The Influence of Motoric Maneuvers on Cervical Vestibular Evoked Myogenic Potentials (cVEMPs).

BACKGROUND: The cervical vestibular evoked myogenic potential (cVEMP) is a vestibular response that is produced by the saccule in response to intense, often low-frequency, short-duration auditory stimuli, and is typically recorded from a contracted sternocleidomastoid (SCM) muscle. Previous research has shown that the amplitude of the cVEMP is related to the amount of SCM electromyographic (EMG) activity. PURPOSE: The aim of this study was to determine the influence of various remote motoric maneuvers on the amplitude of the cVEMP, as well as whether they influence the level of SCM EMG activity. RESEARCH DESIGN: The cVEMP was recorded from the left SCM muscle to left ear stimulation, in response to the SCM condition, as well as three different motoric maneuvers (jaw clench, eye closure, and the Jendrassik maneuver). EMG activity was also varied between 50, 75, and 100% of maximal EMG activity. STUDY SAMPLE: Data from 14 healthy subjects, with a mean age of 25.57 years (standard deviation = 5.93 years), was included in the present study. DATA COLLECTION AND ANALYSIS: Mean latency and amplitude of the cVEMP were compared across the four conditions and varying magnitudes of EMG contraction. SPSS 26 was used to statistically analyze the results. RESULTS: cVEMP latency did not vary across condition. cVEMP amplitude decreased with decreasing EMG magnitude. SCM contraction with jaw clench produced the largest increase in cVEMP amplitude; however, this condition was not significantly different from the SCM condition alone. SCM contraction with the Jendrassik maneuver produced a cVEMP amplitude that was similar and not statistically different from SCM contraction alone, and the addition of the eye closure maneuver to SCM contraction resulted in the lowest cVEMP amplitude, which was found to be statistically different from the standard SCM condition at 100 and 75% EMG activity. The amplitude relationship across the conditions was not found to vary with changes in EMG activity; however, a significant increase in EMG amplitude was found during the 50% muscle contraction condition when subjects performed the Jendrassik maneuver in addition to the standard SCM contraction. CONCLUSIONS: The addition of the eye closure maneuver to SCM contraction resulted in a significant decrease in cVEMP amplitude, while the addition of the Jendrassik maneuver resulted in a significant increase in EMG activity at the lowest level of SCM activation (i.e., 50%). Additional research is necessary to determine how motoric maneuvers influence the cVEMP amplitude, and whether the results are also dependent on how SCM contraction is being produced (e.g., while supine vs. sitting).

Using Unidirectional Rotations to Improve Vestibular System Asymmetry in Patients with Vestibular Dysfunction.

The vestibular system provides information about head movement and mediates reflexes that contribute to balance control and gaze stabilization during daily activities. Vestibular sensors are located in the inner ear on both sides of the head and project to the vestibular nuclei in the brainstem. Vestibular dysfunction is often due to an asymmetry between input from the two sides. This results in asymmetrical neural inputs from the two ears, which can produce an illusion of rotation, manifested as vertigo. The vestibular system has an impressive capacity for compensation, which serves to rebalance how asymmetrical information from the sensory end organs on both sides is processed at the central level. To promote compensation, various rehabilitation programs are used in the clinic; however, they primarily use exercises that improve multisensory integration. Recently, visual-vestibular training has also been used to improve the vestibulo-ocular reflex (VOR) in animals with compensated unilateral lesions. Here, a new method is introduced for rebalancing the vestibular activity on both sides in human subjects. This method consists of five unidirectional rotations in the dark (peak velocity of 320°/s) toward the weaker side. The efficacy of this method was shown in a sequential, double-blinded clinical trial in 16 patients with VOR asymmetry (measured by the directional preponderance in response to sinusoidal rotations). In most cases, VOR asymmetry decreased after a single session, reached normal values within the first two sessions in one week, and the effects lasted up to 6 weeks. The rebalancing effect is due to both an increase in VOR response from the weaker side and a decrease in response from the stronger side. The findings suggest that unidirectional rotation can be used as a supervised rehabilitation method to reduce VOR asymmetry in patients with longstanding vestibular dysfunction.

Learning Effects and the Sensory Organization Test: Influence of a Unilateral Peripheral Vestibular Impairment.

PURPOSE: Healthy young controls exhibit a learning effect after undergoing repeated administrations of the sensory organization test (SOT). The primary objective of the present experiment was to determine if an SOT learning effect is present in individuals with a unilateral vestibular impairment (UVI), and if so, whether it is different from healthy controls. The secondary objective was to determine if the learning effect is dependent on the time frame of repeated SOT assessments. METHOD: Eleven individuals diagnosed with a UVI and 11 controls underwent 6 repetitions of the SOT over 2 visits (3 per visit all within 1 week). A second control group underwent 3 SOT repetitions, with each repetition separated by 1 week, to evaluate the time course of the SOT learning effect. RESULTS: No statistically significant differences were found between the UVI group and the control group. In addition, the magnitude of the learning effect was found to be similar regardless of the length of time that separated the repetitions. CONCLUSIONS: If the SOT is to be used as a measure of improvement, the learning effect should be exhausted (which typically occurs following the third administration) prior to the introduction of therapy. Future research should further investigate the results from those with other vestibular pathologies.

The Influence of Caffeine on Rotary Chair and Oculomotor Testing.

BACKGROUND: When patients are given instructions before vestibular function testing, they are often asked to refrain from ingesting caffeine 24 h before testing. However, research regarding the effects of caffeine on the outcome of vestibular function testing is limited. PURPOSE: To evaluate whether the results from rotational chair tests are influenced by caffeine. RESEARCH DESIGN: Participants were tested after consuming a caffeinated beverage (i.e., coffee containing ∼300 mg of caffeine), as well as after abstaining from caffeinated beverages. The participants underwent oculomotor testing, sinusoidal harmonic acceleration testing, optokinetic testing, visual enhancement/suppression testing, subjective visual vertical/horizontal testing, trapezoidal step testing, and unilateral utricular centrifugation testing. STUDY SAMPLE: Thirty healthy young controls aged 18-40 yr (mean = 23.28 yr; 9 males, 21 females) participated in the study. DATA COLLECTION AND ANALYSIS: Rotational chair tests were completed with the Neuro Kinetics rotary chair (Pittsburgh, PA). VEST 7.0 software was used to collect and analyze the participants' eye movements (I-Portal VOG; Neuro Kinetics). IBM SPSS was used to statistically analyze the results. RESULTS: Statistically significant differences were found for the results from several oculomotor tests (i.e., vertical saccades [SCs], horizontal SCs, and optokinetics), whereas the remaining rotational chair tests did not reveal any statistically significant differences between sessions. If a statistically significant difference was found, the participants were then stratified based on the amount of caffeine they consumed on a daily basis. This stratification was accomplished based on the guidelines from the International Coffee Organization. When the data were analyzed based on the stratified groups, statistically significant results remained in the no/low caffeine intake group, whereas no statistically significant results remained in the moderate/high caffeine intake group. Clinically speaking, the largest effect was seen in those individuals who did not typically ingest large amounts of caffeine, whereas the results were not found to be significantly different in those individuals who were typical caffeine consumers. This strengthens the argument that it is not necessary to require that individuals refrain from consuming caffeinated beverages before oculomotor/rotary chair testing as the results from typical caffeine consumers are not significantly affected. CONCLUSIONS: Although statistically significant results were found for a number of the oculomotor function tests, the ingestion of caffeine had little influence on the clinical interpretation of the responses. Therefore, the results from the present study indicate that it is not necessary to require that healthy young individuals abstain from caffeine before undergoing rotary chair/oculomotor testing. Further research is necessary to determine whether there is also a limited effect of caffeine on rotary chair/oculomotor test results from older individuals, as well as individuals diagnosed with a vestibular impairment.

The influence of caffeine on the sensory organization test.

BACKGROUND: Clinicians often request that patients refrain from consuming caffeinated beverages 24 h before vestibular function testing. However, there is limited research regarding how caffeine may affect the results of these tests. The sensory organization test (SOT) evaluates how well an individual is able to maintain his or her balance during several different conditions that manipulate vestibular, visual, or somatosensory information. PURPOSE: This study evaluated whether caffeine consumption affects the results of the SOT in a group of healthy young adults. RESEARCH DESIGN: Individuals were evaluated under two conditions: (1) after consuming ∼300 mg of caffeine before testing, and (2) without consuming a caffeinated beverage for 24 h before testing. Regular caffeine intake and caffeine withdrawal symptoms were assessed in these individuals. Participants were stratified into a no/low or a moderate/high caffeine intake group through the use of a self-reported 1-week caffeine diary. STUDY SAMPLE: Thirty healthy control participants (mean age = 23.28 yr; males = 9) without any history of vestibular or balance impairment participated in the present study. DATA COLLECTION/ANALYSIS: The NeuroCom SMART Equitest was used to administer the SOT, whereas paired t-tests, completed with IBM SPSS Statistics 20, were used to analyze the data for statistical significance. RESULTS: Analysis of the data revealed a statistically significant difference between the caffeine and no-caffeine sessions during (1) condition 5 (C5): eyes closed, platform sway-referenced; and (2) the total composite score. Statistically significant differences were also noted for the vestibular and somatosensory preference ratios. In general, the participants performed better (i.e., higher equilibrium/composite scores) during the caffeine session. When significant results were found, the participants were stratified by weekly caffeine intake into a no/low caffeine (LC) intake group versus a moderate/high caffeine (HC) intake group. After this stratification, a statistically significant difference remained for C5, the composite score, and the somatosensory/vestibular preference ratios for the LC intake group, whereas no statistically significant results were found in the HC intake group. In addition, further analysis revealed less of a change in the equilibrium score as the amount of weekly caffeine intake increased. Despite these significant results, the mean differences were small in magnitude, and C5, the composite score, as well as the sensory analysis ratios, fell within normal limits for all participants during both sessions. CONCLUSIONS: The ingestion of caffeine did not produce a clinically significant effect in healthy young control participants. Future research is needed to determine if these same results occur in older adults, or in individuals with a history of vestibular impairment.

The auditory steady state response: far-field recordings from the chinchilla.

OBJECTIVE: Previous studies in our lab have found that the presentation of multiple ASSR-generating stimuli results in a decrease in ASSR amplitude when recorded from an electrode implanted in the chinchilla inferior colliculus. The purpose of the present experiment was to determine whether this same effect occurs in far-field recordings, i.e. recordings similar to those made in human subjects. The effect of inhalant anesthesia on ASSR amplitude in response to multiple stimuli was also investigated. DESIGN: Stimuli consisted of three sinusoidally-amplitude modulated tones with carrier/modulation frequencies of (1/.095 kHz), (2/.1 kHz), or (4/.107 kHz). The modulated carriers were presented to the right ear either alone or in combination, while recordings were made from subdermal needle electrodes placed on the head. STUDY SAMPLE: Nine adult chinchillas. RESULTS: A 20%-70% decrease in the response amplitude with the presentation of multiple ASSR-generating stimuli was found, which depended on both carrier frequency as well as stimulus pairing. In general, both the ASSR and the noise floor were reduced under anesthesia. CONCLUSIONS: The time savings obtained from presenting multiple stimuli simultaneously may not be as great as initially predicted, as the time saving is at least partially offset by the observed amplitude reduction.

The vestibular evoked myogenic potential (VEMP): air- versus bone-conducted stimuli.

OBJECTIVE: Several studies have evaluated the effects of different stimulus and recording parameters on the cervical vestibular evoked myogenic potential (cVEMP); however, it is difficult to directly compare these studies as they have all used different recording methods, different sternocleidomastoid (SCM) muscle contraction/electromyography monitoring methods, and different stimulus parameters. DESIGN: : This study made a direct comparison of the cVEMP in response to air-conducted (AC) and bone-conducted (BC) stimuli in the same subjects, using the same stimulus/recording/electromyography monitoring methods. RESULTS: We found that the input/output (I/O) functions were more linear in response to AC stimuli, whereas cVEMPs in response to BC stimuli began to saturate at the highest level. In addition, cVEMP threshold was obtained at a lower stimulus level (i.e., at a lower sensation level) in response to BC stimuli as compared with AC stimuli, and cVEMPs in response to BC stimuli were larger than cVEMPs in response to AC stimuli, which is in agreement with what has been found in previous studies. In addition, this was one of the few studies to evaluate the repeatability of the cVEMP in response to BC stimuli. Interestingly, we found that cVEMP latency in response to BC stimuli was, in most cases, less variable than cVEMP latency obtained in response to AC stimuli, whereas the reverse was true for cVEMP amplitude. We also found that BC masking presented to the forehead affected response amplitude of the AC cVEMP regardless of the specific SCM muscle contraction/toneburst presentation condition. In addition, we found that the ratio of amplitude reduction was greater in the binaural stimulation/bilateral SCM muscle contraction condition as compared with the monaural stimulation/bilateral SCM muscle contraction condition. CONCLUSIONS: The present experiment provided a direct comparison of the cVEMP in response to AC versus BC 500 Hz short-duration toneburst stimuli in the same subjects. The results of the present experiment also provide insight into the laterality of the cVEMP response and reveal that the cVEMP may not be completely ipsilateral (i.e., there may be a form of bilateral interaction that occurs when both sides are stimulated simultaneously). Last, the results indicate that BC stimuli likely activates the saccule as well as the utricle, given that AC VEMPs can be masked by the administration of BC masking noise presented to the midline.

Use of 64-channel electroencephalography to study neural otolith-evoked responses.

BACKGROUND: The vestibular evoked myogenic potential (VEMP) is a myogenic response that can be used clinically to evaluate the function of the saccule. However, to date, little is known about the thalamo-cortical representation of saccular activation. It is important to understand all aspects of the VEMP, as this test is currently used clinically in the evaluation of saccular function. PURPOSE: To identify the areas of the brain that are activated in response to stimuli used clinically to evoke the VEMP. RESEARCH DESIGN: Electroencephalography (EEG) recordings combined with current density analyses were used to identify the areas of the brain that are activated in response to stimuli presented above VEMP threshold (500 Hz, 120 dB peak SPL [pSPL] tone bursts), as compared to stimuli presented below VEMP threshold (90 dB pSPL, 500 Hz tone bursts). Ten subjects without any history of balance or hearing impairment participated in the study. RESULTS: The neural otolith-evoked responses (NOERs) recorded in response to stimuli presented below VEMP threshold were absent or smaller than NOERs that were recorded in response to stimuli presented above VEMP threshold. Subsequent analyses with source localization techniques, followed by statistical analysis with SPM5 (Statistical Parametric Mapping), revealed several areas that were activated in response to the 120 dB pSPL tone bursts. These areas included the primary visual cortex, the precuneus, the precentral gyrus, the medial temporal gyrus, and the superior temporal gyrus. CONCLUSIONS: The present study found a number of specific brain areas that may be activated by otolith stimulation. Given the findings and source localization techniques (which required limited input from the investigator as to where the sources are believed to be located in the brain) used in the present study as well as the similarity in findings between studies employing galvanic stimuli, fMRI (functional magnetic resonance imaging), and scalp-recorded potentials in response to VEMP-eliciting stimuli, our study provides additional evidence that these brain regions are activated in response to stimuli that can be used clinically to evoke the VEMP.

The effects of a second stimulus on the auditory steady state response (ASSR) from the inferior colliculus of the chinchilla.

The auditory steady-state response (ASSR) is an auditory evoked potential which follows the envelope of the stimulus. One of the advantages of the ASSR is that multiple stimulation frequencies can be tested simultaneously. In experiment 1, we evaluated the effects of simultaneously presenting two separate stimuli on ASSR response amplitude. In experiment 2, we evaluated the effects of presenting two ASSR-generating stimuli monotically vs. dichotically, either ipsilaterally or contralaterally to the recording electrode. Recordings were made from the chinchilla inferior colliculi, in response to tonebursts, two-tones, or sinusoidally-amplitude modulated tones. We found that the addition of a second stimulus resulted in a reduction in ASSR response amplitude at moderate to high stimulus levels. The amount of amplitude reduction was typically larger in the monotic (e.g. approximately 50%) vs. dichotic condition (e.g. approximately 10-20%), regardless of whether the responses were recorded ipsilaterally or contralaterally to the ear of stimulus presentation. In conclusion, central as well as peripheral interactions contribute to the reduction in ASSR amplitude in response to the simultaneous presentation of multiple stimuli.