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ACOUSTIC

NEUROMAS

Clinical

Vignette

A

previously healthy 5B-year-old

woman noted left-

sided

hearing

loss

and

mild tinnitus that

had

gradually

worsened over

several

years. ln recent months, she no'

ticed that she

could

no[ use her

le{t

ear

for the

tele-

phone.

ihe believed

that

her right-sided

hearing

was

normal. ihe did

not report

any

other neurologic symp-

toms. Her

n'tedical

history

was

unremarkable.

an

examination,

the

patient was

bright and

alert

with mild

nystagnus

on

left lateral

gaze.

Facial sensa

tion

was intact and there

vvas no facial asymmetry. Her

hearingwas

grosslv

diminished only

in

the

leit

ear.

The

rentainder of her examination

results

were

normal.

Audiometri

c

exami

n ati o

n d e

m

o

n

strated

a

nar

ked

ly

decreased

pure

tone average

and

diminished

speech

discrimination

in the

leit

ear.

Brainstem

auditory

evoked

response

testing showed

prolonged

I ll

latency

on the

left. Cadoliniun-t

lvlRl revea/ed a homoge-

neous/r'enl-rancing

2

x

1.5.n1

nra-s.< in

the

le{t

cerebel

lar

pontine

angle enanating

fron the internal

auditory

canal,

leading to

mild

pontine

distortion.

The

history

of

slowl,v

progressive,

unllateral

hearir-rg

loss

associated

with tinnitus

is consistent

with ihe slow

growth

of

a

benign

acoustic

neu-

roma

(also

called

vesfibular

schwannoma).

These benign

tumors

arise

from

the Schwann

cells of

the

vestibular

nerve

within

the eighth cra-

nial nerve complex.

Acoustic

neuromas

comprise

approximately

6 /

to

8oh

of

primary intracranial

tumors. The

in-

cidence

is

27o within

the

general

population

and

peaks

between

the

ages

of

40

and 60

years.

Un-

less

patients

have

neurofibromatosis type ll, it is

unusual

to

see

acoustic

neuromas

before

the

age

of

20

years.

They

are

predominantly

unilat-

eral lesions,

although

those associated

with

type

ll

neurofibromatosis

often

are

bilateral

(Figure

73-2).

Clinical

Presentation and

Diagnostic

Studies

Although acoustic

neuromas arise

from

the

vestibular

portion

of CN-VIll,

hearing

loss

is

usu-

ally

the

most

prominent

symptom

(Figure

73-2).

Anatomically,

CN-VIl

(facial

nerve) is

closely

re-

lated

to

CN-Vlll;

however, it is unusual

to see

CN-VIl

involvement

as

a

presenting symptom.

Because of

CN-VIll's

relation

to the

vestibular

nerve and

its

proximity

to

the

brainstem and

cerebellum,

patients

with

larger

acoustic

neuro-

mas

often

have

gait

instability

or ataxia initially,

sometimes

as

presenting

signs. With

large tu-

mors, hydrocephalus can

occur

if.

CSF

flow is im-

paired

near

the fourth ventricle.

This

also

can

contribute

to

gait problems

and cause

head-

aches.

Cranial

nerve evaluation

is

of

the

utmost

im-

portance.

Hearing

loss

from

CN-VIll

involve-

ment

is

the

hallmark

of

these

tumors.

Lateral

gaze

nystagmus is

notable

on

extraocular

move-

ment testing.

Acoustic

neuromas

often

cause

pressure

on

the brainstem, affecting

CN-V

with

resultant

ipsilateral facial

sensory

loss

and

dimln-

ished

corneal

reflex.

Larger tumors

may

cause

CN-VIl

impairment

with

ipsilateral facial

weak-

ness.

Lower

cranial

nerve involvement

and

brainstem compression that

produce

hemipare-

sis or

sensory

loss

are unusual.

MRI

is the

best

means

of evaluation.

lts

clarity,

resolution,

and

ability to

scan

in multiple

planes

allow for

3-dimensional assessment. Because

le-

sion size

arrd its

relation to adjacent neurologic

structures, such

as

the

pons

and various cranial

nerves,

often

determine

treaiment, MRI

is

cru-

cial. However,

CT

is

often the

first

study

ob-

tained. Typically,

these

welldemarcated,

homo-

geneously

enhancing

tumors

arise

within

the

cerebellar

pontine

angle and

extend into

the

in-

ternaI

auditory

canal.

624

Nruno-oNcoLocY