The cardiac autonomic control system response to vestibular stimulation in subjects with BPPV compared with healthy controls.

BACKGROUND: Vestibulo-sympathetic reflexes (VSR) demonstrate the autonomic interaction between the vestibular system and the cardiovascular system, however little is known about this interaction in patients with benign paroxysmal positional vertigo (BPPV). Although the main complaint of patients with BPPV is vertigo, additional symptoms such as nausea and sweating raise questions whether patients with BPPV have impaired VSR. OBJECTIVE: The aim of this study is to assess the cardiac autonomic control system (CACS) response to vestibular stimulation in subjects with BPPV compared with healthy controls. METHODS: This is a case-control study. Heart-rate variability (HRV) parameters were monitored in two groups (30-70 years old)- subjects with BPPV (N = 18) and gender and age-matched healthy controls (N = 18), at rest and during vestibular stimulation. RESULTS: No significant differences were found between groups in HRV parameters during rest. Both groups demonstrated an autonomic response to vestibular stimulation, yet no differences were found between groups. CONCLUSIONS: Patients with BPPV seem to have an intact cardiac response to vestibular stimulation. Over-activation of the Vestibular Semicircular Canals, as shown in BPPV, doesn't seem to alter the VSR in the cardiac autonomic control system.

Can Dyssynergia of Vestibulosympathetic and Baroreflexes Cause Vestibular Syncope? The Hypothesis Based on the Velocity-Storage Function.

The mechanism of vestibular syncope, the syncope occurring during the vertigo attacks, remains uncertain. This study aims to clarify the mechanism of vestibular syncope by pursuing the function of vestibular system in cardiovascular autonomic control and by defining neuro-hemodynamic changes in vestibular syncope. By integrating the velocity-storage (VS) circuit in the brainstem and cerebellum, we propose that the vestibular syncope develops as a result of dyssynergia of the vestibulosympathetic and baroreflexes in which centrally estimated downward inertial acceleration during the vertigo attacks acts as a trigger. Recognition of the vestibular disorders as a possible cause of syncope would allow proper managements for prevention of further syncope and related complications in patients with vestibular disorders.

Pulsed Infrared Stimulation of Vertical Semicircular Canals Evokes Cardiovascular Changes in the Rat.

A variety of stimuli activating vestibular end organs, including sinusoidal galvanic vestibular stimulation, whole body rotation and tilt, and head flexion have been shown to evoke significant changes in blood pressure (BP) and heart rate (HR). While a role for the vertical semicircular canals in altering autonomic activity has been hypothesized, studies to-date attribute the evoked BP and HR responses to the otolith organs. The present study determined whether unilateral activation of the posterior (PC) or anterior (AC) semicircular canal is sufficient to elicit changes in BP and/or HR. The study employed frequency-modulated pulsed infrared radiation (IR: 1,863 nm) directed via optical fibers to PC or AC of adult male Long-Evans rats. BP and HR changes were detected using a small-animal single pressure telemetry device implanted in the femoral artery. Eye movements evoked during IR of the vestibular endorgans were used to confirm the stimulation site. We found that sinusoidal IR delivered to either PC or AC elicited a rapid decrease in BP and HR followed by a stimulation frequency-matched modulation. The magnitude of the initial decrements in HR and BP did not correlate with the energy of the suprathreshold stimulus. This response pattern was consistent across multiple trials within an experimental session, replicable, and in most animals showed no evidence of habituation or an additive effect. Frequency modulated electrical current delivered to the PC and IR stimulation of the AC, caused decrements in HR and BP that resembled those evoked by IR of the PC. Frequency domain heart rate variability assessment revealed that, in most subjects, IR stimulation increased the low frequency (LF) component and decreased the high frequency (HF) component, resulting in an increase in the LF/HF ratio. This ratio estimates the relative contributions of sympathetic nervous system (SNS) and parasympathetic nervous system (PNS) activities. An injection of atropine, a muscarinic cholinergic receptor antagonist, diminished the IR evoked changes in HR, while the non-selective beta blocker propranolol eliminated changes in both HR and BP. This study provides direct evidence that activation of a single vertical semicircular canal is sufficient to activate and modulate central pathways that control HR and BP.

The integrated functions of the cardiac autonomic and vestibular/oculomotor systems in adolescents following severe traumatic brain injury and typically developing controls.

INTRODUCTION: The cardiac autonomic control system (CACS) is frequently impaired post-traumatic brain injury (TBI). However, the prevalence of vestibular/oculomotor impairment is less studied. These two systems interact during position change and contribute to blood-pressure regulation through the vestibulo-sympathetic reflex. Aim: To assess the CACS, the vestibular/oculomotor systems and their integrative function in adolescents post-TBI compared to typically-developing (TD) adolescents. Methods: 19 adolescents in the subacute stage following a severe TBI (14-117 days post injury) and 19 age and sex matched TD controls were recruited. Heart Rate Variability (HRV) was assessed at rest and during a modified tilt-test. A quantified version of the Vestibular/Ocular-Motor Screening (VOMS) was also administered. Results: At rest, the TBI group had higher HR and lower HRV values (p < .001). All participants with TBI demonstrated impairments in the VOMS (median of positive tests: 5 [range 2-9]) compared to only 6 out of 19 in the TD participants (median 0 [0-2]) (z = -5.34; p < .001). In response to the modified tilt test, the HRV increased significantly in the lifting period and decreased significantly once in standing only in the TBI group (z = -2.85, p = .025). Conclusion: Adolescents post severe TBI demonstrated impairments in the CACS, positive tests on the VOMS and significantly greater changes in the modified tilt test as compared to TD. Clinical trial gov. number: NCT03215082.

Nondestructive and objective assessment of the vestibular function in rodent models: A review.

The normal function of the vestibular system is crucial for the sense of balance. The techniques used to assess the vestibular function plays a vital role in the research of the vestibular system. In this article, we have systematically reviewed some popular methods employing vestibular reflexes and vestibular evoked potentials for assessing the vestibular function in rodent models. These vestibular reflexes and vestibular evoked potentials to effective stimuli have been used as nondestructive and objective functional measures. The main types of vestibular reflexes include the vestibulo-ocular reflex (VOR), vestibulocollic reflex (VCR), and vestibulo-sympathetic reflex (VSR). They are all capable of indicating the functions of the semicircular canals and otoliths. However, the VOR assessment is much more prevalently used because of the relatively stereotypical inputoutput relationship and simple motion pattern of the ocular response. In contrast, the complicated motion pattern and small gain of the VCR response, as well as the undesired component possibly contributed from the acceleration receptors outside the labyrinths in the VSR response, restrict the widespread applications of VCR and VSR in the assessment of the vestibular system. The vestibular evoked myogenic potentials (VEMPs) and vestibular sensory evoked potentials (VsEPs) are the two typical evoked potentials that have been also employed for evaluating the vestibular function. Through exploiting different types of the VEMPs, the saccular and utricular functions can be evaluated separately. The sound-induced VEMPs, moreover, are capable of noninvasively assessing the unilateral vestibular function. The VsEPs, via the morphology of their signal waveforms, enable the access to the location-specific information that indicates the functional statuses of different components within the vestibular neural pathway.

Vestibulo-sympathetic reflex in patients with bilateral vestibular loss.

This study assessed cardiovascular control during head-down neck flexion (HDNF) in a group of patients suffering from total bilateral idiopathic vestibular loss (BVL) for 7 ± 2 yr. Nine adult patients (age 54 ± 6 yr) with BVL were recruited. Calf blood flow (CBF), mean arterial pressure (MAP), and heart rate (HR) were measured with subjects' eyes closed in two lying body positions: ventral prone (VP) and lateral (LP) on the left side. Vascular resistance (CVR) was calculated as MAP/CBF. The HDNF protocol consisted in passively changing the head position: head up (HU)-head down (HD)-HU. Measurements were taken twice at each head position. In VP CBF significantly decreased in HD (3.65 ± 0.65 mL·min-1·100 mL-1) vs. HU (4.64 ± 0.71 mL·min-1·100 mL-1) (P < 0.002), whereas CVR in VP significantly rose in HD (31.87 ± 6.93 arbitrary units) vs. HU (25.61 ± 6.36 arbitrary units) (P < 0.01). In LP no change in CBF or CVR was found between the two head positions. MAP and HR presented no difference between HU and HD in both body positions. Age of patients did not significantly affect the results. The decrease in CBF of the BVL patients was similar to the decrease observed with the same HDNF protocol in normal subjects. This suggests a sensory compensation for the lost vestibular inputs that could originate from the integration of inputs from trunk graviceptors and proprioceptive and cutaneous receptors. Another possibility is that the HDNF vascular effect is evoked mostly by nonlabyrinthine sensors.NEW & NOTEWORTHY The so-called vestibulo-sympathetic reflex, as demonstrated by the head-down neck flexion (HDNF) protocol, is present in patients with total bilateral vestibular idiopathic loss, equally in young and old subjects. The origin of the sympathetic effect of HDNF is questioned. Moreover, the physiological significance of the vestibulo-sympathetic reflex remains obscure, because it acts in opposition to the orthostatic baroreflex. It may serve to inhibit the excessively powerful baroreflex.

Vestibular neurons with direct projections to the solitary nucleus in the rat.

Anterograde and retrograde tract tracing were combined with neurotransmitter and modulator immunolabeling to identify the chemical anatomy of vestibular nuclear neurons with direct projections to the solitary nucleus in rats. Direct, sparsely branched but highly varicose axonal projections from neurons in the caudal vestibular nuclei to the solitary nucleus were observed. The vestibular neurons giving rise to these projections were predominantly located in ipsilateral medial vestibular nucleus. The cell bodies were intensely glutamate immunofluorescent, and their axonal processes contained vesicular glutamate transporter 2, supporting the interpretation that the cells utilize glutamate for neurotransmission. The glutamate-immunofluorescent, retrogradely filled vestibular cells also contained the neuromodulator imidazoleacetic acid ribotide, which is an endogenous CNS ligand that participates in blood pressure regulation. The vestibulo-solitary neurons were encapsulated by axo-somatic GABAergic terminals, suggesting that they are under tight inhibitory control. The results establish a chemoanatomical basis for transient vestibular activation of the output pathways from the caudal and intermediate regions of the solitary nucleus. In this way, changes in static head position and movement of the head in space may directly influence heart rate, blood pressure, respiration, as well as gastrointestinal motility. This would provide one anatomical explanation for the synchronous heart rate and blood pressure responses observed after peripheral vestibular activation, as well as disorders ranging from neurogenic orthostatic hypotension, postural orthostatic tachycardia syndrome, and vasovagal syncope to the nausea and vomiting associated with motion sickness.NEW & NOTEWORTHY Vestibular neurons with direct projections to the solitary nucleus utilize glutamate for neurotransmission, modulated by imidazoleacetic acid ribotide. This is the first direct demonstration of the chemical neuroanatomy of the vestibulo-solitary pathway.

Cross-coupling vestibular stimulation: motion sickness and the vestibulo-sympathetic reflex.

Motion sickness is associated with a variety of autonomic symptoms, presumably due to proximity or functional interconnectivity between the autonomic centers in the brainstem and the vestibular system. A direct influence of the vestibular system on cardiovascular variables, defined as the vestibulo-sympathetic reflex, has been reported previously. Our aim was to investigate the sudomotor components of the autonomic responses associated with motion sickness during passive cross-coupling stimulation ("roll while rotating"). Healthy subjects (n = 17) were rotated at 40°/s around an earth-vertical yaw axis alone and in combination with sinusoidal roll oscillations (0.2 Hz). Motion sickness was assessed verbally every minute using a 1-10 scale, while recording DC and AC skin conductance levels (SCL) from the forehead. Yaw rotation alone provoked neither motion sickness nor variations of forehead sweating. Yet during cross-coupling stimulation all subjects reported motion sickness. Higher motion sickness scores (>5) were associated with significantly higher amplitudes of AC-SCL events compared to the lower scores (0.22 ± 0.01 vs. 0.11 ± 0.01 µS, respectively). Frequency domain analysis of the AC-SCL events revealed a peak at 0.2 Hz, coinciding with the frequency of the chair rolls. The total power of AC-SCL signals did not match the trend of motion sickness scores across conditions. We conclude that: (1) although SCL is related to motion sickness, it does not follow the perceived sickness closely; (2) the discrepancy between SCL and motion sickness and the rhythmic AC-SCL events could reflect a sudomotor component of the vestibulo-sympathetic reflex.

Effect of betel nut chewing on the otolithic reflex system.

OBJECTIVE: This study investigated the effect of betel nut chewing on the otolithic reflex system. METHODS: Seventeen healthy volunteers without any experience of chewing betel nut (fresh chewers) and 17 habitual chewers underwent vital sign measurements, ocular vestibular-evoked myogenic potential (oVEMP), and cervical VEMP (cVEMP) tests prior to the study. Each subject then chewed two pieces of betel nut for 2min (dosing). The same paradigm was repeated immediately, 10min, and 20min after chewing. On a different day, 10 fresh chewers masticated chewing gum as control. RESULTS: Fresh chewers exhibited significantly decreased response rates of oVEMP (53%) and cVEMP (71%) after dosing compared with those from the predosing period. These abnormal VEMPs returned to normal 20min after dosing. In contrast, 100% response rates of oVEMP and cVEMP were observed before and after masticating chewing gum. In habitual chewers, the response rates of oVEMP and cVEMP were 32% and 29%, respectively, 20min after dosing. CONCLUSION: Chewing betel nuts induced a transient loss of the otolithic reflexes in fresh chewers but may cause permanent loss in habitual chewers. SIGNIFICANCE: Chewing betel nuts can cause a loss of otholitic reflex function. This creates a risk for disturbed balance and malfunction, for instance, during driving.