Short latency responses in the averaged electro-oculogram elicited by vibrational impulse stimuli applied to the skull: could they reflect vestibulo-ocular reflex function?

Direction-dependent excitatory and inhibitory ocular vestibular-evoked myogenic potentials (oVEMP) produced by oppositely directed accelerations along the midsagittal axis of the head [corrected]

Oppositely directed displacements of the head need oppositely directed vestibulo-ocular reflexes (VOR), i.e. compensatory responses. Ocular vestibular-evoked myogenic potentials (oVEMPs) mainly reflect the synchronous extraocular muscle activity involved in the process of generating the VOR. The oVEMPs recorded beneath the eyes when looking up represent electro-myographic responses mainly of the inferior oblique muscle. We aimed: (1) to study the properties of these responses as they were produced by head acceleration impulses to the forehead and to the back of the head; (2) to investigate the relationships between these responses and the 3-D linear head accelerations that might reflect the true stimulus that acts on the vestibular hair cells. We produced backward- and forward-directed acceleration stimuli in four conditions (positive and negative head acceleration impulses to the hairline and to the inion) in 16 normal subjects. The oVEMPs produced by backward- and forward-directed accelerations of the head showed consistent differences. They were opposite in the phase. The responses produced by backward accelerations of the head began with an initial negativity, n11; conversely, those produced by accelerations directed forward showed initially a positive response, p11. There was a high inter-subject correlation of head accelerations along the head anteroposterior and transverse axes, but almost no correlation of accelerations along the vertical axis of the head. We concluded that backward-directed head accelerations produced an initial excitatory response, and forward-directed accelerations of the head were accompanied by an initial inhibitory response. These responses showed dependence on acceleration direction in the horizontal plane of the head. This could be consistent with activation of the utricle.

Vestibular evoked myogenic potentials: past, present and future

Since the first description of sound-evoked short-latency myogenic reflexes recorded from neck muscles, vestibular evoked myogenic potentials (VEMPs) have become an important part of the neuro-otological test battery. VEMPs provide a means of assessing otolith function: stimulation of the vestibular system with air-conducted sound activates predominantly saccular afferents, while bone-conducted vibration activates a combination of saccular and utricular afferents. The conventional method for recording the VEMP involves measuring electromyographic (EMG) activity from surface electrodes placed over the tonically-activated sternocleidomastoid (SCM) muscles. The "cervical VEMP" (cVEMP) is thus a manifestation of the vestibulo-collic reflex. However, recent research has shown that VEMPs can also be recorded from the extraocular muscles using surface electrodes placed near the eyes. These "ocular VEMPs" (oVEMPs) are a manifestation of the vestibulo-ocular reflex. Here we describe the historical development and neurophysiological properties of the cVEMP and oVEMP and provide recommendations for recording both reflexes. While the cVEMP has documented diagnostic utility in many disorders affecting vestibular function, relatively little is known as yet about the clinical value of the oVEMP. We therefore outline the known cVEMP and oVEMP characteristics in common central and peripheral disorders encountered in neuro-otology clinics.

Acoustic clicks activate both the canal and otolith vestibulo-ocular reflex pathways in behaving monkeys

Acoustic activation of the vestibular system has been well documented in humans and animal models. In the past decade, sound-evoked myogenic potentials in the sternocleidomastoid muscle (cVEMP) and the extraocular muscles (oVEMP) have been extensively studied, and their potentials as new tests for vestibular function have been widely recognized. However, the extent to which sound activates the otolith and canal pathways remains controversial. In the present study, we examined this issue in a recently developed nonhuman primate model of acoustic activation of the vestibular system, i.e., sound-evoked vestibulo-ocular reflexes (VOR) in behaving monkeys. To determine whether the canal and otolith VOR pathways are activated by sound, we analyzed abducens neurons' responses to clicks that were delivered into either ear. The main finding was that clicks evoked short-latency excitatory responses in abducens neurons on both sides. The latencies of the two responses, however, were different. The mean latency of the contralateral and ipsilateral abducens neurons was 2.44 +/- 0.4 and 1.65 +/- 0.28 ms, respectively. A further analysis of the excitatory latencies, in combination with the known canal and otolith VOR pathways, suggests that the excitatory responses of the contralateral abducens neurons were mediated by the contralateral disynaptic VOR pathways that connect the lateral canal to the contralateral abducens neurons, and the excitatory responses of the ipsilateral abducens neurons were mediated by the ipsilateral monosynaptic VOR pathways that connect the utricle to the ipsilateral abducens neurons. These results provide new insights into the understanding of the neural basis for sound-evoked vestibular responses, which is essential for developing new tests for both canal and otolith functions in humans.

The role of the superior vestibular nerve in generating ocular vestibular-evoked myogenic potentials to bone conducted vibration at Fz

OBJECTIVE

The n10 component (n10) of the ocular vestibular evoked myogenic potential (oVEMP) to brief bone conducted vibration (BCV) of the forehead at Fz is probably caused by the vibration selectively activating vestibular otolithic receptors. If the n10 is due primarily to utricular activation then diseases which affect only the superior division of the vestibular nerve (SVN) should reduce or eliminate n10.

METHODS

The n10 component of the oVEMP was measured in 13 patients with unilateral SVN but with inferior vestibular nerve function preserved.

RESULTS

We compared the n10 to BCV of these 13 SVN patients to previously published data for healthy subjects and patients after complete unilateral vestibular loss. We found that in 12 out of the 13 patients with SVN, n10 was markedly reduced or absent under the contralesional eye.

CONCLUSION

Since all utricular afferents course in the superior vestibular nerve and in 12/13 of these patients the n10 was reduced we conclude that the n10 component of the oVEMP to BCV is probably mediated by the superior vestibular nerve and probably due to activation of mainly utricular receptors.

SIGNIFICANCE

The n10 appears to be a simple new test of superior vestibular nerve and probably mainly utricular function.

Ocular vestibular evoked myogenic potentials to bone conducted vibration of the midline forehead at Fz in healthy subjects

OBJECTIVE

To provide the empirical basis for using ocular vestibular evoked myogenic potentials (oVEMPS) in response to Fz bone conducted vibration (BCV) stimulation to indicate vestibular function in human subjects. To show the generality of the response by testing a large number of unselected healthy subjects across a wide age range and the repeatability of the response within subjects. To provide evidence that the response depends on otolithic function.

METHODS

The early negative component (n10) of the oVEMP to brief BCV of the forehead, in the midline at the hairline (Fz) is recorded by surface EMG electrodes just beneath the eyes. We used a Bruel and Kjaer 4810 Mini-Shaker or a light tap with a tendon hammer to provide adequate BCV stimuli to test a large number (67) of unselected healthy people to quantify the individual differences in n10 magnitude, latency and symmetry to Fz BCV. A Radioear B-71 bone oscillator at Fz is not adequate to elicit a reliable n10 response.

RESULTS

The n10 oVEMP response showed substantial differences in amplitude between subjects, but is repeatable within subjects. n10 is of equal magnitude in both eyes with an average asymmetry around 11%. The average n10 amplitude for Mini Tone Burst BCV is 8.47microV+/-4.02 (sd), the average latency is 10.35ms+/-0.63 (sd). The amplitude of n10 decreases and its latency increases with age.

CONCLUSIONS

oVEMPs are a new reliable, repeatable test to indicate vestibular and probably otolithic function.

SIGNIFICANCE

This study shows the optimum conditions for recording oVEMPs and provides baseline values for individual differences and asymmetry. oVEMPs can be measured in senior subjects without difficulty.

Ocular vestibular evoked myogenic potentials (OVEMPs) produced by air- and bone-conducted sound

OBJECTIVE

To determine the origin and properties of short latency extraocular potentials produced by activation of the vestibular apparatus using two modes of acoustic stimulation.

METHODS

Extraocular potentials were measured in 10 normal subjects using a bipolar montage to increase selectivity. Three dimensional eye movements were also recorded in five subjects. The subjects were stimulated with both air-conducted (AC) and bone-conducted (BC) sound using a single cycle of a 500Hz sine wave.

RESULTS

Short latency positive and negative potentials that peaked at 8.1-12.7ms for AC and 7.5-13.9ms for BC stimulation were recorded, which were distinct for the two eyes and for the two modes of stimulation. The extraocular potentials began prior to the onset of eye movements, which peaked at 16.5-20.1ms for AC, 17.8-25.0ms for BC stimulation.

CONCLUSIONS

The pattern of short latency eye movements and extraocular potentials induced by AC and BC vestibular stimulation are distinct. As the potentials preceded the eye movements and were not correlated morphologically with them, the source of the observed potentials is not an eye movement and thus we refer to them as ocular vestibular evoked myogenic potentials (OVEMPs).

SIGNIFICANCE

The potentials had properties consistent with modulation of the electromyogenic activity of the extraocular muscles and if interpreted as originating from displacement of the eye will give misleading results. AC and BC acoustic stimulation are likely to activate differing profiles of vestibular end organs.

Bone conducted vibration selectively activates irregular primary otolithic vestibular neurons in the guinea pig

The main objective of this study was to determine whether bone-conducted vibration (BCV) is equally effective in activating both semicircular canal and otolith afferents in the guinea pig or whether there is preferential activation of one of these classes of vestibular afferents. To answer this question a large number (346) of single primary vestibular neurons were recorded extracellularly in anesthetized guinea pigs and were identified by their location in the vestibular nerve and classed as regular or irregular on the basis of the variability of their spontaneous discharge. If a neuron responded to angular acceleration it was classed as a semicircular canal neuron, if it responded to maintained roll or pitch tilts it was classified as an otolith neuron. Each neuron was then tested by BCV stimuli-either clicks, continuous pure tones (200-1,500 Hz) or short tone bursts (500 Hz lasting 7 ms)-delivered by a B-71 clinical bone-conduction oscillator cemented to the guinea pig's skull. All stimulus intensities were referred to that animal's own auditory brainstem response (ABR) threshold to BCV clicks, and the maximum intensity used was within the animal's physiological range and was usually around 70 dB above BCV threshold. In addition two sensitive single axis linear accelerometers cemented to the skull gave absolute values of the stimulus acceleration in the rostro-caudal direction. The criterion for a neuron being classed as activated was an audible, stimulus-locked increase in firing rate (a 10% change was easily detectable) in response to the BCV stimulus. At the stimulus levels used in this study, semicircular canal neurons, both regular and irregular, were insensitive to BCV stimuli and very few responded: only nine of 189 semicircular canal neurons tested (4.7%) showed a detectable increase in firing in response to BCV stimuli up to the maximum 2 V peak-to-peak level we delivered to the B-71 oscillator (which produced a peak-to-peak skull acceleration of around 6-8 g and was usually around 60-70 dB above the animal's own ABR threshold for BCV clicks). Regular otolithic afferents likewise had a poor response; only 14 of 99 tested (14.1%) showed any increase in firing rate up to the maximum BCV stimulus level. However, most irregular otolithic afferents (82.8%) showed a clear increase in firing rate in response to BCV stimuli: of the 58 irregular otolith neurons tested, 48 were activated, with some being activated at very low intensities (only about 10 dB above the animal's ABR threshold to BCV clicks). Most of the activated otolith afferents were in the superior division of the vestibular nerve and were probably utricular afferents. That was confirmed by evidence using juxtacellular injection of neurobiotin near BCV activated neurons to trace their site of origin to the utricular macula. We conclude there is a very clear preference for irregular otolith afferents to be activated selectively by BCV stimuli at low stimulus levels and that BCV stimuli activate some utricular irregular afferent neurons. The BCV generates compressional and shear waves, which travel through the skull and constitute head accelerations, which are sufficient to stimulate the most sensitive otolithic receptor cells.

Function tests of the otolith or statolith system

PURPOSE OF REVIEW

This review aims to provide an overview of recent advances in tests to evaluate otolith function over the last 2 years.

RECENT FINDINGS

Over the last 2 years, many papers have focused on the application of the vestibular evoked myogenic potentials (VEMP). Several aspects are under survey: a search for optimal stimuli, search for normative data, search for which labyrinthine function losses and what kind of pathologies induce abnormal VEMPs. The review shows that some fundamental problems still have to be solved to improve reproducibility and to increase sensitivity. Other research and modelling is performed to find out how the brain distinguishes tilts from translations. Several papers support routine implementation of subjective visual vertical (SVV) measurements (in rest and during centrifugation) in the standard vestibular test battery. Interesting reports mention short latency vestibulo-ocular reflex induced by taps and short auditory stimuli. One report mentions the impact of otolith dysfunction upon spontaneous nystagmus and head shaking nystagmus.

SUMMARY

Although validation is still needed and in progress, the state of the art laboratory should consider the following tests for an evaluation of otolith function as relevant: slow tandem gait, VEMP, SVV during centrifugation.

Vestibular Responses to Sound

Research into vestibular responses to sound has evolved in four stages. The first, largely the work of Tullio in the 1920s, involved inspection of the eye, head, and postural responses to sound of alert animals with surgical fenestrae into various parts of the bony labyrinth. The second, begun in 1964 by Bickford and his group and continued by our group and then by others in the last 10 years, involves the measurement of evoked myogenic potentials to air-conducted and bone-conducted clicks and tones in normal humans. The third, begun by Mikaelian at about the same time as Bickford and continued by McCue, our group, and others, involves electrophysiological recordings of primary vestibular afferent neuron responses to sound in anesthetized animals. The fourth involves measurements of vestibulo-ocular responses to sound in humans with the Tullio phenomenon. It was begun by Minor and his group in 1998 with the observation that sound-induced nystagmus in humans, the Tullio phenomenon, aligned with the rotation axis of the superior semicircular canal. They then showed a defect in the temporal bone between the apex of the superior semicircular canal and the middle cranial fossa, which was the cause of most, if not all, cases of sound-induced nystagmus. Here some of the key observations made in each of these four stages are reviewed.

Short latency disconjugate vestibulo-ocular responses to transient stimuli in the audio frequency range

OBJECTIVE

To determine whether unilateral activation of the vestibular labyrinth by brief air conducted tones can elicit disconjugate reflex eye movements in healthy subjects.

METHODS

40 normal volunteers, one patient with bilateral congenital sensorineural deafness, and four patients with an acoustic neuromas were subjected to monoaural air conducted tones (125 to 6000 Hz; 132 dB SPL; 3-4 ms). Eye movements were recorded by averaged EOG.

RESULTS

The stimuli elicited bi- or triphasic transient EOG responses with a duration of about 10 ms and a 7-8 ms latency in 16 of 40 tested volunteers and in the patient with congenital deafness. In patients with acoustic neuromas the responses were induced only by stimuli to the healthy ear. The responses in the vertical EOG were recorded predominantly from the eye contralateral to the stimulated ear and were negligible ipsilaterally. These responses were similar to those found in patients with only one functioning labyrinth reported in a previous study. In the remaining subjects the responses were absent or barely discernible.

CONCLUSIONS

(1) Disconjugate eye movements in response to brief mechanical stimuli in this audio frequency range always indicated the side of the activated vestibular labyrinth. (2) In contrast to longer stimuli used by conventional vestibular activation methods, brief stimuli may activate only the direct monocular vestibulo-ocular pathway. This may be because the responses elicited by brief stimuli cease well before the slower indirect vestibulo-ocular subsystem can align the eyes.