Abstracts

Rationale: Dravet syndrome (DS) is a severe, infantile-onset epilepsy syndrome commonly due to mutation of the SCN1A gene. DS patients have a high risk of sudden unexpected death in epilepsy (SUDEP), but the mechanisms of death are not well defined. Here we used a mouse epilepsy monitoring unit (EMU) to study sudden death after seizures induced by maximal electroshock (MES) in two mouse strains: 1) A DS model with a mutation of SCN1A (R1407X) that has spontaneous seizures and an incidence of sudden death of 20-40%, and; 2) C57BL6 mice which do not have spontaneous seizures and are not known to have sudden death.Methods: In one set of experiments, DS mice were monitored with video recording for 24 hours per day from postnatal day 16 (P16) to P60, and severe seizures were identified behaviorally (wild running and jumping, or tonic extension). In a second set of experiments, seizures were induced by MES in DS mice (n=13) and C57BL6 mice (n=8) while animals were monitored in a mouse EMU for recording EEG, nuchal EMG, EKG, video, and plethysmography (breathing). In a third set of experiments, seizures were induced by MES in DS mice (n=10) outside of the EMU. A rodent ventilator was used to resuscitate animals if there was a prolonged postictal period without chest movement (>8 seconds).Results: Death occurred after spontaneous seizures in 41% of DS mice (n = 18 of 44). Death always occurred after a severe seizure with tonic extension. In those animals that died, there was a low frequency of spontaneous severe seizures until 1-2 days prior to death. DS mice that did not die had a very low frequency of seizures from P16-P60. MES induced tonic seizures in all DS mice (n=13). However, only three of these DS mice died, and in each case this occurred after the first seizure. The other 10 DS mice did not die despite induction of at least one tonic seizure. In contrast, 100% of C57Bl6 mice died after the first trial of MES, in all cases after a tonic seizure (n=8). When death occurred in DS and C57Bl6 mice, complete respiratory arrest always occurred after the seizure. This was slowly followed by bradycardia, and then asystole after 3-5 minutes. In DS mice that survived, there was either no apnea (n=8) or a short period of apnea followed by autoresuscitation (n=2). A single seizure was induced by MES in a separate cohort of DS mice (n=9). Of these mice, 7 did not have postictal apnea. Two mice had prolonged apnea (>8 s), but death was prevented by mechanical ventilation.Conclusions: These data demonstrate that DS mice are more likely than C57Bl6 mice to have spontaneous seizures and sudden death. However, DS mice are less likely than C57Bl6 mice to have postictal apnea and death in response to MES. In both strains, when death occurred central apnea was followed by bradycardia and asystole. Survival was increased in DS mice by autoresuscitation or mechanical ventilation indicating that bradycardia was not sufficient, by itself, to cause death. These results indicate that prolonged postictal apnea is a biomarker for sudden death induced by MES in these mice.