Sudden unexpected death in epilepsy (SUDEP) is the most common cause of death in chronic refractory epilepsy patients. Dravet Syndrome (DS) is an infantile-onset epilepsy with severe seizures commonly due to mutations of the sodium channel gene SCN1A. DS patients have a high risk of SUDEP, but the mechanisms of death are not well defined. The principal risk factor for SUDEP is a high frequency of seizures. The recent MORTality in Epilepsy Monitoring Unit Study (MORTEMUS) reported the largest series of SUDEP cases while in Epilepsy Monitoring Units (EMUs). Most cases occurred after a generalized seizure, and were associated with both cardiac and respiratory dysfunction. None of the SUDEP cases in the MORTEMUS study had direct measurements of breathing, but visual analysis of video recording suggested that apnea occurred early in the sequence of events preceding death. Since SUDEP is always identified after death and is impossible to predict its incidence, it is not possible to run clinical trials in patients. Therefore, developing animal models of SUDEP is beneficial. In a DS mouse model with an Scn1aR1407X/+ mutation, death occurs after spontaneous and induced seizures. This postictal death is likely to be relevant to the mechanisms of SUDEP in DS patients. In a previous study, electrocardiogram (EKG) recordings from DS mice have shown that postictal death after heat-induced seizures is due to progressive bradycardia and asystole. However, postictal breathing has not been measured during experiments studying postictal death in these mice, so it is unknown if respiratory dysfunction, such as central apnea, contributes to postictal death.
The first goal of this dissertation is to design and develop a mouse EMU that monitored electroencephalogram (EEG), nuchal electromyography (EMG), EKG, video, whole body plethysmography (breathing), body temperature, room temperature, and humidity from mice until the occurrence of postictal death. Using a mouse EMU we sought to evaluate the primary cause of death in multiple mouse strains with seizure-induced death, and to determine whether they have a common final pathway of death. We induced seizures acutely in multiple non-epileptic mouse strains that are prone to sudden death in response to seizures: 1) maximal electroshock (MES)-induced seizures in Lmx1bf/f/p mice, 2) MES-induced seizures in C57Bl6 mice, and 3) audiogenic seizures in DBA/1 mice. These seizures caused immediate and permanent respiratory arrest (terminal apnea) in all 3 strains of mice. In each strain, EKG activity continued for 3 to 5 minutes after terminal apnea. We interpret these data as indicating that the primary cause of postictal death was central apnea, and the resulting hypoxia then caused bradycardia and asystole.
The second goal of this dissertation is to understand the mechanism(s) of SUDEP in DS. Here we found that DS patients have frequent postictal respiratory dysfunction, while cardiac activity was normal. One of these patients who had severe postictal hypoventilation later died of SUDEP. Also, we studied mice with an Scn1aR1407X/+ mutation to determine the role of respiratory dysfunction in postictal death after spontaneous and acutely induced seizures. In DS mice, death occurred after spontaneous, heat-induced, and MES seizures while monitoring in a mouse EMU. Death always occurred after a severe seizure with tonic extension. We found that both non-epileptic and epileptic mice have consistently died by the same primary mechanisms of central apnea. Death could be prevented after heat-induced and MES seizures by mechanical ventilation. We conclude that the primary cause of postictal death was central apnea that began during the seizures and induced secondary bradycardia due to the hypoxia, ultimately leading to terminal asystole.
The final goal of this dissertation is to propose a new alternative dietary therapy for DS patients. Some epilepsy children with refractory seizures, especially DS patients, have been able to reduce their seizures by following a strict high-fat and low-carbohydrate ketogenic diet (KD). Although the exact anti-epileptic mechanisms of KD diet are unknown, producing ketone bodies and creating ketosis have been widely believed to contribute to the anti-epileptic effects. Our collaborator (Toshi Kitamoto, PhD, Univ. of Iowa) has found that a diet containing milk whey was able to prevent seizures in Drosophila with an orthologous sodium channel mutation. Here we tested the effect of both KD and milk whey supplementation on DS mice. Two types of KD (with and without milk additives), KD with glucose water to eliminate ketone formation, milk whey supplementation, and standard diet were administered to growing DS mice age from P16 to P40. Compared to a standard diet, all KDs, KD with glucose water, and milk whey supplementation had beneficial effects on seizure control and prevention of postictal death. Compared to a standard diet, all KDs greatly elevated ketone body levels (β-hydroxybutyrate) and mice consistently weighed less, whereas the milk whey diet had no effect on ketosis or weight. KD with glucose water did not produce high ketone levels, but mice weighed less. These results demonstrate that ketone bodies are not the main reason for the anti-epileptic property of ketogenic diets.
Taken together, these data indicate it is important to obtain data on both cardiac and respiratory function to make conclusions about the mechanisms of SUDEP in both humans and animals. Data in this dissertation show that severe postictal respiratory dysfunction in DS patients may play a major role in causing SUDEP, and may be a biomarker for those at highest risk. Death in DS mice with spontaneous seizures may be directly related to the mechanism of SUDEP in humans. Defining the specific mechanisms of postictal apnea may help to identify methods for prevention of SUDEP. We propose a SUDEP prevention strategy in DS through a new alternative diet supplemented with a milk whey compound. Milk whey supplementation of diet has a great potential to prevent postictal death in an economical and non-invasive manner without detrimental metabolic outcomes, such as ketosis and weight reduction. Milk whey supplementation of diet may be a new treatment to prevent SUDEP in DS patients.