An Experimental Model of Vasovagal Syncope Induces Cerebral Hypoperfusion and Fainting-Like Behavior in Awake Rats

Vagal sensory neurons mediate the Bezold-Jarisch reflex and induce syncope

Visceral sensory pathways mediate homeostatic reflexes, the dysfunction of which leads to many neurological disorders1. The Bezold-Jarisch reflex (BJR), first described2,3 in 1867, is a cardioinhibitory reflex that is speculated to be mediated by vagal sensory neurons (VSNs) that also triggers syncope. However, the molecular identity, anatomical organization, physiological characteristics and behavioural influence of cardiac VSNs remain mostly unknown. Here we leveraged single-cell RNA-sequencing data and HYBRiD tissue clearing4 to show that VSNs that express neuropeptide Y receptor Y2 (NPY2R) predominately connect the heart ventricular wall to the area postrema. Optogenetic activation of NPY2R VSNs elicits the classic triad of BJR responses-hypotension, bradycardia and suppressed respiration-and causes an animal to faint. Photostimulation during high-resolution echocardiography and laser Doppler flowmetry with behavioural observation revealed a range of phenotypes reflected in clinical syncope, including reduced cardiac output, cerebral hypoperfusion, pupil dilation and eye-roll. Large-scale Neuropixels brain recordings and machine-learning-based modelling showed that this manipulation causes the suppression of activity across a large distributed neuronal population that is not explained by changes in spontaneous behavioural movements. Additionally, bidirectional manipulation of the periventricular zone had a push-pull effect, with inhibition leading to longer syncope periods and activation inducing arousal. Finally, ablating NPY2R VSNs specifically abolished the BJR. Combined, these results demonstrate a genetically defined cardiac reflex that recapitulates characteristics of human syncope at physiological, behavioural and neural network levels.

Headache and Autonomic Dysfunction: a Review

PURPOSE OF REVIEW

We explore the anatomy of the central and peripheral autonomic pathways involved in primary headache as well as the mechanisms for secondary headache associated with disorders of the autonomic nervous system. The prevalence and clinical presentation of cranial and systemic autonomic symptoms in these conditions will be discussed, with a focus on recent studies.

RECENT FINDINGS

Several small studies have utilized the relationship between headache and the autonomic nervous system to identify potential biomarkers to aid in diagnosis of migraine and cluster headache. Headache in postural orthostatic tachycardia syndrome (POTS) has also been further characterized, particularly in its association with orthostatic headache and spontaneous intracranial hypotension (SIH). This review examines the pathophysiology of primary and secondary headache disorders in the context of the autonomic nervous system. Mechanisms of headache associated with systemic autonomic disorders are also reviewed.

Venous capacitance and venous return in young adults with typical vasovagal syncope: a cross-sectional study

Vasovagal syncope (VVS) has a high prevalence in the general population and is associated with potential complications. There is limited information on the possible association between venous capacitance (VC) and venous return (VR), important determinants of preload and VVS. Since the tilt test was reported to yield a high rate of false positive results, the aim of this study was to evaluate whether abnormal VC and VR at baseline could predispose individuals to VVS.To this end, 88 young, healthy volunteers were recruited and classified to 26 (29.5%) who experienced typical VVS and 62 (70.5%) who did not. VC and VR were evaluated with a commercial device and plethysmography applied to the elevated legs. Maximum venous outflow (MVO), segmental venous capacitance (SVC) and MVO/SVC ratio were calculated and averaged.No significant differences between MVO (5.0±0.5 vs 5.6±0.8, p>0.05), SVC (6.0±0.5 vs 6.3±0.8, p>0.05) or MVO/SVC ratio (0.83±0.02 vs 0.86±0.03, p>0.05) were observed for the non-VVS and VVS volunteers, respectively. There was a significant association between a higher MVO and SVC values and a larger decrease in diastolic blood pressure with standing, although correlations were weak (R²=0.0582 and 0.0681, respectively).In conclusion, at baseline, VC and VR are not impaired in healthy volunteers with a history of VVS. It remains unknown if similar results would be found in patients with cardiovascular comorbidities. Also, the sensitivity of VC and VR evaluations to identify a predisposition for VVS following physiological provocations merits further study.

A transient decrease in heart rate with unilateral and bilateral galvanic vestibular stimulation in healthy humans

The study of cardiovascular function with galvanic vestibular stimulation has provided evidence on the neural structures that are involved in the vestibulo-autonomic reflex. This study determined if the effect on heart rate using galvanic vestibular stimulation persists after provoking a sympathetic response and if this response differs when using unilateral or transmastoid (bilateral) stimulation. We analysed heart rate and heart rate variability using unilateral and transmastoid galvanic vestibular stimulation combined with cardiovascular reflex evoked by postural change in 24 healthy human subjects. Three electrode configurations were selected for unilateral stimulation considering the anatomical location of each semicircular canal. We compared recordings performed in seated and standing positions, and with unilateral and transmastoid stimulation. With subjects seated, a significant transient decrease in heart rate was observed with unilateral stimulation. With transmastoid stimulation, heart rate decreased in both seated and standing positions. Average intervals between normal heartbeats recorded with stimulation resemble parasympathetic cardiac function induced by auricular vagal nerve stimulation. Our results indicate that unilateral stimulation does not eliminate the natural heart rate increase caused by orthostatic hypotension. In contrast, transmastoid stimulation provoked a transient reduction in heart rate, even when subjects were standing. These responses should be considered while performing experiments with galvanic vestibular stimulation and subsequent effects in cardiac regulation mechanisms.

Cardiovascular correlates of human emotional vasovagal syncope differ from those of animal freezing and tonic immobility

It has been suggested that vertebrate freezing and tonic immobility (TI) represent the antecedents of human emotional vasovagal syncope. When a prey detects an approaching predator, it suddenly interrupts its ongoing activity and behaves according to the predator's distance. A prey enters TI when the fight or flight reaction is not feasible and the animal is captured. TI is defined as a post-contact, all or none, innate immobility reflex response that persists after the end of the prey-predator contact. In humans, vasovagal syncope, a reversible adaptive response frequently associated with fainting, occurs in response to emergency conditions such as strong emotions, orthostatic stress, anoxia, visceral pain and decrease in blood volume. The aim of the present review is to dispute the hypothesis that human vasovagal syncope represents the evolution of the bradycardia observed during freezing in a prey-predator condition in other vertebrates. The hypothesis does not take into account three crucial issues: 1) freezing and TI are defence responses which differ from each other in behavioural, cardiovascular and neurophysiological correlates; 2) the initial phase of vasovagal syncope is associated with tachycardia, whereas freezing is associated with a sudden fast-developing bradycardia; 3) the second phase of vasovagal syncope terminates with a blood pressure collapse, whereas blood pressure levels remain at basal levels during both freezing and TI.

Hemodynamics of tonic immobility in the American alligator (Alligator mississippiensis) identified through Doppler ultrasonography

American alligators (Alligator mississippiensis) held inverted exhibit tonic immobility, combining unresponsiveness with flaccid paralysis. We hypothesize that inverting the alligator causes a gravitationally promoted increase in right aortic blood flowing through the foramen of Panizza, with a concurrent decrease in blood flow through the primary carotid, and thereby of cerebral perfusion. Inverting the alligator results in displacement of the liver, post-pulmonary septum, and the heart. EKG analysis revealed a significant decrease in heart rate following inversion; this decrease was maintained for approximately 45 s after inversion which is in general agreement with the total duration of tonic immobility in alligators (49 s). Doppler ultrasonography revealed that following inversion of the alligator, there was a reversal in direction of blood flow through the foramen of Panizza, and this blood flow had a significant increase in velocity (compared to the foraminal flow in the prone alligator). There was an associated significant decrease in the velocity of blood flow through the primary carotid artery once the alligator was held in the supine position. Tonic immobility in the alligator appears to be a form of vasovagal syncope which arises, in part, from the unique features of the crocodilian heart.

Remote Limb Ischemic Preconditioning Attenuates Cerebrovascular Depression During Sinusoidal Galvanic Vestibular Stimulation via α1-Adrenoceptor-Protein Kinase Cε-Endothelial NO Synthase Pathway in Rats

BACKGROUND

Vasovagal syncope (VVS) is characterized by hypotension and bradycardia followed by lowering of cerebral blood flow. Remote limb ischemic preconditioning (RIPC) is well documented to provide cardio- and neuroprotection as well as to improve cerebral blood flow. We hypothesized that RIPC will provide protection against VVS-induced hypotension, bradycardia, and cerebral hypoperfusion. Second, because endothelial nitric oxide synthase has been reported as a mediator of cerebral blood flow control, we hypothesized that the mechanism by which RIPC primes the vasculature against VVS is via the α1-adrenoceptor-protein kinase Cε-endothelial nitric oxide synthase pathway.

METHODS AND RESULTS

We utilized sinusoidal galvanic vestibular stimulation in rats as a model of VVS. RIPC attenuated the lowerings of mean arterial pressure, heart rate, and cerebral blood flow caused by sinusoidal galvanic vestibular stimulation, as well as improving behavior during, and recovery after, stimulation. RIPC induced elevated serum norepinephrine, increased expression of brain α1-adrenoceptors, and reduced brain expression of norepinephrine transporter 1. Antagonizing adrenoceptors and norepinephrine transporter 1 prevented RIPC protection of cerebral perfusion during sinusoidal galvanic vestibular stimulation.

CONCLUSIONS

Taken together, this study indicates that RIPC may be a potential therapy that can prevent VVS pathophysiology, decrease syncopal episodes, and reduce the injuries associated with syncopal falls. Furthermore, the α1-adrenoceptor-protein kinase Cε-endothelial nitric oxide synthase pathway may be a therapeutic target for regulating changes in cerebral blood flow.