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Neurosurg Clin N Am 19 (2008) 145–174

Microsurgical Anatomy of Acoustic Neuroma

Albert L. Rhoton, Jr, MD * , Helder Tedeschi, MD

Department of Neurological Surgery, University of Florida College of Medicine, Gainesville, FL, USA

Acoustic neuromas, as they expand, may in-

volve a majority of the cranial nerves, cerebellar

arteries, and parts of the brain stem. On the lateral

side, in the meatus, they commonly expand by

enlarging the meatus and may infrequently erode

into the vestibule and cochlea. On the medial side,

they compress the pons, medulla, and cerebellum.

An understanding of microsurgical anatomy is

especially important in preserving the facial and

adjacent cranial nerves, which are the neural

structures at greatest risk during acoustic neu-

roma removal. A widely accepted operative pre-

cept is that a nerve involved by tumor should be

identiﬁed proximal or distal to the tumor, where

its displacement and distortion are 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.

the inferior and superior vestibular nerves ( Fig. 1 ).

The position of the nerves is most constant in the

lateral portion of the meatus, which is divided into

a superior and an inferior portion by a horizontal

ridge, called either the transverse or the falciform

crest. The facial and the superior vestibular nerves

are superior to the crest. The facial nerve is ante-

rior to the superior vestibular nerve and is sepa-

rated from it at the lateral end of the meatus by

a vertical ridge of bone, called the vertical crest.

The vertical crest is also called Bill’s bar in recog-

nition 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 [6] . The co-

chlear 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 ves-

tibular nerve, posterior-superior; and the inferior

vestibular nerve, posterior-inferior.

The anatomy of the region oﬀers the opportu-

nity for three basic approaches to the tumor in the

meatus and cerebellopontine angle. One is di-

rected through the middle cranial fossa and the

roof of the meatus. Another is directed through

the labyrinth and posterior surface of the tempo-

ral bone. The third is directed through the

posterior cranial fossa and posterior meatal lip.

The anatomy presented by all three approaches is

reviewed here.

Meatal relationships

The nerves in the lateral part of the internal

acoustic meatus are the facial, the cochlear, and

Middle fossa approach

This article originally appeared in Otolaryngologic

Clinics of NA: volume 35, issue 2, April 2002; p. 257–

294.

* Corresponding author: Neurological Surgery, Uni-

versity of Florida, Box J-265, JHM Health Center Gain-

esville, FL 32610.

In the middle fossa approach, the meatus is

approached from above, through a temporal

craniotomy located anterior to the ear and above

the zygoma ( Figs. 2 and 3 ) [2,11] . The dura under

the temporal lobe is elevated from the ﬂoor of the

doi:10.1016/j.nec.2008.02.005

neurosurgery.theclinics.com

146

RHOTON & TEDESCHI

Fig. 1. Posterior view into the left cerebellopontine angle and internal acoustic meatus. Insert shows the orientation. (A)

The tentorium (Tent.) is above the trigeminal nerve (V). The facial and vestibulocochlear nerves enter the internal acous-

tic meatus. The posterior surface of the vestibulocochlear nerve is formed by the inferior (VIII I.V.) and superior ves-

tibular (VIII S.V.) nerves. The glossopharyngeal (IX), vagus (X), and spinal accessory nerves (XI) enter the jugular

foramen. The premeatal segment of the anterior inferior cerebellar artery (A.I.C.A.) is not visible because it is anterior

to the nerves. The meatal segment (Mea. Seg.) passes posterior to the nerves and gives rise to the subarcuate artery

(S.A.). The postmeatal segment (Post. Mea. Seg.) passes above the nerves. The insert shows the superior cerebellar artery

(S.C.A.) above the trigeminal nerve, and the posterior inferior cerebellar artery (P.I.C.A.) below the glossopharyngeal

nerve. (B) The posterior wall of the internal acoustic canal has been removed. The facial nerve (VII) is anterior to

the superior vestibular nerve. The subarcuate artery had to be divided to gain access to the posterior wall of the acoustic

canal. The transverse crest (Trans. Crest) separates the superior and inferior vestibular nerves at the lateral end of the

canal. (C) The superior and inferior vestibular nerves have been divided to expose the facial and cochlear nerves (VIII

Co.). The premeatal segment (Pre. Mea. Seg.) gives origin to the internal auditory (I.A.A.) and recurrent perforating

(R.P.A.) arteries. The initial segment of the recurrent perforating artery loops toward the meatus before turning medially

to reach the side of the brainstem. (From Martin RG, Grant JL, Peace D, et al. Microsurgical relationships of the an-

terior inferior cerebellar artery and the facial-vestibuloco-chlear nerve complex. Neurosurgery 1980;6:483–507; with

permission.)

Fig. 2. Middle fossa approach to internal acoustic meatus. (A) Superior view of left temporal bone. Facial canal exposed

proximal and distal to junction with canal of greater petrosal nerve. Internal auditory meatus unroofed. (B) Enlarged view.

Vertical and transverse crests exposed at lateral end of internal auditory meatus. (C) Cochlea exposed in angle between the

groove for the greater petrosal nerve and labyrinthine part of the facial canal. (D) Specimen with nerves intact. Dura and

bone above facial canal and internal acoustic meatus removed. Cochlear nerve exposed medial to geniculate ganglion.

(E) Three semicircular canals, nerves in meatus, and carotid artery exposed. (From Pait TG, Harris FS, Paullus WS,

et al. Microsurgical anatomy and dissection of the temporal bone. Surg Neurol 1977;8:363–91; with permission.)

148

RHOTON & TEDESCHI

Fig. 2 (continued)

MICROSURGICAL ANATOMY OF ACOUSTIC NEUROMA

149

Fig. 2 (continued)

middle cranial fossa until the arcuate eminence

and greater petrosal nerve are identiﬁed. The dis-

tance from the inner table of the skull to the facial

hiatus, through which the greater petrosal nerve

passes, ranges from 1.3 to 2.3 cm (average 1.7

cm) [17] . In separating the dura from the ﬂoor

of the middle fossa, one should remember that

bone may be absent over all or part of the genic-

ulate ganglion. In a previous study of 100 tempo-

ral bones, we found that all or part of the

geniculate ganglion and genu of the facial nerve

was exposed in the ﬂoor of the middle fossa in

15 bones (15%) [19] . In 15 other specimens, the

geniculate ganglion was completely covered, but

there was no bone extending over the greater pe-

trosal nerve. The greatest length of greater petro-

sal nerve covered by bone was 6 mm. More than

50% of the specimens had less than 2.5 mm of

greater petrosal nerve covered.

It is also important to remember that the

petrous segment of the carotid artery may be

exposed without a covering of bone in the ﬂoor of

the middle fossa deep to the greater petrosal

nerve. In a previous study, we found that a 7-mm

length of petrous carotid artery may be

exposed without a bony covering in the area

below where the greater petrosal nerve passes

below the lateral margin of the trigeminal

ganglion [5,12] . The foramen spinosum and mid-

dle meningeal artery and the foramen ovale and

3rd trigeminal division are situated at the anterior

margin of the extradural exposure. The extradural

exposure can usually be completed without oblit-

erating the middle meningeal artery at the fora-

men spinosum. The tensor tympany muscle and

eustachian tube, although not exposed in this ap-

proach, are located beneath the ﬂoor of the mid-

dle fossa roughly parallel to and in front of the

horizontal portion of the petrous carotid (see

Fig. 3 ).

In completing the middle fossa approach, bone

is removed over the greater petrosal nerve to

expose the geniculate ganglion and genu of the

facial nerve. From here the labyrinthine portion

of the facial nerve is followed to the lateral end of

the internal auditory canal by removing bone. The

lateral part of the bone removal is limited

posteriorly by the superior semicircular canal,

which is located a few millimeters behind and is

oriented parallel to the labyrinthine segment of

the facial nerve. The anterior edge of the exposure

is limited by the cochlea, which sits only a few

millimeters anterior to the site of bone removal in

the angle between the labyrinthine portion of the

facial nerve and the greater petrosal nerve. It is

important that the cochlea and semicircular

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