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Retrosigmoid–Transclival Approach
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Pituitary protuberance
ICA bulge
Upper clivus
a
Fig. 28.2 Endoscopic endonasal cadaveric dissection with a 0-degree scope. The posterior wall of sphenoid sinus is shown. Below the
pituitary protuberance and between the two paraclival internal carotid artery bulgings, the upper clivus can be observed.
Lower clivus
C1
a
Fig. 28.3 Endoscopic endonasal cadaveric dissection with a 0-degree angled scope. (a) After skeletonization of the mucosa and
parapharyngeal muscles, the inferior clivus is exposed. (b) After drilling of the fl oor of sphenoid sinus, and partially drilling of the
inferior clivus, the anterior dura of the posterior fossa is exposed. (c) After removal of the dura, the ventral structures of the posterior
fossa are exposed.
ICA bulge
ICA ICA
b
VI CN
b
Upper clivus
ICA
Upper clivus
Middle clivus
Lower clivus
Pituitary gland
ICA
Pituitary gland
III CN III CN
c
ICA
VI CN
c
Basilar artery
Basilar artery
ICA
VI CN
Vertebral a
is represented by the vidian nerve, which points out the
genu between the petrosal and paraclival arteries and
then allows identifying a safe area between the two ICAs
(Fig. 28.4). Intradurally, the middle clivus corresponds to
the pons, the basilar trunk, anterior inferior cerebellar arteries, and the cisternal portion of CN VI (Fig. 28.3c). Finally, the inferior third of the clivus can be identified detaching the rhinopharyngeal mucosa and muscles ( longus
capitis and rectus capitis anterior), below the floor of the
sphenoidal sinus (Fig. 28.3a). This route gives access to
the lower part of the clivus and the craniovertebral junction till, usually, the body of C2. Indeed the lowest point
that can be reached is depicted by an imaginary line passing through the anterior border of nasal bone and the
inferior border of the hard palate.
18
The surgical landmarks
are represented by the inferior turbinate and posteriorly
by the choana, while the eustachian tube is of crucial
relevance to point out the parapharyngeal tract of internal
carotid artery. Thus, the detachment of the rhinopharyngeal mucosa and the parapharyngeal muscles should be
performed medially to the tube to avoid injuries to the
vessel. This approach gives access to a trapezoid space,
wider superiorly and limited by the condyles inferiorly.
The main anatomic landmark of the condylar area is
represented by the hypoglossal canal, which divides the
inferolateral portion of clivus in two segments:
• The superior or tubercular compartment represents
the ventral portion of jugular tuberculum. Indeed, its
lateral limit is represented by the medial portion of
VI
Sympathetic
V2
Vidian
Fig. 28.4 Endoscopic endonasal cadaveric dissection with a
30-degree scope. The course of the vidian nerve is illustrated after
opening of the pterygoid canal. The nerve points at the passage
between the petrosal and paraclival internal carotid artery.
ICA
Clivus
the jugular foramen. Intradurally, it corresponds to
the tracts of CN IX, CN X, and CN XI toward the jugular
foramen and medially to the vertebral artery.
• The condylar compartment is composed by the con-
d
yle itself. Inferiorly limited by the articular surface and
laterally by the CN XII. Intradurally, it corresponds to
first segment after the piercing of the dura of the vertebral artery, which is crossed anteriorly by the cisternal
segment of the hypoglossal nerve.
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To perform the endoscopic endonasal approach to the clivus, we prefer that the patient is placed in a semisitting
position, with the thorax slightly elevated on the operating table. An orotracheal intubation is needed and the
laryngopharynx is packed with gauzes to avoid blood and
fluid leakage. Routinely, we use a neuronavigation system
(StealthStation S7 MEDTRONIC), based on a computed
tomography angiogram (CTA), processed through the
StealthMerge software (MEDTRONIC).
The surgical approach should be selected depending
on the localization of the lesion in the upper, middle,
or lower clivus and on its lateral extension. For cases
limited to the upper clivus, we start displacing the middle and inferior turbinates laterally. When necessary, the
tail of the latter is resected to widen the surgical corridor.
Surgery is performed through both nostrils and with a
four-hand technique, after detaching of the vomer from
its insertion and resecting the posterior aspect of the septum. Afterward, we perform an anterior sphenoidectomy,
and the posterior wall of the sphenoid sinus is exposed
(Fig. 28.5a). Except in presellar or conchal types of sphenoid sinus, for which a neuronavigation system is mandatory, the upper clivus can be easily identified and accessed
through its anatomic landmarks in the posterior wall of
the sphenoidal sinus (Fig. 28.6a). The dorsum sellae lies
behind the pituitary gland; thus, the approach to this
superior part of the upper clivus might require a pituitary
gland transposition. The latter consists in a superior
hypophysopexy,
the floor and the dorsum of the pituitar y fossa. Chordomas
can usually be identified after drilling the upper
clivus as much as necessary, and eventually the dorsum
sellae (Fig. 28.7), while intradural neoplasias require the
opening of the dura wall. For extradural tumors in this
segment of the clivus, the lateral limit of this approach
is represented by the paraclival carotid artery, while
for intradural extension this limit is given by a sagittal
plane passing at the level of the CN III, which cannot
be trespassed to avoid injury to the nerve. To extend
this approach more laterally, an ethmoid- pterygoidsphenoidal route is necessary. For this, an ethmoidectomy, followed by a medial maxillectomy and drilling of the
19
which permits to progressively expose
tip of the vertical process of the palatine bone and of the
medial pterygoid process, is performed. This route allows
facing frontally the cavernous sinus and reaching its compartments lateral to the carotid artery. We adopt this
extension for paramedian tumors with an extensive cavernous sinus involvement, in particular when the tumor
occupies the anteroinferior and/or lateral compartments
or encases the internal carotid artery. The lateral limit of
this extension is given by the lateral wall of cavernous
sinus, where CN III, CN IV, and CN V1 run. To expose the
middle clivus, we perform an inferior extension of the
approach to the upper clivus by drilling off the sellar floor
between the paraclival carotid (Fig. 28.5b). Anatomic
landmarks, eco-Doppler, and neuronavigation system are
routinely adopted to avoid injuries to the internal carotid
artery (Fig. 28.6b, c). Once the dural plane has been
exposed, its opening to address intradural tumors should
be performed carefully to avoid injuries to the CN VI
(Fig. 28.4c). This nerve pierces the dura medially to the
carotid artery above the level of the vertebrobasilar
junction. Many techniques have been proposed to avoid
damages to the CN VI, including intraoperative electrophysiologic stimulation of the dura to recognize its
location, as well as eco-Doppler and/or neuronavigation
to localize the vertebrobasilar artery and thus start opening the dura inferiorly. For extradural lesions, the main
limitation of this approach is given by the paraclival
carotid artery, while for intradural tumors, a sagittal
plane passing through CN VI represents the lateral limit of
this route. Often, tumors of the middle clivus can spread
to the surrounding regions, such as the pterygopalatine of
the infratemporal fossa. To follow the tumors extending
to these areas, a transmaxillo-pterygoid approach is useful. It requires a medial maxillectomy (or an anterior and
medial maxillectomy according to Denker for tumors located more laterally), followed by the opening of the posterior wall of maxillary sinus and drilling of the vertical
process of the palatine bone and of the pterygoid process.
In this approach, particular care should be paid to the
management of the internal maxillary artery, which usually requires a clipping, to avoid intraoperative or postoperative bleeding. The posterior limit of this approach
a b
Fig. 28.5 Intraoperative view. (a) The upper clivus is visible at the posterior wall of sphenoid sinus. (b) B: Drilling of the fl oor of the
sphenoid sinus to expose the middle clivus.
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Fig. 28.6 Navigated scan and
intraoperative view. Three possible
uses of neuronavigation in endoscopic
endonasal approach for posterior
fossa are depicted. (a) Presellar
variation in a pediatric patient with an
upper clivus chordoma.
(b) Localization of the left parasellar
internal carotid artery.
(c) Confi rmation of anatomic
landmarks, such as the left pterygoid
canal.
a
b
c
is given by V3, which is the landmark for the petrosal
portion of the carotid artery, running behind the nerve.
To reach the lower clivus, a further inferior extension of
the middle clivus approach is necessary. Sometimes, for
lesions located far inferiorly, we prefer to retract the soft
palate with two thin rubber nasogastric probes inserted
through the nose and extracted through the mouth. This
maneuver is performed at the beginning of the surgery.
Many techniques to skeletonizing the rhinopharynx
mucosa and dissecting the muscular planes have been
proposed. We prefer an inverted U-shaped flap from
one Rosenmüller fossa to the other. Before surgery, an
angio-CT scan to study the location of the parapharyngeal
carotid artery is of great help and can be recommended. It
is not uncommon to observe a midline loop of the carotid,
which could represent a major caveat for this surgery. The
neuronavigation system and eco-Doppler are valid tools
to identify the vessel while skeletonizing the lower clivus
(Fig. 28.6b). As in the other segments of the clivus, the carotid artery in its parapharyngeal tract represents the main
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lateral extradural limit of this approach, while for intradural lesions, this limit is represented by the plane passing
for the CN IX, X, XI at the jugular tuberculum level and CN
XII at the condylar level. To expand this access laterally,
some authors propose a transcondylar and/or transjugular
tuberculum approach (so-called “far medial”).
sists in the removal of condylar and tubercular regions,
respectively. The transcondylar approach allows maximizing the exposure of the foramen magnum area. The
transjugular–tuberculum approach allows obtaining an
exposure of the medial border of the jugular foramen.
Once the approach has been allowed to face the tumor
as frontally and straightforward as possible, its removal
is performed through the usual microsurgical technique:
the lesion is dissected bimanually from the surrounding
anatomic planes (we prefer to fix the endoscope on the
holder to allow both surgeons to operate with four hands),
and avoiding tractions, it is progressively debulked and
finally resected (Fig. 28.7b–d). When it presents a soft
consistency, tumor removal can be progressively performed with suction, following the chordomas’ typical
11,12
It con-
“termite-like” erosion of the clivus (Fig. 28.7b). Conversely, for tumors with a hard consistency, curettes and dissectors are useful, and for calcified tumors, a diamond
drill can be necessary. Debriders are useful to reduce the
tumor mass, but it should be used carefully, after identifying the vessels and nerves in the surgical field. We do
not suggest using debriders intradurally to avoid an injury
to arteries or nerves. Here, the CUSA (ultrasound) can be
used carefully to debulk the tumor.
The relationship of the tumor with the dura is of crucial importance for the surgical strategy. When there is
no invasion, the tumor should be dissected by this layer
and then resected. In case of extensive dura invasion
by a chordoma for example, it is not always possible
to resect the tumor completely because of their invasiveness. When the dura is trespassed by the tumor, it
is possible to follow its extension, entering in the posterior fossa. If the dural dimple is of small dimension,
a bigger opening can be performed to visualize all the
tumor extension. Furthermore, the tumor should be
carefully dissected from nearby vascular, nervous, and
a b
cd
Fig. 28.7 Intraoperative view. (a) Drilling of the upper clivus. (b) The tumor is visible. The soft consistency allows its resection by
means of the suction. (c) The tumor has an intradural invasion; the dissection from the midbrain is shown. (d) Final exploration of the
surgical cavity to demonstrate the complete removal.
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parenchymal structures, avoiding any tractions, which
could lead to injuries (Fig. 28.7c). Especially for lesions
involving or compressing the brainstem, an electrophysiologic monitoring of somatosensory-evoked potentials and motor-evoked potentials is recommended
to monitor the brainstem.
Cerebrospinal fluid (CSF) leaks need to be repaired. We
usually use mucoperiosteum from middle turbinate and
fascia lata, positioned in a multilayer fashion. The fascia
lata is placed intracranial intradurally and the mucoperiosteum externally. Abdominal fat is suitable to fill the “dead
space” between both, if needed. In case of approaches to
the lower clivus, the U-inverted muscular and mucosal
flap from the nasopharynx can be repositioned with some
stitches to repair the CSF leak. The nasal cavity is filled with
Gelfoam and Merocel is kept in place for the first 3 days.
28.2.2 Retrosigmoid Approach
The patient can be placed in a sitting or supine position, depending on tumor extension, anesthesiological
comorbidities, and surgeon’s preference. The head is
tilted toward the surgeon. The skin incision should be
based on some anatomic landmarks: the tip of mastoid
process and the inion (Fig. 28.8a, b). The transverse sinus
can approximately be represented by an imaginary line
passing through the zygomatic arch and the inion. The
skin incision is C-shaped or slightly anteriorly bent. It is
placed around 4 cm posteriorly to the mastoid process.
The sternocleidomastoid muscle is usually raised with
the skin, while the remaining suboccipital muscles (splenius capitis, semispinalis capitis, longissimus captious,
and rectus capitis) are split in the direction of the fibers
(Fig. 28.8c). The occipital artery runs deep into the digastric
groove. It can be coagulated or ligated to avoid blood loss.
The transverse–sigmoid junction is marked by a constant
emissary vein (Fig. 28.8d). A burr hole is placed just po-
steroinferior to the presumed location of sigmoid–transverse junction, after which a craniotomy or craniectomy
can be performed (Fig. 28.9). Small tears in the transverse
sinus can be controlled by Surgicel packing or cottonoids.
The complete dural exposure should extend from edge of
transverse sinus to the posterior fossa floor and laterally
to the sigmoid sinus. The dura is incised with a C-shaped
cut and opened posteroinferiorly to protect the cerebellum (Fig. 28.10). Immediately after this opening, the
lateral cerebellomedullary cistern is opened to drain CSF
and relax the brain. So far, it is possible to progressively
retract the cerebellum exposing the CPA, the brain stem,
Splenium capitis
a
Sternocleidomastoid
Longissimus
capitis
Splenium
capitis
c
Fig. 28.8 Cadaveric dissection. (a, b) After a linear skin incision, the splenium captis muscle is visible. (c, d) After incision of splenium
capitis, the inner muscular layer is dissected and the occipital artery and emissary vein are shown.
Splenium
capitis
Emissary vein
b
Sternocleidomastoid
Splenium capitis
Longissimus capitis
Semispinalis capitis
d
Digastric sulcus
Mastoid tip
Emissary vein
Occipital artery
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Emissary vein
Digastric sulcus
Occipital artery
(cut)
Asterion
Occipito-mastoid suture
Parieto-mastoid suture
Labdoid
suture
Mastoid tip
Digastric sulcus
Emissary vein
a
Fig. 28.9 Cadaveric dissection. (a) The landmarks for the retrosigmoid approach are dissected (digastric sulcus, mastoid tip, and
emissary vein) and the asterion region is showed. (b) A burr hole is placed just medially and inferiorly to the junction between sigmoid
and transverse sinus.
Digastric sulcus
Sigmoid sinus
a
Fig. 28.10 Cadaveric dissection. (a, b) The burr hole is enlarged by a high-speed drill and a Kerrison rongeur to expose the
transverse and sigmoid sinus. (c) After dural opening, the cerebellum is visible.
Mastoid cell
Transverse-
Tra nsv er se
XI CN
Sigmoid
junction
sinus
IX CN
X CN
b
Foramen lacerum
VII-VIII CN
b
Sigmoid sinus
Digastric
sulcus
c
XI CN
X CN
Transverse sinus
IX CN
VII CN
VIII CN
a
Fig. 28.11 Cadaveric dissection. (a, b) After cerebellum retraction, the 9th–11th cranial nerves (CN) and the 7th and 8th CNs are
visible.
a
Fig. 28.12 Cadaveric dissection. (a,b) After cerebellum retraction, the fi fth cranial nerve and tentorium can also be observed.
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VII-VIII CN
Flocculus
V CN
Dandy’s vein
Tentorium
b
V CN
SCA
IV CN
PCom artery
Tentorium
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VI CN
Petrous bone
VII-VIII CN
Petrous bone
SCA
IV CN
V CN
III CN
PCom artery
b
Tentorium
IX CN
a
Fig. 28.13 Cadaveric dissection. (a,b) The course of the sixth cranial nerve is visible, pointing toward the Dorello canal in the clivus.
(c) The vertebral artery is showed.
V CN
a
Fig. 28.14 Cadaveric dissection. Endoscopic view with a 30-degree scope. (a) The cisternal portion of the fi fth cranial nerve can be
seen. (b) The cerebellar-pontine angle is explored.
XI CN
X CN
b
IX CN
VI CN
VIII CN
c
Internal acustic meatus
XI CN
Petrous bone
Porus acusticus
VII- VIII CN
X CN
IX CN
Vertebral arter y
V CN
VIII CN
and the CNs (Figs. 28.11–28.14). This approach allows
a good maneuverability within these deep regions and
allows accurate neurovascular dissection, especially
when these structures are encased by a tumor. When the
lesion is placed anteriorly to CN planes, such as premeatal petrous meningioma, it can be removed working between CN windows (between CN V and CN VII or between
CN VII–VIII and CN IX–XI), which are normally enlarged
by the tumor.
28.2.3 Endoscopic Endonasal and
Retrosigmoid Approach: Alternative
or Complementary Surgeries?
Considering our experience, we found that endoscopic
endonasal surgery has an elective indication for extradural posterior fossa tumors. The anatomic knowledge and
the use of technological devices, such as neuronavigation
and intraoperative micro-Doppler probe, in this phase
is crucial to avoid major and potentially life-threatening
complications, such as carotid artery injury. For extradural lesions, this route is preferable because it follows the
tumor direction of growth. In these cases, the endonasal
approach presents the advantage of addressing the tumor
directly, avoiding the opening of dural plane, if spared
by the tumor, and neural and vessel structure manipulation. Furthermore, combining multiple endonasal
routes, such as transclival, ethmoid-pterygoid-sphenoidal, and/or transmaxillo-pterygoid approach, a wide and
tailored exposure of the tumor can be obtained. In our
experience, we found that the main partial limitation to
this route for extradural tumors is the internal carotid
course, which should be kept under continuous control
by the surgeon. Conversely, in case of dura trespassing
by the tumor, the endoscopic endonasal approach allows
following the intradural tumor extension, enlarging the
dural opening and removing the mass. The main advantage is avoiding crossing any CNs, thus reducing the risk
of intraoperative damage. Crossing the nerve line exposes
the neural structures to a greater risk of being injured by
instruments when they are not under direct visual control. This advantage also represents the main limitation
of endoscopic endonasal approach for intradural lesion,
because the CNs represent an absolute limitation to this
approach. Thus, for tumors located more posteriorly to
this plane, the retrosigmoid–transcranial approach is the
first choice. For those challenging lesions extending both
ventrally and dorsally to the CNs, a combined approach
can be a satisfactory strategy to limit the surgical morbidity and to increase the tumor resection.
Retrosigmoid and transclival approaches are not alternative but complementary to each other, permitting to resect
safely and effectively different tumors with different locations and relationships with the posterior fossa dura and CN
plane, and to be combined for more challenging lesions.
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28.3 Conclusion
The endoscopic endonasal approach for posterior fossa
tumor is a complex surgery, which should be reserved to
experienced surgeons, skilled in expanded endoscopic
techniques. It requires good knowledge of the anatomic
relationships, in particular regarding the carotid artery,
which represents the main limitation for extradural tumors.
However, the extensibility of this approach permits overwhelming this limit, adopting different corridors, which
allows working around the carotid artery and thus remove
the tumor located laterally or behind. Moreover, intradural
extensions are not a contraindication of this approach.
Conversely, the extension of the tumor beyond the nerve
line is an absolute limitation to endoscopic tumor removal.
For these cases, the retrosigmoid approach or combined
mono- or multi-staged transcranial and endoscopic endonasal approaches should be the first choice.
References
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2008;63(2):299–307, discussion 307–309
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clivus: the transcondylar and transjugular tubercle approaches.
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Neurosurgery 2006; 59(1, Suppl 1):ONS50–ONS57, discussion
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17. Kassam AB, Mintz AH, Gardner PA, Horowitz MB, Carrau RL, Snyderman CH. The expanded endonasal approach for an endoscopic
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Chapter 29
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29.1 Far Lateral Approach 304
Far Lateral-
Craniovertebral
Approach
29.2 Extended Endoscopic
Endonasal Transclival
Approach to the Ventrolateral
Brain Stem 307
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29 Far Lateral-Craniovertebral Approach
Giuseppe Catapano, Matteo de Notaris, Giuseppe Di Nuzzo, Nelido Gonzalez Fernandez, Iacopo Dallan, Alberto Prats-Galino
29.1 Far Lateral Approach
29.1.1 Introduction
Lesions located in the lower clivus and extending until
the anterior margin of foramen magnum have always
presented as a challenge to the neurosurgeon. Chordomas,
chondrosarcomas, and meningiomas can arise from the
clival region and extend toward the anterolateral surface
of the brainstem.
the sellar and suprasellar areas, inferiorly to the foramen
magnum and craniovertebral junction, and laterally to
the middle cranial fossa, tentorium, and cerebellopontine
angle. One single surgical corridor may not allow full
exposure of the entire lesion. The majority of these
lesions can be approached posteriorly by suboccipital,
retrosigmoid, or far lateral craniotomies or anteriorly using
two approaches: the transoral or the endonasal route.
Concerning the far lateral approach, the posterolateral
exposure of the craniovertebral junction has proved
to be very helpful in the management of lesions in this
5
It is an extension of the standard suboccipital
area.
approach, designed to maximize exposure of the
lateroventral craniocervical junction.
of the occipital bone including the posterior aspect of the
occipital condyle and posterior arch of C1.
The main anatomic landmarks are the vertebral artery
and the hypoglossal nerve. Following the basic principle
of cranial base surgery, the angle of view is increased
by bone removal. The initial steps of this approach are
as follows: dissection of occipital–cervical muscles
with the exposition of suboccipital triangle, lateral
suboccipital craniotomy, posterolateral occipital partial
condylectomy, and, finally, exposure of vertebral artery
until its entrance into the dura mater. When the intention
is reaching the anterior and lateral medulla regions, as
in tumors of the lower clivus, the inferior third basilar
artery, or vertebral basilar junction aneurysms, the far
lateral approach allows a tangential, unobstructed view
of the posterolateral cervicomedullary area.
Advances in endoscopic endonasal skull base surgery
have led to the development of new routes to areas
beyond the midline skull base.
base surgery has been enriched by the introduction of
endonasal techniques to access lesions located anterior
or anterolateral to the brainstem. Modifications of the
standard transsphenoidal route, especially with the use of
the endoscope, have allowed additional exposure of the
suprasellar, retrosellar, and retroclival areas, providing
a direct view.
historical target of the endoscopic endonasal approaches
has been the midline skull base, initially limited to the
sellar region and over time expanded to areas from the
cribriform plate down to C2. Anatomic knowledge and
intraoperative image guidance have led to the expansion
of such routes to areas beyond the midline skull base.
Previous studies have shown that the lateral limits of
the endoscopic endonasal transclival approach can be
overcome by additional bone removal, and feasible surgical
corridors to Meckel’s cave, the cerebellopontine angle,
and the ventrolateral brainstem have been described.
1–4
They may expand superiorly to reach
6,7
It allows removal
8
In the past decade, cranial
9,10
Since the beginning of the 1990s, the
11–13
Actually, anatomic
extended endoscopic approaches have demonstrated
the possibility of reaching areas that extend from the
cribriform plate down to the craniovertebral junction and
18
C2
to remove both extradural and intradural tumors via
a pure transclival endoscopic endonasal approach.
14–17
and clinical reports concerning
29.1.2 Surgical Steps
Positioning
An accurate patient positioning is important. The
patient’s head is flexed until the chin is 1 cm from the
sternum, rotated contralaterally to the lesion, and flexed
30 degrees laterally toward the contralateral shoulder,
so that the ipsilateral external auditory meatus and
the mastoid bone are at the highest point, allowing the
angle between the atlas and the foramen magnum to be
increased. The neurosurgeon should pay attention to the
jugular veins to avoid compression, which could cause a
venous return impairment and brain swelling. For this
reason, an axillary roll is placed and the contralateral arm
rests on a Krauss armrest. The elevated arm is distracted
inferiorly toward the foot of the table to provide more
room for the surgeon above the shoulder. All pressure
points are carefully padded with foam or gel pads and the
patient is secured to the operating table with adhesive
tape to allow safe rotation of the table during the
operation to improve the surgeon’s line of sight.
After the patient has been induced general anesthesia
and properly catheterized, the hair should be combed
with a brush used for washing the hands, soaked in
detergent solution so as to facilitate the shaving that
should be performed up to 2 cm from the region of the
surgical incision.
Skin Incision
After the positioning and trichotomy, the marking of
the skin incision is done, in such a way that the two
endings form an imaginary straight line that adequately
simulates the separation of the skin flap and the
consequent bone exposure. The most important external
anatomic landmarks are represented by inion, asterion,
C2 spinous process, and mastoid apex, and these should
be marked. The transverse process of C1, which may be
touched in the middle way between the mastoid tip and
the posterior angle of the mandible, can be localized as
well. The incision begins in the midline, approximately
5 cm below the inion and goes straight upward until 3
cm above the external occipital protuberance. Then
it turns laterally to the asterion and finally it turns
downward and laterally over the sternocleidomastoid
muscle posterior edge, approximately 5 cm below the
mastoid apex (Fig. 29.1). Alternatively, the skin and galea
are elevated first to expose the underlying pericranium
above the superficial neck fascia, which may be harvested
as a fascial graft for later watertight dural closure. The
pericranium and the superficial fascia are then elevated
to expose the underlying musculature and the occipital
19–24
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