-surgery olfactory status and preservation
possibilities must be considered [43]
[44]
[45]. The boundaries for the
transcribriform route include the angle between the posterior frontal plate
and the anterior skull base anteriorly, the connection of ethmoid and
sphenoid planum posteriorly, and the medial orbital walls laterally. Based
on local extension, in selected cases, the resection can be enlarged over
these limits, above the orbits, on the planum sphenoidalis, and on the
posterior plate of the frontal sinus. Moreover, unilateral endoscopic
craniectomy can control selected monolateral lesion, not crossing the nasal
septum [46]. This surgical strategy,
in parallel with good oncologic results once a complete resection is
achieved, allows patients to partially maintain the sense of smell. In
contrast, bilateral resections are followed invariably by anosmia, since the
olfactory organs (fila, bulbs, and tracts) are cut on both sides. The first
phase of surgery consists of the exploration of the nasal fossa to
understand local extension and debulking of the lesion. To expose the
anterior skull base, a bilateral medial maxillectomy, complete
ethmoidectomy, Draf III procedure, transrostral sphenoidotomy, and
septectomy are performed. The bilateral approach allows for a dual nostril,
four-hands technique. Once the bony anterior skull base is exposed, the
ethmoidal arteries must be coagulated and cut. The anterior (and middle if
present), usually runs extracranially, while the posterior runs in a bony
canal between the cribriform plate and planum, which has to be drilled to
expose the artery before the coagulation. Once the area of interest is
exposed, the craniectomy is performed. The resection can be extended to the
dura, falx, and olfactory bulbs and tracts if necessary. This technique
provides a clear view of the gyrus rectus and medial orbital gyrus, which
can be addressed endoscopically too in case of modest infiltration. During
the intracranial phase, vessels like the anterior falcine artery and
frontopolar artery must be managed carefully, to avoid injuries that may
need to be controlled with an open approach [47].
To date, multilayer reconstruction using various grafts and flaps represents
the gold standard [48]
[49]. The geometry of the defect, with
gravity and intracranial pressure pushing on the margins of the plasty, as
well as the rigid buttress offered by the bony boundaries of the resection,
allows for a stabilization of the different layers of the reconstruction.
This determines the low rates of postoperative cerebro-spinal fluid leak
reported in the literature. Fascia lata/iliotibial tract, synthetic, or
hetero-grafts, in combination or not with fat grafts, are usually used for
the intracranial intradural and extradural layers of the reconstruction. As
second or third layer, vascularized mucosal pedicled flaps offer an optimal
option, minimizing leaks and crusting formation. However, since
nasal-ethmoidal tumors have the propensity to affect the nasal septum and
turbinates, local vascularized flaps are often unavailable. In these cases,
the use of a third extracranial fascial layer demonstrated good results in
several experiences in the literature ([Fig. 3]).
Fig. 3 Case of a naso-ethmoidal intestinal type adenocarcinoma
(white stars in figure a and b). The lesion invaded
the nasal septum (white arrow in a) and had a critical
contact on the left lamina cribrosa (white arrow in b). A
bilateral transcribriform approach was therefore performed. Image
d shows the dural defect, with brain (B) exposed
and frontal vessels (white arrow). Image e shows the fascia
lata of the intradural reconstruction (blue star). Image c depicts
the multilayer reconstruction with 3 strata of fascia lata. The
first (blue) is positioned intradurally, the second (red) is
intracranial extradural, the third (green) is extracranial. O –
Orbit.
2.2.3 Transsellar approach
Indications for the transsellar approach are mainly sellar pathologies with
pituitary adenomas being the most frequent encountered lesion. However other
intra – or suprasellar lesions may be operated via the transsellar approach,
eg. craniopharyngiomas with limited suprasellar extension. Other rare
pathologies may be metastatic tumors or lymphomas. According to Cappabianca
[50] the surgical ablative
procedure is divided into 3 phases, the nasal, the sphenoidal and the sellar
phase and is carried out in a very standardized way. The approach should be
as minimal as possible to get an adequate exposure of the pituitary adenoma
or any other lesion, the pathology should be removed with preservation of
the pituitary gland and a thorough reconstruction has to be performed with
closure of the defect.
2.2.3.1 Nasal phase
The operation starts after endoscopic inspection of both nostrils using a 0°
endoscope. For standard pituitary tumors, a mononostril approach may be
performed without the use of a speculum. Some authors prefer a binostril
approach [50]
[51]. Other surgeons propagate the
mononostril endonasal transethmoidal-paraseptal approach to resect pituitary
adenomas [52]. In our experience, the
mononostril approach provides sufficient surgical freedom. Dissection is
done using the three or four hands technique. In the nasal phase the surgeon
usually holds the endoscope in his left hand while the dissecting
instruments are used with his right hand. With this technique a more dynamic
view is provided to assess depth perception. The assistant surgeon uses
suction and clears the endoscope from debris using on demand irrigation. In
our interdisciplinary team the ENT surgeon starts surgery with neurosurgical
assistance. After the endoscope has been introduced into the nose some
landmarks guide the further procedure. The endoscope is advanced between the
inferior turbinate and the nasal septum and the choana is identified. The
sphenoid ostium may by localized very early by elevating the endoscope about
1.0 cm cranial to the choana. The middle turbinate is identified. The
standard approach includes a gentle lateralisation of the middle turbinate
to enlarge the room between the middle turbinate and the nasal septum and to
increase the surgical freedom in the chosen nostril. Care should be taken
during lateral luxation of the turbinates, in order to avoid ethmoidal plate
injuries which may result in cerebrospinal fluid (CSF) leak. If the sphenoid
ostium is covered by either the superior or the supreme turbinate, these can
be gently lateralized or removed. We use circular punches to enlarge the
ostium and then one can already visualize the ipsilateral sphenoid sinus.
From the medial circumference of the sphenoid ostium the mucosa is incised
by a straight upward directed incision towards the posterior part of the
nasal septum. Prior to the incision saline may be injected to elevate the
mucosa at the posterior part of the nasal septum. A monopolar cautery or a
sickle knife can be used for mucosal incision. In cases with extended
approaches or suspected high flow CSF fistula a pediculized flap on the
sphenopalatine artery is dissected according to the size of defect as
described [53]. The posterior part of
the nasal septum is pushed to the contralateral side. The contralateral
mucosa is elevated, and the contralateral ostium is exposed.
2.2.3.2 Sphenoid phase
An anterior sphenoidectomy is performed with the use of a 4mm diamond drill.
It is crucial to widely expose and open the anterior face of the sphenoid in
order to gain a proper working angle for all the instruments when inside the
sphenoid with their tips in the sella. If necessary, the inferior part of
the ipsilateral superior turbinate may be removed and a limited posterior
ethmoidectomy may be performed to extend the degree of surgical freedom.
After the anterior sphenoidectomy (removal of the rostrum) has been
completed, the intrasphenoid septae are removed using a diamond drill. It is
recommended to spare the bony septa for later reconstruction of the sellar
floor. The mucosa is elevated and lateralized or partially resected. If
multiple septae are present, especially horizontal septa, the use of image
guidance may help to define the extent of exposure with the exposition of
the planum sphenoidale as the most cranial landmark. In all cases the
bilateral bony prominence of the carotid arteries and optocarotid recesses
are used as landmarks for identifying the sellar floor ([Fig. 5]).
Fig. 4 rostrally extending inactive pituitary macroadenoma
which was operated via a transplanum transtuberculum approach.
Preoperative contrast enhanced MRI sagittal a and coronal
b plane, c intraoperative supradiaphragmatic view
displaying both A2 arteries (A2) and in the anterior
interhemispheric fissure.
2.2.3.3 Sellar phase
After completing the sphenoid phase, the neurosurgeon and the ENT surgeon may
switch positions and the neurosurgeon continues to perform the surgical
procedure while the assistant surgeon drives the endoscope. We do not use an
endoscope holder during surgery as we feel a dynamic endoscopy may increase
the surgeons’ comfort and the flexibility during intrasellar dissection.
Bimanual dissection is performed. Opening of the sellar floor is performed
using a diamond drill. In cases were the sellar floor is thinned out a hook
or dissector may be used to open the floor. The extent of opening reaches
from the base of the sella to the planum sphenoidale and from one carotid
prominence to the contralateral carotid prominence. Care has to be taken not
to injure the carotid artery as in some occasions no bony coverage of the
carotid artery is present. After removal of the bone a straight sharp dura
incision at the basal part of the sella is performed. The dural opening is
enlarged cranially on both sides laterally using microscissors. Bleeding
from the intercavernous sinus or the cavernous sinus may be stopped with
Floseal or bipolar coagulation. Long straight formed instruments are used to
manipulate through the nose and the tip of the instrument should always be
visualized during each step of surgery. The size of the tumor and the
preoperative tumor location determines the dissection strategy. In
microadenomas the surgeon identifies the tumor according to the preoperative
MRI and a selective adenomectomy is performed. Identification of a
pseudocapsule may facilitate extracapsular dissection [54]. In macroadenomas tumor dissection
usually starts at the basal part of the tumor. Intratumoral debulking may
facilitate the identification of the pseudocapsule and allows subsequent
extracapsular dissection. With various angled curettes a stepwise bimanual
dissection in a caudal to cranial direction is performed. With the suction
in one hand and the curette in the other hand the surgeon may dissect with
only gentle traction while counter traction is applied, as it is usually
applied during transcranial microsurgical procedures, avoiding tearing of
neurovascular structures or the arachnoid at the sellar entrance. The medial
wall of the cavernous sinus has to be identified. Once the intrasellar tumor
has been removed, the suprasellar tumor has to be progressively removed. The
early identification of the diaphragm may prevent CSF fistula. Caution has
to be given to differentiate residual tumor from normal pituitary gland. The
gland is usually more yellow and differs from its consistency. If the tumor
removal is felt to be complete, the angled endoscope should be applied to
have a panoramic view into all directions in order to confirm that no
residual tumor is left behind. If a cavernous sinus infiltration is detected
on preoperative MRI [55] the tumor
resection may be continued by either just following the tumor with the
curette or by removing the medial wall of the cavernous sinus [56]. The main concerns of tumor
resection within the cavernous sinus are carotid artery injury and cranial
neuropathies. Differences in the extent of resections are described
depending on whether the tumor extends into the superior cavernous sinus
(Knosp 3A) and into the inferior cavernous sinus (Knosp Grade 3B) [57]. For tumors extending into the
superior cavernous sinus, rates of endocrine remission/gross-total resection
were significantly higher in grade 3A than in grade 3B pituitary adenomas
[57]. In Knosp Grade 1–3 adenomas,
histological assessment of the medial cavernous sinus wall confirmed
invasion in 93% of nonfunctional adenomas and in 83% of functional adenomas
and all tumors could be completely removed [58]. Safe techniques to maximize the extent of resection through
the medial CS wall while minimizing the risk of cranial neuropathy and blood
loss have been described by several groups [56]
[59]. If the principles
of gentle dissection under constant visualization are applied patients with
secreting adenomas (GH- adenomas and ACTH- adenomas) may benefit from the
resection of medial cavernous sinus tumor extensions without increased
morbidity or mortality. If tumors extend into the lateral cavernous sinus
compartments, adjuvant treatment strategies (e. g. radiosurgery) may be
necessary and resection should be performed by experienced surgeons.
2.2.3.4 Closure and reconstruction techniques
According to the size and the growth direction of the pituitary adenoma, the
likelihood of an intraoperative CSF fistula, especially a high flow, should
be considered in advance. Pituitary adenomas with anterior growth direction
and/ or a very large suprasellar tumor extension carry the risk of
intraoperative CSF leak, which may require special reconstruction.
Therefore, a strategy for proper closure techniques has to be considered
before surgery starts. Sometimes it is easier to dissect the pedicled
mucochondroperiostial flap during the nasal stage of the operation,
especially in patients with abnormalities of the nasal septum, rather than
harvesting a rescue flap after resection of a very large suprasellar
pituitary adenoma. Preparation and draping of the sites were closure
material may be harvested (abdominal fat, fascia lata) is best performed at
the start of surgery rather than at a later phase of surgery. Fascia lata is
a very robust and safe dural reconstruction material, which may be used as
inlay or onlay graft either as autologous transplant or as commercially
available material. Fat should be used to fill large dead spaces and bony
material, which was removed during the approach, supports the reconstruction
if it is lodged against the surrounding bone.
2.2.4 Transplanum-tuberculum approach
Rostrally extending suprasellar lesions ([Fig. 4]) and large suprasellar tumors growing toward or in the
third ventricle may not be accessible through transsellar approach.
Extension of the surgical corridor through the planum sphenoidale and
tuberculum sellae have been shown to allow appropriate dissection of those
lesions and have advantages over transcranial surgery obviating cosmetic
deformity and brain/chiasmal retraction. The rostral extension via
endoscopic endonasal transsellar-transtubercular-transplanum approaches can
provide a safe and feasible route for suprasellar lesions, in subchiasmatic,
suprachiasmatic, and intraventricular regions [60]. Indications for a transplanum
transtuberculum approach are large and rostrally extending pituitary
adenomas, craniopharyngiomas, meningiomas of the planum and tuberculum
sellae. Patient positioning is similar to the position for the transsellar
approach with sharp fixiation in the Mayfield clamp and image guidance is
used for intraoperative validation of anatomical structures. Check
preoperative CT scan for sphenoid sinus anatomy as a conchal type of sinus
may require extensive bone drilling. In selected cases intraoperative
monitoring using SSEP's and VEP's and monitoring of the
oculomotoric cranial nerves may be helpful. The intraoperative use of a
micro doppler is recommended to identify the carotid and ophthalmic artery
and avoid vascular complications. The approach may be limited by medialized
(kissing) carotids with reduced intercarotid distance.
After performing a sellar approach, the bone overlying tuberculum and planum
is drilled using a diamond 4-mm burr. Bone drilling is performed rostrally
up to the level of the posterior ethmoidal arteries. The olfactory apparatus
should be spared and care has to be applied to avoid injury. Important
landmarks during surgery are the carotid protuberance, the medial and
lateral OCR, the middle clinoid process, the clival recess, the optic nerve,
the lateral tubercular crest, the lateral tubercular recess and the distal
osseous arch of carotid sulcus [60]
([Fig. 5]). Bone drilling is done
to egg shell to planum und tuberculum and careful bone removal is performed
using either dissecting instruments or a 1mm 45-degree Kerrison punch. Dural
integrity is ensured at all times. Afterwards the bone overlying the optic
nerve above and lateral to the sella and the tuberculum is removed using
2-mm and 4-mm diamond drills. Continues irrigation with saline is of
importance to avoid thermal damage to the optic nerve. Be aware, sometimes
the ICA is with a very thin or even no bony coverage. A strait midline
durotomy is performed from caudal to cranial to the level of the superior
intercavernous sinus. Than the dura incision in continued above the
intercavernous sinus. The intercavernous sinus is cauterized using bipolar
forceps and sharply divided. If still venous bleeding occurs hemostatic
agents (e. g. Floseal) als Verweis auf das Produkt is used to achieve
sufficient hemostasis. The dura incision of the planum is slightly curved
laterally to both sides and the dural flap is mobilized laterally to
identify the carotid artery with its origin of the ophthalmic artery before
further extending the incision laterally. The arachnoid of the subchiasmatic
cistern is carefully dissected. Care has to be applied to avoid injury to
the superior hypophyseal artery branches supplying the chiasm ([Fig. 6]). Tumor removal is performed
in bimanual dissection procedure as it is performed by transcranial surgery.
The assistant, who has to closely follow the dissection steps and helps to
identify neurovascular structures, drives the endoscope. Central tumor
debulking enables a stepwise extracapsular dissection. Identification of the
pituitary stalk and preservation is of importance. Laterally the
3rd nerve may be identified. If the dissection is performed
backwards the basilar tip with the ventral brainstem, both posterior
cerebral arteries and supracerebellar arteries are encountered ([Fig. 7a–c]). If the lesion extends
into the third ventricle a very careful dissection of the ventricular wall
and the hypothalamus has to be performed ([Fig. 7d]).
Fig. 5 bony exposure transplanum transtuberculum approach.
IMpartant landmarks are both carotid promineces, the medial and
lateral OCR, the tuberculum, the planum. ON optic nerve, lOCR
lateral optocarotid recess, CP carotid prominence, mOCR medial
optocarotid recess, ICA internal carotid artery.
Fig. 6 intradural exposure after performing a transplanum
transtuberculum approach for a suprachiasmatic approach to a lesion
within the third ventricle. C-Chiasm, PS-pituitary stalk, PC
posterior clinoid process, ACoA- anterior communicating artery
complex, LT- lamina terminalis.
Fig. 7 preoperative contrast enhanced sagittal a and
coronal b MRI depicting a recurrent craniopharyngioma after
previous transcranial surgery and radiation therapy with extension
towards the 3rd ventricle, c intraoperative view
on the basilar tip with the left posterior cerebral artery and the
dissection of the tumor capsula from the right posterior artery,
left supracerebellar artery and the left oculomotor nerve, d
view into the 3rd ventricle after tumor removal
displaying the choriod plexus, E postoperative sagittal MRI with fat
graft for obliteration of the dead space.
A suprachiasmatic dissection may be necessary in lesions with suprachiasmatic
extension. Here the anterior communicating artery complex should be
identified and injury to the arteries has to avoided. Be aware of the
recurrent arteries (e. g. artery of Heuber) ([Fig. 6]).
The closure phase may be particularly challenging, due to the big dimension
of the defect and high CSF cisternal flow. In cases of transplanum,
transtuberculum approach, a pedicled mucochondroperiostial flap is usually
elevated during the nasal stage of the operation. Additionally, preparation
and draping of the sites where closure material may be harvested (abdominal
fat, fascia lata) is performed at the start of surgery. A lumbar drain is
placed before surgery starts and is opened during surgery when the bone is
drilled and the dura is incised. A 3-layer reconstruction is usually
performed. Fat is placed to fill the dead space. Fascia lata is a very
robust and safe dural reconstruction material. It can be used as an inlay
plastic and it is placed under the dural rims inside the dura and fixed with
a thin layer of fibrin glue. Then a bony reconstruction is performed and
when possible, the grafted bone from the sphenoid septae or the nasal septum
is lodged against the surrounding bone. If available, a fascia lata piece is
placed over the defect as overlay and the vascularized flap is placed to
cover the whole defect. For the reconstruction of the nasal septum, the
reverse flap and silicon airway nasal splints are used to enhance the
healing of the nasal mucosa.
2.2.5 Transclival approach
Indications for the transclival approach are sellar lesions with caudal
expansion or infrasellar lesions with rostral extension, such us pituitary
adenomas, chordomas or chondrosarcomas Selected cases of intradural
pathologies of the posterior fossa may also be an indication for transclival
approach (eg. (petro)clival meningiomas).
Surgical positioning and preparation is similar to the transplanum,
transtuberculum approach. Image guidance should be applied and a binostril
dissection technique is recommended. After performing the resection of the
anterior wall of the sphenoid sinus in a standardized way using a 4mm coarse
diamond drill the floor of the sphenoid sinus has to be exposed. To gain
access to the caudal parts of the lesions the choana should be drilled and
the sphenoid floor should be thinned to improve the maneuverability of the
instruments according to the borders of the lesion by applying image
guidance.
Key landmarks for the transclival approach are the sellar floor and the
transclival ACI at both sides ([Fig.
8]). The anatomy of the sphenoid sinus must be checked on
preoperative CT, hence the degree of pneumatization of the sphenoid sinus
may help to evaluate the amount of bone drilling.
Fig. 8 intraoperative view of the mid clivus with both
paraclival carotid arteries, S- sella, PC paraclival carotid artery,
C clivus.
The transclival approach can be divided into upper, middle, and lower [61]. For the upper approach, drilling
of the sellar face is performed. The sellar dura mater consists of two
layers. The outer or periosteal layer spans between the anterior surface of
the gland and the parasellar and paraclival ICA, thus forming the anterior
wall of the cavernous sinus. The inner/meningeal layer covers the capsule of
the gland, at the same time forming the medial wall of the cavernous
sinus.
The upper clival approach provides midline access to the interpeduncular
cistern, the basilar apex, the mammillary bodies, and the floor of the third
ventricle. Access to the interpeduncular cistern and midbrain behind the
upper clivus usually requires posterior clinoidectomy, which can be
unilateral or bilateral depending on the pathology. Removal of the posterior
clinoids most of the times requires transposition of the pituitary gland,
which can be done extradurally, interdurally, or intradurally [61]. The interdural pituitary
transposition offers a transcavernous sinus approach to paramedian lesions
within or behind the parasellar space, such as para/retro/suprasellar
extension of chordomas or chondrosarcomas. This approach requires
mobilization or sacrifice of the ipsilateral inferior hypophyseal artery
[61].
A middle transclival approach provides access to the ventral pons and the
prepontine cistern, the basilar trunk and anterior inferior cerebellar
artery, as well as the cisternal segment of the abducens nerve ([Fig. 9]). The sphenoidal clivus is
limited laterally by the paraclival ICAs, and the petroclival fissure.
Laterally, the middle transclival exposure is limited by the interdural
segment of cranial nerve VI.
Fig. 9 mid transclival approach for resection of a clival
meningioma. Preoperative sagittal a and transversal b
contrast enhanced MRI and preoperative CT scan which demonstrates
the relation of the sphenoid sinus to the meningioma c,
d intraoperative view on the ventral pons, ** basisilar
artery, + vertebral artery, postoperative sagittal e and
axial f MRT showing complete resection and postoperative CT
scan demonstrating the bony reconstruction of the clivus
g
The lower transclival approach through the lower segment of the clivus, which
lies below the roof of the choana, exposes the premedullary cistern and
ventral medullary surface, the vertebral arteries, vertebrobasilar junction
and posterior inferior cerebellar arteries, as well as cranial nerves IX–XII
[61]. The lower transclival
approach may be applied to lesions of the craniocervical junction and to
address pathologies below the foramen magnum to the level of the odontoid.
The lower limit of an endonasal lower transclival approach is determined by
the hard palate and the nasopalatine line may be used to estimate the lowest
extent of resection.
2.3 Coronal approaches
2.3.1 Medial maxillectomies, pterygopalatine and infratemporal
fossae
Coronal approaches offer access to skull base lesions with lateral extension.
Different subsites can be considered surgical target in case of direct
involvement, as well as “doors” to be crossed to reach posterior lesions. In
this view, the anatomical structures of major interest are the maxillary
sinus, the pterygoid process, the orbit, the sphenoid walls, the petrous
bone, and the occipital condyle.
The maxillary sinus gives access to the pterygopalatine and infratemporal
fossae posteriorly, and the approaches encompassing the endoscopic
disassembling its medial wall are usually referred as endoscopic medial
maxillectomies. These can be classified into 4 types, going from a less
invasive to a more invasive resection, following a modular dissection [62].
Type A endoscopic medial maxillectomy expects an inferior uncinectomy and has
as anatomical boundaries the insertion of the inferior turbinate inferiorly,
the orbital floor superiorly, the descending palatine canal posteriorly, and
the nasolacrimal duct anteriorly. In Type B endoscopic medial maxillectomy,
the inferior turbinate is removed, sparing the Hasner’s valve. The portion
of the medial wall behind the nasolacrimal duct is completely removed,
reaching the floor of the nasal cavity. With Type C maxillectomy, the
nasolacrimal duct and the remaining anterior portion of the medial wall are
resected. Type D endoscopic medial maxillectomy, also referred as Denker’s
or Sturmann-Canfield operation, expects the removal of the anteromedial
angle of the maxillary sinus, with the infraorbital foramen as lateral
limit. The different procedures offer incremental grades of endoscopic
exposure, in medio-lateral and cranio-caudal direction. In particular, type
D medial maxillectomy provides complete access to the posterior and lateral
walls of the sinus, orbit floor, and alveolar and zygomatic recesses. A
variation of these procedures is the prelacrimal approach, which consists in
the removal of the medial bony edge of the piriform aperture, preserving the
lacrimal duct which is displaced medially and repositioned at the end of the
procedure [63]
[64]
[65]
[66]
[67]. This technique reduces the risk of
lacrimal stenosis and allows a similar endoscopic view as Type C and Type D
maxillectomies, while providing limited working volume and postoperative
surveillance.
Once the posterior walls of the maxillary sinus are exposed, infracranial
spaces such as the pterygopalatine and infratemporal fossae can be
controlled. The first is a narrow cavity whose limits are the pterygoid
process posteriorly, the posterior wall of the maxillary sinus anteriorly,
the perpendicular process of the palatine bone anteromedially, and the
coronal plane connecting the lateral pterygoid lamina and the posterior wall
of the sinus (pterygomaxillary fissure), laterally. It is a crossroad
between the sinonasal cavity, the infratemporal fossa, and the orbital
cavity, containing many neurovascular structures: V2 and pterygopalatine
ganglion, along with their branches, and the pterygopalatine tract of the
internal maxillary artery with its collateral vessels.
A wide variety of lesions can develop from the pterygopalatine fossa, mainly
represented by juvenile angiofibromas [68]
[69], vidian and
maxillary nerve schwannomas [70]
[71]
[72]
[73], fibro-osseous
lesions, and hemangiomas [74].
The opening of the fossa starts with the removal of the posterior wall of the
maxillary sinus and the perpendicular process of the palatine bone, paying
attention not to damage the periosteum. Usually, the dissection is started
at the sphenopalatine foramen. The lateral limit of the fossa is found
following the infraorbital canal reaching the posterior wall of the
maxillary sinus posteriorly [75]. The
entrance points the pterygomaxillary fissure. The periosteum is then incised
and carefully removed with the underlying fat to expose progressively the
pterygopalatine tract of the internal maxillary artery and its collaterals,
the nervous structures placed in front of the pterygoid process, and the
cranial insertion of the lateral pterygoid muscle on the homonym bony lamina
and great wing of the sphenoid. Posteriorly, the vidian nerve and foramen
rotundum are exposed once the content of the fossa is removed or transposed
laterally [76].
The infratemporal fossa is an anatomical space whose boundaries are the
greater wing of sphenoid bone and the squamous portion of the temporal bone
superiorly, the lateral pterygoid plate and the maxillary tuberosity
medially, the zygomatic arch, temporal fossa, and the mandibular ramus
laterally, and the upper parapharyngeal space posteriorly. It communicates
with adjacent spaces: the temporal fossa laterally and the pterygopalatine
fossa medially [77]
[78]. Due to these close relationships,
lesions arising outside the infratemporal fossa can invade it, typically
benign lesions from the pterygopalatine fossa. Lesions directly originating
from the fossa can spread externally too, like V3 schwannomas and sarcomas
[79]. Three dissection corridors
within the infratemporal fossa have been described [80]. The lateral corridor exposes the
coronoid process through the temporal muscle bellies. The middle corridor
reaches the front of the temporomandibular joint, using the space between
the temporal and lateral pterygoid muscles. The medial corridor runs between
the lateral pterygoid muscle and the lateral pterygoid plate, exposing the
mandibular nerve, middle meningeal artery, and the temporomandibular
joint.
When dealing with pterygopalatine and infratemporal fossae lesions, the
vascularization of the tumor must be considered. In fact, juvenile
angiofibromas [69] need preoperative
embolization for a safe and complete removal. Therefore, as with almost all
clinical entities involving the skull base, multidisciplinary management is
crucial, with surgery performed in referral centers equipped with all the
necessary facilities [81] ([Fig. 10]).
Fig. 10 Case of recurrent inverted papilloma of the maxillary
sinus, with anterior attachment (images a and b).
Image b shows a defect of the anterior wall of the maxillary
sinus (white arrow), due to previous transvestibular surgeries. A
type D endoscopic maxillectomy, encompassing the removal of
the anteromedial angle of the maxillary bone was performed. Image
c shows the premaxillary soft tissues (PMT), partially
removed for oncologic reasons, and the complete exposure of the
maxillary sinus. The white arrow indicates the alveolar recess, the
arrow points the zygomatic recess. The star in image d
indicates the lacrimal sac, sectioned, with the lacrimal stent
passing through.
2.3.2 Transpterygoid approaches
The dissection of pterygoid process of the sphenoid provides access to
several anatomical areas. The upper transpterygoid approach allow reaching
the sphenoid floor medially, the greater wing of the sphenoid bone
laterally, the superior orbital fissure superiorly, and the scaphoid fossa
inferiorly. The lower transpterygoid approach gives exposure of the base of
the pterygoid process superiorly, the lateral pterygoid muscle laterally,
the pterygomaxillary junction inferiorly, and the nasopharynx medially. The
transpterygoid approaches exploit the corridor provided by the maxillary and
sphenoid sinuses; thus, a medial maxillectomy (usually type B-C) and
sphenoidal opening (usually via a transrostral approach) are usually
necessary propaedeutical phases.
Upper transpterygoid approach
The upper transpterygoid approach is usually performed to deal with
lesions involving the lateral aspect of the cavernous sinus and Meckel’s
cave (typically pituitary tumors, meningiomas, and neurogenic tumors),
as well as the lateral recess of the sphenoid sinus (typically
meningoceles). The approach encompasses the removal of the
posterior-medial wall of the maxillary sinus and the orbital process of
the palatine bone to expose the contents of the pterygopalatine fossa.
Once the neurovascular network is dissected, the contents of the fossa
are displaced laterally or removed, based on oncologic needs. The vidian
nerve can either be preserved or sacrificed, and it can be used to
identify the internal carotid artery at the genu, posteriorly. The
inferior orbital fissure can also be dissected, providing an advantage
in exposing the mandibular nerve, from the gasserian ganglion to the
infratemporal fossa [82]
[83]
[84]. Drilling the base of the pterygoid, in combination with
the removal of the anterior wall of the sphenoid sinus, allows for
exposure of the lateral wall of the sphenoid sinus as well as its
lateral recess. Based on the pathological scenario, the approach can be
combined with a classic transsellar approach, extended laterally.
The anatomy of the area is particularly complex, making surgery
especially challenging. Specific technological tools assist the surgeon.
When dealing with the cavernous sinus, venous bleeding must be
controlled with hemostatic agents, and the use of the Doppler allows for
the correct detection of the internal carotid artery. Neuromonitoring is
mandatory to minimize morbidity related to cranial nerves, especially
the III, IV, and VI, in cavernous sinus dissection.
Lower transpterygoid approach
The dissection of the caudal portion of the pterygoid process gives
access to the upper portion of the parapharyngeal space. This is an
inverted pyramid shaped volume [77], whose boundaries are the prevertebral space posteriorly,
infratemporal fossa and submandibular fossa anterolaterally, the parotid
space along with nodal levels IB and IIA posterolaterally, and the
nasopharynx and oropharynx medially [85]. It can be divided into upper, middle, and lower portions
based on horizontal axial planes passing through the lower border of the
lateral pterygoid lamina and the mandibular angles [85]
[86]. Furthermore, it can also be segmented in prestyloid and
retrostyloid compartments, by a plane passing through the styloid
process [77]. The area of major
clinical interest which can be targeted endoscopically is the upper
parapharyngeal space, further divided in medial and lateral, with the
corresponding surgical corridors. The medial approach provides control
of the eustachian tube and associated muscles, while the lateral one
allows a correct visibility of the internal carotid artery and jugular
foramen, and associated neurovascular structures.
Typical lesions involving the area are nasopharyngeal cancers.
Undifferentiated nasopharyngeal carcinomas are usually addressed
endoscopically in case of recurrence after (chemo)-radiotherapy. For
other histologies, like salivary gland cancers and melanomas, endoscopic
surgery may represent the primary treatment option, followed or not by
adjuvant treatment [87]
[88]
[89]
[90]
[91].
The nasopharyngeal endoscopic resection (NER) is the transnasal approach
designed to resect nasopharyngeal lesion involving the upper
parapharyngeal space [87]. The
main surgical challenge is to avoid injuries to the parapharyngeal tract
of the internal carotid artery, while achieving free surgical margins.
When performing NER, it is crucial to recognize bony landmarks,
understand fascial planes, adopt the correct endoscopic perspective, and
utilize preoperative imaging and navigation tools to locate and preserve
the internal carotid artery. The procedure starts with the exposure and
lateralization of the content of the pterygopalatine fossa via a
transmaxillary approach. The medial pterygoid lamina is removed to
expose the medial pterygoid muscle, tensor veli palatini and levator
veli palatini muscles, and eustachian tube. The internal carotid artery
lies behind the tube, pointed by the lateral pterygoid lamina. The
vidian nerve represents a useful landmark for the foramen lacerum. The
dissection of tissues medial and inferior to it, is safe from injuries
of the carotid, except for cases of prominent kinking.
In extended cases, the trans-nasal route is combined to other approaches
(i. e. transoral, transorbital, transcervical, and transpetrosal),
integrated in a multiportal surgical strategy [92] ([Fig. 11]).
Fig. 11 Case of recurrent nasopharyngeal carcinoma. The
primary lesion underwent radio-chemotherapy. The patient
developed recurrent disease on the right Rosenmuller’s fossa
(arrows in image a-b and star in image c).
The patient underwent endoscopic nasopharyngectomy type 3, with
the sectioning of the Eustachian tube (white arrow in d).
MS – Maxillary sinus; NS – Nasal Septum; SpS – Sphenoid
Sinus.
