Atypical language localization in right temporal lobe epilepsy: An fMRI case report

Discordant Wada and fMRI language lateralization: a case report

Functional MRI (fMRI) is gaining importance in the preoperative assessment of language for presurgical planning. However, inconsistencies with the Wada test might arise. This current case report describes a very rare case of an epileptic patient who exhibited bilateral distribution (right > left) in the inferior frontal gyrus (laterality index [LI] = -0.433) and completely right dominance in the superior temporal gyrus (LI = -1). However, the Wada test revealed a dissociation: his motor speech was located in the left hemisphere, while he could understand vocal instructions with his right hemisphere. A clinical implication is that the LIs obtained by fMRI should be cautiously used to determine Broca's area in atypical patients; for example, even when complete right dominance is found in the temporal cortex in right-handed patients. Theoretically, as the spatially separated functions of motor speech and language comprehension (by the combined results of fMRI and Wada) can be further temporally separated (by the intracarotid amobarbital procedure) in this case report, these findings might provide direct support to Broca's initial conclusions that Broca's area is associated with acquired motor speech impairment, but not language comprehension per se. Moreover, this current finding supports the idea that once produced, motor speech can be independent from language comprehension.

Language lateralization differences between left and right temporal lobe epilepsy as measured by overt word reading fMRI activation and DTI structural connectivity

In cases of brain disease such as temporal lobe epilepsy (TLE), damage may lead to functional reorganization and a shift in language dominance to homolog regions in the other hemisphere. If the effects of TLE on language dominance are hemisphere-focused, then brain regions and connections involved in word reading should be less left-lateralized in left temporal lobe epilepsy (lTLE) than right temporal lobe epilepsy (rTLE) or healthy controls, and the opposite effect should be observed in patients with rTLE. In our study, functional magnetic resonance imaging (fMRI) showed that patients with rTLE had more strongly lateralized left hemisphere (LH) activation than patients with lTLE and healthy controls in language-related brain regions (pars opercularis and fusiform gyrus (FuG)). Corresponding with this difference, diffusion tensor imaging (DTI) found differences in connectivity indicative of patients with lTLE having greater tract integrity than patients with rTLE in the right hemisphere (RH) uncinate fasciculus (UF), inferior longitudinal fasciculus (ILF), and inferior fronto-occipital fasciculus (IFOF) using the network-based statistic analysis method. The UF, ILF, and IFOF tract integrity have previously been associated with lexical (whole-word) processing abilities. Multivariate distance matrix regression provided converging evidence for regions of the IFOF having different connectivity patterns between groups with lTLE and rTLE. This research demonstrates language lateralization differences between patient groups with lTLE and rTLE, and corresponding differences in the connectivity strength of the ILF, IFOF, and UF. This research provides a novel approach to measuring lateralization of language in general, and the fMRI and DTI findings were integral for guiding the neurosurgeons performing the TLE resections. This approach should inform future studies of language lateralization and language reorganization in patients such as those with TLE.