66
Early warning of neurologic compression, retrac-
tion, or ischemic injury can be provided by elec-
trophysiologic monitoring of somatosensory,
motor, and brainstem auditory evoked potentials.
Free running and stimulated electromyographic
monitoring of lower cranial nerve function may
be also useful, especially in the setting of a mass
lesion causing displacement of normal structures.
The patient is placed in the three-quarter prone
position, the side of the lesion uppermost, which
preserves alignment of the craniocervical junc-
tion, allows the cerebellar hemisphere to fall
away, and maintains a low risk for venous air
embolism. A gel roll is placed below the axilla
and a cushion between the legs, and the depen-
dent arm is supported in a sling beyond the end of
the operating table.
The Mayfield clamp is placed with the single
pin 2 cm superior and anterior to the ipsilateral
pinna and the paired pins 2 cm superior to the
contralateral pinna, all points maintaining a low
profile. Placement of the pins is critical should
the procedure require access to the occipital
artery for a bypass graft. The head is fixed with
incorporation of four movements (Fig. 5.1): (1)
anteroposterior flexion to uncover the suboc-
cipital region and rostral clivus, (2) contralat-
eral flexion to increase working space beside
the ipsilateral shoulder, (3) contralateral rota-
tion to bring the suboccipital surface uppermost
in the field, and (4) upward translation to par-
tially sublux the ipsilateral atlanto-occipital
joint and facilitate drilling of the condyle if
needed. The ipsilateral shoulder is gently pulled
toward the patient’s feet while avoiding exces-
sive traction on the brachial plexus, and the
entire body secured with adhesive tape to allow
for side-to- side rotation of the table. The table
is placed in reverse Trendelenburg, elevating
the head slightly above the heart, to decrease
cerebral venous congestion.
Incision and Muscle Dissection
A hockey stick incision is started in the midline at
the level of the C2 spinous process, and extended
superiorly to 2 cm above the inion (Fig. 5.2a, b).
It is continued laterally above the superior nuchal
line to a point directly superior to the mastoid
process and then turned inferiorly to end at the
mastoid tip. The occipital artery is preserved if
required later in the procedure, as is pericranium
for potential duraplasty. Dissection through the
midline aponeurosis decreases muscle trauma
and allows for early identification of the C1 lam-
ina at a distance from the vertebral artery
(Fig. 5.2c). The trapezius and sternocleidomas-
toid muscles overlie the semispinalis capitis and
splenius capitis, which together conceal the sub-
occipital triangle. The superficial muscles are
reflected as a single flap, maintaining a cuff along
the superior nuchal line for a tight closure to
decrease risk of cerebrospinal fluid leak
(Fig. 5.2d). The muscle mass is elevated in the
subperiosteal plane and swept laterally to expose
the mastoid process and digastric groove
(Fig. 5.2e). The attachments of longissimus capi-
tis and posterior belly of digastric muscle are
then released. As the technique aims to approach
the ventral brainstem from a lateral trajectory,
adequate inferolateral retraction is necessary to
prevent encroachment upon the exposure by
muscle bulk; hooked retractors on elastics serve
this function better than hinged self-retaining
retractors.
Incisions including the linear/curvilinear ret-
romastoid incision and the S-shaped incision,
beginning in the retromastoid region and extend-
ing medially to the midline, reduce the lateral
bulk by directing the exposure through the mus-
culature. However, these approaches increase
muscle trauma and likelihood of sectioning the
occipital artery and nerve, and place the verte-
bral artery at greater risk of injury as bony land-
marks are not readily identified. Alternatively,
the lateral muscle bulk can be decreased by indi-
vidually identifying and dividing the sternoclei-
domastoid, splenius capitis, and semispinalis
capitis and reflecting the muscles medially,
although this approach increases the risk of
wound dehiscence.
As the subperiosteal dissection continues
inferiorly, the occipital attachments of the
rectus capitis posterior minor, rectus capitis
posterior major, and obliquus capitis supe-K. Au et al.