Laryngeal reflex apnea in neonates: effects of CO2 and the complex influence of hypoxia

Utilizing multimodal imaging to visualize potential mechanism for sudden death in epilepsy

Sudden death in epilepsy or SUDEP is a fatal condition that accounts for more than 4000 deaths each year. Limited clinical and preclinical data on sudden death suggest critical contributions from autonomic, cardiac, and respiratory pathways. A potential mechanism for such sudden and severe cardiorespiratory dysregulation may be linked to acid reflux-induced laryngospasm. Here, we expand on our previous investigations and utilize a novel multimodal approach to provide visual evidence of acid reflux-initiated cardiorespiratory distress and subsequent sudden death in seizing rats. We used systemic kainic acid to acutely induce seizure activity in Long Evans rats, under urethane anesthesia. We recorded electroencephalography (EEG), electrocardiography (ECG), chest plethysmography, and esophageal pH signals through a multimodal recording platform, during simultaneous fast MRI scans of the rat stomach and esophagus. MRI images, in correlation with electrophysiology data were used to identify seizure progression, stomach acid movement up the esophagus, cardiorespiratory changes, and sudden death. In all cases of sudden death, esophageal pH recordings alongside MRI images visualized stomach acid movement up the esophagus. Severe cardiac (ST segment elevation), respiratory (intermittent apnea) and brain activity (EEG narrowing due to hypoxia) changes were observed only after acid reached larynx, which strongly suggested onset of laryngospasm following acid reflux. The complementary information coming from electrophysiology and fast MRI scans provided insight into the mechanism of esophageal reflux, laryngospasm, obstructive apnea, and subsequent sudden death in seizing animals. The results carry clinical significance as it outlines a potential mechanism that may be relevant to SUDEP in humans.

Laryngeal Chemoreflex in Health and Disease: A Review

The larynx plays a key role in airway protection via the laryngeal chemoreflex (LCR). This involuntary reflex can be evoked when hazardous substances activate mucosal receptors, which send signals to be processed within the brainstem. Although the LCR is meant to be protective, the reflex can become hyperstimulated, even to benign stimuli, which can result in pathological disorders, such as chronic cough and inducible laryngeal obstruction. In this review, we will outline the mechanism of the LCR and its associated pathological disorders.

Ictal activation of oxygen-conserving reflexes as a mechanism for sudden death in epilepsy

OBJECTIVE

To test the hypothesis that death with physiological parallels to human cases of sudden unexpected death in epilepsy (SUDEP) can be induced in seizing rats by ictal activation of oxygen-conserving reflexes (OCRs).

METHODS

Urethane-anesthetized female Long-Evans rats were implanted with electrodes for electrocardiography (ECG), electrocorticography (ECoG), and respiratory thermocouple; venous and arterial cannulas; and a laryngoscope guide and cannula or nasal cannula for activation of the laryngeal chemoreflex (LCR) or mammalian diving reflex (MDR), respectively. Kainic acid injection, either systemic or into the ventral hippocampus, induced prolonged acute seizures.

RESULTS

Reflex challenges during seizures caused sudden death in 18 of 20 rats-all MDR rats (10) and all but two LCR rats (8) failed to recover from ictal activation of OCRs and died within minutes of the reflexes. By comparison, 4 of 4 control (ie, nonseizing) rats recovered from 64 induced diving reflexes (16 per rat), and 4 of 4 controls recovered from 64 induced chemoreflexes (16 per rat). Multiple measures were consistent with reports of human SUDEP. Terminal central apnea preceded terminal asystole in all cases. Heart and respiratory rate fluctuations that paralleled those seen in human SUDEP occurred during OCR-induced sudden death, and mean arterial pressure (MAP) was predictive of death, showing a 17 or 15 mm Hg drop (MDR and LCR, respectively) in the 20 s window centered on the time of brain death. OCR activation was never fatal in nonseizing rats.

SIGNIFICANCE

These results present a method of inducing sudden death in two seizure models that show pathophysiology consistent with that observed in human cases of SUDEP. This proposed mechanism directly informs previous findings by our group and others in the field; provides a repeatable, inducible animal model for the study of sudden death; and offers a potential explanation for observations made in cases of human SUDEP.

Mechanisms and prevention of acid reflux induced laryngospasm in seizing rats

OBJECTIVE

Recent animal work and limited clinical data have suggested that laryngospasm may be involved in the cardiorespiratory collapse seen in sudden unexpected death in epilepsy (SUDEP). In previous work, we demonstrated in an animal model of seizures that laryngospasm and sudden death were always preceded by acid reflux into the esophagus. Here, we expand on that work by testing several techniques to prevent the acid reflux or the subsequent laryngospasm.

METHODS

In urethane anesthetized Long Evans rats, we used systemic kainic acid to acutely induce seizure activity. We recorded pH in the esophagus, respiration, electrocorticography activity, and measured the liquid volume in the stomach postmortem. We performed the following three interventions to attempt to prevent acid reflux or laryngospasm and gain insights into mechanisms: fasting animals for 12 h, severing the gastric nerve, and electrical stimulation of either the gastric nerve or the recurrent laryngeal nerve.

RESULTS

Seizing animals had significantly more liquid in their stomach. Severing the gastric nerve and fasting animals significantly reduced stomach liquid volume, subsequent acid reflux, and sudden death. Laryngeal nerve stimulation can reverse laryngospasm on demand. Seizing animals are more susceptible to death from stomach acid-induced laryngospasm than nonseizing animals are to artificial acid-induced laryngospasm.

SIGNIFICANCE

These results provide insight into the mechanism of acid production and sudden obstructive apnea in this model. These techniques may have clinical relevance if this model is shown to be similar to human SUDEP.

Sex-Specific Consequences of Neonatal Stress on Cardio-Respiratory Inhibition Following Laryngeal Stimulation in Rat Pups

The presence of liquid near the larynx of immature mammals triggers prolonged apneas with significant O2 desaturations and bradycardias. When excessive, this reflex (the laryngeal chemoreflex; LCR) can be fatal. Our understanding of the origins of abnormal LCR are limited; however, perinatal stress and male sex are risk factors for cardio-respiratory failure in infants. Because exposure to stress during early life has deleterious and sex-specific consequences on brain development it is plausible that respiratory reflexes are vulnerable to neuroendocrine dysfunction. To address this issue, we tested the hypothesis that neonatal maternal separation (NMS) is sufficient to exacerbate LCR-induced cardio-respiratory inhibition in anesthetized rat pups. Stressed pups were separated from their mother 3 h/d from postnatal days 3 to 12. At P14-P15, pups were instrumented to monitor breathing, O2 saturation (Spo2), and heart rate. The LCR was activated by water injections near the larynx (10 µl). LCR-induced apneas were longer in stressed pups than controls; O2 desaturations and bradycardias were more profound, especially in males. NMS increased the frequency and amplitude of spontaneous EPSCs (sEPSCs) in the dorsal motor nucleus of the vagus (DMNV) of males but not females. The positive relationship between corticosterone and testosterone observed in stressed pups (males only) suggests that disruption of neuroendocrine function by stress is key to sex-based differences in abnormal LCR. Because testosterone application onto medullary slices augments EPSC amplitude only in males, we propose that testosterone-mediated enhancement of synaptic connectivity within the DMNV contributes to the male bias in cardio-respiratory inhibition following LCR activation in stressed pups.

Prenatal nicotinic exposure prolongs superior laryngeal C-fiber-mediated apnea and bradycardia through enhancing neuronal TRPV1 expression and excitation

Maternal cigarette smoke, including prenatal nicotinic exposure (PNE), is responsible for sudden infant death syndrome (SIDS). The fatal events of SIDS are characterized by severe bradycardia and life-threatening apneas. Although activation of transient receptor potential vanilloid 1 (TRPV1) of superior laryngeal C fibers (SLCFs) could induce bradycardia and apnea and has been implicated in SIDS pathogenesis, how PNE affects the SLCF-mediated cardiorespiratory responses remains unexplored. Here, we tested the hypothesis that PNE would aggravate the SLCF-mediated apnea and bradycardia via up-regulating TRPV1 expression and excitation of laryngeal C neurons in the nodose/jugular (N/J) ganglia. To this end, we compared the following outcomes between control and PNE rat pups at postnatal days 11-14: 1) the cardiorespiratory responses to intralaryngeal application of capsaicin (10 µg/ml, 50 µl), a selective stimulant for TRPV1 receptors, in anesthetized preparation; 2) immunoreactivity and mRNA of TRPV1 receptors of laryngeal sensory C neurons in the N/J ganglia retrogradely traced by 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate; and 3) TRPV1 currents and electrophysiological characteristics of these neurons by using whole-cell patch-clamp technique in vitro Our results showed that PNE markedly prolonged the apneic response and exacerbated the bradycardic response to intralaryngeal perfusion of capsaicin, which was associated with up-regulation of TRPV1 expression in laryngeal C neurons. In addition, PNE increased the TRPV1 currents, depressed the slow delayed rectifier potassium currents, and increased the resting membrane potential of these neurons. Our results suggest that PNE is capable of aggravating the SLCF-mediated apnea and bradycardia through TRPV1 sensitization and neuronal excitation, which may contribute to the pathogenesis of SIDS.-Gao, X., Zhao, L., Zhuang, J., Zang, N., Xu, F. Prenatal nicotinic exposure prolongs superior laryngeal C-fiber-mediated apnea and bradycardia through enhancing neuronal TRPV1 expression and excitation.

Infants dying suddenly and unexpectedly share demographic features with infants who die with retinal and dural bleeding: a review of neural mechanisms

The cause of death in infants who die suddenly and unexpectedly (sudden unexpected death in infancy [SUDI]) remains a diagnostic challenge. Some infants have identified diseases (explained SUDI); those without explanation are called sudden infant death syndrome (SIDS). Demographic data indicate subgroups among SUDI and SIDS cases, such as unsafe sleeping and apparent life-threatening events. Infants dying suddenly with retinal and dural bleeding are often classified as abused, but in many there is no evidence of trauma. Demographic features suggest that they may represent a further subgroup of SUDI. This review examines the neuropathological hypotheses to explain SIDS and highlights the interaction of infant oxygen-conserving reflexes with the brainstem networks considered responsible for SIDS. We consider sex- and age-specific vulnerabilities related to dural bleeding and how sensitization of the dural innervation by bleeding may influence these reflexes, potentially leading to collapse or even death after otherwise trivial insults.

Sudden Unexpected Death in Fetal Life Through Early Childhood

In March 2015, the Eunice Kennedy Shriver National Institute of Child Health and Human Development held a workshop entitled "Sudden Unexpected Death in Fetal Life Through Early Childhood: New Opportunities." Its objective was to advance efforts to understand and ultimately prevent sudden deaths in early life, by considering their pathogenesis as a potential continuum with some commonalities in biological origins or pathways. A second objective of this meeting was to highlight current issues surrounding the classification of sudden infant death syndrome (SIDS), and the implications of variations in the use of the term "SIDS" in forensic practice, and pediatric care and research. The proceedings reflected the most current knowledge and understanding of the origins and biology of vulnerability to sudden unexpected death, and its environmental triggers. Participants were encouraged to consider the application of new technologies and "omics" approaches to accelerate research. The major advances in delineating the intrinsic vulnerabilities to sudden death in early life have come from epidemiologic, neural, cardiac, metabolic, genetic, and physiologic research, with some commonalities among cases of unexplained stillbirth, SIDS, and sudden unexplained death in childhood observed. It was emphasized that investigations of sudden unexpected death are inconsistent, varying by jurisdiction, as are the education, certification practices, and experience of death certifiers. In addition, there is no practical consensus on the use of "SIDS" as a determination in cause of death. Major clinical, forensic, and scientific areas are identified for future research.