Purpose: To assess the prevalence and attributes of atypical language lateralization (ALL) in patients with left mesial temporal lobe epilepsy associated with hippocampal sclerosis (MTLE-HS).
Methods: We recruited consecutive patients with left MTLE-HS, who had undergone resective surgery and had pathologically proven HS. Based on the Wada test, language lateralization was classified into typical (left hemispheric) or atypical (right hemispheric or codominant). We assessed the attributes of patients with ALL using univariate and multivariate analyses.
Results: Of 124 patients with left MTLE-HS, 23 (18.5%) had ALL. ALL occurred more frequently in patients with severe initial precipitating injury (IPI), early onset of epilepsy, and a short latent period between IPI and onset of habitual seizures. ALL was more common in patients with bitemporal and extratemporal interictal epileptiform discharges (IEDs) on electroencephalogram (EEG) and extratemporal changes on magnetic resonance imaging (MRI). On multivariate analyses, the age at onset of habitual seizures <6 years, atypical IPI, nonunilateral temporal IEDs, and extratemporal MRI abnormalities independently predicted ALL. The likelihood of ALL was very low ( approximately 1%) when all of these four risk factors were absent, whereas it was very high (>95%), if any three or all four of them were present.
Conclusions: ALL occurs in one-fifth of patients with left MTLE-HS. ALL is more frequent in those with structural or functional extrahippocampal involvement and early onset of epilepsy interrupting the development of normal language networks. Because ALL is uncommon in those with damage/dysfunction restricted to the hippocampus, the hippocampus itself may have only a limited role in determining language lateralization.