Do ictal slow waves correlate with forgetting?

Many patients with epilepsy suffer from memory dysfunction. Are the seizures the reason for poor memory function? Studies investigating the long-term impact of seizures on memory conclude that in most epilepsy syndromes it is unlikely that memory performance gets worse over time. But there is an exception: Frequently occurring seizures of a specific form, so-called clonic-tonic seizures, over a long interval of time are likely to damage the patient`s brain. In addition, patients with seizures originating in the left part of the brain showed increased forgetting of contents when a seizure occurred between learning and recall. Slow brain activity correlates with loss of consciousness during epileptic seizures but also with disturbed memory functions in seizures without loss of consciousness. We think that slow activity during and after epileptic seizures could be a correlate of a disturbing mechanism in the consolidation of memory entries. To test this assumption, we need to control for other factors which could jeopardize or sustain memory processes, such as the beneficial effect of sleep on memory, epileptiform events that occur between seizures, and mechanisms that occur during encoding, consolidation, or retrieval of memory entries. Controlling these and other factors is possible in a video-EEG-monitoring unit, where patients are observed and evaluated for a week on 24 hours per day. We will assess 100 patients with surface recordings of electric brain activity (the electroencephalogram, EEG) and 20 patients with invasive recordings (i.e., electrodes implanted into the patient`s brain) by a schedule of 6 repeated memory tests including 3 days and 3 nights. We will test if properties extracted from the EEG during and between the eventually occurred seizures are related to memory performance. The results of this study are not only relevant for each patient`s perspective on the impact of the seizures on cognition. The results could also serve as a justification for therapies and incubate new theories about the relationship between memory consolidation and states of unconsciousness such as seizures or sleep.

Memory of patients with epilepsy is often impaired. Could seizures be the cause of these memory problems? Long-term studies found that for most patients there is no general decline of memory because of seizures, unless patients experienced a high number of tonic- clonic seizures over a long period of time. Short-term effects of seizures in patients with temporal lobe epilepsy suggest that their memory for words is impaired when a seizure occurs between learning and remembering. During seizures with loss of consciousness it is possible to detect slow brain activity by recording the electroencephalogram (EEG). These slow waves (max. 4 Hz) are also a characteristic sign of deep sleep - the time when we consolidate memory. But they are also present when humans are in a state of disordered consciousness, such as deep sleep, unresponsive wakefulness syndrome, or the minutes after a tonic-clonic seizure. We assumed that these slow waves could be a sign for processes that impair memory. In order to test this assumption we examined 119 patients in the video-EEG monitoring unit of the University clinic for Neurology Salzburg. On four consecutive days we examined the patients twice a day. We examined motor learning, verbal memory, and episodic memory, which is the memory for first-person experiences. Episodic memories were generated by immersing participating patients into a specially developed virtual town on a computer. We examined whether seizures, and specifically those which exhibit slow waves in the EEG, would disturb retention of memory. In contrast to our expectations, patients did not perform worse in motor learning, verbal memory, or episodic memory with seizures, with or without slow waves. However, motor learning benefitted from sleep if no seizures occurred during sleep. Additionally, we found an interesting interaction between seizures and antiepileptic dugs. In general, antiepileptic drugs had a negative effect on episodic memory. This result suggests that the effect of antiepileptic drugs is larger than the effect of seizures. However, when patients experienced seizures while the dose of antiepileptic medication was low, the memory performance was lower than when patients experienced seizures and dose of antiepileptic medication was high. This result suggests that possibly a higher readiness for seizures affects learning and remembering. There exist numerous different antiepileptic drugs. Future research should examine whether this interrelation between seizures and drug load is stronger for some drugs than for others.