Shift-back of right into left hemisphere language dominance after control of epileptic seizures: Evidence for epilepsy driven functional cerebral organization

Abstract

Atypical, i.e. right hemisphere language dominance is frequently observed in early onset left hemisphere epilepsies. In left mesial temporal lobe epilepsy, where eloquent cortex is not directly involved, it is a matter of debate, to which degree atypical language dominance is driven not only by morphological lesions but also by epileptic dysfunction, and whether atypical dominance is hardwired or not. Taking this as the background this study evaluated the hypothesis that epilepsy driven atypical dominancy might be reversible when seizures are successfully controlled. This was evaluated in patients with left mesial temporal lobe epilepsy, who were atypically language dominant by means of language fMRI before surgery, and became seizure free after left selective amygdalo-hippocampectomy. Three out of 53 consecutive atypically dominant patients with chronic epilepsy fulfilled these criteria. Postoperative follow-up language fMRI indicated reversal of right into left dominance in one patient going along with unexpected losses in verbal memory performance. The two other patients experienced unchanged or even enhancement of the pre-existing dominance pattern, going along with consistent postoperative performance changes in cognition. The data thus provide supporting evidence that atypical language dominance can indeed be functionally driven and moreover that in at least some patients, right hemispheric language can shift-back to the left hemisphere when the driving factor, i.e. seizures, becomes successfully controlled. The results have clinical implications for outcome prediction after brain surgery in atypically dominant patients with epilepsy. However, further research in larger groups of atypically dominant patients is required to identify the conditions under which atypical dominance becomes hardwired and when not.