Ординатура / Офтальмология / Английские материалы / The Neurology of Eye Movements_Leigh, Zee_2006

into a particular position in the orbit (the null point or zone). For patients with congenital nystagmus, there is usually some horizontal eye position in which nystagmus is minimized, and the eyes of patients with downbeat nystagmus may be quieter in up gaze. In practice, however, patients use head turns to bring their eyes to the quietest position, and only rarely are prisms that produce a conjugate shift helpful.

A different approach has been to use an optical stabilization device that negates the visual effects of eye movements.1196 This system consists of a high-plus spectacle lens worn in combination with a high-mi- nus contact lens. The system is based on the principle that stabilization of images on the retina could be achieved if the power of the spectacle lens focused the

primary image close to the center of rotation of the eye. However, such images are defocused, and a contact lens is required to extend back the focus onto the retina. Since the contact lens moves with the eye, it does not negate the effect of retinal image stabilization produced by the spectacle lens. With such a system it is possible to negate about 90% of the visual effects of eye movements.836 The system has several limitations, however. One is that it disables all eye movements (including the vestibulo-ocular reflex and vergence), so it is only useful while the patient is stationary and viewing monocularly. Another is that with the highest-power components (contact lens of —58.00 diopters and spectacle lens of +32 diopters), the field of view is limited. Some patients with ataxia or tremor (such as those with multiple

462 The Diagnosis of Disorders of Eye Movements

sclerosis) have difficulty inserting the contact lens. However, initial problems posed by rigid polymethyl methacrylate contact lenses have been overcome by develop-

ment of gas-permeable or even soft contact lenses.1505 Most patients do not need

the highest-power components for oscillopsia to be abolished and vision to be useful. In selected patients, the device may prove useful for limited periods of time, such as the duration of a television program. The effects of this optical system should be differentiated from that of simply wearing contact lenses, which appear to suppress congenital nystagmus, not owing to the mass of the lenses, but probably through stimulation of trigeminal afferents.367 The main therapy for latent nystagmus consists of measures to improve vision, especially patching for amblyopia in children.1439

Procedures to Weaken the

Extraocular Muscles

BOTULINUM TOXIN AS TREATMENT OF NYSTAGMUS

Injection of botulinum toxin into either the extraocular muscles or retrobulbar space has been used to temporarily reduce

or abolish acquired nystagmus (Fig. 10-17).304'610 Several studies have re-

ported that some patients gain improved, more stable vision.839'1137'1190'1386 Less of-

ten, botulinum toxin has been used to treat congenital or latent nystagmus.237'872 Common side effects are ptosis and diplopia, which may be more troublesome than the visual consequences of the nystagmus. Rarer complications include persistent filamentary keratitis.1386

A major limitation of botulinum toxin treatment for nystagmus is that it also impairs normal eye movements, static eye position being affected longer than the effect on saccades,14'674 which become hypometric (Fig. 10-17). Impairment of the vestibulo-ocular reflex causes patients to complain of blurred vision, oscillopsia, or vertigo when they walk. Another effect occurs in patients who habitually view with the injected, paretic eye. After several

days, adaptive changes take place (i.e., increased innervation to compensate for extraocular muscle weakness). Thus, for example, saccadic adaptation is apparent in the noninjected eye as hypermetric saccades (Fig. 10-17). In addition, the nystagmus itself may increase in the noninjected eye.

In summary, botulinum toxin may abolish nystagmusand improve vision in some patients and may be acceptable to patients who are prepared to view monocularly, but its limited period of action and side effects often reduce its therapeutic value.

SURGICAL PROCEDURES

FOR NYSTAGMUS

Three surgical procedures on extraocular muscles have been proposed as treatment for selected patients with congenital nystagmus; none have been properly evaluated for acquired nystagmus. One procedure is the Anderson-Kestenbaum operation,^4'763 which aims to move the attachments of the extraocular muscles so that the null point corresponds to the eyes' new central position. It is best planned by measuring the nystagmus at different gaze angles so that the surgeon

can calculate what is required to shift the position of the null point.360'1549 In prac-

tice, the Anderson-Kestenbaum procedure not only shifts and broadens the null zone but also decreases nystagmus outside of the null zone. However, it is of uncertain value in the treatment of acquired forms ofnystagmus.

The Cuppers procedure aims to diverge the eyes.315-1257 It may be helpful in pa-

tients with congenital nystagmus that is suppressed during fixation of near targets, and who have stereopsis. Studies comparing these two methods indicate that either the divergence procedure or combined operations give better visual improvement

than the Anderson-Kestenbaum procedure alone.734'1257'1549

A third surgical procedure for congenital nystagmusconsists of large recession of the horizontal rectus muscles.542'609'1441 Modest improvement of visual acuity is reported, but further studies are required to establish the role of this procedure and

determine whether weakening the extraocular muscles will induce adaptive changes that will cause the nystagmus to increase again.

Recently, the mechanisms by which these operations may damp congenital nystagmus have been re-evaluated. L. F. DelFOsso has suggested that simply detaching the muscles, dissecting the perimuscular fascia and then re-attaching them at the same site on the globe may suppress congenital nystagmus;352 experimental studies support this hypothesis.620a Such a procedure might have its effects by altering proprioceptive input from the pallisade organs that lie close to the attachment points (see Extraocular Proprioception in Chap. 9).352 Further studies are required to establish the clinical value of the procedure.

There is a consensus that neurosurgery does have a role in the therapy of the nystagmus associated with the Arnold-Chiari syndrome. Suboccipital decompression has been reported to improve downbeat nystagmus and prevent progression of other neurologic deficits.1073'1313 Surgical treatment of superior oblique myokymia is discussed in Chap. 9.

Application of Somatosensory

or Auditory Stimuli to

Suppress Nystagmus

Following up on the finding that wearing contact lenses may suppress congenital nystagmus,367 it was documented that electrical stimulation or vibration over the forehead may suppress the oscillations in some patients.1279 These effects may be exerted via the trigeminal system, which receives extraocular proprioception (discussed in Chap. 9). Acupuncture administered to the neck muscles may suppress congenital nystagmus in some patients, by a similar mechanism.143'679 Biofeedback has also been reported to help some patients with this condition.2'256 However, the role of any of these treatments outside the laboratory has yet to be demonstrated, and controlled trials are needed to evaluate these and other measures reported to improve congenital nystagmus.425a

SKEW DEVIATION AND THE OCULAR TILT REACTION (OTR)

Clinical Features of Skew

Deviation and OTR

Skew deviation is a vertical misalignment of the visual axes caused by a disturbance of prenuclear inputs (see VIDEOS: "Skew deviation" and "Wallenberg's syndrome"). A torsional and horizontal deviation may be associated. The hypertropia may be nearly the same in all positions of gaze (concomitant) or vary with eye position (nonconcomitant); sometimes it may even alternate with eye position (e.g., right

hypertropia on right gaze, left hypertropia on left gaze).744'972-1189'1527 When the

skew deviation is nonconcomitant, and especially if the pattern of misalignment resembles that of an individual muscle palsy, it may be difficult to differentiate from a vertical extraocular muscle palsy; coexisting signs of central neurologic dysfunction usually clarify the localization. Skew deviation has been reported in association with a variety of abnormalities in the vestibular

periphery, the brain stem, or the cerebellum.170'1723'738'973 It has also been reported

as a reversible finding associated with raised intracranial pressure due to supratentorial tumors or pseudotumor.482 In infants, the presence of a skew deviation may be the harbinger of a subsequent horizontal strabismus.659

In some patients, skew deviation is associated with ocular torsion (cyclodeviation) and a head tilt, the ocular tilt reaction (OTR)—see Figure 10-18. The OTR is

commonly tonic (sustained),168'172a but may be paroxysmal.602'1118 Rarely, the

skew deviation may slowly alternate or vary in magnitude over the course of a few minutes.296'547'948 Usually, any pathologic head tilt (ear to shoulder) is contralateral to the hypertropic eye, and the ocular torsion is such that the upper poles of the eyes rotate toward the lower ear. This is in contrast to physiologic counterrolling in response to an induced head tilt, when the ocular torsion is such that the upper poles of the eyes rotate toward the higher ear. The ocular torsion may be dissociated be-

Figure 10-18. The ocular tilt reaction represented as a "motor compensation" of a lesion-induced apparent eye-head tilt (dashed line), and which would be opposite in direction to the apparent tilt. The eyes and head are continuously adjusted to what the lesioned brain computes as being vertical. (Courtesy Dr. Thomas Brandt, Munich, Germany.)

tween the two eyes.169'498 Torsional nystagmus (Display 10-4)is commonly associated with acute skew deviation.60'498 Patients with OTR also show a deviation of the subjective visual vertical.168^170'172

The OTR is usually attributed to an imbalance in otolith-ocular and otolith-collic reflexes; these are part of a phylogenetically old righting response to a lateral tilt of the head. In lateral-eyed animals, tilting the head laterally around the longitudinal (anterior-posterior) axis causes a disjunctive, vertical (skew) deviation (one eye goes up, the other down) that acts to hold the visual axis of each eye close to the horizon. In human subjects, who are frontaleyed, a static head tilt (ear to shoulder) causes sustained conjugate counterrolling

of the eyes (ocular torsion) equal to about 10% of the head roll;60'285 thus the static

ocular response does not compensate for the head tilt and is thought to be vestigial. In normal subjects there may be skewing during rotation of the head around its roll (anterior-posterior) axis but the amount is small, idiosyncratic, and dependent upon the viewing distance.703 In contrast, peripheral or central lesions that disrupt otolithic inputs often cause large amounts of skew deviation (as much as 7°) and ocular torsion (as much as 25°). An imbalance

Topologic Diagnosis of Skew

Deviation and the OTR

Acute peripheral vestibulopathy—lesions affecting the vestibular organ or its nerve— can cause skew deviation and the complete

OTR, based upon an imbalance in inputs from the utricles.575'1149'1203'1493 The OTR

may also occur as a component of the Tullio phenomenon, which is characterized by sound-induced vestibular symptoms.392'947 It occurs in patients with a perilymph fistula, or with abnormalities of the ossicular chain and its connection with the membranous labyrinth. In one well-studied patient, stimulation of the left ear with a specific auditory tone caused a head tilt to the right, left hypertropia, intorsion of the left eye, and extorsion of the right eye;392 this effect was ascribed to mechanical stimulation of the left utricle by a hypermobile stapes. These results are consistent with

the effects of experimental stimulation of the otoliths313 and the utricular nerve,1346

which causes ipsilateral hypertropia and conjugate counterrolling.

The utricle projects predominantly to the ipsilateral lateral vestibular nucleus, and the saccule to the vestibular y-group. Thus, disease of the vestibular nuclei (e.g., as part of Wallenberg's syndrome—lateral medullary infarction) may also cause skew deviation with hypotropia on the side of the lesion.389 In addition, some patients show an ipsilateral head tilt and disconjugate ocular torsion. The latter is an excylotropia, with excyclodeviation of the ipsilateral, lower eye, but small or absent

incyclodeviation of the contralateral, higher eye.169'960

Patients with cerebellar lesions may also show a skew deviation.971'973'1427'1527 Some

of these patients show an alternating skew deviation that changes with the direction of horizontal gaze; usually there is a hyperdeviation of the abducting eye. This abnormality, too, is analogous to a phylogenetically old, otolith-mediated, righting reflex present in lateral-eyed animals. In this case, however, the reflex is related to the ocular motor response that compensates for fore and aft pitch of the head when the eyes are directed laterally in the orbit.1527 Although involvement of the brain stem is also likely in some of these patients, skew deviation has been reported in patients who appear to have pure cerebellar disease.971 This suggests that, just as the cerebellum governs the semicircular canal-ocular reflex, it also influences the otolith-ocular reflexes.89 Indeed, downbeat nystagmus (Display 10-2), which is sometimes attributable to disease of the flocculus, commonly coexists with skew deviation.

Utricular projections from the vestibular nuclei probably cross the midline and ascend in the medial longitudinal fasciculus. Therefore, unilateral internuclear ophthalmoplegia is often associated with a skew deviation, usually an ipsilateral hypertropia.

In the midbrain, otolith projections contact the third and fourth nerve nuclei, and the interstitial nucleus of Cajal (INC) (see Display 6-6). Mesencephalic lesions in

or around the INC may cause skew devia- tion738,744 and the oTR.168'172a'573 When

the head tilt is sustained (tonic), it is contralateral to the side of the lesion; usually there is also a hypertropia that is ipsilateral to the lesion and a conjugate cyclotorsion, with the ipsilateral eye intorting and the contralateral eye extorting. Associated defects of vertical eye movements and oculomotor or trochlear nerve function,

including seesaw nystagmus, are com- mon 33,578,1035 Combined fascicular or nu-

clear trochlear lesions and prenuclear lesions in the midbrain may cause extorsion of the contralateral eye and contralateral OTR.385'390 Some patients present with a

paroxysmal skew deviation (with or without a head tilt).602'1118 In one patient with a

clearly defined lesion close to the right INC, episodes of contralateral hypertropia and ipsilateral head tilt occurred, suggesting an irritative mechanism.602 This interpretation of the findings in paroxysmal skew deviation is supported by the results of electrical stimulation near the INC in monkeys, which produces an ocular tilt reaction that consists of depression and extorsion of the ipsilateral eye

and elevation and intorsion of the contralateral eye.1478 With the head free to

move, an ipsilateral head tilt also occurs.1477 In humans, stimulation in the region of the INC causes an ipsilateral ocular tilt reaction.885 A microvascular compression syndrome has also been suggested as a cause of paroxysmal skew deviation with torsional nystagmus.1331

Midbrain lesions may also be associated with a periodic alternating skew deviation that

alternates or varies in magnitude over the course of a few minutes.296'547'948 The peri-

odicity of the phenomenon is reminiscent of periodic alternating nystagmus(Display 10-5), and the two phenomena have been reported to coexist.865 Skew deviation may occasionally be seen as a feature of epilepsy, presumably on the basis of excitation from the cerebral hemispheres to the portions of the vestibular nuclei that mediate otolith-ocular reflexes.499

DISEASE OF THE

VESTIBULAR PERIPHERY

Disease of the vestibular organ or its nerve may cause vertigo, oscillopsia, nystagmus, and the ocular tilt reaction. Vestibular nystagmus and the ocular tilt reaction are described above; vertigo and oscillopsia are discussed below.

Vertigo andDizziness

Dizziness is a common symptom, but one that is frequently not diagnosed satisfactorily, particularly in the elderly.67-328'797

46 The Diagnosisof Disordersof Eye Movements

Whether a patient is complaining of vertigo or some other type of dizziness (such as presyncopal faintness, loss of stable balance, or lightheadedness) can often be determined by taking a careful history and determining whether a series of provoca-

2).401,1530 These procedures include testing for orthostatic hypotension, the Valsalva maneuver, tragal compression, presentation of loud tones with an audiometer, mastoid vibration, hyperventilation, positional testing, sudden head or body turns, rotation in a swivel chair, and any other stimulus that the patient has identified as producing the dizziness. Many patients who present with the complaint of dizziness have a psychological disorder such as agoraphobia, acrophobia, phobic postural vertigo syndrome, or vestibular symptoms that are a component of panic attacks or depression.165'491'666 Some of these patients have had vestibular disorders in the past, or have a coexisting vestibular disorder, and may show abnormalities on vestibular function tests.636'666'690

Vertigo is defined here as an illusory sensation of motion of self or of the environment. Rotational vertigo usually indicates disease of the semicircular canals or their central connections. Linear vertiginous sensations such as translation (e.g., lateropulsion and levitation) or body tilt occur with disease of the otoliths or their central connections. The sensation of vertigo is often associated with vegetative symptoms: nausea, weakness, and diaphoresis. Oscillopsia, on the other hand, usually refers to illusory movements of the seen environment that are often to and fro; it is absent with the eyes closed.

Not all cases of vertigo are due to disease. Certain individuals are prone to develop vertigo, unsteadiness, or malaise with motion, at height, and when assuming certain postures. Vertigo in these situations and in motion sickness probably occurs because of a mismatch between vestibular and other sensory inputs.167 For diagnostic purposes, it is helpful to differentiate between acute, recurrent, and posturally induced vertigo.

Clinical Features of Acute

Peripheral Vestibulopathy

Sudden loss of tonic neural input from one labyrinth or vestibular nerve causes acute vertigo, nystagmus, and postural instability (Display 10-15). The nystagmus (Display 10-1) is typically mixed horizon- tal-torsional, with slow phases directed toward the side of the lesion. The nystagmus is more marked on looking in the direction of the quick phases, following Alexander's law. Quantitative three-dimensional recordings of the response to head rotations in different planes in patients with acute vestibular neuritis suggest that the brunt of the pathology is in the superior division of the vestibular nerve.436'1500 The direction of the spontaneous nystagmus alone, however, cannot be used confidently to predict which canals are involved, perhaps because some adaptation and rebalancing of tonic levels in the vestibular nuclei may have taken place. Following vestibular neurectomy, the direction of nystagmus may be influenced by whether some canal afferents run in the spared cochlear division of the eighth cranial nerve.161

Often the patient cannot decide the direction of perceived rotation. This may be because labyrinthine signals suggest rotation in one direction but inappropriate vestibular eye movements cause visual sensations that, when self-referred, imply turning in the opposite direction. It is helpful, therefore, to inquire about the direction of rotation of the body when the eyes are closed, which is away from the side of the lesion. Usually, the acutely vertiginous patient will lie on one side, with the affected ear uppermost; it has been suggested that this allows otolith influences, which centrally converge with semicircular canal inputs, to reduce the nystagmus caused by imbalance of the semicircular canals.469

Caloric testing is the most reliable method of confirming that the vestibular imbalance is peripheral in location. In patients with spontaneous nystagmus, irrigation with a warm stimulus of the ear to which slow phases of nystagmus are directed is especially important; lack of any

effect upon the spontaneous nystagmus implies disease affecting the stimulated ear. The head thrust maneuver, with head rotation toward the side of the paretic labyrinth, is also a reliable sign of unilateral loss of labyrinthine function. Rotational testing in patients with acute peripheral lesions shows decreased and asymmetric gain and decreased time constant of the VOR. Quantitative aspects of the changes in vestibular responses with peripheral lesions are discussed further in Laboratory Evaluation of Vestibular and Optokinetic Function in Chap. 2.

Acute Vertigo

INFECTIONS CAUSING ACUTE VERTIGO

When acute vertigo occurs without auditory or neurologic disturbances (Table 10-9), particularly in children and young

adults, it is usually ascribed to a viral disturbance of the vestibular nerve; it is often referred to as vestibular neuronitis, vestibular neuritis, or vestibular neurolabyrinthitis.^27'992

Although a definite etiology is not proven in most cases, the histopathology is compatible with a viral affliction.82 As discussed above, the brunt of the pathology seems to be in the superior division of the vestibular nerve. Sometimes such vertigo occurs in epidemics, but the responsible agent is usually not identified. Mumps, measles, and infectious mononucleosis are among the infections that may be suspected if acute vertigo is accompanied by deafness. Experimental studies of viral infection of the inner ear have shown a selective vulnerability to specific viruses of the cochlea, labyrinth, or eighth nerve ganglion.329 One well-recognized cause is herpes zoster, which produces not just vertigo but also a burning pain in the ear followed by a vesicular eruption in the external auditory canal and concha. Deaf-

Table 10-9. Etiology of Vertigo

Acute Vertigo159'572

Physiologic vertigo:167 motion sickness, height vertigo, postural vertigo (on head extension or bending forward at the waist)

Infection of the labyrinth, the vestibular nerve, or both:229-992 by virus,327-329 including

zoster;45'799-1112 acute and chronic bacterial infections; syphilis; Lyme disease681-796'1177

Meniere's syndrome37-1240

Trauma: by head injury,325'437'600-1045.1278.1385-1473 complication of ear surgery575

Perilymph fistula458'1114.1292'1454 Otosclerosis1204-1369

Congenital anomalies of the inner ear89-982-1239 Vestibular atelectasis941-1018 Vestibular-masseter syndrome646

Cogan's syndrome253'599-6083-902'1242-1436

Occlusive or hemorrhagic vascular disease of the inner ear38-451-541-1241

Brain stem hemorrhage, ischemia, and infarction (e.g., Wallenberg's syndrome)68-441-526'664

Cerebellar hemorrhage766 or infarc-

tion28'246'247-1019-1472

Arnold-Chiari malformation (Valsalva-induced vertigo)23-1487

Multiple sclerosis480

Tumors of the brain stem,307 cerebellum,403-548 eighth cranial nerve,957 and petrous bone, including glomus tumors1309

Epilepsy92'496'721-779-1011'1233'1296-1328

Drugs and toxins621-672-974-1249

Recurrent Vertigo

Meniere's syndrome37-79-1064-1240

Syphilis1489

Recurrent Vertigo—continued

Perilymph fistula457-458-696-1114-1238'1292-1454

Otosclerosis1060'1204'1369

Autoimmune conditions125'592'921'1398-1438 including Cogan's syndrome,253-599'6083'902'

1242,1436 Susac's syndrome,66-1022-1344 and giant cell arteritis926

Benign paroxysmal vertigo of child-

hood443'811'1062'1401 Epilepsy92'496'721'779-1011'1233-1296-1328

Migraine and its variants including basilar artery migraine71-131'314'589'1221-1433

Familial vertigo, ataxia, and nystag- mus85,90,175,226,486,1411,1413

Hypothyroidism'30

Brain stem ischemia68-441'451'526'541'1019-1028 Multiple sclerosis480

Posterior fossa tumors593 Microvascular compression124'171'975 Vestibular atelectasis941'1018

Central angioma821

Recurrent idiopathic vestibulopathy1198

Positionally Induced Vertigo

Benign paroxysmal positional vertigo ("cupulolithiasis" and "canalolithiasis")173

Alcohol436"'952 Central causes:

Cerebellar infarcts1210 Cerebellar tumors548-1460 Multiple sclerosis480-729 Brain stem ischemia541

Arnold-Chiari malformation and other craniocervical anomalies86

ness, ipsilateral facial pain, and facial

paralysis may also occur (Ramsay-Hunt syndrome).799-1112 Enhancement of the fa-

cial and vestibulocochlear nerves on MRI has been reported.905 Bacterial infection of the middle ear and serous otitis media remain common causes of vertigo, especially in children.

TRAUMA CAUSING VERTIGO

Acute vertigo may be associated with head trauma.325'457-600-1045 The injury is often

mild; frequently the patient also complains

of headache and difficulty with concentration. Posttraumatic vertigo is commonly caused by whiplash injuries incurred in rear-end automobile accidents. About50% of such patients show abnormalities on vestibular testing, such as reduced caloric responses, positional nystagmus, and occa-

sionally increased, "hyperactive" vestibular responses.444-600-1045-1385 High-impact

aerobics has been implicated in patients with otherwise unexplained vestibular symptoms.1463 Temporal bone fractures

are often associated with vertigo and vestibular damage.600-1473

It has been suggested that disturbance of cervical muscle afferents might be the cause of vertigo in some patients (cervical vertigo}. In support of this idea, injection of local anesthetic into the neck of volunteer subjects produces a sensation of being drawn toward the side of the cervical injection, with ataxia but not nystagmus; however, nystagmus does appear when monkeys are injected with a local anesthetic in their neck muscles.333 Radical neck surgery can lead to abnormal rotational vestibular responses.705 Vibration of neck muscles can lead to illusions of motion in normal subjects723 and to nystagmus in patients with unilateral loss of function.1501 Thus, there is a potential substrate for cervical influences on vestibular sensation, but further studies are required to establish cervical vertigo as a clearly identifiable clinicalentity.

Trauma may also cause vertigo by creating a fistula between the perilymph and middle ear. Perilymph fistula may follow mastoid or stapes surgery, minor head trauma (e.g., from diving into a swimming

pool), barotrauma (high altitude or underwater),673'939'1114 strenuous exercise, sup-

pressed sneezing,1244 and air travel.696 A useful clinical test consists of applying manual pressure over the tragus or applying pressure to the tympanic membrane with the pneumatic otoscope; a positive result is indicated by the production or exacerbation of vertigo or the elicitation of nystagmus (Hennebert's sign). A positive Hennebert's sign is not specific for an oval or round window fistula, however. Other causes include fistulas involving any of the semicircular canals, or abnormal connections between the stapes footplate and the otoliths, including vestibulofibrosis and a hypermobile stapes. Pressure sensitivity may also occur in Meniere's syndrome, when the otolith organs become dilated and abut the stapes footplate. A positive Hennebert's sign has also been associated with bilateral vestibular loss.15 Pressure-induced signs can sometimes be documented by recording eye movements or measuring body

sway as pressure on the tympanic membrane is increased.1051'1278 Patients with fis-

tula may complain of imbalance, positional vertigo, nystagmus, and hearing loss.

The Tullio phenomenon comprises vestibular symptoms that include vertigo, oscillopsia, nystagmus (Fig. 10-3), the OTR, and postural imbalance induced by auditory stimuli (see VIDEO: "Tullio phenomenon"). It is usually due to perilymph fistula, but subluxation of the stapes foot-

plate and other ear pathology may be re- sponsible.187'278'392'1051-1094'1188 The symp-

toms may be due to abnormal stimulation of the semicircular canals or of the otoliths. Patients may have an increased click-evoked sacculocollic reflex, as reflected by increased surface EMG activity over the sternocleidomastoid muscle.283 A recently identified cause of the Tullio phenomenon, with pressure sensitivity and Valsalva-induced symptoms is dehiscence of the roof of the superior semicircular canal.947 Such patients also have vertigo and nystagmus induced by vibration of the mastoid bone. The bony abnormality can be identified on coronal and transverse CT scans of the petrous bone; in particularly bothersome cases, plugging of the superior canal is an effective treatment. Perilymph fistulas of the round or oval window often resolve spontaneously, but sometimes surgical repair is necessary. Some patients with posttraumatic vertigo develop benign paroxysmal positional vertigo (BPPV).

A spontaneous oval or round window fistula has also been invoked as a cause of unexplained vertigo and dysequilibrium.458'1454 Unfortunately, there are no reliable diagnostic tests for this syndrome. Many patients in whom no other cause for their vestibular symptoms is uncovered have undergone an exploratory tympanotomy and patching of the oval and round windows, even if no fistula is clearly identified. We suspect that only a small percentage of patients in this category are helped

surgically; the problem is how to identify this subgroup.1238

TOXIC CAUSES OF VERTIGO

The most common toxic cause of acute vertigo is ethyl alcohol. It is well known that positional changes exacerbate the vertigo of a hangover. The reason may be that alcohol diffuses into the cupula and