Abstract
When evaluating patients with dizziness, knowledge of basic vestibular physiology and bedside examination techniques is critical to reach the correct diagnosis. During the vestibular examination, the clinician should look for signs of static and dynamic vestibular imbalance in the semicircular canal and otolith function. Here we outline the key components of the clinical vestibular examination considering recent technical advances in video-oculography and mobile devices such as tablet computers. Using these portable technologies, the vestibular examination can be quantified at the bedside to improve diagnostic accuracy and clinical management. In this process, a careful and thorough clinical evaluation of all subtypes of eye movements, stance, and gait is critical because of the close anatomic and physiologic connections among vestibular, postural, and ocular motor functions.
Keywords
Dizziness, Examination, Head impulse, Nystagmus, Ocular motor, Vestibular, VOR
Acknowledgments
The recordings in Figures 2.1–2.9 were performed as part of the AVERT clinical trial (NIH/NIDCD U01 DC013778). We thank the study’s Principal Investigator, Dr. David Newman-Toker and the rest of the AVERT team for providing these resources.
Introduction
Dizziness is a common complaint across many clinical settings and often an intimidating challenge for physicians to diagnose despite modern advances in medical technology and imaging. Lack of a systematic approach to examination of the vestibular system is often to blame for unnecessary tests and incorrect diagnoses. However, an understanding of basic vestibular and ocular motor physiology, when applied with targeted history taking and proper examination techniques, can usually steer clinicians to the correct diagnosis at the bedside. This is particularly important when evaluating patients with acute vestibular symptoms because the pressing question in such cases is whether the underlying cause is anatomically “peripheral” (affecting the labyrinth or the vestibular nerve) or “central” (affecting the vestibular projections or brain networks involved in ocular motor function, postural control, or perception of spatial orientation). Such a diagnostic challenge translates to whether the patient has a relatively benign, usually self-limited condition or a serious and potentially life-threatening injury. The suspicion of a central lesion should initiate emergency procedures including evaluation and treatment for stroke (see Chapter 16 ). On the other hand, peripheral vertigo is not life threatening, even though it can be severely disabling. Usually in patients with peripheral vestibular disorders, timely rehabilitation and proper therapeutic maneuvers enhance recovery and assure an optimal clinical outcome.
In this chapter we first outline physiologic principles underlying the evaluation of the vestibular system and then review the key elements of examination in the context of recent technical advances in video-oculography (VOG) and mobile devices, such as tablet computers and smartphones. Using these portable technologies, the vestibular examination can now be quantified at the bedside to improve diagnostic accuracy and clinical management. In this process, a careful and thorough evaluation of eye movements is critical for diagnosis because of the close anatomic and physiologic connections between the vestibular and ocular motor systems. Clinical or “bedside” vestibular and ocular motor evaluations must always be put into the context of a thorough history and general neurologic examination, and when needed, other neurootologic laboratory testing, including formal tests of hearing and vestibular and balance function.
Physiologic Principles for Vestibular Examination
In normal individuals, when the head is still, both vestibular nerves and the vestibular nuclei on either side of the brainstem have equal resting discharge and thus there is no perception of head motion or need for any vestibulo-ocular compensation. Movement of the head toward one side excites the labyrinth on that side and inhibits the labyrinth on the other side, e.g., right head rotation stimulates the right lateral semicircular canal and inhibits the left. Such a change in the balance of activity between the vestibular nuclei leads to the perception of head motion and also activates the vestibulo-ocular reflex (VOR) in the opposite direction of the head movement so that the eyes can maintain fixation on the intended visual target. The slow-phase eye movements during the VOR compensate for horizontal (yaw), vertical (pitch), or torsional (roll) head rotations. Normally, the VOR matches the direction and speed of the head movement during yaw and pitch rotations. In contrast, during rotation around the roll axis, the VOR is less than compensatory, but this apparently is of less functional importance for central vision because images still remain on or close to the fovea.
A knowledge of the geometric arrangement of the semicircular canals aids accurate interpretation of VOR function. The lateral (horizontal) canal lies orthogonal to the sagittal plane and makes a 30-degree angle with the true horizontal plane. The anterior (superior) and posterior canals are oriented roughly vertically, almost orthogonal to each other, and each makes a roughly 45-degree angle with the sagittal plane. During rotation around the roll axis, eye movements are aligned with head movements, but the gain (eye movement/head movement) is lower (dynamic roll VOR gain is roughly 50%).
Following a unilateral vestibular injury, the balance in the levels of tonic activity between the vestibular nuclei must be restored to eliminate spontaneous nystagmus and postural imbalance, a process that may take days to complete. With a complete unilateral peripheral vestibular loss, the nystagmus is unidirectional, with the fast component beating away from the side of the lesion irrespective of the gaze position. There is often a torsional component with the top pole of the eyes beating away from the side of the lesion. With partial lesions the patterns of nystagmus vary depending on how vestibular inputs from the semicircular canals are affected, but a pure vertical or a pure torsional spontaneous nystagmus usually points to a central lesion ( Table 2.1 ). If nystagmus is peripheral in origin, the horizontal component is more intense when the gaze is pointed in the direction of the fast phase; i.e., opposite, or away from the side of the lesion (Alexander’s law; Fig. 2.1 ). With central lesions in the cerebellum or brainstem, the nystagmus does not always follow Alexander’s law and can be more intense when looking in the direction of the slow phase. In patients with central disorders, there may be a superimposed component of nystagmus related to impaired eccentric gaze holding in which there is right-beating nystagmus upon right gaze and left-beating nystagmus upon left gaze.
| Peripheral Vestibular Lesion | Nystagmus/Direction |
|---|---|
| Horizontal SCC | Horizontal/beating away from the side of the lesion |
| Anterior SCC | Upbeat and torsional/top pole beating away from the side of the lesion |
| Posterior SCC | Downbeat and torsional/top pole beating away from the side of the lesion |
| Anterior + Horizontal SCCs | Horizontal, upbeat, and torsional/all beating away from the side of the lesion |
| Posterior + Horizontal SCCs | Horizontal, downbeat, and torsional/all beating away from the side of the lesion |
| Anterior + Horizontal + Posterior SCCs | Horizontal and torsional/both beating away from the side of the lesion |
Patients with loss of vestibular function, especially in cases with bilateral loss, often have difficulty with tasks during head movements, for example, reading street signs while driving or reading labels on items while walking down the aisles in the grocery store. Such a “dynamic” vestibular imbalance can be brought out during the bedside examination, especially in response to high-acceleration or high-frequency stimuli such as head impulses. These stimuli normally rely mostly on excitation rather than inhibition of the labyrinths. Therefore, in patients with a unilateral loss the abnormality is most apparent when the head is rotated toward the side of the impaired labyrinth. Head shaking and vibration over the skull are other maneuvers that can bring out a vestibular deficit, which manifests as a nystagmus with a slow phase toward the side of the vestibular deficit. These provocative stimuli are especially important when examining patients with chronic unilateral vestibular loss in whom naturally occurring central repair mechanisms have already compensated for the static vestibular tone asymmetry and eliminated the spontaneous nystagmus. In such cases, if the level of activity from the paretic side is suddenly restored after central adaptation has taken place, an excessive vestibular tone may arise on the paretic side and result in a new imbalance in the levels of tonic activity between the vestibular nuclei. This new imbalance can lead to “recovery” nystagmus in which slow phases are directed toward the intact ear.
The central vestibular system within the brainstem and cerebellum also has an important role in improving low-frequency sustained responses of the VOR, for example, during prolonged, constant-velocity head movements. This mechanism is called “velocity storage,” important for understanding the pathophysiology of head shaking–induced nystagmus and the changes in the time constant of vestibular responses, as reflected in the duration of the VOR response to a constant-velocity rotation.
Just as a tone imbalance in the semicircular canals causes spontaneous nystagmus, imbalance in the otolith and especially utricular pathways can induce the ocular tilt reaction (OTR). The OTR consists of a lateral head tilt (ear to shoulder), vertical misalignment of the eyes (skew deviation with the eye on the side of the higher ear being relatively higher in its orbit than the eye on the side of the lower ear), and a torsional deviation of both eyes with the top poles rotating toward the side of the lower eye (ocular counter-roll or OCR). There is often an associated perceptual tilt of the visual world toward the side of the lower ear. As mentioned, the OTR reflects a vestibular tone imbalance from the otolith organs analogous to the spontaneous nystagmus that occurs with a tone imbalance from the semicircular canals.
Vestibular Examination in Acute-Onset Dizziness, Vertigo, and Imbalance
Patients with acute vertigo often have nausea or vomiting, unstable gait, nystagmus, and are intolerant of head motion. With such a clinical presentation, called the acute vestibular syndrome , the vital question at the bedside is to correctly distinguish benign, usually peripheral dizziness (e.g., vestibular neuritis) from more serious and potentially life-threatening “central” dizziness (e.g., brain infarction). Overall, approximately 25% of patients presenting to the emergency department with acute vestibular syndrome are found to have posterior circulation strokes. Traditionally, clinicians rely on the general, non-ocular-motor neurologic examination to distinguish central and peripheral causes of vertigo. However, only a small fraction of stroke patients presenting with dizziness present with such focal neurologic signs (roughly 19% of patients with stroke, excluding those with truncal ataxia). Examination of the VOR and evaluation of eye movements, posture, and balance are the most sensitive and accurate approach to the correct diagnosis in patients with acute-onset vertigo. In the following section we review the key parts of the bedside examination that can distinguish between peripheral and central lesions and help diagnose the cause of dizziness in these patients.
Vestibulo-ocular Examination
Head impulse test, evaluation of nystagmus, and test of skew deviation
A battery of ocular motor examinations including H ead I mpulse test, evaluation of N ystagmus, and T est of S kew deviation (HINTS) helps distinguish between peripheral and central causes in patients with the acute vestibular syndrome. HINTS is highly sensitive in detecting central lesions (sensitivity, 96.8%; specificity, 98.5%) with a diagnostic accuracy higher than that of the diffusion brain MRI within the first 48 hours from the onset of symptoms. The components of the HINTS examination—nystagmus, head impulse test, and skew deviation—are discussed in more detail later in this chapter. Using this bedside battery, one should suspect central lesions in patients with an intact head impulse test (indicating normal peripheral vestibular function), direction-changing nystagmus (i.e., right beating on right gaze and left beating on left gaze), or skew deviation of the eyes. On the other hand, the combination of abnormal head impulse test, direction-fixed, primarily horizontal nystagmus (obeying Alexander’s law) and absence of skew deviation usually suggests a peripheral vestibular lesion ( Table 2.3 ).
| Peripheral Vestibular Nystagmus | Central Nystagmus |
|---|---|
| Mixed, horizontal torsional beating away from the lesion | Mixed, pure torsional, or pure vertical |
| Increase with gaze toward the fast phase (Alexander’s law) | May increase with gaze away from the fast phase (anti-Alexander’s law) |
| Strongly suppressed by visual fixation | May be suppressed by visual fixation |
| Does not change direction with change in gaze position | May change direction (e.g., gaze-evoked or rebound nystagmus) |
| Lesion Site | Spontaneous Nystagmus | Direction-Changing Nystagmus | Head Impulse Test | Ocular Tilt Reaction |
|---|---|---|---|---|
| Labyrinth or vestibular nerve | Contralesional | None | Positive HIT, ipsilesional | Ipsiversive (skew is rare) |
| Vestibular nucleus | Contralesional | contra > ipsi | Positive HIT, ipsilesional | Ipsiversive |
| Nucleus prepositus hypoglossi | Ipsilesional | ipsi > contra | Positive HIT, contralesional | Ipsiversive |
| Inferior cerebellar peduncle | Ipsilesional | None | Negative HIT | Contraversive |
| Flocculus | Ipsilesional | ipsi > contra (weak) | Positive HIT, contralesional | Contraversive |
| Tonsil | Ipsilesional (weak) | ipsi > contra | Negative HIT | Contraversive |
| Nodulus | Ipsilesional | None | Negative HIT | Contraversive |
Peripheral versus central nystagmus ( Table 2.2 )
In patients with acute vestibular syndrome, spontaneous nystagmus is often observed at the bedside. As mentioned earlier, nystagmus caused by a peripheral vestibular lesion is mainly horizontal and often mixed with a torsional component. The slow phase of the nystagmus is directed toward the side of the lesion (i.e., the side of vestibular weakness), and the fast phase is away from the side of the lesion. This peripheral nystagmus does not change direction; however, the intensity of nystagmus increases at gaze positions in the direction of the fast phase, away from the side of the lesion (i.e., the nystagmus conforms to Alexander’s law) ( Fig. 2.1 ). In addition, peripheral nystagmus is often weakened or suppressed by visual fixation and is enhanced or brought out by removing visual fixation using Frenzel goggles, VOG, occlusive ophthalmoscopy, or simply by looking at the movement of the corneal bulge under the closed eyes. Visual suppression of the torsional component of nystagmus is relatively poor compared with that of the horizontal or vertical component in humans. Thus the true direction of the nystagmus is best evaluated in the absence of visual fixation.
Central lesions, too, may lead to horizontal, vertical, or torsional nystagmus. Spontaneous pure vertical or pure torsional nystagmus is often associated with central lesions, but it is not a sensitive sign because only about 9% of patients with central lesions present with pure vertical or torsional nystagmus. Peripheral nystagmus is usually suppressed by visual fixation. This finding is not highly specific for peripheral lesions, as visual fixation may still weaken or suppress nystagmus in patients with small cerebellar or brainstem infarcts.
The effect of gaze position on nystagmus can help distinguish between central and peripheral causes. This can be simply evaluated by having the patient change the position of their eyes in the orbit to the far gaze positions. Nystagmus from peripheral causes is usually unidirectional (i.e., right or left beating) and conforms to Alexander’s law. Nystagmus from central causes, on the other hand, may not remain unidirectional or may not conform to Alexander’s law and thus can become more intense at gaze positions in the opposite direction of the fast phase. Patients with central lesions also often have nystagmus that changes direction with gaze position. This “gaze-evoked” nystagmus beats in the same direction as the eccentric position of the eyes in the orbit (e.g., right beating in right gaze and left beating in left gaze) ( Fig. 2.2 ). Such “direction-changing” nystagmus, primarily caused by lesions involving the gaze-holding networks within the brainstem or cerebellum, is an important sign of central lesions in the acute vestibular syndrome (sensitivity of approximately 21% and specificity of 100%). Note that low-velocity spontaneous nystagmus from peripheral vestibular lesions might appear only at gaze positions in the direction of the fast phase because of Alexander’s law. This unidirectional peripheral nystagmus should not be confused as direction-changing central nystagmus.
In addition, in some patients with central lesions, the velocity of direction-changing nystagmus may differ considerably depending on the gaze position. Such nystagmus has been reported in cerebellopontine angle tumors (affecting both the cerebellum and the vestibular nerve) as well as in acute cerebellar and brainstem strokes with a combined peripheral vestibular and central involvement. This type of nystagmus, known as Bruns nystagmus, reflects the combination of a vestibular imbalance and a disturbed gaze-holding network. Bruns nystagmus is of low frequency and large amplitude in gaze positions toward the side of the lesion and is of high frequency and small amplitude in gaze positions away from the side of the lesion.
Sometimes patients with gaze-evoked nystagmus on eccentric gaze may have transient nystagmus when the eyes return to the straight-ahead (i.e., central) position. This “rebound” nystagmus has a fast phase beating opposite to the previous gaze direction. Rebound nystagmus is often encountered in patients with cerebellar or brainstem dysfunction and is likely related to a bias caused by adaptive mechanisms counteracting the centripetal drift during the gaze-evoked nystagmus. Transient physiologic nystagmus may occur at extreme eccentric gaze positions in normal individuals. This physiologic, end-gaze nystagmus is different from the pathologic, gaze-evoked nystagmus, which usually is sustained and triggered at relatively modest eccentric gaze positions (e.g., 25 degrees away from the central position), and in addition may be followed by rebound nystagmus when the eyes return to the central position. Strong downbeat nystagmus in the straight ahead position or even only on lateral gaze is also pathologic and implies a central origin, especially with lesions involving the vestibulocerebellar pathways. Combined downbeat and horizontal gaze-evoked nystagmus is known as “side-pocket nystagmus.”
Head impulse test
The bedside head impulse maneuver can detect peripheral vestibular loss involving the function of the semicircular canals and the cranial nerve VIII. The patient is instructed to look at a visual target, and the examiner quickly turns the head with brief, high-acceleration rotations of approximately 10 degrees excursion (peak velocity of at least 150 degrees/second). Head rotation normally generates an imbalance in the resting vestibular tone by affecting the level of activity from the semicircular canals, which depends on the direction and velocity of head rotation. At high velocities, the vestibular input from the side opposite to the direction of rotation is fully inhibited and the VOR depends primarily upon excitatory inputs from the labyrinth on the side of rotation. Accordingly, a vestibular loss on the side of rotation results in a VOR that does not match the high-velocity head movement and thus a corrective “catch-up” or “refixation” saccade must be triggered during or after head rotation to keep the eyes on the visual target. Usually patients with central lesions that spare vestibular pathways have a normal head impulse response. On the other hand, an abnormal head impulse response can be seen with lesions involving the vestibular nucleus, nucleus prepositus hypoglossi, CN VIII fascicles in the brainstem, cerebellar flocculus, or the vascular territory of the anterior inferior cerebellar artery (AICA) that includes both labyrinth and vestibulocerebellum. Therefore, although seemingly counterintuitive, a normal head impulse test points to a central lesion in patients with the acute vestibular syndrome (sensitivity of 93% and specificity of 100%).
Head impulses are performed horizontally with right and left rapid head rotations to examine the function of the lateral semicircular canals. For examination of the vertical canals, head impulses are performed in the coplanar canal orientations: right anterior/left posterior and left anterior/right posterior planes. In some patients after recovery from vestibular loss, preprogrammed “covert” saccades are generated during head impulses that compensate for the defective VOR. Covert saccades are difficult to detect with the naked eye, and VOR function may seem intact in these patients. These covert saccades may be converted to more easily seen, “overt” saccades (that occur after the head movement is completed) by making the direction and amplitude of the head movement unpredictable during the head impulse testing.
Test of skew deviation
Skew deviation is a sign of static imbalance in the otolith-ocular inputs with a vertical misalignment of eyes that is relatively “concomitant,” i.e., the degree of misalignment changes little with gaze position. Skew deviation can usually be detected using the alternate cover test while the patient is looking at a visual target. During this test, the examiner covers the eyes alternately while looking for the corrective vertical eye movement in the uncovered eye. The corrective movement of the uncovered eye indicates that it was not visualizing the target and hence there is an underlying vertical misalignment. Skew is more common in central lesions involving the otolith-ocular pathways and overall is more enduring and larger compared with skew caused by peripheral vestibular lesions. Thus skew deviation is a valuable sign to detect central lesions in the acute vestibular syndrome (sensitivity of 25% and specificity of 96%). Skew can also help localize central lesions in combination with other neurologic or ocular motor findings. Because fibers of the otolith-ocular pathways cross at the mid-pons, skew often presents with the lower eye on the side of the lesion in caudal pontomedullary lesions and with the higher eye on the side of the lesion in rostral pontomesencephalic lesions.
HINTS plus hearing loss
Hearing loss is often thought to point to a peripheral labyrinthine lesion in patients with acute vestibular syndrome. However, new-onset hearing loss with vertigo should always raise suspicion for central lesions and specifically infarcts within the AICA distribution that supplies the cerebellum and brainstem as well as the labyrinth. Accordingly, HINTS plus acute hearing loss has a better sensitivity for detecting a central etiology in patients with acute vestibular syndrome (96.5% HINTS vs. 99.1% HINTS plus).
Vestibulo-ocular Examination and Video-oculography
With the advent of the new-generation, light-weight VOG goggles, examination of the vestibular system can be easily quantified and analyzed at the bedside. Using this technology, the video head impulse test (vHIT) can provide quantitative assessments of VOR function for individual horizontal and vertical semicircular canal planes (see Chapter 8 ). The head sensor on the VOG goggles can measure head velocity, and the video cameras on the goggles can track eye movement by detecting the pupil or iris pattern. The eye and head recordings can then be used to measure VOR gain, although one must always be aware of artifacts induced by slippage of the goggles, or when the recordings are unreliable because a sick patient cannot cooperate with the examination. Because the eye and head movements are recorded simultaneously, vHIT traces can detect covert saccades during head impulses, which may be hard to see with the naked eye ( Fig. 2.3 ). VOG can also be used to quantify spontaneous nystagmus or positional nystagmus and the ocular motor components of the OTR, such as skew deviation ( Fig. 2.4 ). Such a quantitative VOG examination makes a “telemedicine” approach possible, enabling remote consultation for dizzy patients seen in outpatient settings (e.g., HINTS examination using VOG).
Balance and Gait Evaluation
Evaluation of balance is an essential part of the vestibular examination. Patients with acute-onset vertigo, however, are highly sensitive to motion and often cannot tolerate standing or walking. The severity of vertigo affects postural control and can cause imbalance irrespective of the cause of dizziness. Thus in patients with acute vestibular syndrome it is often not possible to perform a thorough balance evaluation. Nevertheless, simple tests such as sitting or standing up can still be useful to screen for severe truncal ataxia. Patients with peripheral vestibular loss can still rely on their visual and somatosensory inputs to stand and walk with slight assistance despite ongoing vestibular symptoms. If they cannot do so, one must suspect a central etiology.
Vestibular Examinations in Chronic Dizziness
Static Vestibular Evaluation
Spontaneous nystagmus from unilateral peripheral vestibular loss weakens over time and may not be observed at the bedside, as the central vestibular tone is rebalanced between the vestibular nuclei in the process of recovery. However, if this central rebalancing is incomplete, an underlying peripheral vestibular imbalance can often be brought out by removing visual fixation or using provoking maneuvers that can unmask the persisting asymmetry in the peripheral vestibular inputs. Frenzel goggles are used traditionally for removing visual fixation during vestibular examination. The goggles are fitted with magnifying lenses (+20 diopters) and internal lighting to illuminate the patient’s eyes so the examiner can see any spontaneous nystagmus when the patient has no visual fixation. Frenzel goggles are best applied in an otherwise dark room. VOG goggles equipped with infrared cameras are also useful for bedside vestibular examination. They allow magnified viewing of the eyes on a television or computer screen while visual fixation is removed with a cover occluder. In addition, the ocular motor findings can be viewed and recorded at the bedside. Another method for removing visual fixation is with an ophthalmoscope, as subtle nystagmus can be detected by looking at the fundus. Here the examiner can remove visual fixation by covering the other eye and asking the patient to keep the eyes straight ahead while the examiner looks at the fundus (i.e., occlusive ophthalmoscopy). The optic disk is then observed for abnormal movements. During ophthalmoscopy the direction of horizontal and vertical movements of the fundus are opposite to the direction of the globe, as the optic disc is behind the axis around which the eye is rotating. Also, magnifying cardboard goggles that are now widely available for virtual reality applications can be used for removing visual fixation and simultaneously recording eye movements via smartphones. Another less expensive method is to use a magnifying glass or a plastic sheet magnifier to remove visual fixation and examine one eye while covering the other eye. A penlight-cover test can also be used during which visual fixation is removed by shining a “blinding” penlight in one eye while the other eye is covered.
Provoking Maneuvers
Head shaking
In patients with unilateral vestibular loss, an asymmetry in vestibular inputs during high-velocity head rotations leads to an asymmetric accumulation of activity in the central velocity-storage mechanism within the vestibular nuclei. As a result, a vigorous nystagmus appears immediately following head shaking with slow phases directed toward the affected side, which sometimes is followed by a less intense reversal phase with slow phases directed toward the intact side. This reversal phase reflects the effect of short-term central adaptation mechanisms. In patients with Ménière’s disease, however, the direction of head shaking–induced nystagmus does not have a localizing value as the nystagmus can be related to increase or decrease of vestibular function or central vestibular adaptation related to recovery. The head shaking test is also performed with removing visual fixation. The head is rotated repeatedly within the patient’s comfortable range of motion at a frequency of 3 cycles/second for approximately 15 seconds.
The head shaking–induced nystagmus may also occur in central disorders. For example, vertical nystagmus after horizontal head shaking, known as “perverted” or “cross-coupled” nystagmus, indicates a central disorder. Head shaking–induced nystagmus from central causes is usually due to lesions involving the vestibulocerebellum (i.e., within the flocculus/paraflocculus, nodulus, or ventral uvula), vestibular nuclear complex, or its connections within the brainstem involved in the velocity-storage mechanism. Head shaking–induced nystagmus may also occur in the lateral medullary (Wallenberg) syndrome, beating in the opposite direction of the spontaneous nystagmus. These findings are related to unilateral loss of cerebellar inhibition over the velocity-storage mechanism within the vestibular nucleus. Head shaking–induced nystagmus is usually suppressed by pitching the head forward in peripheral vestibular lesions but not in patients with central vestibular lesions.
With vertical head shaking, vertical nystagmus is rarely seen, as the velocity storage for the vertical VOR is weaker than for the horizontal VOR. However, because the orientation of the posterior semicircular canal is tilted toward the horizontal plane, activation of the posterior canals also contributes to the horizontal VOR. Thus an asymmetric contribution to the velocity-storage mechanism during vertical head shaking may produce a horizontal nystagmus. Vertical semicircular canal function can also be examined with circular head shaking, i.e., circular rotation of the head in a clockwise or counterclockwise direction. With circular head shaking, the anterior and posterior canals are stimulated together, which results in torsional nystagmus. The absence of nystagmus after circular head shaking implies loss of function in the vertical semicircular canals.
Skull vibration
The vibration test is performed with removing visual fixation while a small handheld muscle massager is applied over each of the mastoid bones and then the vertex. Vibration (e.g., 60–100 Hz) over the mastoid bones can induce robust nystagmus in patients with an underlying vestibular imbalance. In normal individuals, however, there is often either no nystagmus or provoked nystagmus beating to the side of the vibration (i.e., right beating with vibration over the right mastoid and left beating with vibration over the left mastoid). With unilateral peripheral vestibular loss, the vibration-induced nystagmus beats away from the side of the lesion irrespective of the side of vibration on the skull (i.e., slow phase toward the side of the lesion). In these patients, nystagmus is provoked in at least two of the three vibration sites (i.e., mastoids and the vertex) and the slow-phase velocity correlates with the unilateral canal weakness in the caloric test. Spontaneous nystagmus from peripheral vestibular loss is often intensified with skull vibration. On the other hand, vibration-induced nystagmus beating in the opposite direction of spontaneous nystagmus has been reported with lateral medullary infarction. When vertical nystagmus is triggered by vibration over the skull, a central lesion should be suspected, for example, in patients with Wernicke disease. Unlike head shaking, vibration-induced nystagmus is not mediated through the velocity-storage mechanism because it ceases as soon as the vibration ends.
Hyperventilation
The hyperventilation test is also performed with removing visual fixation while the patient repetitively breathes deeply through the mouth for about 40 seconds. With peripheral vestibular injuries such as vestibular schwannoma or neurovascular compression, the alkalosis and consequent changes in calcium currents can improve conduction on demyelinated nerves, resulting in a transient increase of neural activity on the side of the lesion. This new imbalance provokes nystagmus with the slow phase directed toward the intact ear. In patients with vestibular neuritis, however, such nystagmus can be contralesional or ipsilesional depending on when the patient is tested in the course of recovery (see recovery nystagmus discussed earlier in the section on the physiologic principles). In the chronic stage, the slow phase of the nystagmus is often directed toward the side of the vestibular loss. Hyperventilation can also enhance or induce downbeat nystagmus in cerebellar disorders, craniocervical junction anomalies, or in patients with perilymph fistula. Also, in superior canal dehiscence, hyperventilation can excite the dehiscent superior canal and provoke downbeating, torsional nysgamus.
Dynamic Vestibular Evaluation
Dynamic visual acuity
Patients with vestibular loss often have oscillopsia that is brought on or exacerbated by head movement. Dynamic visual acuity can be used to evaluate such visual symptoms. First, visual acuity is measured with the head motionless in the upright position. The head is then rotated repeatedly at a high frequency (2 cycles/second) in the horizontal or vertical plane while the patient reads a visual acuity chart. Dynamic visual acuity is abnormal if there is a loss of more than two lines on the visual acuity chart compared with the measurement with the head stationary. In patients with bilateral vestibular loss, there is often a loss of more than four lines. Dynamic visual acuity can also be used to assess VOR recovery and track the effect of vestibular rehabilitation.
Slow and fast head rotations
In addition to the head impulse test, VOR can be evaluated with slow head rotations. During slow head rotations, both low-frequency VOR and smooth pursuit maintain the eyes on a visual target. Therefore, catch-up saccades during slow head rotation are the sign of peripheral vestibular loss combined with central ocular motor dysfunction; for example, in patients with spinocerebellar ataxia type 3 (SCA-3), or patients with cerebellar ataxia, neuropathy, and vestibular areflexia syndrome (CANVAS). The VOR can also be examined during ophthalmoscopy while the patient is looking at a distant target using the other eye. The examiner gently shakes the head horizontally or vertically (2 cycles/second) while looking at the optic disc. With normal vestibular function the optic disc remains steady, but in patients with vestibular loss it oscillates during the head shaking.
Another useful test of VOR function at the bedside is the modified “suppression” head impulse paradigm (SHIMP; Fig. 2.5 ). Here, the examiner quickly turns the head while the patient is looking at a head-mounted target (i.e., the visual target moves with the head as opposed to a stationary target fixed with respect to the patient during the head impulse test). Patients with vestibular loss are able to maintain visual fixation on the head-mounted target without corrective saccades, as the eyes do not move in the opposite direction of the head rotation due to the defective VOR. Healthy individuals, however, have to make catch-up saccades to regain the target after the head turn, as the intact VOR drives the eyes off the head-mounted target during the fast head rotation. Thus, in contrast to vHIT in which the compensatory saccades indicate vestibular loss, the catch-up “anticompensatory” saccade in the direction of the head movement is the sign of a normal VOR during SHIMP. In addition, unlike HIT, corrective saccades in SHIMP usually appear after the end of the head movement (i.e., no covert saccades). Therefore, when covert saccades contaminate VOR traces during HIT, SHIMP can be used to eliminate them and allow accurate measurement of the gain of the slow phases of the VOR. Likewise, SHIMP can be used to measure VOR gain in patients with spontaneous nystagmus. In these patients, with head rotation toward the side of the vestibular loss, the fast phase of the spontaneous nystagmus is in the opposite direction of the corrective saccades during SHIMP. In contrast, during HIT the fast phase of nystagmus and corrective saccades occur in the same direction and are difficult to distinguish. Therefore, SHIMP can be used to overcome measurement error of VOR gain resulting from covert saccades and spontaneous nystagmus.
