Measurement of interhemispheric time differences in generalised spike-and-wave.

We have compared interhemispheric time differences (ITD) calculated by coherence/phase and linear and non-linear correlation analyses of generalised spike-wave episodes in 30 patients (15 with primary generalised epilepsy (PGE), 5 with secondary generalised epilepsy (SGE) and 10 with a lateralised epileptogenic area). Most cases were recorded during routine departmental EEGs. No significant difference was found between measures of interhemispheric synchrony (IS) (proportion of ITDs that are synchronous) calculated using the linear and the non-linear correlation techniques, although the latter gave slightly more (average 7%) valid ITD estimates. This suggests that the non-linear correlation technique does not provide significantly more time difference information than its linear counterpart. Similarly, no significant differences in IS values were identified between the coherence/phase and the correlation techniques. For all 3 techniques, IS values greater than 50% were derived for most patients with PGE and SGE; the proportion of patients with a lateralised epileptogenic area showing this was smaller. However, broad overlap in the distribution of IS values between the 3 groups suggests that individual patients cannot reliably be distinguished using these methods.