Skull Base Surgery of the Posterior Fossa

209

Approaches to the Dorsal Midbrain

The dorsal midbrain can be approached using the
midline SCIT approach or the posterior
interhemispheric transtentorial approach [ 116 ].
These approaches expose the distal most aspect
of the PCA and SCA and can be used to approach
aneurysms involving these vessels, to perform
technically challenging occipital artery-to-PCA
or -SCA bypasses, to remove AVMs involving
the incisura, cerebellar hemisphere, and poste-
rior midbrain, and to remove cavernous malfor-
mations at the level of the colliculi, using the
intercollicular safe-entry zone. We prefer to use
the SCIT approach over the posterior inter-
hemispheric transtentorial approach to avoid
the scenario where there may be a rich venous
network draining into the superior sagittal sinus
posteriorly.

Approaches to the Ventral Pons

The ventral pons is encased by the clivus, and
ventral pontine lesions are usually not approached
using a direct anterior route but rather by a ven-
trolateral approach. More recently, endonasal
endoscopic, transclival approaches have been
reported for resecting ventral pontine/posterior
fossa lesions. Care must be selected when
approaching vascular lesions using this approach,
and the use of these approaches should be limited
to surgeons with extensive experience
[ 117 – 120 ].

Approaches to the Lateral Pons

The posterior petrosal and retrosigmoid
approaches are the workhorses for exposure of
the lateral pons [ 121 ]. These approaches allow
the surgeon to release cerebrospinal fluid from
the cerebellopontine angle, thereby allowing for
cerebellar relaxation and exposure of the entire
lateral surface of the pons. These exposures allow
for visualization of the lateral SCA, the AICA, and
the entire BA for treating aneurysms involving

these vessels (Fig. 14.9). The additional exposure

of the lateral pons allows for surgical removal of

AVMs and fistulas on this lateral surface

(Fig. 14.10). These exposures also allow the sur-

geon to enter the pons at three safe-entry zones:

the peritrigeminal, the supratrigeminal, and the

lateral pontine or middle cerebellar peduncle

safe-entry zones (Fig. 14.11). These safe-entry

zones permit safe entry into the lateral pons for

resecting intrinsic lesions. At our institution, we

rarely use petrosal approaches and instead most

often use the retrosigmoid approach and the

middle cerebellar peduncle safe-entry zone to

remove lateral pontine cavernous malforma-

tions [ 121 ].

Approaches to the Dorsal Pons

The dorsal pons is approached using a suboccipi-

tal craniotomy. The suboccipital craniotomy can

be used for resecting posterior pontine/cerebellar

AVMs and cavernous malformations in this

region. When combined with opening of the tela

and the velum interpositum, the suboccipital

telovelar approach permits the dorsal pons to be

visualized to the level of the foramen of Luschka.

The telovelar approach allows the surgeon to

expose the superior fovea safe-entry zone to

resect lesions at the level of the facial colliculus

(Fig. 14.12) [ 122 ]. In addition to this safe-entry

zone, the surgeon may use the suboccipital

approach to expose the median sulcus of the

fourth ventricle, as well as the suprafacial collic-

ulus and infrafacial colliculus safe-entry zones

[ 86 ]. In general, we prefer to avoid incising the

floor of the fourth ventricle in the midline when

possible to avoid injury to the calamus

scriptorius.

Approaches to the Ventral Medulla

Similar to the ventral pons, direct approaches to

the ventral medulla are seldom necessary. Lesions

at the level of the ventral medulla can be readily

exposed laterally or dorsolaterally.

14 Microsurgical Management of Posterior Fossa Vascular Lesions