Skull Base Surgery of the Posterior Fossa

185

The posterior fossa is a special surgical situation
where space is limited, and vital structures such
lower cranial nerves and the brainstem are in
close proximity. Successful surgical resection of
metastatic tumors in the posterior fossa requires
careful study and examination of tumor volume,
location, and neighboring structures. The primary
surgical objective should be safe gross total resec-
tion (GTR) for any metastatic lesion without
incurring new neurological deficits.

Preoperative Care

Preoperatively, patients with posterior fossa met-
astatic lesions are started on dexamethasone to
control vasogenic edema and brain swelling.
Intraoperatively, mannitol, 3% saline, and mild
hyperventilation may be required to relax the cer-
ebellum during the dural opening. Patients who
present with features of hydrocephalus and ven-
tricular obstruction may have received an EVD
preoperatively. Alternatively, a prophylactic burr
hole can be placed intraoperatively 7 cm superior
and 3 cm lateral to the inion to aid in placement
of a ventricular drain for intracranial fluid man-
agement throughout the case. Surgical neuronav-
igation can aid in this process as necessary.

Surgical Approaches

to the Posterior Fossa

Many surgical corridors to posterior fossa lesions

have been described [ 45 – 51 ]. For the purposes of

this chapter, we describe approaches that can be

applied to metastatic lesions of the vermis, cere-

bellar hemispheres, and often deeper anatomy

including the cerebellar peduncles, depending on

the use of stereotactic guidance as necessary to

define acceptable boundaries of resection.

Approaches to the midline vermis, as well as the

cerebellar hemispheres, can follow standard

practices of suboccipital craniotomy and expo-

sure. Lateral lesions, such as those near the cere-

bellopontine angle or jugular foramen, as well as

deeper or anteriorly distributed metastases are

accessed via retrosigmoid or skull base

approaches. Frameless stereotaxis guided by

thin-sliced MRI for real-time image guidance is

used to avoid damaging important structures dur-

ing approach and to ensure efficient and thorough

tumor resection.

Posterior fossa craniectomy or craniotomy

may be appropriate for single or multiple meta-

static lesions that are surgically resectable. A

recent retrospective, multivariate analysis of 88

patients undergoing surgical removal of meta-

static posterior fossa lesions highlights a poten-

tial difference in patient outcome resulting

from choice of surgical approach. The authors

report a lower incidence of postoperative com-

plications (12.5%) in patients receiving a crani-

otomy rather than craniectomy (34.6% overall

complication rate). However, the relatively

small number of patients and single-institution

analysis precludes any strong conclusions

based on this work. Importantly, mortality was

unaffected by surgical approach [ 52 ]. Another

group arrived at a similar conclusion in the

pediatric population [ 53 ].

Proper patient positioning for posterior fossa

craniotomies is important to provide clear work-

ing space, microscope visibility, and maneuver-

ability of surgical instrumentation. Historically,

three major positions have been used for surgical

access to the posterior fossa: a park bench (lateral

oblique), prone (or modified prone), or sitting

position. Although the latter provides excellent

Table 13.1 Recursive partitioning analysis classification
scale for brain metastasis

Class Patient characteristics

Proportion of
patients
I KPS ≥ 70 20%
Controlled primary disease
Age <65 years
No evidence of extracranial
metastases
II KPS ≥ 70 65%
Uncontrolled primary
disease
Age ≥65 years
Other extracranial
metastases present
III KPS <70 15%
Data from Gaspar et al. 1997, 2000 [ 37 , 86 ]
Note that the majority of patients fall into Class II, where
the prognostic benefit of any given form of therapeutic
modality is more ambiguous.
KPS Karnofsky performance scale

13 Metastasis to the Posterior Fossa