Intracranial CArtography of Cortical Contribution to Respiratory Load Compensation in Epilepsy

Overview

30% of patients with epilepsy suffer from drug-resistant seizures and have a greater risk of premature mortality than the general population. Among all causes of death, the most frequent is SUDEP, for sudden and unexpected death in epilepsy patients. SUDEP typically occurs after a nocturnal seizure, and primarily results from a postictal central respiratory dysfunction in patients with generalized convulsive seizure (GCS), suggesting the critical role of seizure-related impairment of breathing control, and underscoring the importance of monitoring and preventive interventions during the post-ictal phase. Most of patients with drug-resistant seizures demonstrate transient peri-ictal apnea and hypoxemia especially in the aftermath of a GCS. Experimental and clinical data suggest that most SUDEP primarily result from a fatal seizure-related respiratory arrest 5. Apnea was the primary cause of death in several epilepsy models. In patients whose SUDEP had occurred during long-term video-EEG monitoring, we observed fatal postictal central apnea after a nocturnal GCS in all SUDEP. Accordingly, it is currently hypothesized that in a subgroup of patients, repetition of seizures may contribute to chronic alteration of respiratory regulation which may increase the risk of fatal postictal central respiratory arrest. Central regulation of autonomic function is ensured by the so-called Central Autonomic Network (CAN), which anatomy in humans has primarily been investigated in neuroimaging studies or using intraEEG (iEEG) data in patients with drug-resistant focal epilepsy undergoing presurgical evaluation with intracerebral electrodes. Central regulation of breathing primarily rely on brainstem, especially the preBötzinger complex for rhythm generation and the retrotrapezoid nucleus and dorsal raphe for chemoreception, especially ventilatory response to hypercapnia. However, through an intricated structures connecting these regions, this respiratory signal projects to a network of cortical and subcortical regions mainly including the limbic and sensorimotor cortical areas. Studies in patients undergoing iEEG reinforced the role of limbic and paralimbic structures, with transient central apnea elicited by direct electrical stimulation of amygdala, hippocampus, anterior parahippocampal, and antero-mesial fusiform gyri. However, our group also reported transient hypoxemia could be elicited by cortical direct electrical stimulation outside the temporo-limbic structures, most commonly after stimulation of the perisylvian cortex. Importantly, our group recently showed that involvement of this perisylvian cortex in the epileptogenic zone is a strong risk factor of SUDEP, reinforcing the importance of further studying its integration in the cortical control of respiration. The involvement of cortical control of ventilation is particularly important to ensure expiratory load compensation, a typical situation after GCS, which is associated with airway obstruction, especially when the face is positioned into the pillow. This cortical component of the physiological response to experimental expiratory loads was investigated in healthy subjects through the study of EEG activity during an expiratory load compensation protocol. Accordingly, EEGs were processed by ensemble averaging expiratory time-locked segments and examined for pre-expiratory EEG potentials, defined as a slow negative shift from the baseline signal preceding expiration, and suggestive of cortical preparation of expiration. Expiratory load compensation was associated with EEG premotor potential presumably involving the supplementary motor area. However, because of the limited spatial resolution of scalp EEG, the organization of cortical neural sources involved in this expiratory load compensation or during response to hypercapnia, especially the interaction between the premotor cortex, the sensorimotor cortical areas and the perisylvian cortex is unknown.