Chapter 46. Vestibular Testing

Before performing any vestibular test, taking a thorough medical history and ascertaining the patient's symptoms constitute the first steps in caring for a patient with a vestibular disorder. Sometimes the patient history alone may suggest a diagnosis.

Symptoms

Taking a patient history should include determining the patient's symptoms, including balance, hearing, vision, somatosensation, and motor function. The first task for a neurotologist is to allow the patient to describe what he or she senses. The clinician may help the patient in choosing the correct terms to describe his complaints.

Vertigo

Vertigo can be described as an unreal sense of rotationary movement. It should be distinguished from dizziness, which describes any kind of altered sense of orientation. A history of vertigo is of great value in identifying the presence of vestibular pathology but not in localizing its origin. Vertigo results from impaired tonic symmetry in the inputs of the vestibular nuclei. Therefore, a vestibular lesion can occur anywhere within the vestibular end-organs, the vestibular nuclei, the cerebellum, the pathways connecting these structures in the brainstem, and, rarely, within the cortex.

The differentiation between peripheral and central nervous system (CNS) lesions may be based on detailed features of vertigo, even though these features may not apply to every patient. The clinician should determine whether the vertigo occurs in episodes or continuously. If it is episodic, it should be ascertained how often the episodes occur and how long they last. In peripheral causes, vertigo occurs in episodes with an abrupt onset. It disappears in varying time periods, from seconds to days, based on the underlying pathology. The origin of intensive, episodic vertigo that lasts up to a minute is more likely benign paroxysmal positional vertigo (BPPV) if it is provoked with particular positions. Another cause of brief but recurrent vertigo or dizziness, especially if precipitated by body straining, is perilymph fistula. Vertigo that lasts 2–20 minutes is consistent with a transient ischemic attack, which affects the posterior circulation if it is associated with visual deficits, ataxia, and localized neurologic findings. Meniere disease causes recurrent vertigo attacks that can last between 20 minutes and 24 hours. An isolated attack of vertigo that lasts more than 24 hours is suggestive of vestibular neuronitis. Autonomic symptoms such as nausea, vomiting, and sweating are common presenting symptoms. Generally, the more intense symptoms a patient has, the more likely it is that the vertigo is caused by a peripheral lesion.

Lightheadedness

Lightheadedness describes the sensation of unsteadiness and falling or the symptoms similar to those preceding syncope, such as blurred vision and faded facial color. It should be distinguished from both vertigo and visual disorientation. Most often, lightheadedness occurs with nonvestibular causes such as cardiac or vasovagal reflex.

Imbalance

Imbalance is described as the inability to maintain the center of gravity. It causes the patient to feel unsteady or as if about to fall. The causes may be sensory or motor.

Other Symptoms

The physician should also ascertain the presence of other associated symptoms such as hearing loss, tinnitus, and facial weakness. A positive history of precipitating factors (eg, rapid head movement) may lead the clinician to variants of BPPV. However, identifying the factors that induce vertigo may not be helpful in distinguishing peripheral lesions from CNS lesions because vertigo precipitated by rapid head movements may result from either decompensated peripheral vestibular lesions or CNS lesions. The physician should ascertain whether the patient has a history of falling with no loss of consciousness; this symptom may be associated with Meniere syndrome. Determining whether noise is a precipitating factor may be useful in identifying Tullio phenomenon. A history of brief episodes of vertigo induced by Valsalva-like maneuvers, which increase middle ear pressure, may be indicative of a perilymph fistula, Chiari malformation, or dehiscence of the superior semicircular canal (SCC). Figure 46–1 shows downbeating nystagmus induced by hyperventilation in a patient with superior canal dehiscence syndrome.

Drug Use

Determining the patient's drug history and current drug use (prescription or other) is crucial in evaluating dizziness. Vestibulotoxic drug intake may cause bilateral vestibular end-organ damage, which results in oscillopsia.

Psychological Factors

The clinician should also query patients about psychological factors. The specific site where dizziness occurs should be identified. Panic attacks or agoraphobia may be suspected if lightheadedness occurs in crowded areas or public places.

Family History

A positive family history of a balance disorder may contribute to the diagnosis, especially in Meniere syndrome, neurofibromatosis, migraine, and a narrow endolymphatic duct.

Physical Exam

The physical examination of a patient with a balance disorder should begin with a complete ear, nose, and throat exam. A detailed neurotologic examination should also be performed; it should include an evaluation for nystagmus and oculomotor function, as well as positional tests, postural control tests, and a cranial nerve examination.

Testing & Evaluation

Oculomotor Function Tests

Oculomotor function is tested by asking the patient to gaze at the tip of the clinician's index finger. The clinician should first hold her or his finger 25 cm away from the patient's eyes and then move it laterally and vertically, which is the tracking function. The clinician should assess whether the patient's eye movements are conjugate or disconjugate. In testing the horizontal tracking function, anything other than a smooth horizontal eye movement is assumed to be indicative of vestibulocerebellar pathology. During the vertical tracking test, a superimposed horizontal eye movement (ie, a saccadic intrusion) may occur in patients with a central oculomotor lesion. An imbalance in the tonic levels of activity that underlies the otolith-ocular reflexes leads to static ocular torsion, head tilt, and a skew deviation, which is a vertical misalignment of the eyes that is observed upon switching the cover from one eye to the other.

Nystagmus Testing

In assessing for the presence of nystagmus, the clinician should be aware of possible changes in findings at the time of either the acute or chronic phase of the vertigo or dizziness.

Spontaneousnystagmus

Spontaneousnystagmus is identified by having the patient wear Frenzel glasses. If nystagmus is found, the direction of its fast phase, frequency, and amplitude are noted. Determining the characteristics of the nystagmus would give the physician an overall indication, before electronystagmographic testing is performed, if there is an obvious asymmetry in the vestibular system. If primary positional nystagmus is purely vertical or purely torsional, a CNS disorder, usually in the vestibulocerebellum, the vestibular nuclei, and their connections within the interstitial nucleus of Cajal in the midbrain, is likely. Figure 46–2 shows downbeating nystagmus in a patient with diffuse cerebellar atrophy. Spontaneous nystagmus that is peripheral in origin is characteristically diminished by visual fixation and increased only when fixation is canceled.

Gaze Nystagmus

Gaze nystagmus is identified by holding the index finger at off-center positions. Central origin nystagmus may change its direction with different gaze positions. The direction of peripheral origin nystagmus is fixed in all gaze positions. A low-velocity, direction-fixed nystagmus (ie, 1–2°/s) or a direction-changing, gaze-evoked nystagmus, both of which present only in darkness, can occur as a nonspecific finding both in nonsymptomatic individuals and patients with organic peripheral or central vestibular lesions. While gazing at a distant object, the passive rotation of a patient's head at the frequency of 1 Hz over 20 seconds causes a patient with oscillopsia to make saccadic corrections and to view the object as no longer being stationary.

Head-Shaking Nystagmus

Head-shaking nystagmus is evaluated in the same way as gaze nystagmus; however, in head-shaking nystagmus, patients either wear Frenzel glasses or close their eyes. The frequency and speed of the patient's head shaking should be maintained at sufficiently high levels (at least 160°/s) to elicit the nonlinearity of the diseased vestibular labyrinth. The direction of head-shaking nystagmus may be toward either the side with the lesion or the side without it, and it may be monophasic, biphasic, or triphasic. If a head-shaking nystagmus beats toward the side without the lesion in a patient with no spontaneous nystagmus, the presence of a statically compensated peripheral lesion should be considered.

Nonlinearity Testing

Dynamic nonlinearity in the SCCs can be tested at the bedside by observing the effect of head rotations on eye movements. With this test, the malfunction of individual canals is examined by applying high-acceleration head thrusts, with the eyes beginning about 15° away from the primary position in the orbit and the amplitude of the head movement such that the eyes end near the primary gaze position. The patient is asked to fix his or her gaze on the examiner's nose. Any corrective saccade shortly after the end of the thrusts is a sign of an inappropriate and compensatory slow-phase eye movement. Each canal can be tested in its plane.

Fistula Testing

The presence of a fistula is suspected if nystagmus occurs or if the patient perceives movement of a visual target that is fixed after applying positive pressure to the outer ear canal. A positive test result (ie, Hennebert sign) suggests either a perilymph fistula or Meniere disease. Tullio phenomenon occurs in the same clinical entities when a loud noise is applied. Hyperventilation may induce symptoms in patients with anxiety and phobic disorders, but it seldom produces nystagmus.

Positional Tests

Positional tests can be described as either dynamic or static. Static positional tests are discussed in the Electronystagmography section of this chapter.

The dynamic positional test is called the Dix-Hall-pike maneuver. This test is performed to elicit typical nystagmus of BPPV of the vertical SCCs. The patient may be asked to wear Frenzel glasses. In the test, the patient sits on the examination table with his head rotated 45° from the sagittal plane to one side. The patient is then moved quickly into a position where his head hangs over the edge of the table. After a 20-second waiting period, if nystagmus is not observed, the patient is returned to his initial sitting position. The patient then rotates his head 45° from the sagittal plane to the alternate side. Then he is again brought quickly into a position where his head hangs over the edge of the table. Nystagmus is again sought. If rotational or torsional nystagmus is observed in any of the head-hanging positions, then typical nystagmus reversal is expected when the patient returns to the initial sitting position. The horizontal variant of BPPV is investigated in a different way; the patient is placed in the supine position with head raised 30° by the clinician. Horizontal geotropic or ageotropic nystagmus is identified when the clinician rotates the patient's head to both sides with nystagmus observation time interval. The side of the lesion is determined based on the intensity of horizontal nystagmus produced by head movement toward each side. It is the side of lesion to which head movement creates more intense nystagmus.

Visual Acuity Testing

Visual acuity is the patient's ability to read an eye chart while his or her head is moving. The head is rotated passively at a frequency of 1–2 Hz/s. A drop in acuity of two lines or more from the baseline suggests an abnormal vestibuloocular reflex gain.

Postural Control Tests

The examination of postural control includes the following tests: (1) the Romberg test, (2) the pastpointing test, (3) the tandem gait test, and (4) the Fukuda stepping test. Postural control tests are considered to have mild sensitivity and specificity in identifying lesions. Depending on the nature and phase of the pathology, the side of the lesion cannot reliably be identified from these tests. Excessive swaying toward one side in the Romberg test, deviation to one side in the pastpointing test, or rotation to one side in the Fukuda stepping test may all indicate either a paretic lesion of the labyrinth in that side or an irritative lesion in the opposite side. The patient may show sway, rotation, or deviation toward the unaffected side if the peripheral lesion is at the compensated phase.

Romberg Test

During the Romberg test, which is used to identify vestibular impairment, the patient is asked to stand still with eyes closed and feet together. An increased sway or fall toward either side is considered abnormal. The Romberg test can be made more sensitive by asking the patient to stand with the feet in a heel-to-toe position and with arms folded against the chest.

Pastpointing Test

The patient and clinician both stand facing each other; they then stretch their arms forward with index fingers extended and in contact with one another. The patient is asked to raise his arms up and bring his index fingers again into contact with the clinician's index fingers, which are fixed. The patient performs this movement 2–3 times with eyes open; later, the patient repeats the same maneuver with eyes closed. Deviation to one side is considered abnormal.

Tandem Gait Test

The patient is asked to take tandem steps with eyes closed. Healthy individuals can take at least 10 steps without deviation. Patients with vestibular disorders fail this test.

Fukuda Stepping Test

The patient is asked to march in place with eyes closed. After 50 steps, a rotation >30° toward one side is considered abnormal.

Cranial Nerve Evaluation

An evaluation of cranial nerve function may reveal hypoesthesia of the outer ear canal and an absent corneal reflex, as found in acoustic neuromas. Facial nerve paralysis may be associated with herpes zoster oticus. Eye muscle restrictions may be elicited by evaluating the functioning of cranial nerves III (oculomotor nerve), IV (trochlear nerve), and VI (abducens nerve) before the electronystagmogram.

Electronystagmography (ENG) is the fundamental test and the first step in a vestibular testing battery to evaluate the vestibuloocular reflex in patients with a balance disorder. It is based on recording and measuring eye movements or eye positions in response to visual or vestibular stimuli.

Equipment

Standard ENG equipment consists of the following components: (1) an amplifier for amplification of the corneal-retinal potential that occurs following eye movement, (2) band-pass and notch filters, (3) a signal recorder, (4) a light array, and (5) water and air caloric stimulators. The techniques available to record eye movements are electrooculography (EOG), infrared recording, magnetic search coil, and video-recording systems.

An ENG analysis consists mainly of three tests: (1) oculomotor tests, (2) positional tests, and (3) caloric tests. Before each test, the system needs to be calibrated to maintain accuracy. The calibration is performed via a saccade test that is discussed in the section on oculomotor tests.

Utility of Electronystagmography

ENG is very useful in diagnosing vestibular pathology. No other test provides information on the site of the lesion. The data obtained from an ENG test battery support the diagnoses of horizontal BPPV, vestibular neuronitis, Meniere disease, labyrinthitis, and ototoxicity. With acoustic neuromas, it may be helpful to predict the nerve from which the tumor originates; caloric weakness may be associated with a tumor that originates from the superior vestibular nerve. ENG may also predict whether the patient will experience vertigo after acoustic tumor removal. However, relying on ENG alone to identify lesions in the CNS would not be appropriate.

Abnormal findings in ENG testing do not necessarily indicate a definite CNS lesion. One study investigated the ratio of patients with abnormal results as reported by magnetic resonance imaging (MRI) to patients with abnormal ENG findings in different age groups and found a better correlation between MRI and ENG findings in a group of elderly patients. Overall, MRI confirmed a central lesion in 52% of patients with abnormal ENG findings. In contrast, ENG findings were abnormal in 15 of 21 patients (71%) with an abnormal MRI. In two recent studies, only 30–37% of the patients with abnormal ENG findings had abnormal MRI scans.

Oculomotor Tests

Oculomotor tests measure the accuracy, latency, and velocity of eye movements for a given stimulus. The standard oculomotor test battery includes saccade tests, smooth pursuit tests, optokinetic nystagmus testing, gaze tests, and fixation suppression testing. All oculomotor tests are performed with the patient seated upright, with the head stabilized. For oculomotor tests, the ENG device should have a light array on which LED (light-emitting diodes) are given as a stimulus. The light array may be rotated vertically for calibration purposes as well as for testing vertical saccades. The center of the light arrays should be at the same level as the patient's eyes.

Saccade Test

Saccades are rapid eye movements that bring objects in the periphery of the visual field onto the fovea. The latency of saccades is very brief. Because peak velocity can be as high as 700°/s, vision is not clear during saccadic movement. Saccades are controlled by the occipitoparietal cortex, the frontal lobe, the basal ganglia, the superior colliculus, the cerebellum, and the brainstem.

To test saccadic eye movement, the patient is asked to follow the LED with as much accuracy as possible. The LED flashes sequentially in two positions: at the center of the array and then 15–20° to the right or left from the center. The interval between flashes is usually a few seconds. The test is repeated vertically.

Three parameters are of clinical significance in evaluating saccades: latency, peak eye velocity, and accuracy of the saccades.

Latency is the time difference between the presentation of a target and the beginning of a saccade. The mean latency is 192 ± 32 ms in normal subjects. Abnormalities in latency include prolonged latency, shortened latency, and differences in the latency between the right eye and the left eye. These abnormalities are observed in the presence of neurodegenerative disease.

The peak velocity is the maximum velocity that eyes reach during a saccadic movement. It ranges from 283°/s to 581°/s for 20° of amplitude in normal subjects. Abnormalities in the saccadic velocity are slow saccades, fast saccades, or a difference in the velocity between the right eye and the left eye. Reasons for saccadic slowing include the use of sedative drugs, drowsiness, cerebellar disorders, basal ganglia disorders, and brainstem lesions. Fast saccades can be observed in calibration errors and eye muscle restrictions. The asymmetry of velocity is observed in internuclear ophthalmoplegia, eye muscle restrictions, ocular muscle palsies, and palsy of cranial nerves III and VI (the oculomotor and abducens nerves, respectively).

Accuracy is the final parameter in the evaluation of saccades. Saccadic accuracy is determined by saccadic movement by comparing the patient's eye position relative to the target position. Figure 46–3 provides a record of normal saccadic movement with accurate square tracing. If the saccadic eye movement goes farther than the target position, it is referred to as a hypermetric saccade (or overshoot dysmetria). If the saccadic movement is shorter than the target position, it is referred to as hypometric saccade (or undershoot dysmetria). Undershooting by 10% of the amplitude of the saccade may be observed in healthy subjects, whereas hypermetric saccades rarely occur in healthy subjects. Inaccurate saccades suggest the presence of a pathologic condition in the cerebellum, brainstem, or basal ganglia.

Smooth Pursuit Test

Smooth pursuit is the term used to describe eye movement that is created when the eyes track moving objects. Similar central pathways to those of saccadic movement produce smooth pursuit movement. The neural pathways serving the “pursuit system” are distributed in the cortical and subcortical areas of the brain. Smooth pursuit function also involves the fovea.

In a commonly used stimulus paradigm of the smooth pursuit test, the LED moves back and forth between two points on a light bar at a constant frequency and velocity. The patient is asked to follow this moving target. The frequency of the test stimulus should be between 0.2 and 0.8 Hz/s. A typical pursuit velocity is between 20°/s and 40°/s. Performance declines with higher velocities and increasing patient age.

The primary parameters for evaluation are gain, phase, and trace morphology. Gain is the ratio of peak eye velocity to the target velocity. For a stimulus of 0.5 Hz with a sweeping amplitude of 40°, a gain >0.8 is considered normal. A low gain is suggestive of a CNS disorder. Phase is the difference in time between eye movement and target movement. Under optimal conditions, healthy subjects can track a target with a phase angle of 0°. The level of attention and drugs affecting the CNS can destroy pursuit performance.

A morphologic assessment of the trace is also important. Figure 46–4 shows a record of normal tracking eye movement. A morphologic abnormality is referred to as a staircase of saccades, in which the trace shows staircase-like eye movement while the target is followed. Pursuit traces can be impaired symmetrically or asymmetrically. An asymmetrically impaired pursuit is more suggestive of a CNS lesion than is a symmetrically impaired pursuit. Acute peripheral vestibular lesions can also impair smooth pursuit contralateral to the affected side when the eyes are moving against the slow phase of a spontaneous nystagmus.

Optokinetic Nystagmus & Optokinetic Afternystagmus

Optokinetic nystagmus (OKN) is an involuntary oculomotor response to a moving target that fills at least 90% of the visual field.

Equipment

The best optokinetic stimulator is a 360° turning cloth drum with black and white stripes. Because this drum can be unwieldy, it is preferable to use an optokinetic projector.

Test Administration

The normal response to an optokinetic stimulator is a smooth eye movement that follows the direction of the visual stimulus both clockwise and counterclockwise. OKN aims to stabilize the visual field onto the retina. OKN is produced by cortical and brainstem structures, which is the same as the pursuit. Optokinetic afternystagmus (OKAN) is a form of nystagmus that is produced by the brainstem after a 10-second, constant-velocity optokinetic stimulus. It lasts about 30 seconds.

OKN can be stimulated with a constant target speed between 20°/s and 60°/s or a sinusoidal target speed of up to 100°/s. Each target speed needs to be repeated in both clockwise and counterclockwise directions. The patient is asked to gaze straight ahead, while the target is moved in front of his or her field of vision. The type of stimulus chosen is presented for 1 minute. When a constant-velocity optokinetic stimulus is used, the patient's eyes reach a constant velocity after 10 seconds of stimulation in one direction. Once this stimulus is discontinued, the room light is turned off and the recording is continued for OKAN until the OKAN is decayed. The same stimulus is then applied in the opposite direction. It should be noted that a sinusoidal stimulus cannot be used to test OKAN.

Parameters of Optokinetic Nystagmus

In testing of OKN, the most useful parameters are gain and phase.

Gain

The normal value of gain is ≥0.5, as well as symmetry on both sides (ie, in both eyes) for a stimulus of 60°/s. A gain in OKN may be reduced symmetrically or asymmetrically. A symmetrically reduced gain is observed in visual disorders, fast-phase disorders, and congenital nystagmus. Unilateral parietal-occipital lesions cause an asymmetrically reduced gain.

Phase

As a testing parameter, phase is applied only for the sinusoidal stimulus of OKN. The testing of OKN is less sensitive than a pursuit test. The sensitivity and specificity of OKN elicited by stimulation of the full visual field are 46% and 92%, respectively, which is superior to the sensitivity and specificity of OKN elicited by stimulation of the partial visual field.

Parameters of Optokinetic Afternystagmus

The testing of OKAN is evaluated by three parameters: the initial velocity, the time constant, and the slow-cumulative eye position. The initial velocity is calculated from the OKAN at the 2nd second. This initial velocity is approximately 10°/s for a stimulus of 60°/s. The time constant is the length of time required for the slow-phase velocity to decline to 37% of the initial velocity. The slow cumulative eye position is a function of both the initial velocity and the time constant. Because it shows less intersubject variability compared with the other two, it is the most useful parameter. The normative value of the slow cumulative eye position varies among the vestibular laboratories. Abnormalities in OKAN present as symmetrically reduced OKAN, which is bilateral; asymmetrically reduced OKAN; and hyperactive OKAN. A complete bilateral loss of OKAN is observed in a bilateral vestibular loss, which may be either peripheral or central. Asymmetry in OKAN is indicative of a unilateral vestibular loss. Hyperactive OKAN may be seen in mal de debarquement syndrome.

Gaze Test

The gaze test is performed by recording eye movements, while the patient fixes his vision on the center of a target; the patient then fixes his gaze 30–40° to the right, to the left, and then above and below the center of the target. The patient's gaze, as well as a recording of that gaze, is sustained for at least 30 seconds. A gaze test may reveal peripheral or CNS lesions that are either vestibular or nonvestibular in origin. It may also reveal either congenital or spontaneous nystagmus. Patients with gaze nystagmus cannot maintain stable conjugate eye deviation away from the primary position; therefore, the focus of the patient's vision is brought back to the center by resetting the corrective saccades. Vestibular spontaneous nystagmus is seen during and after unilateral vestibular dysfunction and beats away from the afflicted side. It is seen as a horizontal nystagmus in an ENG recording, but it is actually both horizontal and torsional in nature. The intensity of vestibular spontaneous nystagmus increases when the patient's gaze is directed toward the direction of the nystagmus.

A typical gaze-evoked nystagmus that is peripheral in origin is unidirectional on a horizontal plane; it is both horizontal and torsional. Its intensity increases when gaze is directed toward the direction of the nystagmus. A gazeevoked nystagmus with a CNS origin may change direction with the patient's gaze. A nystagmus that results from a vertical gaze is always suggestive of CNS lesions.

Gaze nystagmus may be classified as symmetric, asymmetric, rebound, or disassociated. In symmetric gaze nystagmus, the eyes move in equal amplitude in both directions. The ingestion of drugs that affect the CNS, as well as multiple sclerosis, myasthenia gravis, and cerebellar atrophy, may all cause symmetric gaze nystagmus. Asymmetric gaze nystagmus is indicative of a lesion within the brainstem or the cerebellum. Rebound nystagmus begins in lateral gaze positions and reverses its direction to the primary position, even though there is no evidence of nystagmus in the primary position at the beginning of testing. It is also a strong indicator of cerebellar or brainstem lesions. Dissociated (disconjugate) nystagmus is the difference in eye movements during the gaze. It results from lesions of the medial longitudinal fasciculus.

Fixation Suppression Testing

Spontaneous nystagmus is determined by placing the patient, with eyes closed, in a totally darkened room without any visual or positional stimuli. If spontaneous nystagmus is found, its slow-phase velocity is recorded. The patient is then asked to fixate on the center of a visual target (central gaze). The ratio of the slow-phase velocity with fixation to the slow-phase velocity without fixation is then calculated. This calculation provides a fixation suppression index. This index should be <50%. Nystagmus that results from a peripheral origin decays to more than 50% with fixation. Figure 46–5 shows the effect of fixation on a spontaneous nystagmus that is peripheral in origin.

Positional Tests

The purpose of positional testing is to determine the effect of different stationary head positions (and not head movements) on eye movements. The assumption of these tests is that the patient's nystagmus is generated as a result of the orientation of the patient's head to gravity. The patient is asked to wear Frenzel glasses (or the test can be performed while the patient's eyes are closed), and the patient is brought slowly into the following positions: the patient's head (1) is turned right and then left while sitting, (2) is turned right and then left in the supine position, (3) is turned right and then left in a decubitus position, and (4) hangs straight down. Each position is maintained for at least 20 seconds. Positional nystagmus may be intermittent or persistent, and the direction may be fixed or changing.

The identification of positional nystagmus is not a localizing finding since it may be observed in patients with both peripheral and CNS lesions. Two features may help to distinguish the positional nystagmus that results from a peripheral lesion from one that results from a central lesion: (1) positional nystagmus caused by a peripheral lesion is suppressed by fixation; (2) the direction changing nystagmus may be indicative of a CNS lesion. The clinician must be careful about the contamination of spontaneous nystagmus with positional changes. If persistent nystagmus is noted, it should be observed for at least 2 minutes. This observation is especially important with periodic alternating nystagmus, in which the nystagmus reverses direction every 2 minutes. It is found that this type of nystagmus is caused by CNS lesions.

Caloric Tests

Caloric tests are based on comparing magnitude of the induced nystagmus on the right and left sides. Since the outer ear canal is close to the horizontal SCC, most of the response origins come from the horizontal SCC. Therefore, the nystagmus is horizontal. The temperature gradient produced by a cold stimulus causes the cupula to move away from the utricle, thereby creating a nystagmus that beats toward the opposite side. A warm stimulus causes the endolymph to rise, resulting in a nystagmus that beats toward the stimulus side.

Caloric testing is an important tool in assessing the vestibular system. It allows for the separate stimulation of each ear. Therefore, it provides data about the site of the lesion. However, there are some disadvantages of this test. Heat transfer from the ear canal to the horizontal SCC may vary among individuals, depending on the differences in the temporal bone pneumatization among patients. Another disadvantage is the fact that a caloric stimulus can provide a means of evaluating the vestibular response, but at only one frequency. The last disadvantage is that the caloric test allows only for the evaluation of the horizontal SCC.

Equipment

The caloric test uses a caloric stimulator, either a water or air irrigator, in addition to the EOG recording equipment. Two types of water stimulators are available: open loop and closed loop. The difference between the two stimulators is where the water circulates. An open-loop stimulator delivers water directly into the outer ear canal. In closed-loop systems, the water circulates in an expandable rubber medium to preserve its temperature. Open-loop systems are thought to provide more reliable and reproducible results than closed-loop systems. Caloric testing with either air or a closed-loop water stimulus should be reserved for patients who have a tympanic membrane perforation.

Test Administration

The patient should be in the supine position, with his head tilted 30° upward to bring the horizontal SCC into the earth vertical position—this position makes the horizontal SCC more sensitive. The test can be performed with either a bithermal or a monothermal caloric stimulus. The bithermal caloric test provides the most useful data on the vestibular system, which is stimulated by warm and cold water or air. To enhance the nystagmus response, mental tasks are given to the patient during the test. The recording can be performed with the patient's eyes opened in total darkness, with his eyes opened and wearing Frenzel glasses, or with his eyes closed.

In performing caloric testing, temperatures 7 C below and above body temperature (30°C and 44°C) are used as cold- and warm-water stimuli. A total volume of 250 mL of water is given to the outer ear canal over a period of 30–40 seconds. As an alternative to the water stimulus, two air stimuli that are 24°C and 50°C, respectively, are used with a flow rate of 8 L/min for 60 seconds. Four caloric stimuli are given with an interval of no less than 5 minutes to prevent superimposition or conflicting responses. The following order of stimuli is preferred: (1) right–warm, (2) left–warm, (3) right–cold, and (4) left–cold. In response to the caloric stimulus, the nystagmus begins just before the end of the caloric stimulus and reaches a peak at approximately 60 seconds of stimulation; it then slowly decays over the next minute. When it reaches its peak, patients are asked to fixate their eyes on a central point to check the fixation suppression index.

Testing Parameters

The most reliable and consistent parameter is the peak slow-phase velocity of the induced nystagmus. The peak slow-phase velocity is averaged over a 10-second period and is calculated for each side.

The values obtained for each caloric stimulus are placed into equations, with each used for specific conditions that define vestibular function. Unilateral weakness (ie, canal paresis) indicates a significantly weak response on one side relative to the other. It is formulated as follows:

(R 30°C + R 44°C) − (L 30°C + L 44°C) × 100% ÷ (R 30°C + R 44°C + L 30°C + L 44°C)

The difference between the sides ≥20–25% indicates the presence of a unilateral weakness. However, normative data for this critical percentage should be determined for each laboratory. Unilateral weakness is not a localizing finding and may be caused by lesions from the labyrinth to the root entry zone of the eighth cranial nerve (ie, the vestibulocochlear nerve) in the brainstem, such as Meniere disease, labyrinthitis, vestibular neuronitis, acoustic neuromas (and other tumors pressing on the eighth nerve), and multiple sclerosis.

Directional preponderance (ie, unidirectional weakness) refers to a condition in which the mean-peak, slow-phase velocity of the nystagmus beating toward one side is significantly greater than the mean-peak, slow-phase velocity of the nystagmus beating toward the opposite side. It is determined by the following equation:

(R 30°C + L 44°C) − (R 44°C + L 30°C) × 100% ÷ (R 30°C + R 44°C + L 30°C + L 44°C)

A difference >20–30% assumes the existence of a directional preponderance. This critical percentage should be determined by a testing laboratory. The directional preponderance is often associated with a spontaneous nystagmus because a spontaneous nystagmus enhances the nystagmus beating toward its direction and eliminates the nystagmus beating toward the opposite direction. The directional preponderance simply shows the existence of bias in the tonic activity of the vestibular system. However, the directional preponderance is considered to reflect an asymmetry in the dynamic sensitivity between the left and the right medial vestibular neurons, as opposed to the reason behind the spontaneous nystagmus, which is reflected in asymmetry in the resting activity. The directional preponderance is a poor localizing finding. It may be observed in lesions from the labyrinth to the cortex. The directional preponderance is toward the lesion site for labyrinth and eighth nerve lesions, and toward the uninvolved site for lesions of the brainstem and cortex. It is controversial that a directional preponderance without a spontaneous nystagmus is suggestive of a CNS disorder. One retrospective study showed that 5% of patients with an isolated directional preponderance had a CNS lesion. Other patient groups had peripheral lesions or no definite diagnosis. Directional preponderance and unilateral weakness may be observed together, which is suggestive of acute unilateral peripheral lesions.

Caloric weakness may be found in both sides, which is referred to as bilateral weakness. The level of response that is considered a bilateral weakness varies based on the normative data. However, several physicians give their own normative measurements. For both sides, the total response to a warm stimulus (<11°/s) and the total response to a cold stimulus (<6°/s) are considered bilateral weakness. Patients with bilateral weakness often present with oscillopsia. A bilateral weakness is often associated with vestibulotoxic antibiotherapy or bilateral Meniere disease. However, it is also observed in patients with lesions of the vestibular nuclei, Lyme disease, Cogan syndrome, pseudotumor cerebri, and neurodegenerative diseases of the brainstem and cerebellum.

Hyperactive caloric responses may also be observed. The numeric criteria for these responses varies among laboratories from 40°/s to 80°/s. Hyperactive caloric responses are associated with a cerebellar lesion or atrophy due to removal of the cerebellar inhibitory effect on the vestibular nuclei.

Failure (ie, an abnormal finding) of the fixation suppression test may be found in the caloric test. The patient is asked to fixate on a central point during the peak caloric response. Vestibular nystagmus is normally suppressed by visual fixation. The fixation index expresses this attenuation quantitatively, which is the difference between the slow-phase velocity in the dark and in the light divided by the slow-phase velocity in the dark. The normal value for the visual suppression of the caloric response is >50%. If it fails—that is, <50%—impaired fixation suppression results. Cerebellar lesions affecting the flocculus cause impaired fixation suppression.

The inversion of caloric nystagmus is observed in patients with a tympanic membrane perforation. It occurs because of the cooling effect of the evaporation of moisture in the middle ear mucosa when warm air is used as a caloric stimulus.

Premature caloric reversal is the finding that can be observed in patients with Friedreich ataxia and brainstem lesions. The normal caloric response starts to decay at 90 seconds of the stimulation and disappears after 200 seconds, with a nystagmus beating toward the opposite side. In premature caloric reversal, nystagmus reversal occurs earlier than 140–150 seconds. It is worth noting that one should not refer to a preexisting spontaneous nystagmus as a premature caloric response.

The rotary chair test, which is also referred to as rotational testing, is used to evaluate the pathway between the horizontal SCC and the eye muscles. This pathway is known as the horizontal vestibuloocular reflex because the patient is positioned so that only the horizontal SCC is stimulated. Rotational testing has three main functions: (1) to confirm the bilateral impairment of horizontal functioning of the SCC, (2) to provide evidence of a central vestibular dysfunction, and (3) to quantify the progress of a known vestibulopathy.

Equipment

A typical rotary chair test consists of a stimulus device, a response recording and its analysis, a light-proof booth, a video camera, and a two-way communication system. The stimulus device is a chair whose rotational speed is precisely controlled by a computer within certain speed and frequency limits. The response recording is made with electrodes, which are placed to record horizontal eye movements.

Stimulus Types

Rotary chair testing includes basically two types of stimuli. Different protocols for each type of stimulus are used in different laboratories. One type of stimulus is called a sinusoidal harmonic acceleration; a series of rotational stimuli is used at the octaves of frequencies from 0.01 to 1.28 Hz, to the right and to the left. The velocity of the chair is set at 50–60°/s. The rotational stimulus at a given frequency is used for multiple cycles. The second type of stimulus is a velocity step test, which applies a series of velocities. The test is started with an acceleration impulse of 100°/s2 until a fixed, desirable rotational stimulus of 60–180°/s is achieved. Once the fixed velocity has been reached and applied for 46–60 seconds, the chair is decelerated to 0°/s, with the same magnitude of the acceleration. The test is then repeated in the opposite direction.

After each stimulus protocol, the computer detects slow-component eye velocity, omitting the fast component of the induced nystagmus. During the tests, the eye position, eye velocity, and chair velocity (ie, head velocity) are monitored.

Test Administration

The patient sits in the chair with his or her head secured in the head support. The seatbelt should be fastened. Eye movements are recorded with an infrared camera or electrodes placed lateral to both outer canthi. The patient should be informed of the stimulus type to be given and instructed not to move throughout the test unless told to do so. Throughout the test, the patient should be kept mentally alert with arithmetic tasks (eg, counting cities or states in alphabetical order). Before the rotational test, the system should be calibrated. The test is performed in total darkness, with the patient's eyes opened. Throughout the test, the patient's eyes should be monitored with a video camera. A two-way communication system is used to give instructions and arithmetic tasks to the patient.

Testing Parameters

In a sinusoidal harmonic acceleration test, there are three parameters to be evaluated: phase, gain, and symmetry data.

Phase

The phase demonstrates a timing relationship between the head velocity and the slow-component eye velocity through the frequencies tested. The difference between the two is defined as the phase angle and is expressed in a measure of degrees. The fact that the eye velocity is greater than the head velocity is known as the phase lead. The opposite is known as the phase lag. For maintaining the position of objects in the retina, the eye velocity needs to be equal to the head velocity. Under this circumstance, the phase angle would be 180°. An alternative measure of the phase angle is the time constant of the response, which is inversely correlated to it.

Gain

The gain is the ratio of slow-component eye velocity to the head velocity, which represents the response capability of the vestibular system through the frequencies tested. The values outside of the normal range, based on normative data, are considered abnormal as long as the system is calibrated and the patient is alert.

Symmetry Data

Symmetry data demonstrates whether slow-component eye velocities are equal on both sides.

The main testing parameter in the velocity step test is the time constant, which is the time needed for slow-component eye velocity to decline to 37% of the initial value. The second parameter of the velocity step test is the gain, whose definition is the same as its counterpart in the sinusoidal harmonic acceleration test.

In a sinusoidal harmonic acceleration test, an increased phase angle may imply a peripheral system insult or, less commonly, vestibular nucleus involvement. A decreased phase angle may imply a lesion in the cerebellum. Low gain is consistent with bilateral peripheral insult; high gain may be seen in cerebellar lesions. Asymmetry can be suggestive of the involvement of a central or decompensated peripheral vestibular system. If the central system is intact, a paretic lesion where either asymmetry or an irritative lesion in the opposite side is present is likely. It is analogous with the directional preponderance in the caloric test. In the velocity step test, an acute peripheral insult results in low gain and a shortened time constant of the response to the rotational stimulus toward the side of the lesion. Figure 46–6 presents a comparative analysis of caloric and rotary chair testing.

The relationship between the horizontal vestibuloocular reflex generated by the rotational stimulus and the caloric stimulus is significant. There is no linear relationship between the gain and canal paresis because as the magnitude of canal paresis increases, the gain decreases somewhat and then remains stable. However, the time constant decreases proportionally with increasing canal paresis.

High-frequency rotational tests are tools for testing horizontal and vertical vestibuloocular reflexes generated by the patient making active or passive head movements like those encountered in everyday life. High-frequency and high-acceleration rotational stimuli are expected to unmask the inherent asymmetry in the vestibular system. The tests have some advantages over the rotary chair test. For example, the equipment is more affordable. Testing time is very short (almost 20 seconds). The equipment is not heavy machinery in contrast to the rotary chair. The tests investigate vestibuloocular reflex by means of active or passive rotationary head movements in both horizontal and vertical planes.

Equipment

The two types of high-frequency rotational tests are the head thrust test and the head (vestibular) autorotation test (VAT). The equipment is almost the same. The equipment mainly consists of skin electrodes or one of the other eye-movement recording systems, a software calculating gain and phase data, and a headband carrying a motion sensor for detecting head movements.

Test Administration

There is a target in front of the patient who is sitting upright. The patient keeps his or her eyes on the target during the test. The main difference between the two tests is the way of applying stimulus. Stimuli in the head thrust test are passive (applied by the examiner). Stimuli in the VAT are actively generated by the patient but based on computer-driven tonal stimuli. For the VAT, the patient shakes his or her head like “no” (a movement in horizontal plane) and then “yes” (a movement in vertical plane) while looking at the target. Frequency of the head movement is sinusoidally increased from 2 to 6 kHz in accordance with auditory stimulus. For the head thrust test, rotational stimuli are given manually at an unpredictable onset time and in a randomly varied direction; 20–40 head thrusts are analyzed.

Testing Parameters

There are two parameters for each test: (1) gain, phase, and symmetry data for the VAT and (2) gain and response delay for the head thrust test. Description of the gain and the phase is the same as in the sinusoidal harmonic acceleration test (rotary chair test). The response delay is the time between the onsets of head and eye movements.

For healthy subjects, the gain is almost 1 at low frequencies and declines somewhat at the higher frequencies. The phase is almost zero (ie, 180°) at the low frequencies and lags somewhat at the higher frequencies. In case of vestibular lesion, the most common pattern is decreased gain (below 0.7 when especially ipsilesional head movement is performed) and/or increased phase angle. The response delay does tend to be longer. It was shown that sensitivity to identify an abnormality in the vestibuloocular reflex is higher for the head thrust test than for the VAT because of the reflex augmentation in predictable stimulus paradigm used in the latter. A comparative study showed that the VAT provides additional information that could be missed by the caloric testing in diagnosing an abnormality in vestibuloocular reflex caused by a labyrinthine lesion and vestibular schwannoma. Figure 46–7 denotes horizontal VAT results of a patient who had unilateral weakness in caloric test.

Subjective visual vertical and horizontal tests are measures of otoliths, and especially of utricular function. The bilateral gravitational input from the otoliths dominates the patients' perception of vertical and horizontal positions. To test for the subjective visual vertical or the subjective visual horizontal, the subject sits with head fixed in an upright position and looks at an illuminated line (either on a computer display or projected with a laser galvanometer system) in complete darkness. The subject is asked to adjust the line several times from starting positions at different angles to his subjective visual vertical or subjective visual horizontal. In acute peripheral vestibular lesions, including the utricles, there is a typical deviation of the subjective visual vertical or subjective visual horizontal of about several degrees to the affected side. With central compensation, gradual improvement occurs in the patient's perception of tilt.

Computerized dynamic posturography is an established test of postural stability. It is an important tool to quantitatively assess individual and integral patterns of visual, proprioceptive, and vestibular signal processing, as well as overall balance function, in response to simulated tasks similar to those encountered in daily life.

Equipment

The computerized dynamic posturography test described here is the EquiTest platform (Neurocom International, Inc.).

Computerized dynamic posturography measures the force applied by the body to a platform equipped with strain gauges. The device, which is controlled by a computer, measures postural sway in several test conditions and allows for the manipulation of somatosensory and visual feedback. The information obtained with this test includes the vertical and horizontal shear forces generated by the patient during postural sway. Ground reaction forces are used to infer particular types of postural sway.

The test includes some requirements for testing personnel and patients. The patient should be able to stand still, unassisted and with eyes open, for at least 1 minute. The safety harness should be appropriately fastened so that the patient can move freely with no external support. The patient's feet and medial malleolus should be placed at designated points on the force plate.

Testing consists of three main protocols: (1) the sensory organization test, (2) the posture-evoked response, and (3) motor control tests. Of the three tests, the sensory organization test is the most useful in the assessment of patients with vestibular disorders.

Sensory Organization Test

The sensory organization test evaluates whether a patient with a balance disorder appropriately does utilize visual, vestibular, and somatosensory cues, and picks the appropriate cue under conflicting conditions to maintain balance.

Sensory Conditions

The sensory organization test includes six sensory conditions of gradually increasing difficulty that disrupt somatosensory cues, visual cues, or both. In sensory condition 1, the patient is asked to stand still with eyes open. The support surface and visual surround are fixed. Sensory condition 2 is like sensory condition 1 except that the patient's eyes are closed. In sensory condition 3, the support surface is fixed. The visual surround leans forward, which is called sway referenced. The patient keeps eyes open. Sensory condition 4 requires the patient to stand on the tilted support surface with eyes open and the visual surround fixed. Sensory condition 5 is like condition 4 except that the patient's eyes are closed. In sensory condition 6, the support surface is tilted and the visual surround leans forward, which means that the visual and support conditions are sway referenced.

Test Administration

This protocol consists of three repetitions of each sensory condition. Before each trial, the patient is asked to stand as still as possible and ignore the visual surround and the support surface motions. During each condition of the sensory organization test, force plates monitor the sway of the patient's center of gravity for periods of 20 seconds. Stability is quantified by an equilibrium score that is the percentage expression of the ratio of anteroposterior peak-to-peak sway amplitude during the trials to the theoretical anteroposterior limits of the stability. Equilibrium scores near 100% show little sway, whereas scores closer to zero are associated with a sway near the limits of the stability. Theoretical limits of stability are calculated on the basis of the maximum backward and forward center of gravity sway angles to which healthy subjects can move without losing balance.

Testing Parameters

The primary testing parameters are the composite equilibrium score and the sensory analysis.

Composite Equilibrium Score

The composite equilibrium score, which is a weighted average of all trials, provides an overall idea of the patient's balance performance. Abnormally low scores may be associated with either a malingering circumstance or vestibular, somatosensory, or visual dysfunction.

Sensory Analysis

The sensory analysis quantifies the differences in the equilibrium scores between two conditions. The equilibrium score for sensory analysis is the average of each of the three trials of the conditions 1–6. Differences are sought in four ratios: (1) the somatosensory ratio, (2) the visual ratio, (3) the vestibular ratio, and (4) the vision preference ratio. The vestibular dysfunction pattern should also be considered in the sensory analysis.

Somatosensory Ratio

In the somatosensory ratio, a comparison is made of the equilibrium scores of conditions 1 and 2. Abnormally low ratios are associated with dysfunction of the somatosensory system.

Visual Ratio

The visual ratio is the ratio of the equilibrium scores of conditions 1 and 4. Low ratios are associated with a poor processing of visual cues.

Vestibular Ratio

The vestibular ratio compares the equilibrium scores of conditions 1 and 5. Low scores are considered indicative of a dysfunction of the vestibular system (Figure 46–8B).

Vision Preference Ratio

The vision preference ratio compares the sum of the equilibrium scores of conditions 3 and 6 with the sum of the equilibrium scores of conditions 2 and 5. It tests whether the patient uses inappropriate and inaccurate visual cues. Low ratios are considered an abnormal preference of visual inputs. Normal subjects suppress inaccurate visual inputs, whereas a patient with a vision preference shows unsteadiness when many stimuli are moving simultaneously. A free-fall is usually enough to rule out an exaggeration or malingering circumstance because it is difficult for a patient to fall freely without a causative disorder.

The vestibular dysfunction pattern in sensory analysis is seen in bilateral vestibular loss or decompensated unilateral vestibular loss. In these cases, the equilibrium scores are expected to be within the normal range for conditions 1 through 4, but below the lower limit of the range for conditions 5 or 6 (or both) (Figure 46–8A). However, the vestibular dysfunction pattern alone is not enough to make a distinction between peripheral and central vestibular lesions. An abnormal vision preference usually occurs in patients following head trauma. It can be associated with a vestibular dysfunction pattern, depending on whether vestibular compensation develops. Multisensory dysfunction patterns, including combinations of vestibular and vision systems or vestibular and somatosensory systems, suggest CNS lesions.

Utility of Computerized Dynamic Posturography

The clinical usefulness of computerized dynamic posturography in neurotologic practice has been assessed in the literature. It is agreed that this test is quite useful in the following conditions and situations: (1) chronic disequilibrium, (2) persistent dizziness or vertigo despite treatment, (3) patients with normal results in other vestibular tests, (4) measuring the baseline postural control prior to treatment, (5) monitoring the results of vestibular ablative treatments, and (6) selecting the most useful rehabilitation strategy. The identification of malingering individuals is possible with this test. Strong indicators of a poor response are (1) a substandard performance on the sensory organization test in conditions 1 or 2 (or both), with great intertrial differences; (2) a relatively better performance in conditions 5 and 6 compared with that in conditions 1 and 2; (3) circular sway without falling; (4) exaggerated motor responses to small platform translations; and (5) inconsistent motor responses to small and large, forward and backward platform translations. However, computerized dynamic posturography alone does not localize and lateralize the site of the lesion and cannot aid in establishing a diagnosis. Moreover, the results of this test may be at odds with the results of the ENG and rotary chair test because computerized dynamic posturography assesses the vestibulospinal and postural control systems, whereas the other two rely on the vestibuloocular reflex.

The vestibular evoked myogenic potentials (VEMP) are short latency electromyograms that are evoked by acoustic stimuli in high intensity and recorded from surface electrodes over the tonically contracted sternocleidomastoid muscle. The origin of VEMP has been shown to be the saccule. The response pathway consists of the saccule, inferior vestibular nerve, lateral vestibular nucleus, lateral vestibulospinal tract, and sternocleidomastoid muscle. The test provides diagnostic information about saccular and/or inferior vestibular nerve function. An intact middle ear is required for the response quality.

Equipment and Recording

A commercially available evoked potential unit can be used for recording the VEMP. The patient is tested while seated upright with the head turned away from the tested ear to increase tension of the muscle. A noninverting surface electrode is placed at the middle third of the sternocleidomastoid muscle. An inverting electrode is located at the sternoclavicular junction. A ground electrode is placed on the forehead. Rarefaction click (at 100 dB normalized hearing level [nHL]) or tone-burst stimuli (2-0–2 ms at 500 or 750 Hz and 120 dB peak sound pressure level [SPL]) are delivered monoaurally at the rate of 5/s. Electromyogenic activity of the muscle is amplified (×5000) and bandpass filtered from 10 to 2000 Hz. Analysis window is adjusted to 100 ms. Responses are averaged over a series of 128 or more based on the response stability.

Waveform of the Response

The VEMP waveform is characterized by a positive peak (P13 or wave I) at 13 (13–15) ms, and a negative peak (N23 or wave II) at 23 (21–24) ms. Peak-to-peak amplitude of P13–23 is measured. Exemplary traces are shown in Figure 46–9.

Stimulus intensity, stimulus frequency, and tonic electromyogenic activity may affect response amplitude but not response latency. Click-evoked VEMP threshold ranges from 80 to 100 dB nHL in subjects with normal audiovestibular function. In tone burst-evoked VEMP recordings, robust responses are obtained with 500, 750, and 1000 Hz tone bursts, and thresholds ranges from 100 to 120 dB peak SPL across frequency.

Parameters and Evaluation

For clinical purposes, the main interest is amplitude and threshold asymmetries between the right and left sides. However, controlling the tonic state of the sternocleidomastoid muscle is important for the accurate interpretation of interaural amplitude difference. Asymmetry ratio (AR) between the right and left ears is formulated as follows:

AR = 100 (AL − AR)/(AL + AR)

where AL and AR indicate peak-to-peak amplitude of P13 and 23, respectively. When the ratio exceeds 36%, it is interpreted as an indicator of saccular hydrops and called “augmented VEMP.” The augmented VEMP has been reported in almost 33% of affected ears. This finding was found to correlate with flat and high-frequency hearing loss. VEMP may also be absent in up to 54% of Meniere patients. The absent VEMP correlates with low scores in sensory condition 5 of posturography and also low-frequency hearing loss, which is an indicator of apical hydrops. The test is also useful in detecting vestibular schwannoma originating from the inferior vestibular nerve. In this circumstance, elevated VEMP threshold or absent VEMP is expected. In vestibular neuronitis, VEMP may be absent. Superior canal dehiscence syndrome causes lowered VEMP thresholds or increased amplitudes. In otosclerosis, absent VEMP is expected. However, despite conductive hearing loss, if there is still normal VEMP, one should suspect the superior canal dehiscence syndrome rather than otosclerosis. Latency of the VEMP peak was focused on less than the parameters aforementioned. However, prolonged P13 latency was reported to correlate with retrolabyrinthine lesions such as large vestibular schwannoma and multiple sclerosis.