Balance Function Tests

Colin L. W. Driscoll
J. Douglas Green Jr

Balance is maintained through complex interaction of the vestibular, visual, and somatosensory information that is combined within the brainstem to generate a motor response correcting the perturbation. Abnormalities within any portion of the system can cause a sensation of imbalance or dizziness. Dizziness is one of the most common reasons for seeking medical evaluation, and the otorhinolaryngologist is often the primary contact. Evaluating patients for dizziness can be frustrating for both patient and physician. The symptoms are difficult for patients to describe, the differential diagnosis is broad, and many tests have to be considered. An understanding of the balance tests currently available and the pathophysiologic principles on which they are based improves treatment of these challenging patients. The particular approach to dizziness is affected by type of practice, available resources, and patient population.
The main goals of the diagnostic evaluation are to determine the location and severity of lesions within the balance system and to help formulate and guide a treatment plan. The etiologic evaluation of the balance system dysfunction necessitates compilation of other data, including those from a comprehensive history and physical examination and other directed laboratory and radiologic examinations. In most cases, clues revealed during the history and physical examination lead to a diagnosis, and treatment can begin without additional testing. Balance function testing should be interpreted in light of the history and physical exam findings and often provides confirmatory information as to pathophysiologic processes involved. Formal balance function testing provides information as to the site and side of the lesion, to ascertain who is likely to benefit from vestibular rehabilitation, to assess recovery of vestibular function, and to document contralateral function when a destructive procedure is contemplated. Clinical decision support systems have been described to help office personnel determine which patients may need balance function testing (1).
This chapter reviews tests for assessing the balance system (Table 131.1). These balance system tests, usually administered by an audiologist or trained technician, require specialized equipment and dedicated space. The basic physiologic principles of the tests are reviewed.

Electronystagmography
The electronystagmography (ENG) test battery is the workhorse of balance function testing. It is a combination of tests that together provide complementary information about the vestibular and oculomotor systems. Rather than a diagnosis, the tests provide data that must be used in conjunction with the findings of the history, physical examination, and other studies to determine a diagnosis. The ENG battery typically consists of the vestibular and oculomotor tests listed in Table 131.2.
The ENG tests are based on the well-studied neurophysiologic characteristics of the vestibulo-ocular reflex. Motion of the head, or simulated motion through labyrinthine stimulation, produces compensatory eye movement. These eye movements are observable and recordable responses that provide the basis for interpretation of the various tests. Changes in eye position are recorded by means of taking advantage of the natural difference in electrical charge between the cornea (+) and the retina (-), the corneal-retinal potential. Electrodes record the changes in potential when the eye moves [electrooculography (EOG)]. The advantages of this recording technique are low cost, ease of administration, noninvasiveness, avoidance of head restraint, and extensive experience in interpreting the data. The technique does have technologic limitations. The signal is susceptible to changes in skin resistance due to perspiration, interference from eye-blink artifacts, and a poor signal-to-noise ratio. Although abnormal eye movements can be revealed by means of direct visualization during testing, it is preferable in most cases to have quantifiable data to characterize the abnormalities. Furthermore, some parts of the test require that the eyes be closed to prevent fixation suppression. Nystagmus is the primary ocular movement measured and is defined by its direction (horizontal, vertical, or torsional) and velocity (degrees per second). Direction is determined by the fast component, and velocity is calculated from the slow-phase component.
New technologies for observing and recording eye movement continue to be developed (2,3). Video ENG systems, vidoenystagmography, are now available that use small video cameras to observe and monitor horizontal, vertical, and torsional eye movement during vestibular evaluations (Fig. 131.1). With this system, tests that require the patient to move are performed with a lightweight goggle assembly. Pursuit, gaze, and saccade testing can be performed with an oculomotor module with tracking targets for each eye. The eyes are illuminated with near-infrared light that is not visible to the patient but allows the cameras to pick up and project images of the eyes on video monitors. This system has several advantages over traditional ENG testing techniques. Nonelectrode eye movement recording eliminates artifacts, the need for frequent recalibration, and impedance testing. Vertical eye movements can be accurately recorded. Torsional eye movements can be visualized and recorded without fixation suppression. Disconjugate eye movements are more easily identified. A portable unit allows testing at home, in an intensive care unit, or at any other remote location. The disadvantages are high cost, need to wear goggles, and unfamiliarity with the equipment. Most companies that manufacture ENG equipment now offer a videonystagmography system to make use of these advantages.
Virtual reality testing systems are being developed and promise to provide a means to produce visual stimuli that were previously impossible (4). Images can be produced on a visual display, and because the system is software driven, the images can be manipulated quickly and with tremendous flexibility. For example, cross-axis stimulation (head motion in one plane and eye motion in another) becomes easy to perform.
The sequence of ENG tests is important to prevent obtaining misleading results. For example, testing for benign paroxysmal positional vertigo is performed at the beginning of the test series to avoid fatiguing of the response. The tests must be performed with proper attention to detail and accurate calibration. Patient anxiety, fatigue, lack of cooperation, and medications can adversely affect test results.
Interpretation of ENG results is critical to proper diagnosis and treatment and should be performed by qualified audiologists and physicians who are trained and familiar with the equipment being used for testing. Increasingly, specialists in other fields, chiropractors, and poorly trained medical doctors are investing in ENG equipment in hopes of financial gain, often without knowledge of how to interpret the ENG test results and how to apply them in clinical practice. It is essential that otorhinolaryngologists offering ENG testing be familiar with interpretation of the results. Several excellent courses are available to otorhinolaryngologists offering ENG testing within their offices.

Spontaneous and Gaze Nystagmus
Spontaneous nystagmus refers to nystagmus that is present without visual or vestibular stimulation. Spontaneous nystagmus can sometimes be seen only with loss of visual fixation (e.g., milder forms of spontaneous vestibular nystagmus) or may be seen with both eyes open and with loss of visual fixation (e.g., congenital nystagmus and severe vestibular nystagmus). Spontaneous nystagmus can be observed both at the bedside and in the vestibular laboratory. However, although reduction of visual fixation can be achieved easily in the laboratory using infrared video goggles or EOG with eye closure, at the bedside, achieving a reduction in visual fixation while still maintaining an ability to observe eye movements can be challenging. Observing a patient's eye with an ophthalmoscope while the other eye is occluded allows the examiner to assess spontaneous nystagmus with reduced visual fixation. The most common type of spontaneous nystagmus, that is, spontaneous vestibular nystagmus, occurs with unilateral peripheral vestibular lesions. Spontaneous vestibular nystagmus is always unidirectional and increases when the patient gazes in the direction of the quick component of the nystagmus. This gaze dependent change in nystagmus intensity is called “Alexander's Law.” As noted previously, loss of visual fixation also increases the magnitude of spontaneous vestibular nystagmus. Thus, judicious use of gaze direction and presence or absence of visual fixation can aid the examiner both at the bedside and in the laboratory in judging whether or not a spontaneous nystagmus is a result of a vestibular abnormality. Failure of fixation suppression is highly suggestive of a central pathologic condition.
Gaze nystagmus, also known as gaze-evoked nystagmus, is a bidirectional nystagmus with right beating nystagmus on right gaze and left beating nystagmus on left gaze. Many patients with gaze-evoked nystagmus also will manifest an up-beating nystagmus on upward gaze. Note that down-beating nystagmus in any gaze position, even in downward gaze, is not considered a component of gaze-evoked nystagmus and should be regarded as a manifestation of a central nervous system abnormality at the level of the craniocervical junction unless proven otherwise. Gaze-evoked nystagmus can be seen in normal individuals when horizontal gaze exceeds 30 degrees from the straight-ahead position. Thus, it is best to limit the amount of gaze deviation when assessing a patient for gaze-evoked nystagmus to less than 30 degrees. Gaze-evoked nystagmus occurs as a result of inadequate gaze-holding, thereby leading to a slow drift of the eyes back toward the straight ahead gaze position with the drift interrupted intermittently by rapid, nystagmus fast phases in the direction of gaze. Bidirectional gaze-evoked nystagmus is always a result of a central nervous system abnormality and never is the result of a peripheral vestibular abnormality. There are many etiologies for gaze-evoked nystagmus. The most common cause of gaze-evoked nystagmus is a medication effect (e.g., from anticonvulsants).

Positional and Positioning Tests
Positional tests are designed to detect the response to changes in direction of gravitational force. The patient is moved slowly into a series of stationary positions with the eyes closed, and presence or absence of nystagmus is assessed. The nystagmus can be either fixed or direction changing. If nystagmus is elicited in the positional tests, the entire body is turned to determine whether neck torsion is responsible. Interpretation of results is controversial and necessitates consideration of the number of positions that elicit nystagmus and the velocity of the nystagmus. Positional nystagmus of peripheral origin can fatigue with repeated testing, is usually direction fixed, and often is associated with caloric weakness. The direction of nystagmus caused by a peripheral lesion typically does not change independently of head movement (5). Direction-changing nystagmus without an accompanying change in head position indicates the presence of a central disorder.
The positioning test used most often is the Dix-Hallpike maneuver. The patient is rapidly moved from sitting to supine with the head turned and hanging below the level of the table. If nystagmus is elicited, the maneuver is repeated to determine the existence of fatigability. A response that fatigues suggests a peripheral problem. The patient's eyes are kept open to allow the examiner to evaluate for torsional nystagmus, which can indicate benign positional vertigo due to loose otoconia within the labyrinth. Torsional nystagmus cannot be recorded with conventional ENG. Head hanging to the right produces counterclockwise torsional nystagmus, and head hanging to the left produces clockwise torsional nystagmus. A variant of benign positional vertigo affecting the horizontal semicircular canal produces pure horizontal nystagmus.

Bithermal Caloric Tests
Bithermal caloric tests are used to evaluate the function of the horizontal semicircular canals. Changes in temperature stimulate fluid flow (equivalent to a very slow frequency of only 0.002 to 0.004 Hz) within the horizontal semicircular canal; if the system is functioning, nystagmus is elicited. The very slow frequency of stimulation is not a condition normally experienced during daily life. Each ear is tested independently, and the responses are compared.
Caloric testing is performed with the patient supine and head elevated 30 degrees. The external auditory canal is irrigated directly with 250 mL of water at 7 degrees above and below body temperature for 30 seconds. An alternative is to place a small, distensible balloon in the ear canal and fill it with water (closed-loop system). Ocular movements are recorded for approximately 2 minutes, beginning with stimulation. Fixation suppression is evaluated during this time. The slow-phase velocity of elicited nystagmus is calculated and recorded as an objective measure of the response. Warm and cool air irrigation or a closed-loop system can be substituted for direct irrigation if the tympanic membrane is perforated. The responses of the right and the left ears are compared. A difference greater than 20% usually is considered abnormal and is reported as left- or right-sided weakness. The total right-beating responses is compared with the total left-beating responses, and the result is reported as a right or left directional preponderance. A difference greater than 30% is considered significant. Abnormal directional preponderance without unilateral weakness suggests a central pathologic condition. The results indicating central or peripheral disorders are summarized
A monothermal caloric screening test has been suggested to minimize procedure time and patient discomfort. This test has been criticized for a high false-negative rate. However, the results of a study by Jacobson et al. (6) suggest that if appropriate failure criteria are used, the sensitivity reaches 93% and the specificity 98%. Patients who fail the screening test need conventional bithermal testing.
Patients with a complete unilateral or bilateral caloric loss should be tested with ice caloric irrigation of the affected ear(s). Frequently, nystagmus can be elicited with this stronger stimulus. Ice caloric stimulation is uncomfortable for the patient and should be limited in use. It should be noted that the absence of caloric response to warm, cool, or ice water irrigations cannot be taken as an indication of complete lack of function. This should be confirmed by rotational chair testing.

Electronystagmography Fistula Test
If a fistula exists between the middle ear space and the inner ear fluid, application of positive or negative pressure in the external ear canal may produce a fluid shift resulting in nystagmus. Objective nystagmus or a clear subjective response suggests a perilymph fistula or dehiscence of the horizontal or superior semicircular canal.

Saccadic System
The saccadic system is used to move a target from theperiphery of the retina to the fovea. Targets more than20 degrees from the line of sight are normally located by means of a combination of eye and head movement, called gaze saccade (7). This test is performed while the patient sits facing a light bar and keeps his or her head stationary. Lights on the bar are activated to the left and right of center by 10 to 20 degrees, and the patient is asked to shift gaze to each new target. Electrooculographic recording techniques are used. Latency, maximum velocity, duration, gain (overshoot or undershoot), refixation saccade, glissade (postsaccadic drift, or slip of the eye), and other variables are measured and observed.
Abnormal saccades can be caused by lesions in a wide variety of locations. However, they are most commonly caused by a central pathologic disorder rather than a peripheral vestibular one. An abnormal saccadic response can be caused by lesions in the cerebellum (dorsal cerebellar vermis), brainstem (particularly the paramedian pontine reticular formation and medial longitudinal fasciculus), or ocular muscles and nerves. Cerebellar lesions typically manifest as saccadic overshoot or undershoot dysmetria. Dysmetria is an error in range, rate, or direction in performance of precision voluntary movement. It usually is found through past pointing in a finger-to-nose test (8). Different patterns of abnormalities and testing paradigms can help to localize a lesion (9). Age, fatigue, lack of attention, sedatives, drug intoxication (phenytoin), and other factors can cause slow saccades or increase latency.

Pursuit System
The smooth pursuit system stabilizes images of moving objects on the fovea (7). Healthy persons can pursue an object moving at a velocity of about 30 degrees per second. At faster rates, the image can slip, and a corrective saccade is used to catch up. Smooth pursuit usually is tested with sinusoidal stimuli, such as a pendulum, light-emitting diodes, or a laser, with a frequency range of 0.2 to 0.7 Hzand a horizontal range less than 20 degrees to the left and right. Electrooculographic techniques are used to record gain, phase lead or lag, symmetry, and other variables. Because the smooth pursuit system is distributed throughout the brainstem and cerebellum, anatomic localization is not possible (9). If age, alertness, medications, and congenital nystagmus are eliminated as causes of abnormal test results, a central cause is suggested. Saccadic pursuit suggests cerebellar disease. Reduced bilateral gain can be caused by a brainstem lesion. Patients with Alzheimer disease and schizophrenia can perform worse with sinusoidal stimuli than with fixed velocity stimuli (9).

Optokinetic System
The optokinetic system differs slightly from the pursuit system in that it allows persons to keep the visual field in focus while they or most of the environment are moving. The vestibulo-ocular reflex alone is not adequate to generate compensatory eye movements at low frequency. In clinical testing, this effect is achieved by keeping the patient stationary and moving the environment. About 90% of the visual field must be filled with the target to avoid simply testing the pursuit system. A rotating drum with thin vertical stripes or stripes projected on the wall are common stimuli. The patient is instructed to look straight ahead. As the targets pass by, there should be small amplitude excursions of the eye (stare nystagmus). If the patient fixes on the target, longer excursions occur, and the pursuit system is being tested (look nystagmus) (9). Separating the smooth pursuit system and the optokinetic system can be even more difficult. Experiments indicate that both systems are responsible for eye movement during stimulation. If, however, the lights are extinguished, the smooth pursuit system response drops to zero almost instantly. With the lights out, patients continue to have nystagmus for approximately 25 seconds. It is believed that this nystagmus [optokinetic afternystagmus (OKAN)] is caused by the optokinetic system alone (7). Because of the contribution of the pursuit system, it is not possible to conclude that the optokinetic system is normal without having nystagmus for approximately 25 seconds following optokinetic stimulation.
The optokinetic system is widely distributed throughout the brainstem and cerebellum; therefore, abnormalities are not site specific. Absence or asymmetry of the OKAN occurs with peripheral vestibular lesions. Bilateral labyrinthectomy profoundly reduces or eliminates OKAN (10). These findings suggest an interaction between the vestibuloocular reflex and the optokinetic system (7). The OKAN becomes asymmetric after unilateral vestibular ablation; the result is stronger and more prolonged nystagmus directed at the side of the lesion (10). A problem is that the sensitivity in identifying unilateral vestibular disease is not high (10). In a study involving 12 patients who had undergone removal of acoustic neuroma, OKAN helped identify the side of the lesion for only 7 of the patients (10). Baloh et al. (11) found abnormal test results among patients with cerebellar atrophy.
Optokinetic testing is not routinely used but can be indicated as a confirmatory examination when abnormal pursuit is identified. Combining the results of OKAN with those of rotary testing can be helpful in determining the side of the lesion. Another potential use might be to exclude vestibular disease, especially for patients who cannot undergo caloric testing (10). As more is learned about the production of optokinetic nystagmus, clinical usefulness can expand.

Rotary Chair, Sinusoidal Harmonic Acceleration
Rotational testing paradigms offer the theoretic ability to test the semicircular canals under more physiologic, that is, higher frequency, conditions than can be achieved with caloric testing. The stimulus can be precisely controlled and is repeatable. Because both ears are tested simultaneously, the results reflect the integrated function of both ears (12). In this test, the rotary chair is oscillated from side to side at a series of preprogrammed rates (Fig. 131.2). The acceleration frequencies tested range from 0.01 to1.28 Hz with a maximum velocity of 50 degrees per second.
More rapid rotational velocities may result in head slippage adversely affecting test results. An infrared camera is mounted to the chair to monitor the patient and eye movement. Mental alerting tests are used, as in ENG. As the chair begins to rotate to the right, standard EOG techniques are used to record slow compensatory eye movement to the left. Saccadic eye movement returns the eye to a central position. This movement is eliminated mathematically, and slow-phase movement is compared with chair movement. Three basic characteristics are measured—phase, gain, and symmetry of eye movement. Testing is comfortable for the patient and takes approximately 15 minutes.
Interpretation of the results requires integration with complementary data and the clinical history. Symmetry represents a comparison between the peak slow-wave velocity when the patient is rotated to the left and the peak slow-wave velocity with rotation to the right. For a patient with acute, uncomplicated unilateral peripheral weakness, the symmetry measure shows weakness on the affected side. A problem is that symmetry can be misleading in many cases. Spontaneous nystagmus, irritation of the labyrinth, vestibular compensation, and cerebellar lesions can produce erroneous data. The value of symmetry alone as an indicator of the side of a lesion is controversial. Caution is needed in interpretation of the results. Symmetry often improves with compensation after a vestibular insult and can be useful for monitoring recovery (11). Asymmetry or directional predominance can also be seen withmigraine-associated dizziness, a common form of dizziness.
The phase variable is the relation between maximum chair velocity and maximum slow-phase velocity. Eye velocity typically leads chair velocity, the so-called phase lead. The phase lead often is exaggerated among patients with central or peripheral vestibular disease. If abnormal, this value is nonlocalizing. Some laboratories also include a step test in addition to sinusoidal acceleration testing described previously. In the step test, the system time constant is measured by varying the speed at which the chair rotates. This test can be performed simultaneously with the sinusoidal acceleration testing, providing additional vestibulo-ocular reflex information.
Gain is the ratio of maximum eye velocity to maximum chair velocity. A gain of one indicates that slow-phase eye velocity equals chair velocity and is opposite in direction. If there is no eye movement, the gain is zero. A problem is that gain can fluctuate markedly with changes in alertness. Consistent testing is critical to obtain valid results. The gain values used in calculating phase and symmetry must be accurate. Low gain values alert the physician that the results may be inaccurate. Depressed gain values under good testing conditions suggest bilateral peripheral lesions. Abnormally high gain can indicate the presence of a cerebellar lesion that is decreasing vestibular inhibition. Another caution is that there can be considerable differences in results between laboratories, depending on the specific data analysis algorithms used and operator intervention (13).
Rotary testing is useful to (a) monitor changes in vestibular function over time, especially bilateral lesions or lesions due to vestibular toxicity, (b) monitor compensation after acute injury, and (c) identify residual labyrinthine function for patients with no response during caloric testing or low-frequency rotary chair testing (12,14).
Off-vertical-axis rotation is a variation of rotary chair evaluation (15). The testing procedure is similar, except that the chair can be tilted 30 degrees. Earth horizontal axis (barbecue spit) rotation is another variation. The proposed advantage is that otolith function is incorporated into the response. The role of this type of testing is being investigated. Data suggest that a single labyrinth is sufficient to produce normal semicircular canal–otolith interaction; therefore, the value of the test may be limited (15). Another modification is to test unilateral otolith-ocular response. During constant angular rate rotation, the patient is displaced laterally on the rotating turntable, so that one labyrinth becomes aligned with the rotary axis and the second (eccentric) labyrinth is solely exposed to inertial acceleration (16).

Vestibular Autorotation Test
The vestibular autorotation test is a method of evaluating the vestibuloocular reflex (head-shaking nystagmus) that has distinct clinical and practical advantages over rotary chair testing and ENG (2,17,18,19,20,21). Patients are wired with conventional EOG electrodes to record horizontal and vertical eye movement and are fitted with a lightweight headband containing an EOG amplifier and rotational velocity sensor. The patient is instructed to fix on a target and to rotate the head in synchrony with an auditory cue. The auditory cue accelerates to a maximum of 6 Hz and maintains this velocity for 13 seconds for a total test time of 18 seconds. The test is repeated three times in the horizontal and vertical dimensions. Phase, gain, and symmetry data are collected over the frequency range of 2 to 6 Hz (17).
The vestibular autorotation test has several attractive characteristics. The vestibular system is evaluated at frequencies more physiologic than the ultralow frequencies used in conventional ENG or rotary chair testing (20,22,23,24). The vestibular autorotation test can be performed efficiently, does not require dedicated space, is portable, allows testing in the vertical plane, and is well tolerated by patients. A potential disadvantage is that it includes the cervical-ocular reflex, but this is thought to be an unimportant contribution at the frequencies used. Vestibular autorotation testing also requires patients to rotate their heads appropriately, which may be problematic for elderly patients or patients with cervical pathology. Interpretation of the results is the same as with ENG and rotary chair testing. The clinical value of headshake nystagmus in evaluating dizziness has been challenged. Jacobson et al. (21) reported a sensitivity of only 27% and a specificity of 85%.

Dynamic Posturography
Computerized dynamic posturography came into the clinical realm in 1985 with the introduction of the Equitest system developed by NeuroCom. Posturography consists of two tests—the sensory organization test and the movement coordination test. The sensory organization test protocol calls for evaluation under six conditions in which sensory and proprioceptive inputs are varied (Fig. 131.3). During the sensory organization test portion, the patient's anterior and posterior body sway is recorded, and a performance index is calculated on a scale of 0% to 100% (fall, 0; no sway, 100). During the movement coordination test, the patient stands quietly on a platform, with the visual surround fixed (Fig. 131.4). The platform then undergoes a series of translational and rotational movements. The principal measurement recorded is the latency to onset of active recovery from destabilizing perturbation. Amplitude and symmetry of the neuromuscular responses also are recorded.
Several classification systems to assist with interpretation of results have been proposed (25). Table 131.4 summarizes one system (9). Although these patterns seem to suggest the site of the lesion, the test is not designed or suited for this purpose. A patient with abnormal scores on tests 5 and 6 can have a central or peripheral vestibular disorder. Patients with a compensated unilateral peripheral vestibular dysfunction have normal performance on tests 5 and 6. A patient with abnormal scores on conditions 1, 2, 3, and 4 is more likely to have a nonvestibular lesion or functional disorder (aphysiologic sway). A diagnosis of functional disorder is suggested when test results on the more difficult conditions (tests 4, 5, and 6) are equal to or better than those recorded during testing of the easier conditions (tests 1, 2, and 3).
The postural evoked responses in the movement coordination test portion of the evaluation are useful in validating the sensory responses and in identifying a central pathologic disorder. The tests are based on the automatic muscle responses thought to be triggered by proprioceptive changes. The response requires normal muscle, nerve, spinal cord, cerebellar, brainstem, and cortical function and is thus a diffusely distributed response. It is likely that the response is modulated by vestibular and other sensory input.
The most common test for perilymphatic fistula is ENG of the vestibuloocular reflex. Dynamic posturography offers an alternative testing strategy that eliminates potentially confounding effects from the visual and proprioceptive systems. Dynamic posturography can eliminate visual and somatosensory input and therefore better isolate the vestibular apparatus. Positive and negative pressures can be applied to the external auditory canal by means of standard tympanometry techniques, and postural adjustments can be analyzed (23). This test can be more sensitive than the traditional ENG fistula test (26,27). However, lack of a definitive test for the existence of a perilymph fistula impedes development of a clear relation.
Posturography can be combined with standard electro-myographic techniques. Electromyography is beneficial in identifying particular muscle response patterns triggered by changes in visual or somatosensory input controlled by the computerized dynamic posturography apparatus. These testing strategies are most useful in research aimed at understanding the complex interactions that allow maintenance of postural control.
Posturography is especially beneficial in documenting overall postural stability and in identifying particular balancing strategies. It is useful for vestibular rehabilitation and for monitoring improvement or decompensation. The computerized dynamic posturography evaluation is comfortable for patients, and they feel that their balance is being thoroughly tested. The test results correlate more closely than those of other balance function test results with the Dizziness Handicap Inventory, which is a subjective measure of overall balance limitations (28).

Pediatric Testing
Vestibular testing of children presents several challenges. Mental alerting, lack of tolerance of uncomfortable stimuli, inability to follow verbal commands, and inability to remain still all make testing difficult. Modifications of the standard adult ENG battery allow oculomotor, positional, and caloric testing (29). Simultaneous minimal caloric irrigations improve tolerability by reducing the degree of induced nystagmus and time of the procedure. Of course, the data obtained are not as informative as those provided by alternate binaural testing. Closed-loop irrigation systems can improve tolerance. Vestibular autorotation testing can prove useful in children, but more experience is needed. Rotational chair testing can be performed on most children, and very young children can sit on a parent's lap. Computerized dynamic posturography can be performed on older children, but normative data are not available. Most testing protocols can be shortened to minimize testing time and improve compliance.

Vestibular Evoked Myogenic Potentials
Standard caloric testing performed as part of an ENG battery evaluates the superior vestibular nerve because the lateral semicircular canal is being stimulated. A relatively new test can be performed using equipment traditionally used for auditory brainstem response testing to evaluate the function of the inferior vestibular nerve. Colebatch and Halmagyi (30) described the electromyographic response of the neck musculature to a loud, ipsilateral, broad band click. The afferent limb of the response is thought to be due to stimulation of the saccule with transmission via the inferior vestibular nerve to the brainstem and vestibular nuclei. The efferent limb of the response is via the spinocerebellar tract with the response typically recorded on the ipsilateral sternocleidomastoid muscle. The desire for inferior vestibular nerve evaluation has greatly increased the use of this test, which is termed the vestibular evoked myogenic potential (VEMP).
As experience with VEMP accumulates, the limitations and benefits of testing are becoming more apparent. Patients older than 60 years may have a reduced response in VEMP amplitude, which is thought to potentially be due to deterioration of saccular function (31). Pathologic changes in the brainstem, such as multiple sclerosis, may result in delayed potentials with variability noted in the response of the test to common conditions such as acoustic neuromas, Ménière disease, and vestibular neuritis (32). Surprisingly, early Ménière disease frequently demonstrates normal VEMP responses with progressive loss of the potentials with disease progression.

Dynamic Visual Acuity
Movement of the head causes significant retinal slip and loss of visual acuity unless the vestibulo-ocular reflex produces an appropriate compensatory response. Patients with dizziness frequently complain of visual blurring and unsteady visual sensations, particularly with head movement. The lack of visual stability on the retina can be quite disconcerting to patients and may prove dangerous forpatients during driving. Several commercial devices are available that can assess dynamic visual acuity. These tests use a rotational velocity sensor in combination with a computer screen to evaluate vision during head movement, giving a functional assessment of the vestibulo-ocular reflex in both horizontal and vertical planes (33).
Age does seem to affect dynamic visual acuity results with older patients demonstrating reduction in visual stability. Although some variability has been noted, the test does show good sensitivity and specificity in distinguishing normal patients from those with abnormalities.Patients with bilateral vestibular deficits were shown to have a greater degree of reduction in comparison with unilateral vestibular deficits and patients with nonvestibular dizziness. Surprisingly, oscillopsia did not correlate with abnormalities on vertical dynamic visual acuity in patients with bilateral vestibular loss. This seems to be due to the central preprogramming and the predictability of the head movements. Recent studies demonstrate recovery of the dynamic visual acuity following deterioration after traumatic brain injury associated with dizziness (34).