Sudden unexpected death in epilepsy (SUDEP) remains a perplexing and tragic phenomenon, accounting for a significant proportion of deaths in individuals with medically refractory epilepsy. Despite ongoing research, the mechanisms underlying SUDEP are not fully understood.
Accumulating evidence suggests that postictal apnea, characterized by the absence of breathing following a seizure, plays a crucial role in many SUDEP cases.
This article delves into the intricate details surrounding SUDEP, exploring the relationship between postictal apnea and seizures, the involvement of the amygdala, and potential mechanisms contributing to this phenomenon.
SUDEP has been reported to account for a wide range of epilepsy-related deaths, making up between 10% to 50% of such cases.
Those most vulnerable to SUDEP are individuals with pharmacoresistant epilepsy, particularly those considered for epilepsy surgery or those who continue to experience seizures even after surgical interventions. Insight into SUDEP has been gleaned from rare cases observed in epilepsy monitoring units (EMUs), where comprehensive assessments of electroencephalographic, cardiac, and respiratory functions are possible.
One striking observation in these cases is postictal apnea, which manifests as a cessation of breathing following a seizure, persisting for minutes and sometimes leading to terminal asystole and death. Studies have also identified near-SUDEP cases and individuals who eventually succumbed to SUDEP, highlighting periods of persistent postictal apnea and breaths with markedly reduced tidal volumes.
These findings collectively indicate that postictal apnea is a significant contributor to SUDEP. However, the precise mechanisms underlying postictal apnea remain a mystery.
Ictal Apnea vs. Postictal Apnea
Distinctions between ictal apnea (apnea occurring during a seizure) and postictal apnea (apnea following a seizure) are critical to understanding SUDEP. Ictal apnea, though more common, is generally less severe than postictal apnea and is estimated to occur in a substantial percentage of seizures.
Studies involving intracranial electroencephalography (iEEG) and continuous respiratory monitoring have associated seizure propagation to the amygdala with ictal apnea. Electrical stimulation of focal amygdala sites has been shown to reproduce this phenomenon, indicating a role for the amygdala in inhibiting respiration during seizures.
However, it is essential to differentiate between ictal and postictal apnea and their potential interplay. Notably, our data suggests that they are related, with postictal apnea consistently following ictal apnea in observed cases.
This suggests that ictal apnea may lead to postictal apnea or that postictal apnea may represent a more severe form of ictal apnea that persists after seizure termination. Understanding this relationship is crucial in unraveling the mechanisms driving these respiratory disturbances.
Postictal Apnea and Seizure Types
While some have posited that postictal apnea is exclusive to generalized convulsive seizures, our findings challenge this notion. Focal seizures, which are more localized than generalized seizures, were found to induce both ictal and postictal apnea, suggesting that postictal apnea can occur following focal seizures.
However, it’s important to acknowledge that the majority of SUDEP cases are associated with postictal apnea following generalized convulsive seizures. The circumstances surrounding these seizures, such as obtundation and mechanical airway obstruction, can increase the risk of postictal apnea leading to death.
Exploring the Role of the Amygdala
To shed light on the mechanisms of postictal apnea and its potential connection to SUDEP, our study focused on 20 patients with intractable epilepsy undergoing iEEG monitoring. We aimed to identify brain sites associated with postictal apnea by recording iEEG and breathing during stimulation-evoked seizures.
Electrical stimulation below seizure threshold was used to identify focal sites influencing breathing, particularly persistent apnea akin to SUDEP cases. Machine learning techniques were then employed to identify a common region critical for postictal apnea.
Stimulation within this subregion led to apnea that persisted beyond the stimulation window. This finding not only reaffirms the amygdala’s role in inhibiting breathing but also highlights its significance in postictal apnea. Importantly, this study is the first to identify a specific brain site associated with postictal apnea in both pediatric and adult epilepsy patients.
Mapping Brain Connections and Mechanisms
To unravel the neural pathways underlying postictal apnea, we employed a novel approach combining direct electrical brain stimulation with functional magnetic resonance imaging (fMRI). Stimulation of the pAIR site was found to affect BOLD activity in the pons, medulla, and ventral insula, regions associated with respiratory control and interoceptive processing.
Decreased BOLD activity in the pons and medulla may indicate reduced neural activity in crucial areas responsible for respiratory rhythm generation. Inhibitory input to preBötC neurons, located in the medulla, may be particularly vulnerable to prolonged suppression, delaying the resumption of breathing.
Another crucial aspect of this study involved examining the impact of CO2 sensitivity on postictal apnea. Amygdala stimulation resulted in reduced respiratory sensitivity to CO2, potentially implicating CO2-sensitive neurons in the retrotrapezoid nucleus and serotonergic neurons in the medulla. Extended suppression of these neurons could interfere with the potent ability of CO2 to stimulate breathing, possibly contributing to postictal apnea.
Emotional Aspects of Postictal Apnea
Air hunger, the primal sensation triggered by rising CO2 levels when breathing is impaired, plays a significant role in postictal apnea. This sensation encompasses fear, arousal, and alarm. Intriguingly, patients with postictal apnea due to amygdala stimulation do not experience air hunger, the emotional response to it, or even awareness that they have stopped breathing despite elevated CO2 levels.
The ventral insula, known for processing interoceptive signals, may hold the key to understanding this phenomenon. Increased BOLD activity in the ventral insula following pAIR site stimulation suggests a possible mechanism by which amygdala stimulation suppresses air hunger.
Challenges and Future Directions
This study, while providing valuable insights, is not without limitations. The relatively small sample size reflects the inherent challenges of recruiting epilepsy patients undergoing invasive monitoring. Furthermore, the controlled environment of the EMU differs from real-world situations in which most SUDEP cases occur. Our inability to target specific neurons with electrical stimulation leaves room for potential influence from adjacent sites or white matter tracts.
Despite these limitations, our findings open doors to further exploration of the amygdala’s role in postictal apnea and SUDEP. Future research may examine differences in vulnerability to postictal apnea among patients and the potential longitudinal risk of SUDEP in those exhibiting persistent apneas. Understanding these complexities is essential for developing effective prevention and management strategies for SUDEP, ultimately enhancing the quality of life for individuals with epilepsy.
Sudden unexpected death in epilepsy remains a challenging and enigmatic issue within the medical community. While the majority of SUDEP cases involve postictal apnea, the mechanisms underlying this phenomenon have remained elusive. This study sheds light on the intricate relationship between seizures, apnea, and the amygdala, providing valuable insights into the potential causes of postictal apnea and its connection to SUDEP.
Further research is needed to unravel the complexities surrounding this devastating condition, with the hope of reducing the risks and improving the lives of those living with epilepsy.