Epileptic signatures - outside of epileptic seizures

Epileptic activity in the brain is characterized by recurring high amplitude signals in the electroencephalogram (EEG), or in recordings of local field potentials. This “signature” of neuronal activity is very different from activity found under healthy conditions. Between epileptic episodes, however, electrophysiological recordings of neuronal activity may appear to be completely normal. Signs of permanent pathological network conditions, however, should be found also in activity between epileptic episodes. The same holds true for neuronal activity leading to epileptic states. Researchers from Freiburg have now shown that the coupling of brain structures in activity between epileptic episodes is already measurably different from the coupling under healthy conditions.

In research published in the  journal Epilepsia, Ulrich Froriep and his colleagues from the Bernstein Center, the Neurocenter of the University Medical Center and the IMTEK in Freiburg made use of a mouse model of temporal lobe epilepsy to search for such signatures. These mice, which are artificially turned epileptic, reproduce several aspects of epilepsy that are found in humans. This comprises, for example, recurring epileptic activity and characteristic cell loss at different sites of the hippocampus, a region that is frequently associated with mesiotemporal lobe epilepsy. This particular class of epilepsy is often resistant to pharmacological treatment in humans.

In healthy mice, activity in a prominent frequency range called ‘Theta band activity’ (4-8 Hz) was synchronized across subfields of the hippocampus and the area from which these nerve cells receive their input, the so-called entorhinal cortex. In contrast, in epileptic mice the timing of oscillations between the hippocampus and the entorhinal cortex was conspicuously shifted. Otherwise, these periods of brain activity seemed completely normal.
Using a computational model of these neural networks, Froriep and his co-authors could relate this result to cell loss in a small region of the hippocampus.

While their study used mice, further analyses of the interaction between different regions of coupled networks during these periods might prove useful to diagnose epilepsy in human patients.
 

Original publication:

Ulrich P. Froriep, Arvind Kumar, Delphine Cosandier-Rimélé, Ute Häussler, Antje Kilias, Carola A. Haas and Ulrich Egert (2012) Altered theta coupling between medial entorhinal cortex and dentate gyrus in temporal lobe epilepsy. Epilepsia (Early View) DOI: 10.1111/j.1528-1167.2012.03662.x

Image caption:

These cross-correlograms show the synchronization state of entorhinal cortex and hippocampus in epileptic (orange) and healthy (blue) mice. While the structures would be synchronous under healthy conditions, indicated by the peaks around 0 ms lag, this relation is shifted by ~25 ms in the epileptic case.