How do the temporal lobes communicate in medial temporal lobe seizures?

Introduction. Spatial and temporal patterns of the spread of partial epileptic seizures depend on the site from which they originate. Characterising seizure propagation patterns may help to better define the seizure focus. In medial temporal lobe epilepsies, seizure propagation to the contralateral temporal lobe is especially studied. State of the art. Intracranial EEG records permit more precise definition of patterns of contralateral propagation in medial temporal lobe epilepsies, Several pathways have been implicated, sometimes differently in distinct studies, in propagation to the contralateral temporal lobe: the three commissures (the corpus callosum, the ventral hippocampal commissure and the anterior commissure, which link the temporal lobes) and an indirect circuit via the frontal lobes. Delays measured for contralateral propagation of a seizure of temporo-limbic origin vary significantly around a mean value of about 30 seconds. This slow spread may depend in part on the relatively small size of human commissural projections from the amygdalo-hippocampal formation, which largely originate in the presubiculum. However, a larger commissural projection pathway originates in the paralimbic medial temporal lobe and electrical stimulation of the anterior perirhinal cortex can induce contralateral epileptic discharges with delays as short as 100 ms (Adam et al., 2004). Thus, seizure activity emerging from anterior paralimbic regions can propagate rapidly to the contralateral medial temporal lobe via the anterior commissure. Perspectives. While the propagation of medial temporal lobe seizures is still debated, further studies are merited since the extent and speed of spread governs the electro-clinical semiology of seizures and our ability to identify their initiation site. Conclusions. We review anatomical (Demeter et al., 1990) and neurophysiological (Wilson et al., 1990) data for a dual inter-temporal propagation of medial temporal lobe seizures at different speeds and via different pathways.