Eloquent and epileptogenic brain: advantages of navigated brain stimulation in focal neocortical epilepsy?

Quality of presurgical evaluation in focal extratemporal epilepsy surgery is highly dependent on precise structural and functional identification of the epileptic focus. Navigated brain stimulation (NBS), which combines the spatial information of MRI and the functional possibilities of cortical stimulation, might improve reliability and cost-effectiveness by complementing existing preoperative procedures. In this study we suggest that NBS can identify and locate eloquent and epileptogenic areas of the brain.

In the rare case of an 8-year-old boy with intractable focal epilepsy, accompanied by somatosensory auras and motor seizures as well as epilepsia partialis continua (EPC), we performed NBS-guided transcranial magnetic stimulation (TMS) over both hemispheres. EMG was recorded from the first dorsal interosseus, abductor pollicis brevis and abductor digiti minimi muscles of both hands. The muscle evoked potential (MEP) amplitude was defined by peak-to-peak amplitude measures from belly-tendon surface recordings.

One singular MEP response could be elicited in the unaffected hemisphere, whereas the affected hemisphere showed extensive MEPs even after a 50% reduction of maximum stimulator output. Latency stratification differentiated responses of motor from sensory origin.

We argue that NBS can identify an epileptogenic zone with high spatial precision over an inconspicuous cortex. The results are in line with the subject's clinical presentation and with a transient pathology found two years prior to presentation. This implies that NBS might be a promising new method to identify eloquent and epileptogenic areas in presurgical evaluation.