Skull Base Surgery of the Posterior Fossa

18

Further drilling of mastoid air cells uncovers the
compact bone of the digastric ridge inferiorly. The
fallopian (or facial) canal can then be expected to
course anteriorly to and in the same plane as a line
joining the incus and the digastric ridge.

Transcrusal and Translabyrinthine
Approaches
The translabyrinthine approach involves drilling
the semicircular canals to expose the IAC, vesti-
bule, and the different nerves directed to the
ampullae (Fig. 1.9b). The IAC is accessed
through the superior ampulla, and the superior
vestibular nerve is encountered first posterior to
Bill bar. This approach increases the surgical
access to the anterolateral brainstem and clivus at
the expense of hearing. The transcrusal, or partial
translabyrinthine approach, has been devised to
preserve most of the surgical advantages of the
translabyrinthine approach without necessarily
sacrificing hearing. Although it only entails drill-
ing of the superior and posterior semicircular
canals and their common crus, the risk of hearing
loss is still significant.

Transcochlear and Transotic
Approaches
In the transcochlear approach, the posterior
petrous bone is completely drilled as well as parts
of the tympanic bone [ 17 ] (Fig. 1.9c, d). To this
end, the external auditory canal (EAC) is opened,
the middle ear is accessed through the facial
recess anterior to the facial nerve, and the latter is
completely skeletonized. The GSPN and the
chorda tympani are disconnected from the facial

nerve to allow a posterior transposition of the

nerve, which will result in permanent weakness.

The cochlea is drilled above the petrous carotid

artery. The transcochlear approach results in the

widest access to the ventral brainstem with infe-

rior extension to the jugular bulb. Because this

approach carries the highest risk of facial nerve

palsy, the transotic variant has been described to

avoid manipulation of the nerve.

Combined Supra-/Infratentorial

Petrosal Approaches

The petrosal exposures can be extended with a

temporal craniotomy that crosses the transverse-

sigmoid junction. The middle fossa dura is care-

fully incised while avoiding injury to the vein of

Labbe before dividing the superior petrosal sinus

and cutting the tentorium to communicate the

posterior and middle fossae (Fig. 1.9e–h). The

temporal lobe, tentorium, cerebellum, and sig-

moid sinus can then all be retracted together to

augment the combined presigmoid and subtem-

poral exposure [ 18 ].

Exposure of Jugular Foramen

The presigmoid mastoidectomy may be carried

inferiorly and combined with a neck dissection to

widely expose the jugular bulb and the infralaby-

rinthine area [ 19 ] (Fig. 1.10). The postauricular

incision is extended inferiorly to expose the

carotid artery, internal jugular vein, and the lower

cranial nerves in the upper cervical region. The

spinal accessory nerve may pass either anterior or

posterior to the internal jugular vein near the C1

transverse process, and care must be taken not to

Fig. 1.9 (continued) of attack to the anterior brainstem
and petroclival region. (c) Transposition of the facial
nerve has been performed before drilling of the cochlea.
(d) The transcochlear approach maximizes the presig-
moid exposure of the deep anterior structures such as the
trunk of basilar artery, the abducens nerve, and the clivus.
(e) Left combined supra-/infratentorial petrosal approach.
The mastoidectomy is combined with a temporal craniot-
omy. The superior petrosal sinus and the tentorium are cut
to join the middle and posterior fossae, and the sigmoid
sinus can be then be mobilized posteriorly to widen the

presigmoid corridor. (f) View through a left retrolabyrin-

thine combined approach. (g) View through a left trans-

labyrinthine combined approach. The vein of Labbe

draining into the transverse-sigmoid junction must be

identified and preserved. Its drainage into the superior

petrosal sinus requires careful planning to preserve venous

outflow. (h) View through a left transcochlear combined

approach, which provides an extensive exposure ranging

from the petrous carotid laterally, basilar trunk and pons

anteriorly, oculomotor nerve superiorly, and jugular bulb

inferiorly

J. Basma and J. Sorenson