Abstracts

Magnetoencephalography (MEG) may be used as a non-invasive tool for determination of hemispheric language dominance /delineating spatiotemporal patterns underlying language processing. Healthy right handed subjects are left hemisphere dominant for language in 94%, but a higher degree of atypical language representation was shown in patients (pts) with epilepsies in a previous fMRI study (1). We prospectively investigated HLD with MEG in 10 rh pts with medically intractable focal epilepsies.
10 pts underwent MEG language mapping during pre-surgical evaluation using whole-head 306-channel MEG/simultaneous 64-channel EEG. All pts were right-handed (Edinburgh Handedness Inventory). A visually presented verbal memory task presenting 240 stimuli, previously shown to activate language areas, was used. Equivalent Current Dipoles (ECD) based on a spherical head model were fitted for each hemisphere using sequential single dipole fitting with a time range of 150ms [ndash] 800ms and time step of 5ms. Only dipoles with a goodness of fit (Gof) of over 70% were considered for analysis. The Laterality Index (LI) was calculated for each pt using the formula (2):
LI = Number of (# )Dipoles L Hemisphere - # Dipoles R Hemisphere
Six out of 10 pts (60%) showed left hemispheric language dominance (HLD). Four pts showed right HLD (40%) (table 1). Five of the 6 left HLD pts were suffering from left hemisphere epilepsy (HE) one from right HE. One of the 5 pts with left HE previously showed an inconclusive Wada test result. The left HLD determined with MEG was confirmed with fMRI using a verb generation paradigm in this particular pt. Three of the 4 pts showing right HLD suffered from left HE and one suffered from right HE.
We found a higher percentage of atypical right-hemisphere language representation in pts with focal intractable epilepsies than would be expected in a population of neurologically normal controls. This suggests that epilepsy might play a role in reorganization of cortical language areas. MEG seems to be a promising method for the determination of hemispheric language dominance in pts with focal epilepsies. Data will be confirmed in a larger population and correlated with Wada test results.
LITERATURE
1. Springer J et al: Language dominance dominance in neurologically normal and epilepsy subjects: a functional MRI study. [italic]Brain[/italic] 1999;122:2033-46
2. Papanicolaou AC et al: Magnetoencephalographic mapping of the language-specific cortex. [italic]J Neurosurg[/italic] 1999;90:85-93[table1]
[Supported by: MIND Institute]