s1042368008000077.pdf

Neurosurg Clin N Am 19 (2008) 239–250

Retrosigmoid Approach for Acoustic

Tumor Removal

Noel L. Cohen, MD *

Department of Otolaryngology, New York University School of Medicine, New York, NY, USA

COMMENTARY

transverse sinus to close to the jugular bulb, expos-

ing the dura behind the sigmoid. A crescentic bone

flap ( O / ¼ 2.5 cm) is removed and preserved and

the dura opened, creating an anterior-based C

flap, and the CSF is decompressed. Retractors are

not necessary but the cerebellum and any visible tu-

mor are protected.

Dural flaps are raised on the posterior face of the

petrous bone over the lAC, which is then drilled ap-

proximately 300 degrees around at the porus and

less so laterally, out to þ /-9 mm from the porus.

The internal dura of the lAC is opened horizontally,

the various nerves identified, a monitoring electrode

is placed on the cochlear nerve and the tumor

removed.

Closure: perimeatal cells, if any, are waxed, fat

placed and glued in the bony defect, the petrous

dural flaps are closed, as is the posterior fossa

dura with the help of a temporalis fascia graft.

The bone flap is replaced with titanium mini plates.

The perilabyrinthine and retrofacial mastoid cells

are waxed, and abdominal fat is glued in the mas-

toid so as not to inhibit incus motion. The Palva

flap is closed and the scalp closed in layers.

This is an update to the approach and technique as

described in the original article that follows.

Noel L. Cohen, MD, J J Thomas Roland, Jr, MD,

John S. Golfinos, MD

The technique for this approach to the posterior

cranial fossa continues to change and evolve, par-

tially due to changing technology such as sophisti-

cated monitoring techniques, and partially the

result of on-going critical evaluation of complica-

tions and results of the surgery.

The main indication for the retrosigmoid ap-

proach remains the patient with useful hearing

and an acoustic neuroma less than 15 mm extra

canalicular. Occasionally, it is used in the case of

a tumor, which is entirely intracanalicular, in which

it is deemed wiser (eg, the older patient) not to per-

form a middle fossa approach. The translabyrinthine

approach is used in virtually all patients with poor

hearing and/or larger tumors, in which hearing pres-

ervation is not a reasonable goal.

The surgery is performed as follows:

Monitoring: EMG based VII, BAER, SSEP, trans-

cranial facial MEP. Position, usually supine with

head rotated away and semi-fixed with tape. Occa-

sionally, lateral with head turned and rigidly fixed

(for the ‘‘fullback’’ patient). The incision currently

used is a ‘‘gentle hockey-stick.’’ An anteriorly-based

skin and soft-tissue flap is turned, and then a pericra-

nial anteriorly-based (Palva) flap is raised. Barbless

fishhooks are used for retraction. Mannitol is admin-

istered intravenously. The mastoid is drilled and the

sigmoid sinus skeletonized from the level of the

The retrosigmoid approach to the posterior

fossa is a modiﬁcation of the traditional neuro-

surgical suboccipital craniotomy. The suboccipital

craniotomy gives a wide view of the posterior

fossa and has been the mainstay of access to this

area by neurosurgeons since it was ﬁrst described

by Fraenkel and colleagues [1] in 1904 ( Fig. 1 ). In

fact, Woolsey had performed the ﬁrst operation in

1903, to be followed very shortly by Krause in

1905 [2] .

Cushing [3] then described his operation, con-

sisting of a large, ‘‘crossbow’’ bilateral exposure

of the posterior fossa, operating on the patient

in the prone position. He advocated a subtotal

removal. Dandy [4] then brought the operation

This article originally appeared in Otolaryngologic

Clinics of NA; Volume 25, issue 2, April 1992; p.

295–310.

* Corresponding author. NYU School of Medicine

Department of Otolaryngology, 550 First Avenue,

New York, NY 10016.

doi:10.1016/j.nec.2008.02.006

neurosurgery.theclinics.com

240

COHEN

Fig. 1. Fraenkel’s original article.

into the more modern period, using a unilateral

approach in 46 cases, with a mortality of only

10.8% ( Fig. 2 ). Considering that the operation

was done with primitive inhalation anesthesia,

without the use of the operating microscope, ade-

quate lighting, microsurgical instrumentation, or

intraoperative monitoring, this was quite a re-

markably low mortality rate. He was an advocate

of total removal, accepting the almost inevitable

loss of facial nerve function (44 of 46 cases).

Interestingly Dandy reported that there was

‘‘good hearing’’ in 34 of his cases preoperatively.

Traditionally the suboccipital approach was

performed in the seated position and consisted of

a long straight incision extending well into the

neck; elevation of the nuchal muscles from the

posterior aspect and undersurface of the occipital

bone; and removal of a large segment of the bone,

extending laterally to the sigmoid sinus, medially

to the midline, superiorly to the transverse sinus,

RETROSIGMOID APPROACH FOR ACOUSTIC TUMOR REMOVAL

241

Fig. 2. Dandy’s 1941 technique.

and inferiorly to the foramen magnum. Following

opening of the dura, a segment of cerebellum was

resected to gain access to the cerebellopontine

angle, after which the tumor could be removed

[5–8] .

The suboccipital operation underwent many

modiﬁcations, ultimately resulting in what has

come to be called the retrosigmoid-transmeatal

operation [9–13] . This approach involves a small

curved or angulated incision, stopping well above

the undersurface of the skull, no dissection of nu-

chal muscle from the bone, a limited removal of

bone behind the sigmoid sinus, and no resection

of the cerebellum. This is combined with exposure

of the internal auditory canal by removing the

posterior wall: hence the term retrosigmoid-

transmeatal.

The surgical team consists of a neurosurgeon,

neurotologist, anesthesiologist, and monitoring

electrophysiologist, in addition to the usual nurs-

ing personnel. The neurosurgical team prepares

the patient, makes the incision, performs the

craniotomy, and exposes the tumor or the poste-

rior face of the petrous bone as well as the VIIth

and VIIIth cranial nerves in the cerebellopontine

angle. If the tumor is large, the team debulks it

and partially dissects it oﬀ the brain stem.

Following this, the neurotologist opens the in-

ternal auditory canal and removes the remainder

of the tumor.

Following tumor removal and plugging of the

internal auditory canal, the closure is performed

by the neurosurgical team. The patient goes to the

neurosurgical intensive care unit for postoperative

242

COHEN

care. This team approach to the surgery allows the

patient to beneﬁt from the experience and exper-

tise of both disciplines as well as avoiding fatigue

on the part of the surgeon.

The neurosurgical head holder is used to ﬁx the

patient’s head position as well as allowing the

surgeon to be seated closer to the operative ﬁeld.

The patient receives intravenous antibiotics, man-

nitol, and steroids; a nonmuscle-relaxant anesthe-

sia technique is used. In addition to the usual

anesthesia monitoring, we routinely monitor the

facial nerve with the electromyogram (EMG)-

based nerve integrity monitor. We also record

the contralateral auditory brain stem response to

monitor brain stem integrity in larger tumors.

When hearing is to be preserved, we prefer to

monitor the VIIIth cranial nerve potential by

placing an electrode directly on the surface of

the VIIIth cranial nerve. If this is not feasible,

ipsilateral auditory evoked responses are

recorded. The incision consists of an L-shaped

or C-shaped ﬂap based anteriorly and centered at

the approximate location of the transverse sinus

( Fig. 3 ). The ﬂap is retracted forward with barb-

less ﬁshhooks. Soft tissues are elevated oﬀ the

bone, and a craniotomy measuring approximately

3 to 5 cm in diameter is performed. The anterior

border is the sigmoid sinus, whereas the superior

border is the transverse sinus ( Fig. 4 ). Often retro-

sigmoid mastoid air cells are entered, and these

are ﬁlled with acrylic to seal them.

An anteriorly based dural ﬂap is then incised

and sutured forward, exposing the cerebellum.

With the patient in the three quarters prone

position, the head slightly elevated, and mannitol

given intravenously, the cerebellum generally

tends to fall away from the tumor once the

Technique

The operation is performed with the patient in

the horizontal position, either in the so-called

park bench (three quarters prone) position or in

the supine otologic position with the head turned

away from the operator. We prefer the former

because it allows a more oblique posterior ap-

proach, giving better access to the lateral end of

the internal auditory canal. The supine position

limits the obliquity of the approach, giving good

access medially to the cerebellopontine angle but

not to the lateral end of the internal auditory

canal. This in turn limits the ability to visualize

the lateral end of the tumor, resulting in either

blind dissection or incomplete tumor removal.

Using the three quarters prone position, the

operator is viewing the internal auditory canal

from a posterior position without the head being

rotated on the cervical spine. This allows visual-

ization of the bone as the drilling proceeds and

enables the operator to see a blue line as the

posterior semicircular canal is approached. In

addition, drilling can be carried as far laterally

as is necessary to expose either the lateral end of

the internal auditory canal or the lateral end of

the tumor.

Fig. 3. Incision for retrosigmoid approach.

RETROSIGMOID APPROACH FOR ACOUSTIC TUMOR REMOVAL

243

Fig. 4. Dural ﬂap elevated.

is taken to remain well within the capsule of the

tumor to avoid injury to the facial nerve and brain

stem. The ultrasonic aspirator need not be used

when dealing with tumors that are less than 15 mm

in diameter as measured from the porus. Obvi-

ously in the case of intracanalicular tumors,

cerebellar retraction is minimal, and the tumor

can usually be excised in one piece.

If necessary, the tumor is dissected oﬀ the

cerebellum as well as the brain stem, depending on

the size of the tumor. The facial nerve can be

identiﬁed at or near the brain stem at this point,

and if hearing preservation is the goal, an elec-

trode can be placed on the cochlear nerve as well

( Fig. 6 A). When attempting hearing preservation,

it is important to limit electrocautery as much as

possible, depending on gentle blunt dissection to

tease blood vessels away from the tumor and the

VIIIth cranial nerve, while using Gelfoam to

stop light venous oozing.

The dura over the posterior face of the petrous

bone is coagulated using monopolar current and

then excised after curettage. Alternately a laterally

based dural ﬂap can be raised [14] . Bleeding is

controlled with the cautery. A cutting bur is

used to outline the approximate location of the in-

ternal auditory canal, following which diamond

burs are used for the ﬁnal drill out. The magnetic

resonance image (MRI) is used as a guide to the

length of tumor extension into the internal audi-

tory canal, and every attempt is made not to

open into the posterior semicircular canal or ves-

tibule, since this would probably result in the total

loss of hearing. The endolymphatic sac and duct

can also be used as landmarks: it is imperative

subarachnoid space is opened. Usually the cere-

bellum need only be very lightly supported

( Fig. 5 A). This then allows opening into the cere-

bellopontine cistern, with release of the cerebro-

spinal ﬂuid. The cerebellum is protected with

Gelfoam (Upjohn, Kalamazoo, Michigan), a rub-

ber dam, or Silastic sheeting ( Fig. 5 B). It need be

retracted only for a larger tumor.

If the tumor is bulky, it may decompressed

with the use of an ultrasonic aspirator at this

point. We no longer use the carbon dioxide laser

to debulk the tumor because the ultrasonic

aspirator is much more rapid, causes no heating

or charring of the tissue, does not require any

bulky addition to the operating microscope, and

avoids the potential danger of accidental burns

inherent in all laser surgery. The rapidity of tumor

removal can be varied by adjusting the power of

the ultrasonic output as well as the suction. Care

Fig. 5. (A) Tumor exposed; (B) cerebellum retracted.

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