The focus
is on the anatomy of acoustic schwannoma
because this is the most common tumor to
occur in the cerebellopontine angle. These
anatomic considerations are divided into
sections dealing with the relationships at
the lateral end of the tumor in the meatus
and those on the medial end of the tumor at
the brain stem. The anatomy of the region
offers the opportunity for three approaches
to the tumor in the cerebellopontine angle.
One is directed through the posterior
cranial fossa and posterior meatal lip.
Another is directed through the labyrinth
and the posterior surface of the temporal
bone. The third is directed through the
middle cranial fossa and the roof of the
internal acoustic canal. The anatomy
presented by all three approaches is
reviewed.
Acoustic
schwannomas, as they expand, may involve a
majority of the cranial nerves, cerebellar
arteries, and parts of the brain stem. On
the lateral side, in the internal acoustic
canal, they commonly expand by enlarging the
meatus. On the medial side, they compress
the pons, medulla, and cerebellum. A widely
accepted operative precept is that a nerve
involved by tumor should be identified
proximal or distal to the tumor, where its
displacement and distortion is the least,
before the tumor is removed from the
involved segment of nerve. Considerable
attention has been directed to the early
identification of the facial nerve distal to
the tumor at the lateral part of the
internal acoustic canal, whether the
operative route be through the middle fossa,
labyrinth, or posterior meatal lip. Less
attention has been directed to
identification at the brain stem on the
medial side of the tumor. These anatomic
considerations are divided into sections
dealing with the relationships at the
lateral end of the tumor in the meatus, and
those on the medial end of the tumor at the
brain stem.
Meatal
Relationships
The nerves
in the lateral part of the internal acoustic
canal are the facial, the cochlear, and the
inferior and superior vestibular nerves. The
position of the nerves is most constant in
the lateral portion of the canal, which is
divided into a superior and an inferior
portion by a horizontal ridge, called either
the transverse crest or the falciform crest.
The facial and the superior vestibular
nerves are superior to the crest. The facial
nerve is anterior to the superior vestibular
nerve and is separated from it at the
lateral end of the canal by a vertical ridge
of bone, called the vertical crest. The
vertical crest is also called Bill's bar in
recognition of William House's role in
focusing on the importance of this crest in
identifying the facial nerve in the lateral
end of the canal. The cochlear and inferior
vestibular nerves run below the transverse
crest, with the cochlear nerve being located
anteriorly. Thus, the lateral meatus can be
considered to be divided into four portions,
with the facial nerve being anterior
superior, the cochlear nerve anterior
inferior, the superior vestibular nerve
posterior superior, and the inferior
vestibular nerve posterior inferior.
Because
acoustic schwannomas most frequently arise
in the posteriorly placed vestibular nerves,
they usually displace the facial and
cochlear nerves anteriorly. Variability in
the direction of growth of the tumor arising
from the vestibular nerves may result in the
facial nerve being displaced, not only
directly anteriorly, but also anterior
superiorly or anterior inferiorly. Because
the facial nerve always enters the facial
canal at the anterior superior quadrant of
the lateral margin of the canal, it is
usually easiest to locate it here, rather
than at a more medial location, where the
degree of displacement of the nerve is more
variable. The cochlear nerve also lies
anterior to the vestibular nerve and will be
stretched around the anterior half of the
tumor.
Nervus
Intermedius
The
filaments of the nervus intermedius are also
stretched around an acoustic schwannoma. The
nervus intermedius is divisible into three
parts: a proximal segment that adheres
closely to the vestibulocochlear nerve, an
intermediate segment that lies free between
the eighth nerve and the motor root of the
facial nerve, and a distal segment that
joins the motor root to form the facial
nerve. Some nerves are adherent to the
eighth nerve throughout their entire course
in the posterior cranial fossa and can be
found as a separate structure only after
opening the internal acoustic meatus. In
most instances the nerve is a single trunk,
but in some cases it is composed of two to
four rootlets. It most frequently arises at
the brain stem anterior to the superior
vestibular nerve as a single large root and
in the meatus, lies anterior to the superior
vestibular nerve. When multiple rootlets are
present, they may arise along the whole
anterior surface of the eighth nerve:
however, they usually converge immediately
proximal to the junction with the facial
motor root to form a single bundle that lies
anterior to the superior vestibular nerve.
Brain
Stem Relationships
There is a
consistent set of neural, arterial and
venous relationships at the brain stem that
facilitate identification of the nerves on
the medial side of the tumor.
Neural
Relationships
The neural
structures most intimately related to the
medial side of an acoustic schwannoma are
the pons, medulla, and cerebellum. The
landmarks on these structures that are
helpful in guiding the surgeon to the
junction of the facial nerve with the brain
stem are the pontomedullary sulcus; the
junction of the glossopharyngeal, vagus, and
accessory nerves with the medulla, the
foramen of Luschka and its choroid plexus
and the flocculus.
Pontomedullary
Sulcus
The facial
nerve arises from the brain stem near the
lateral end of the pontomedullary sulcus.
This sulcus extends along the junction of
the pons and the medulla, and ends
immediately in front of the foramen of
Luschka and the lateral recess of the fourth
ventricle. The facial nerve arises in the
pontomedullary sulcus 1- to 2 mm anterior
to the point at which the vestibulocochlear
nerve joins the brain stem at the lateral
end of the sulcus. The interval between the
vestibulocochlear and facial nerves is
greatest at the level of the pontomedullary
sulcus and decreases as these nerves
approach the meatus.
Glossopharyngeal,
Vagus, and Accessory Nerves
The facial
nerve has a consistent relationship to the
junction of the glossopharyngeal, vagus and
accessory nerves with the lateral side of
the medulla. The facial nerve arises 2 to 3
mm above the most rostral rootlet
contributing to these nerves. A helpful way
of visualizing the point where the facial
nerve will exit from the brain stem, even
when displaced by tumor, is to project an
imaginary line along the medullary junction
of the rootlets forming the
glossopharyngeal, vagus and accessory
nerves, upward through the pontomedullary
junction. This line, at a point 2 to 3 mm
above the junction of the glossopharyngeal
nerve with the medulla, will pass through
the pontomedullary junction at the site
where the facial nerve exits from the brain
stem. The glossopharyngeal and vagus nerves
are seen and should be protected carefully
below the lower margin of the tumor in both
the translabyrinthine and retrosigmoid
approaches.
Cerebellar-Brain
Stem Fissures
Acoustic
schwannomas are closely related to the
cerebellopontine and cerebellomedullary
fissures, the clefts formed by the folding
of the cerebellum around the pons and
medulla. The cerebellopontine fissure is a
V-shaped fissure formed by the folding of
the petrosal surface of the cerebellum
around the lateral side of the pons and
middle cerebellar peduncle. The petrosal
surface is the cerebellar surface that faces
the posterior surface of the petrous bone,
and is the cerebellar surface that is
compressed by an acoustic schwannoma. The
cerebellopontine fissure has a superior limb
situated between the rostral half of the
pons and the superior part of the petrosal
surface, and an inferior limb located
between the caudal half of the pons and the
inferior part of the petrosal surface. The
apex of the fissure is located laterally
where the superior and inferior limbs meet.
The V-shaped area between the superior and
inferior limbs, which has the middle
cerebellar peduncle in its floor,
corresponds to the area that is called the
cerebellopontine angle. The trigeminal,
abducens, facial, vestibulocochlear,
glossopharyngeal. and vagus nerves arise
between the superior and inferior limbs of
the fissure. The facial and
vestibulocochlear nerves arise just anterior
to the inferior limb of the fissure and just
below the middle cerebellar peduncle. The
trigeminal nerve arises near the superior
limb of the fissure.
The
cerebellomedullary fissure, the cleft
between the cerebellum and medulla that
extends upward between the cerebellar tonsil
and the medulla, communicates with the
inferior limb of the cerebellopontine
fissure near the lateral recess of the
fourth ventricle. Several structures related
to the lateral recess and the foramen of
Luschka project into the cerebellopontine
angle near the facial and the
vestibulocochlear nerves.
Foramen
of Luschka, Choroid Plexus, and Flocculus
The
structures related to the lateral recess of
the fourth ventricle that have a consistent
relationship to the facial and
vestibulocochlear nerves are the foramen of
Luschka and its choroid plexus and the
flocculus. The foramen of Luschka is
situated at the lateral margin of the
pontomedullary sulcus, just dorsal to the
junction of the glossopharyngeal nerve with
the brain stem, and immediately
posteroinferior to the junction of the
facial and vestibulocochlear nerves with the
brain stem. The foramen of Luschka is
infrequently well visualized. However, there
is a consistently identifiable tuft of
choroid plexus that hangs out of the foramen
of Luschka and sits on the posterior surface
of the glossopharyngeal and vagus nerves
just inferior to the junction of the facial
and vestibulocochlear nerves with the brain
stem.
Another
structure related to the lateral recess is
the flocculus. It is a fan-shaped cerebellar
lobule that projects from the margin of the
lateral recess into the cerebellopontine
angle. The flocculus, together with the
nodule of the vermis, forms the primitive
flocculonodular lobe of the cerebellum. The
flocculus is attached to the rostral margin
of the lateral recess and foramen of
Luschka. The flocculus is continuous
medially with the inferior medullary velum,
a butterfly-shaped sheet of neural tissue
that forms on the surface of the nodule and
sweeps laterally above the tonsil to form
part of the inferior half of the roof of the
fourth ventricle. The lateral part of the
inferior medullary velum narrows to a
smaller bundle, the peduncle of the
flocculus, which fuses to the rostral margin
of the lateral recess and foramen of
Luschka. The flocculus projects from the
peduncle of the flocculus into the
cerebellopontine angle just posterior to the
site at which the facial and
vestibulocochlear nerves join the
pontomedullary sulcus.
