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 Clinical
Features
The
differential diagnosis of cerebellopontine
angle lesions includes, in order of
occurrence, acoustic neurinomas,
meningiomas, epidermoid tumors and arachnoid
cysts. Significantly less common lesions
include neurinomas of other cranial nerves,
lipomas, glomus tumors and vascular lesions.
Acoustic
neurinomas arise from the Schwann cells of
the vestibular nerve. The vestibular nerve
is ensheathed in oligodendrocytes for much
of its course through the cerebellopontine
angle. However, as the nerve enters the
internal auditory meatus the
oligodendrocytes are replaced by Schwann
cells in a region known as the zone of
Obersteiner-Redlich. This transitional zone
usually lies at the mouth of the internal
auditory meatus and thus Schwann cells
invest the vestibular nerve along virtually
all of its length within the canal. It is
these cells within the canal which are
thought to give rise to the acoustic
neurinoma.
A history
of progressive unilateral hearing loss,
usually over many months and sometimes
years, is the hallmark of an acoustic
neurinoma. In most cases it is associated
with tinnitus. As the tumor enlarges, the
patient complains of unsteadiness and loss
of balance. True rotational vertigo is rare.
The facial nerve usually functions normally
until the tumor reaches a large size. When
nerve function is compromised, it is usually
mild. Total facial paralysis is rare.
Involvement of the trigeminal nerve likewise
occurs late and is seen primarily in tumors
more than 3 cm in diameter. As the tumor
grows upward into the superior aspect of the
cerebellopontine angle, it encroaches upon
the trigeminal nerve, producing a gradual
decrease of the corneal reflex and facial
analgesia and anaesthesia. Tic douloureux
occurs rarely.
It is
unusual for patients with an acoustic
neurinoma to present with complaints of
swallowing dysfunction or hoarseness and
lower cranial nerve involvement is unlikely
unless the tumor is large. Cerebellar
symptoms and signs also occur late in the
clinical course of these tumors and are
often found in association with compromised
function of cranial nerves. Papilledema and
symptoms of hydrocephalus can also be
present and are usually secondary to
compression of the brain stem and the fourth
ventricle by a large tumor.
Meningiomas are the second most frequent
tumor of the cerebellopontine angle. They
constitute 3 to 13 percent of
cerebellopontine angle tumors. These tumors
produce the same general symptoms and signs
as do acoustic tumors, with several
exceptions. Often these lesions originate
from the superior-anterior lip of the porus
acousticus, and are associated with early
involvement of the seventh nerve. Hearing
loss, however, occurs later. Thus, in terms
of facial and auditory function, meningiomas
are the exact opposite of acoustic tumors.
Involvement of the posterior root of the
fifth cranial nerve may lead to numbness of
the face and ticlike symptoms. These
symptoms, preceding hearing loss, suggest
that a meningioma may be present or, less
likely, a trigeminal neurinoma. Meningiomas
also cause a higher incidence of lower
cranial nerve abnormalities compared to
acoustic tumors. The growth downward of
these lesions results in hoarseness,
numbness of the throat or complaints of
difficulty swallowing. As with acoustic
tumors, large meningiomas can produce
cerebellar symptoms and signs or
hydrocephalus with increased intracranial
pressure.
Epidermoid
tumors and arachnoid cysts are both rare
lesions of the cerebellopontine angle,
accounting for 2 to 6 percent and 1 to 3
percent of all lesions, respectively.
Epidermoid tumors are benign and grow
slowly. They can present with multiple
cranial nerve abnormalities or cerebellar
symptoms and signs which develop over a
number of years. Patients with arachnoid
cysts can present with a complaint of
unilateral hearing loss, headache or
imbalance. Facial or trigeminal nerve
dysfunction can occasionally be observed. |
Anatomy
The
cerebellopontine angle is an inverted triangular
cistern in which the fifth, seventh and eighth
cranial nerves, along with the anterior inferior
cerebellar artery (AICA) and the superior petrosal
vein are located. From a surgeon' s viewpoint, the
cistern is bounded laterally by the back wall of the
petrous bone, medially by the pons and cephalad by
the tentorium. which forms the base of the triangle.
This cistern communicates freely with the other
cerebrospinal fluid (CSF) spaces within the
posterior fossa, including a small diverticulum
which extends into the porus acusticus.
At the upper
aspect of the cistern, the fifth cranial appears as
a broad white band, extending from the lateral
aspect of the pons into Meckel's cave. The superior
petrosal vein lies at the upper posterior edge of
this nerve, and drains from the superior aspect of
the cerebellum to the superior petrosal sinus. This
vein is usually 1 to 2 mm in diameter and at times
may be made up of a cluster of veins.
The seventh and
eighth nerves course laterally from the
pontomedullary junction to the internal auditory
canal. They cross the cistern as an apparent single
nerve, which is composed of four discrete nerves:
the superior and inferior vestibular nerves, the
cochlear nerve and the facial nerve. When viewed
from the suboccipital approach. the vestibular
nerves form the posterior aspect, or the portion
closest to the surgeon. The facial nerve makes up
the anterior superior portion within this bundle and
the cochlear division of the eighth nerve makes up
the anterior inferior portion. When one looks into
the posterior fossa from the extreme lateral aspect
of a suboccipital approach, the sixth nerve is
occasionally seen, coursing from its origin at the
pontomedullary junction to its entrance into the
dura of the clivus (Dorello's canal). In situations
where the tumor has rotated and displaced the brain
stem, this nerve may be confused with the seventh
nerve, inasmuch as it exits on the same plane as the
seventh nerve and enters the dura at the same level
as the internal auditory canal.
The ninth, tenth
and eleventh nerves, although not specifically
within the cerebellopontine angle cistern, are found
immediately below its inferior margin. The most
superior of these nerves, the ninth, appears round
and shiny and is made up of a single filament. The
tenth nerve consists of multiple filaments that are
flat, whereas the eleventh nerve is unique in having
a spinal root traversing the foramen magnum.
The anterior
inferior cerebellar artery has a variable location
within the cistern. In acoustic tumors, this vessel
is usually located in the arachnoid over the cleft
between the cerebellum and the dome of the tumor.
Bilateral
Acoustic Tumors
Bilateral acoustic
tumors are pathognomonic of central
neurofibromatosis. In general, the goals for surgery
are preservation of brain stem function followed by
preservation of facial nerve function and hearing.
It is not wise to remove both tumors at one
operation. In general, the larger tumor is operated
on first. Removal of the tumor is carried out using
the technique outlined above. The patient only
returns for surgery on the second side after
completely recovering from the first procedure. This
includes wound healing as well as recovery of facial
nerve function. In the event of facial nerve
paralysis following the first operation, the second
one is delayed until the nerve recovers or a facial
reanimation procedure can be performed. In general,
tumor removal should be carried out as soon as the
tumors are found because removal of smaller tumors
is associated with better results for hearing
preservation.
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