The Acoustic (Vestibulocochlear) Nerve

Some information about hearing may be obtained simply by
observation and gauging the patient’s ability to understand soft and
loud tones and low and high pitches; note signs of deafness, such as a
tendency to turn the head when listening, lip reading, or speaking with
a loud voice. Any history of hearing difficulty, such as trouble using
the telephone or hearing conversation in noisy environments, or
complaints from family members, should prompt a careful evaluation.
Before testing hearing, otoscopic examination should be done to ensure
the tympanic membrane is intact, and to exclude the presence of wax,
pus, blood, foreign bodies, and exudate. The mastoid region should be
examined for swelling and tenderness.

Conductive hearing loss is that due to impaired
conduction of sound to the cochlea and may be due to occlusion of the
external auditory canal, middle ear disease (e.g., otitis), or
abnormality of the ossicular chain (e.g., otosclerosis). Sensorineural
hearing loss is that due to disease of the cochlea (e.g., Ménière
disease) or eighth cranial nerve (e.g., acoustic neuroma). Central
hearing loss is that due to disease of the central pathways. Central
hearing loss is very rare because of the bilaterality and redundancy of
the auditory system; unilateral lesions of the central auditory
pathways typically do not cause any deficit detectable by routine
clinical testing.

There are many ways to assess hearing at the bedside.
All are crude compared to the information that can be obtained with a
formal audiogram. Bedside clinical testing of hearing may theoretically
use any available instrument that is capable of making a sound. Because
the ability to hear and understand speech is the most important
functional aspect of audition, whispered voice is useful; clinically
significant hearing loss is usually detectable using this simple
modality. In certain types of deafness, loss of speech discrimination
is of clinical significance, even though pure tone and even speech
thresholds are normal. One key to the effective use of whisper is
unpredictability of the stimulus, for example, the numbers, “1, 2, 3”
in one ear and “7, 8, 9” in the other. Monosyllables are preferable to
common stock questions such as “How are you?” in which hearing a small
part may enable the patient to “hear” the rest in context. Alternating
words and numbers is a challenging test of hearing. Other useful sounds
for bedside testing include finger rub and pure tones created by a
tuning fork.

Detailed testing of hearing is done monaurally, ideally
while occluding the opposite ear, as by pressing the tragus over the
canal, and the patient is asked to compare the sound intensity between
the two ears. The examiner may also compare the distance from each ear
at which a sound of the same intensity can be heard.

Tuning forks—typically 128, 256, or 512 Hz—are sometimes
used to give more specific information and to assess air conduction
(AC) and bone conduction (BC). The patient may be asked to compare the
loudness of the vibrating fork in the two ears, or the examiner may
compare the distance on each side at which the fork begins or ceases to
be heard. The examiner with good hearing may compare the patient’s air
and bone conduction with his own. In evaluating BC, be certain the
patient hears rather than feels the tuning fork.

The Rinne test compares the patient’s AC and BC; it can
be done in at least two ways. An activated fork may be placed first on
the mastoid process, then immediately beside the ear (or vice versa),
and the patient asked which is louder; it should always be louder by
the ear. The more time-consuming, traditional method is to place the
tuning fork on the mastoid and when no longer heard there move it
beside the ear, where it should still be audible. The fork should be
heard twice as long by AC as by BC. The Rinne test is normal or
positive when AC is better than BC. In conductive hearing loss, AC is
impaired but BC is preserved; sound is not conducted normally through
the canal or from the tympanic membrane through the ossicular chain to
the cochlea, but the sensorineural mechanisms are intact. In
sensorineural hearing loss (SNHL), both AC and BC are impaired while
retaining their normal relationship of AC better than BC

In the Weber test, a vibrating tuning fork is placed in
the midline on the vertex of the skull. It may be placed anywhere in
the midline, over the nasal bridge, forehead, or maxilla, but works
best over the vertex. Normally, the sound is heard equally in both ears
or seems to resonate somewhere in the center of the head; it is “not
lateralized.” In conductive hearing loss, the sound is heard better on
(“lateralized to”) the involved side. In sensorineural deafness, the
sound is heard best in the normal ear.

In summary, with unilateral conductive hearing loss
(CHL) there is primarily loss of AC; BC is preserved or even
exaggerated and the Weber lateralizes to the involved side (Table 13.1).
With unilateral SNHL, AC and BC are both diminished, but AC remains
better than BC, and the Weber lateralizes to the normal ear. With CHL,
low tones are lost, as are some of the broad or flat consonants and
vowels such as m, n, l, r, o, and u. Impairment of speech
discrimination parallels the loss for pure tones. There is no
recruitment, and tone decay is normal. Patients with CHL tend to hear
speech better in a noisy background than in a quiet setting. In SNHL,
the hearing loss is worse for higher frequencies, and there is greater
difficulty with sibilants, sharp consonants, and short vowels (e.g., in
the words sister, fish, twenty, water, and date). Auditory reflex
responses that

produce
a blink or reflex eye closure in response to a loud, sudden noise are
occasionally useful in evaluating hearing in children, patients with
altered mental status, and in hysteria or malingering. Mixed hearing
loss, with elements of both CHL and SNHL, is not uncommon.

Dysfunction of the cochlear nerve and its connections
usually causes either diminution or loss of hearing (hypacusis or
anacusis), with or without tinnitus. Conductive hearing loss is due to
interference with the transmission of sound to the cochlea.
Sensorineural hearing loss is due to disease of the cochlea or its
central connections. In essence, CHL is due to disease external to the
oval window, and SNHL is due to disease central to the oval window.
Some causes of hearing loss are listed in Table 13.2.

Sensorineural hearing loss may be due to disease of the
cochlea (end-organ deafness), such as in Ménière disease, or to disease
of CN VIII or more central structures (retrocochlear), as in acoustic
neuroma. Audiographic findings typical of cochlear disease are loss of
acuity for pure tones with a parallel impairment of speech
discrimination, recruitment, and tone decay. Recruitment is an abnormal
loudness of sounds due to cochlear dysfunction, which can cause a
paradoxical increase in the perception of louder sounds, sometimes
accompanied by sound distortion. Retrocochlear lesions tend to cause a
loss of speech discrimination out of proportion to the loss for pure
tones, no recruitment, and abnormal auditory adaptation by tone decay.

The cochlear and vestibular nerves run together in a
common sheath from the brainstem to their respective end organs, and
disorders of the eighth nerve between the cochlea and brainstem may
cause hearing loss. Some disease processes affect both divisions
peripherally (e.g., labyrinthitis) or centrally (e.g., brainstem
neoplasm). In its course across the cerebellopontine angle (CPA), the
most important disorder to affect both divisions is a neoplasm.
Acoustic neuroma (acoustic

neurinoma,
acoustic schwannoma) is the most common, but neurofibroma, meningioma,
facial nerve schwannoma, cholesteatoma, epidermoid cyst, and other
tumors may arise in the CPA as well. Other conditions that may cause
both hearing loss and vertigo include Ménière disease, labyrinthitis,
viral infection (especially herpes), trauma, meningitis, vascular
occlusion (internal auditory or anterior inferior cerebellar), Susac
syndrome, Cogan syndrome, Fabry disease, perilymphatic fistula, toxins,
and drugs.

Nonorganic hearing loss refers to hearing loss in the
absence of any organic disease, or hearing loss that is exaggerated. It
is more common for real hearing loss to be exaggerated in severity than
for it to be feigned with entirely normal hearing. In most instances
nonorganic hearing loss is a transient symptom related to acute
emotional stress. It may be partial or total, unilateral or bilateral.
It is often bilateral and total, and the patient makes no attempt to
hear what is said or to read the speaker’s lips. The mainstay of
diagnosis is inconsistency in the performance on hearing tests and the
absence of verifiable abnormalities on objective tests. Psychogenic
hearing loss may be associated with other nonorganic symptoms, such as
mutism and blindness. Patients simulating bilateral deafness do not
behave as a deaf person does. Deaf individuals usually raise their
voices during conversation and keep their eyes fixed on the speaker’s
face and lips, watching for any gesture that may help understanding. A
deaf man eager to hear will automatically turn his best ear toward the
speaker. Experienced lip readers have difficulty with sound-alike
words; the dissembler may do better than expected because the words are
actually heard.

Many tests have been devised for detection of unilateral
nonorganic deafness. The diagnosis is best made audiometrically;
discrepancies and inconsistencies on repeated audiometric examinations
are typical. With some trickery, a stethoscope—with one earpiece
occluded—can be put with the occluded earpiece in the good ear and the
open earpiece into the bad ear, to demonstrate that the “deaf” ear can
hear. In the yes-no test, the examiner whispers into the patient’s deaf
ear after instructions to “say yes if you hear it and no if you don’t.”

Tinnitus is spontaneous noise in the ears originating
inside the head. There are many types, and the causes are protean. In
many cases no precise etiology can be established. The most common
identifiable cause is noise exposure. Objective tinnitus refers to
noise audible to both the patient and the examiner, as occurs in
carotid stenosis. Most tinnitus is subjective tinnitus. It may vary in
pitch and intensity, and may be continuous or intermittent. It may be
described in many ways, such as ringing, buzzing, blowing, whistling,
swishing, or roaring. Tinnitus is commonly associated with deafness. It
is common in presbycusis and in other types of SNHL, and is a fairly
constant feature of otosclerosis. Most cases are due to disease of the
cochlea or eighth nerve; some are due to CNS disease. Tinnitus is often
more noticeable at night when environmental noises are diminished, and
it may interfere with sleep. To the patient, tinnitus may be more
distressing than the accompanying deafness, and it may cause depression
in elderly individuals.

Pulsatile tinnitus is synchronous with the pulse; it is
in reality a bruit. Causes include carotid stenosis, arteriovenous
malformations, particularly of the dura, glomus tumors, venous hums,
and hypertension. Pulsatile tinnitus is fairly common in pseudotumor
cerebri, and it occurs occasionally in increased intracranial pressure
of other origins. The perilymphatic duct connects the perilymph-filled
spaces of the cochlea and an extension of the subarachnoid space in the
region of the jugular foramen. Through this channel, pulsations in the
subarachnoid space are transmitted to the cochlea. Vascular tinnitus
may occasionally be affected by carotid artery compression. Rhythmic
tinnitus not synchronous with the pulse may occur with palatal
myoclonus (palatal microtremor).

Other causes of tinnitus include cerumen impaction,
medications (particularly ototoxic drugs), Ménière disease, acoustic
neuroma, and Arnold-Chiari malformation. Muscle spasm, contraction of
the tensor tympani, nasopharyngeal sounds, and temporomandibular joint
clicking may also simulate tinnitus. Tinnitus may be psychogenic.
Bizarre types of tinnitus may occur with pontine and cerebral lesions.
Auditory hallucinations may occur in lesions of the temporal lobe;
these are frequently epileptic auras. More bizarre hallucinations occur
in psychotic and drug-induced states.

The conditions that may present as dizziness range from
trivial to life threatening, and are often difficult to evaluate and
manage. The nebulousness of the patient’s description of dizziness
often produces frustration on the part of the clinician, yet in few
other conditions are the historical details so pivotal in correct
diagnosis. Fortunately, the truly serious conditions that present as
dizziness are rare. The first step in understanding the symptom is to
have the patient describe what he means by “dizziness.” Patients use
the word dizzy to describe vertigo, as well as a number of other
sensations, such as lightheadedness or giddiness, sometimes referred to
as pseudovertigo. Concomitant dysfunction in several systems may cause
dizziness. Conflicting sensory information may certainly cause
dizziness; the sensory mismatch from watching a motion picture with
dramatic movement in the visual panorama while sitting in a stationary
seat illustrates the effect.

In a classic study of 100 dizzy patients in an
ambulatory setting, the causes were as follows: vestibulopathy (54),
psychiatric disorders (16), multifactorial (13), unknown (8),
presyncope (6), dysequilibrium (2), and hyperventilation (1) (Drachman
DA, Hart CW. An approach to the dizzy patient. Neurology
1972;22:323-334). The most common treatable conditions were benign
positional, or benign paroxysmal positional, vertigo (BPV or BPPV), and
psychiatric disorders. Other studies have shown a similar distribution.
Some of the causes of dizziness are listed in Table 13.3. Table 13.4
lists causes of dizziness due to labyrinthine or vestibular pathway
dysfunction. Before discussing vestibular disease, some discussion of
nonspecific dizziness is warranted, since patients with such complaints
make up a large proportion of the dizzy population.

Cerebral hypoperfusion produces a sensation of
lightheadedness, drunkenness, or impending syncope without spinning,
whirling, or any illusion of environmental motion. Such hypoperfusion
may occur under a variety of circumstances, all of which may lead the
patient to seek medial attention because of “dizziness.” In
hyperventilation syndrome (HVS), hypocapnia induced cerebral arterial
constriction and the resultant hypoperfusion induces lightheadedness
along with other symptoms, such as chest pain, headache; numbness and
tingling of the hands, feet, and circumoral region; and occasionally
outright syncope. Frequently, patients are unaware of their
overbreathing, but the high minute volume of respiration produces
dryness of the mouth, which the patient may describe spontaneously or
respond to on specific questioning. Induced hyperventilation may
reproduce the symptom complex. Orthostatic hypotension due to drugs,
prolonged standing, dehydration, increased vagal tone, or dysautonomia
likewise may present as lightheadedness or faintness. Accompanying
symptoms are few, and only a careful history eliciting the relationship
of the dizziness to posture will make the diagnosis. Global cerebral
hypoperfusion may also result from decreased cardiac output via any
number of mechanisms; arrhythmia is the primary concern.

Elderly patients “deafferented” because of separate
disease processes affecting different sensory systems may present with
complaints of vague dizziness, unsteadiness, and difficulty with
balance, particularly when turning (multiple sensory defect vertigo).
Patients can apparently tolerate problems with any one afferent system,
but when multiple systems are involved imbalance and dizziness result.
The term presbylibrium has been applied to poor balance due to aging.

Numerous terms have been employed to describe the
clinical phenomenology of vestibular disease; not all are helpful.
Vertigo is the sensation of environmental motion (spinning, whirling,
lateropulsion, tilt). The term “true vertigo” is sometimes used to
describe this symptom. When true vertigo is present, the problem is
usually an acute peripheral vestibular disturbance. Objective vertigo
creates the sensation that the environment is spinning, whereas
subjective vertigo creates the sensation that the patient is spinning.
The absence of true vertigo does not exclude peripheral vestibular
disease, especially if bilateral pathology exists, such as in
ototoxicity due to drugs. Central vertigo is due to CNS disease;
peripheral vertigo is due to disease of the peripheral vestibular
apparatus or its connections. Patients with CNS lesions may not have
true vertigo. Acoustic neuroma causes a gradual unilateral loss of
vestibular function and is more prone to cause imbalance than true
vertigo. Some other serious conditions may present as dizziness without
true vertigo, such as cardiac dysrhythmias and dysautonomic
orthostasis. Physicians should not make too much of the presence or
absence of true vertigo in judging how seriously to take a patient’s
complaint of dizziness.

Dizziness may be present constantly or intermittently.
If intermittent, as in BPPV, one of the most common causes of
dizziness, the episodes may occur so frequently that the initial
description may lend the impression the symptoms are constant. If the
episodes are intermittent, the duration of the attacks is important.
Attack duration is one of the most important features in distinguishing

between
central and peripheral vertigo. In vertigo due to BPPV, the attacks
last 10 to 30 seconds; in other peripheral vestibulopathies, such as
Ménière disease, the attacks last hours; and in vertebrobasilar
insufficiency, the episodes last for minutes. Exploring the
precipitating factors is very helpful. Dizziness may be provoked by
head or body movement, standing, lying down or occur spontaneously. The
presence of associated symptoms, such as nausea, vomiting, staggering,
deviation of the eyes, disturbances of balance, prostration, tinnitus,
hearing loss, or loss of consciousness, is important. Table 2.8 reviews some of the pertinent history to explore in a dizzy patient.

Useful bedside testing of vestibular function includes
assessment of vestibulospinal reflexes (past pointing, Romberg, Fukuda
stepping test), tests of vestibulo-ocular reflexes (oculocephalic
reflex, head thrust test, dynamic visual acuity, and caloric
responses), and searching for nystagmus (spontaneous, positional, or
after head shaking). In recalling the expected pattern of responses to
some of these, remember that the vestibular system tends to push (eyes,
limbs, body) to the opposite side; when the system is in balance the
eyes are midline and the limbs and body can accurately find a target.
When disease is present the involved labyrinth is usually hypoactive
and the uninvolved labyrinth pushes toward the abnormal side. The
bedside tests of vestibular function are listed in Table 13.5.

Past pointing is a deviation of the extremities caused
by either cerebellar or vestibular disease. It is a poor term because
it implies the patient points beyond the target, when in fact it simply
means the patients misses the target, most often to one side or the
other. Testing is usually done with the upper extremities. A quick and
effective technique is simply to have the patient close his eyes while
doing traditional cerebellar finger-to-nose testing. If past pointing
is present, the limb will deviate to the side of the target because of
the absence of visual correction. This method will usually bring out
past pointing if it is present. The traditional method is to have the
patient extend the arm and place his extended index finger on the
examiner’s index finger; then with eyes closed raise the arm directly
overhead; then bring it back down precisely onto the examiner’s finger.

Cerebellar and vestibular past pointing have different
patterns. With acute vestibular imbalance, the normal labyrinth will
push the limb toward the abnormal side, and the patient will miss the
target to one side. The past pointing will always be to the same side
of the target and will occur with either limb. With a cerebellar
hemispheric lesion, the ipsilateral limbs have ataxia and
incoordination; past pointing occurs only with the involved arm and may
be to the side of the lesion or erratically to either side of the
target. In vestibulopathy, the past pointing is with both limbs, and to
the same side of the target, in cerebellar disease, it occurs only with
one upper extremity, and to either side of the target with no
consistent pattern. The lower limbs would behave similarly but are more
difficult to test. In vestibulopathy, after a period of compensation
the past pointing disappears and may even begin to occur in the
opposite direction.

Romberg’s test is described in more detail in Chapter 33.
In brief, the Romberg compares balance as the patient stands with eyes
open and eyes closed. The feet should be brought as close together as
will allow the patient to maintain eyes open balance. A normal
individual can stand feet together and eyes open without difficulty,
but not all patients can. The critical observation is eyes open v. eyes
closed. Inability to maintain balance with eyes open and feet together
is not a positive Romberg. In unilateral vestibulopathy, if balance is
lost with eyes closed the patient will tend to fall toward the side of
the lesion, as the normal vestibular system pushes him over. If the
patient has spontaneous nystagmus due to a vestibular lesion, the fall
will be in the direction of the slow phase. In peripheral vestibular
disease the direction of the fall can be affected by changing head
position; the patient will fall toward the abnormal ear. With a right
vestibulopathy and facing straight ahead, eye closure will cause the
patient to fall to the right; looking over his right shoulder, he will
fall backward; and looking over his left shoulder, he will fall
forward. The sharpened Romberg (tandem Romberg), which is done by
having the patient stand tandem with eyes closed, may be useful in some
circumstances.

The patient with an acute vestibulopathy may have
difficulty with tandem gait with a tendency to fall to the side of the
lesion, but normal straightaway walking may appear unimpaired because
visual cues compensate for the vestibular abnormality. But straightaway
walking with eyes closed may be informative. A normal individual can
walk without visual clues well enough to point his index finger at the
palm of the examiner’s hand, close his eyes, walk along a path of 20 ft
or so, and then touch the finger to the examiner’s palm. The patient
with acute vestibulopathy may drift toward the side of the lesion and
end up well off the target, the gait equivalent of past pointing.

The Fukuda stepping test is analogous. The patient, eyes
closed, marches in place for one minute. A normal individual will
continue to face in the same direction, but a patient with acute
vestibulopathy will slowly pivot toward the side of the less active
labyrinth. In the star walking test, the patient, eyes closed, takes
several steps forward then several steps backward, over and over. A
normal individual will begin and end oriented approximately along the
same line. A patient with acute vestibulopathy will drift toward the
less active side walking forward, and continue to drift during the
backward phase. The resulting path traces out a multipointed star
pattern. As with past pointing, the direction of gait drift, pivoting
on the stepping test, and similar findings do not reliably indicate the
side of the lesion in patients with chronic vestibulopathy after
compensation has occurred.

The vestibulo-ocular reflex (VOR) serves to move the
eyes at an equal velocity but in the direction opposite the head
movement; this keeps the eyes still in space and maintains visual
fixation while the head is in motion. There are several ways to examine
the VOR, including the doll’s eye test, head thrust test, dynamic
visual acuity, and calorics.

The oculocephalic response is primarily useful in the
evaluation of comatose patients. Turning the head in one direction
causes the eyes to turn in the opposite direction. This response
indicates that the

pathways
connecting the vestibular nuclei in the medulla to the extraocular
nuclei in the pons and midbrain are functioning and that the brainstem
is intact. In an alert patient, visuomotor and ocular fixation
mechanisms come into play, limiting the drawing of any conclusions
about vestibular function.

The doll’s eye test utilizes side-to-side head movements
in a comatose patient; the head thrust test is done in an awake
patient. Abrupt, rapid movements are made in each direction while the
patient attempts to maintain fixation straight ahead, as on the
examiner’s nose. The ocular smooth pursuit mechanism cannot compensate
for head movements done at such high velocity, but normally the VOR
will maintain fixation and the eyes will hold on target. When the VOR
is impaired, the compensatory eye movement velocity is less than the
head movement velocity; the eyes lag behind the head movement and a
corrective “catch-up” saccade must be made to resume fixation in the
eccentric position.

The ability of the VOR to maintain ocular fixation means
that a patient can read even while shaking the head to and fro. The
dynamic visual acuity test is performed by obtaining a baseline acuity,
and then determining the acuity during rapid head shaking. Degradation
by more than three lines on the Snellen chart suggests impaired
vestibular function.

Caloric responses are frequently used to check for
brainstem integrity in comatose patients. Ice water instilled into one
ear canal will abruptly decrease the tonic activity from the labyrinth
on the irrigated side. Cold calorics in a comatose patient with an
intact brainstem cause tonic deviation of the eyes toward the side of
irrigation as the normally active labyrinth pushes the eyes toward the
hypoactive, irrigated labyrinth. In an awake patient, cold calorics
cause nystagmus with the fast component away from the irrigated side
because the cerebral cortex produces a compensatory saccade that jerks
in the direction opposite the tonic deviation. The familiar mnemonic
“COWS” (cold opposite, warm, same), refers to the fast phase of the
nystagmus, not to the tonic gaze deviation. Nystagmus is seen only when
the cortex is functioning normally. Warm water irrigation has opposite
effects. Bilateral simultaneous cold calorics induce tonic downgaze,
warm calorics upgaze.

In comatose patients, large volumes of ice water, 30 cc
to 50 cc, are commonly used since it is imperative to elicit the
response if it is present. Calorics can also be done to assess
vestibular function in dizzy patients, either using much smaller
volumes, 2 cc to 10 cc of an ice and water slush (minicalorics), or
larger volumes of water less cold. The latency to onset and the
duration of the nystagmus elicited is compared on the two sides. A
difference of more than 20% in nystagmus duration suggests a lesion on
the side of the decreased response.

Whether in comatose or awake patients, the head is
positioned so as to bring the horizontal canal into a position to
elicit a maximal response. In comatose patients lying supine this is
with the head flexed 30 degrees to bring the horizontal canal vertical.
For awake patients, the same position may be used, or the head may be
extended with the seated patient looking at the ceiling.

Nystagmus is discussed in greater detail in Chapter 10.
The characteristics of vestibular nystagmus will be briefly reviewed.
Nystagmus due to vestibular disease may occur spontaneously or be
produced by various maneuvers.

The slow phase of spontaneous vestibular nystagmus is
usually in the direction of the lesion, with the fast phase away,
because an acute vestibular lesion typically causes hypoactivity of the

labyrinth.
The findings are similar to those produced by ice water irrigation of
the ear, and are due to the normal labyrinth pushing the eyes toward
the diseased side with the cortex generating a corrective saccade away
from the abnormal side. Because of the influence of the three different
semicircular canals, vestibular nystagmus may beat in more than one
direction, the summation of which creates an admixed rotatory component
rarely seen with other conditions. Vestibular nystagmus typically is
fine, often present but easily overlooked in primary position. Third
degree nystagmus (fast component opposite to the direction of gaze)
rarely occurs with any other nystagmus type. Vertigo, deafness, and
tinnitus also help mark nystagmus as vestibular. When evaluating
nystagmus, the presence of a torsional component suggests a peripheral
origin. The amplitude increases with gaze in the direction of the fast
phase. Peripheral vestibular nystagmus (i.e., that due to disease of
the labyrinth or eighth nerve) is markedly inhibited by visual
fixation. Inhibiting fixation with Frenzel lenses (strongly convex
spectacles that block visual fixation) will often make the nystagmus
more obvious by both blocking fixation and magnifying the eyes. When
Frenzel lenses are used, torsional nystagmus is often more prominent
because vertical and horizontal nystagmus are more easily suppressed by
visual fixation. Another technique is to have the patient close the
eyelids gently, then partially lift one lid and look for abnormal
movements of the scleral vessels. The fundoscopic examination may bring
out subtle vestibular nystagmus. The dim lighting lessens visual
fixation, and the disc is magnified. A rhythmic jerking of the disc to
the patient’s right indicates left beating nystagmus. Formal eye
movement studies are done in darkness with recording of the eye
movements by electrodes in order to remove the effects of visual
fixation. Failure of visual fixation to suppress nystagmus suggests the
nystagmus may be of central origin, usually a cerebellar or brainstem
lesion. The spontaneous nystagmus due to a central lesion may be purely
horizontal or purely vertical.

Nystagmus can sometimes be induced by having the patient
rapidly turn the head back and forth with the eyes closed for about 30
seconds, then opening the eyes (head-shaking nystagmus). No nystagmus
occurs in normal individuals, but patients with vestibular imbalance
may have brief spontaneous nystagmus beating away from the abnormal
side. Alternately, the patient may wear Frenzel lenses while shaking
the head. Spontaneous nystagmus can occasionally be produced by tapping
on the head or by low frequency vibration applied to the mastoid.
Hyperventilation may also help bring out vestibular nystagmus.

When not present spontaneously, nystagmus can sometimes
be elicited by placing the patient’s head into a particular position.
To perform the Dix-Hallpike (Hallpike or Nylen-Bárány) maneuver, the
patient is moved from a seated position to a supine position with the
head extended 45 degrees and turned 45 degrees to one side so that one
ear is dependent. In BPPV, nystagmus begins after a latency of about 3
to 10 seconds, occasionally as long as 40 seconds; persists for 20 to
30 seconds, rarely as long as a minute; and then gradually abates even
though the head remains in the provoking position. The nystagmus is
commonly torsional with the fast component toward the dependent ear.
The response is usually much more dramatic in one particular head
position. Typically, the patient will experience whirling, occasionally
nausea, and rarely vomiting. The response is fatigable, and repeating
the maneuver several times consecutively provokes less of a response
each time until eventually the nystagmus and vertigo are nil. This type
of positional nystagmus is most often due to peripheral vestibular
disease. Although rare, positional vertigo can occur with a central
lesion, especially one near the fourth ventricle, but the
characteristics of the nystagmus are different. With a central lesion,
there may be no latency, and the nystagmus often begins as soon as the
head is placed in the provoking position. Central positional nystagmus
is typically vertical (either up- or downbeating), without the rotatory
component seen with peripheral lesions. In addition, the nystagmus and
associated symptoms may persist for a prolonged period, longer than 30
to 40 seconds, sometimes continuing as long as the head position is
maintained. With central lesions there may be a mismatch in

the
severity of the nystagmus, vertigo, and nausea, in contrast to
peripheral lesions where nystagmus, vertigo, and nausea are generally
of comparable intensity.

Positional nystagmus can be divided into a paroxysmal
type that is fleeting, fatigable, often difficult to reproduce, and
associated with prominent vertigo, and a static type that does not
fatigue, persisting as long as the head is maintained in the provoking
position, often with little associated vertigo. The static type can
occur with either central or peripheral vestibular lesions, but a lack
of visual suppression increases the likelihood of a central lesion.

The characteristics of peripheral versus central positional nystagmus and related findings are summarized in Table 13.6.

The primary manifestation of disorders of the vestibular
nerve is vertigo and related symptoms such as imbalance. Vertigo will
be used in this discussion as a surrogate for all similar symptoms. One
of the primary concerns when dealing with a vertiginous patient is to
separate central vertigo, due to CNS disease, from peripheral vertigo,
due to peripheral vestibular disease. Disease of the

peripheral
vestibular apparatus or eighth cranial nerve produces peripheral
vertigo. Disease of the central vestibular connections produces central
vertigo. The vestibular nuclei lie within the CNS in the dorsolateral
medulla; disease there may act like either peripheral or central forms.
Central vertigo is much less common than peripheral. Certain features
are helpful in making the distinction. Central vertigo is typically
less severe, and other neurologic signs and symptoms are usually
present. Peripheral vestibular disorders cause more nausea, vomiting,
and autonomic symptoms than do central disorders. Imbalance tends to be
more severe with central lesions, and the patients are often unable to
stand or walk. Associated symptoms are helpful if present. Aural
symptoms (hearing loss, tinnitus, pain or fullness in the ear) suggest
a peripheral cause. Facial weakness or numbness occurs with lesions
involving the eighth nerve in the cerebellopontine angle. Processes in
the brainstem typically cause prominent neighborhood signs; isolated
vertigo is rare. Occasionally, vertigo can be a manifestation of
disease of the more rostral vestibular pathways, including the temporal
lobe.

Distinguishing central from peripheral nystagmus is a
common clinical exercise. The most helpful features are of course the
presence of aural symptoms and signs in peripheral nystagmus and the
presence of CNS symptoms and signs with central nystagmus. Peripheral
vestibular nystagmus does not change direction, although it may vary in
amplitude depending on the direction of gaze, and is strongly
suppressed by visual fixation. Central nystagmus typically changes
direction and may not be affected by visual fixation.

Because of the effects of fixation and other
compensatory mechanisms, peripheral nystagmus is seldom prominent after
the first 12 to 24 hours, but central nystagmus may persist for weeks
or months. The vestibular apparatus pushes not only the eyes, but also
the limbs and the body to the opposite side. With acute peripheral
vestibulopathy, the patient will past point, fall on Romberg, turn on
the stepping test, and drift walking eyes closed in the direction of
the nystagmus slow phase. Failure to follow these rules (e.g., past
pointing in the direction of the fast phase) suggests a central lesion,
but can occur with a compensated peripheral lesion. Peripheral
nystagmus is often positional, and the vertigo and vegetative symptoms
are in proportion to the nystagmus. With positional nystagmus, latency
to onset, fatigability, and adaptability all support a peripheral
process. Minimal vertigo with prominent nystagmus, or lack of latency,
fatigability, and adaptability suggests a central process. Peripheral
nystagmus often has a rotary component, and the horizontal nystagmus
beats in the same direction in all fields of gaze (may even be third
degree). Central nystagmus tends to change directions. Visual fixation
inhibits peripheral nystagmus but has no effect on central nystagmus.

Involvement of the vestibular nuclei may cause vertigo
that has central features. Common processes that involve the brainstem
and that are likely to cause vertigo include ischemia, demyelinating
disease, and neoplasms. Less common brainstem lesions causing central
vestibular dysfunction include arteriovenous malformation,
syringobulbia, hematoma, and spinocerebellar degeneration. Lesions in
the cerebellopontine angle affect both the auditory and vestibular
portions of CN VIII.

Vertebrobasilar transient ischemic attacks, or
“vertebrobasilar insufficiency,” commonly causes vertigo, most often
along with other signs and symptoms. Rare patients may have transient
vertigo without accompanying symptoms. A syndrome of acute vertigo
mimicking labyrinthitis may occur with acute cerebellar infarction or
hemorrhage. The most common misdiagnosis in one series of patients with
cerebellar hematoma was labyrinthitis. In acute cerebellar lesions, the
patient will tend to fall toward the side of the lesion on Romberg
testing; the nystagmus may also be maximal with gaze toward the lesion.
As a result, the patient may fall in the direction of the fast phase,
opposite the pattern seen in acute peripheral vestibulopathy.

Dizziness, vertigo, and dysequilibrium occur commonly in
patients with multiple sclerosis (MS). In one instance, an MS lesion in
the medulla caused clinical symptoms mimicking vestibular neuronitis.
However, MS patients are not immune from developing the common syndrome
of BPPV, and peripheral vestibulopathy may be a more common cause of
vertigo in such patients than MS exacerbation.

There is a relationship between migraine and episodic
vertigo. Motion sensitivity is common in migraineurs, and episodic
vertigo occurs in as many as 25%. Isolated attacks of vertigo have been
labeled as a migraine equivalent. Occasionally, migraine patients may
also develop cochlear symptoms, perhaps due to spasm of the
labyrinthine microvasculature. Migraine can mimic Ménière disease. In
some patients there may be a channelopathy involved.

Disorders of the peripheral vestibular apparatus are the most common cause of vertigo and related symptoms. Table 13.4
lists some of the causes of peripheral vertigo. In BPPV, the most
common peripheral vestibulopathy, vertigo is induced by assumption of a
particular head position or by rapid head movement. Classically, such
patients experience vertigo when first lying down or when rolling over
in bed at night, bending over, or looking up. Benign paroxysmal
positional vertigo attacks are brief, generally 10 to 30 seconds, and
frequent. BPPV probably results from otoliths that have become detached
from the macula of the utricle and formed free floating debris that
settles into the posterior canal, the most dependent portion of the
vestibular labyrinth. Movement of the debris causes the attacks of
vertigo. The Dix-Hallpike maneuver causes the debris to move and
reproduces the symptoms. The disorder predominantly affects the
posterior semicircular canal; about 5% involve the horizontal canal and
about 1% the anterior canal. Common identifiable antecedents are head
trauma and viral neurolabyrinthitis. Rarely, patients with posterior
fossa tumors have a clinical picture nearly identical to BPPV.

In vestibular neuronitis or labyrinthitis, more severe
attacks prostrate the patient for several days. Although often used
interchangeably, technically labyrinthitis is accompanied by cochlear
dysfunction, whereas vestibular neuronitis is purely vestibular. Mild,
brief attacks of vertigo similar to BPPV may plague the patient for
months to years after seeming recovery. Some instances of apparent
labyrinthitis are likely due to internal auditory artery ischemia. In
Ménière disease, the attacks of vertigo typically last several hours
and patients describe other symptoms, either along with the vertigo or
independently, including hearing loss (classically fluctuating),
tinnitus, and a sensation of vague pain or fullness in the ear. Inner
ear disease can occasionally cause drop attacks.