Relax,but Don't Forget to Breathe: Ictal Central Apnea and SUDEP

Abstract

Don't Forget to Breathe

Liška K, Pant A, Jefferys JGR. Diaphragm relaxation causes seizure-related apnoeas in chronic and acute seizure models in rats. Neurobiol Dis. 2024;203:106735. doi:10.1016/j.nbd.2024.106735

Background: Ictal central apnoea is a feature of focal temporal seizures. It is implicated as a risk factor for sudden unexpected death in epilepsy (SUDEP). Here we study seizure-related apnoeas in two different models of experimental seizures, one chronic and one acute, in adult genetically-unmodified rats, to determine mechanisms of seizure-related apnoeas. Under general anaesthesia rats receive sensors for nasal temperature, hippocampal and/or neocortical potentials, and ECG or EMG for subsequent tethered video-telemetry. Tetanus neurotoxin (TeNT), injected into hippocampus during surgery, induces a chronic epileptic focus. Other implanted rats receive intraperitoneal pentylenetetrazol (PTZ) to evoke acute seizures. In chronically epileptic rats, convulsive seizures cause apnoeas (9.9 ± 5.3 s; 331 of 730 convulsive seizures in 15 rats), associated with bradyarrhythmias. Absence of EEG and ECG biomarkers exclude obstructive apnoeas. All eight TeNT-rats with diaphragm EMG have apnoeas with no evidence of obstruction, and have apnoea EMGs significantly closer to expiratory relaxation than inspiratory contraction during pre-apnoeic respiration, which we term “atonic diaphragm”. Consistent with atonic diaphragm is that the pre-apnoeic nasal airflow is expiration, as it is in human ictal central apnoea. Two cases of rat sudden death occur. One, with telemetry to the end, reveals a lethal apnoea, the other only has video during the final days, which reveals cessation of breathing shortly after the last clonic epileptic movement. Telemetry following acute systemic PTZ reveals repeated seizures and seizure-related apnoeas, culminating in lethal apnoeas; ictal apnoeas are central - in 8 of 35 cases diaphragms initially contract tonically for 8.5 ± 15.0 s before relaxing, in the 27 remaining cases diaphragms are atonic throughout apnoeas. All terminal apnoeas are atonic. Differences in types of apnoea due to systemic PTZ in rats (mainly atonic) and mice (tonic) are likely species-specific. Certain genetic mouse models have apnoeas caused by tonic contraction, potentially due to expression of epileptogenic mutations throughout the brain, including in respiratory centres, in contrast with acquired focal epilepsies. We conclude that ictal apnoeas in the rat TeNT model result from atonic diaphragms. Relaxed diaphragms could be particularly helpful for therapeutic stimulation of the diaphragm to help restore respiration.

Commentary

Breathing is often synonymous with life itself, and, with few exceptions, we breathe from the moment we are born until our death. Humanity has recognized this since time immemorial; in Genesis of the Abrahamic faiths, God breathes life into Adam, and in Sanskrit, the word Prana, which is often taken to mean “life force,” literally translates to breath. Whether or not we contemplate its significance, breathing remains essential to life. The ability to practice, contort, and adapt our breathing, unlike our cardiac output, relies on the fact that our central nervous system creates and controls breathing rhythm. Breathing rhythm appears to be generated in the ventral lateral medulla. ¹ However, its generation is complex and relies on input from other structures. When these inputs are disturbed, for instance, during central congenital hypoventilation syndrome or opiate overdose, the brain can “forget” to breathe and cause death.^2,3

Sudden Unexpected Death in Epilepsy (SUDEP) is the most common fatal complication of epilepsy. Although the nature of SUDEP is elusive, available data indicates that respiratory arrest (aka terminal apnea) is often the ultimate cause of death. The MORTEMUS study, an international multicenter investigation that analyzed nine cases of SUDEP occurring in epilepsy monitoring units (EMUs), demonstrated that terminal apnea (aka, respiratory arrest) occurred before terminal asystole minutes later. ⁴ This suggests that, at least in some cases, SUDEP is primarily a respiratory-driven event.

Clinical investigation of SUDEP mechanisms is not tenable, which has led many researchers to utilize animal models of epilepsy, such as transgenic mice that represent patient populations (eg, NaV1.1 haploinsufficient, Scn8a mutant),^5,6 inbred mouse lines (eg, DBA/1 & 2), ⁷ or inducible fatal seizures such as maximal electroshock or pentylenetetrazole (PTZ) injection.^6,8 In these cases, fatal seizures were almost always described as “tonic” (ie, exhibiting hindlimb extension and excessive muscle tone) and were followed by respiratory arrest that preceded cardiac arrest. This sequence of events roughly aligns with evidence from the MORTEMUS study, but it does not represent much diversity in the mechanism of this crucial terminal apnea.

Recently, Liska and colleagues developed more diversity by using two adult Wistar rat models of SUDEP, including one model that utilizes tetanus neurotoxin (TeNT) injected into the hippocampus to produce chronic, spontaneous seizures and another model where high dose (75 mg/kg) PTZ is injected to induce acute seizures that are routinely fatal. ⁹ In addition to rats adding species diversity, they enable more detailed physiological measurements. Notably, both sets of rats are instrumented with electromyogram (EMG) of the diaphragm (d-EMG)—a challenging technique that is mostly prohibitive in the mouse models described above—in addition to electroencephalogram (EEG), electrocardiogram (ECG), and nasal thermistors to measure breathing. In some rats, the authors recorded non-respiratory-related dorsal subcutaneous muscle (s-EMG) for comparison with the d-EMG.

Liska and colleagues recorded hundreds of spontaneous convulsive seizures in the TeNT rats—about half of these seizures present with ictal apnea. A few apneas were associated with the tonic phase; however, most were not associated with a tonic phase, nor tonic diaphragm contraction, and the authors referred to these apneas as “atonic.” Two of the 26 TeNT rats experienced SUDEP-like deaths. Only one event had biosignals recorded at the time of fatality, but this did not include d-EMG. In the single fatality with breathing and ECG recorded it was clear that terminal apnea occurred after a seizure and preceded terminal asystole. The authors concluded there were no tonic phase and proposed that tonic diaphragm contraction was unlikely. Interestingly, the one TeNT rat died 108 min after the last seizure, with numerous short and long apneas occurring during that time, leading up to the terminal apnea.

In the PTZ-injected rats, all rats died from terminal apnea that occurred during and/or immediately after seizures. In most cases, like the TeNT rats, these fatal apneas are described as atonic, although an example of this data is not shown. In less than a third of cases, tonic apneas were observed that lead to fatality, as demonstrated in mice. An example of this latter case is shown. Interestingly, and as the authors point out, the tonic apnea proceeds to an atonic apnea, but not until after a few breaths in between.

Before assessing the differences between this data and previous studies, it is essential to acknowledge that the general sequence of these SUDEP-like events is reproduced: terminal apnea precedes terminal asystole. The fact that this is conserved among rats, mice, and humans suggests that this sequence, however it is formed, is likely a common cause of SUDEP.

However, the one TeNT rat and the majority of PTZ rats died from atonic seizures. This differs from what has been observed in mouse models, where hindlimb extension coincides with fatal seizures.^5–8 In the PTZ rats that did experience tonic apneas, the tonic apnea proceeded to an atonic apnea after the seizures. However, in the PTZ rats, breathing resumes for a brief period prior to the initiation of the atonic apnea, which is not observed in mouse models.^5,6,8 Taken together, the evidence suggests that the presence of seizures that produce tonic diaphragm contraction is not necessary for SUDEP-like events to occur.

Only two other studies have examined respiratory muscles during experimental seizure-induced apnea. In 2021, my group published recordings of diaphragm EMG during audiogenic seizures of mice harboring germline Scn8a mutations. ⁶ These experiments identified tonic diaphragm contraction during apnea; however, none of these seizures were fatal. Fatal seizures in these mice did present with apnea initiated during the tonic phase, but the diaphragm was not recorded in these cases. Earlier last year, Umeza and colleagues also performed d-EMG in a smaller cohort of Wistar rats after PTZ injection (60 and 75 mg/kg). They found that all fatalities involved tonic contraction of the diaphragm, followed by brief breathing recovery and then an atonic apnea that led to death. ¹⁰ These results appear identical to what Liska and colleagues see in a minority of cases. Why Umeza and colleagues do not observe any fatal seizures with solely atonic apnea is unclear, but it does support the uncoupling of tonic and atonic apneas. Perhaps tonic apnea is a common, but not universal, byproduct of the seizure state that creates terminal apnea and is more or less common depending on circumstances.

Understanding the mechanism of fatal apnea will be important in the development of treatments for SUDEP. As a comparison, prescribing CPAP to a patient with central (not obstructive) sleep apnea would be unproductive. Similarly, treatments for the fatal apnea involved in SUDEP will need to take the mechanisms of apnea into account. Liska and colleagues' demonstration that sometimes the brain simply forgets to breathe (ie, atonic apnea) suggests that suppression of central breathing drive due to seizures may occur in the absence of seizure spread to the brainstem (ie, the likely mechanism of tonic apnea). Questions remain: under what circumstances is SUDEP initiated by a tonic apnea? Is there an underlying neural mechanism that creates both atonic and tonic apnea that could be targeted? The answers to these questions will likely be multifactorial and complicated. However, understanding these mechanisms, no matter how diverse, will help us to create the appropriate therapies to reduce the occurrence of this fatal comorbidity of epilepsy.

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.