Absence seizures occur most commonly in children as staring spells lasting 5-10 seconds, with rhythmic ?spike-wave? discharge (SWD) on electroencephalography (EEG). They can occur up to hundreds of times per day and are not benign, with deficits in attention and psychosocial function in some cases persisting into adulthood or even after seizure suppression. The mechanisms by which absence seizures impair cognition are not known. One intriguing but little-studied aspect of absence seizures is the fact that some episodes impair and others spare behavioral responses even within the same individual. The relationship between variable absence behavioral severity and neuronal activity may provide fundamental insights into the pathophysiology of seizures. Prior human studies and animal models have shown widespread EEG and fMRI increases as well as decreases during absence seizures. We recently found in a large patient sample that absence seizures with more severely impaired behavior had larger fMRI and EEG amplitude in widespread brain networks. We also found that abnormally enhanced fMRI synchrony persists between bilateral cortical regions even when seizures are not present in patients and in animal absence models. The neuronal basis for these changes both at rest and during SWD is not known, but our recent work in rodent absence models and normal conditions suggests that the amplitude of fMRI signals is related to changes in the total population activity of neurons. Therefore our central hypothesis is that the severity of absence seizures is determined by a combination of the number of neurons involved and their firing pattern in widespread brain networks before and during seizures. An important limitation of previous work has been anesthetic agents, which markedly alter fMRI responses and the excitability of neurons. This may explain why most animal models show cortical fMRI increases during SWD whereas human studies show a predominance of sustained cortical fMRI decreases. Because of this discrepancy the neuronal basis of physiology changes in absence seizures remains uncertain. We recently habituated genetic absence epilepsy rats of Strasbourg (GAERS) to allow awake-head fixed experiments. Initial measurements in this new model show sustained cortical fMRI and CBF decreases during SWD much more closely resembling humans. We now plan complementary high spatiotemporal resolution experiments in this awake model including fMRI, electrophysiology and behavior to fully understand the neuronal basis of variable severity in absence seizures.
Our aims are to first image the networks involved at baseline and during severe versus mild SWD, and to relate the neuroimaging changes to spike and wave amplitude on EEG. Second, we will use multiunit and neuronal ensemble recordings to determine the neuronal basis of severe versus mild SWD. Third, we will relate the physiological severity of SWD to behavior through auditory detection and response tasks. The integration of information across these levels will increase our understanding of neuronal changes in absence epilepsy potentially leading to new treatment options.
Absence epilepsy causes significant cognitive and behavioral impairment during seizures which may persist even when seizures are not occurring. The severity of absence seizures varies, and understanding the neuronal basis for this variable severity will guide the development of novel targeted treatments to prevent seizures and improve long-term cognition and behavior.
