Stability of source estimates in rolandic spikes.

The interictal spike discharges present in rolandic epilepsy has a dominant horizontal dipolar topography, centered near the rolandic area. In order to examine the generator configuration of this focus, we investigated the variation of spike topography in 20 children with rolandic epilepsy of childhood by the dipole localization method (DLM). A quantitative measure ("stability index" or SI) of the degree of source fluctuation was devised, based on the consistency of the source parameters (location, direction and magnitude) over contiguous time points. A high SI was associated with overlapping source locations and parallel directions over many time points, while a low SI was seen with poor and variable solutions. At the peak and trough of the spike, the corresponding sources were found to have different locations. If the patients were separated into those with and without neurological findings, the mean times at which stable sources existed were significantly different: 35 and 150 ms respectively after the spike apex. Further, the group without abnormal neurological findings tended to have higher values of SI. These results suggest that such stability analysis allows measurement of the spatial and temporal extents of source estimates. These results further support the hypothesis that the topographic behaviour of a spike focus is closely associated with its clinical characteristics.