Abstracts

Rationale: In response to median nerve stimulation, patients with intractable epilepsy demonstrate a consistent 20ms response. This response is a robust finding in patients with epilepsy, brain tumors, AVM, infarcts and subdural hematomas. We sought to examine the responses in patients with intractable epilepsy to determine whether they showed a consistent pattern of activation or demonstrated variability possibly due to factors such as age, age at onset, duration of epilepsy, or location of the epileptogenic zone.Methods: Thirty-seven patients with intractable epilepsy who were being considered for epilepsy surgery underwent MEG to assist in identifying the epileptogenic zone: 19 were felt to have seizures lateralized to the left hemisphere. All cases ranged in age from 18 to 63. The groups (Left and Right) did not differ in terms of age, age at onset or duration of epilepsy. For median nerve stimulation, a small electrical current was applied in an alternating fashion to the right and left hand until a twitch was perceived, with an inter-stimulus interval of approximately 371ms. Results: Prior research (Kawasura et al., 1995) has noted that the response at 20ms usually has an anterior orientation, and then reverses to a posterior orientation. Following this phase reversal, the next activation may or may not be anterior, and if there is a fourth activation, it may or may not be posterior. Within the first 100ms following stimulation, the patients demonstrated a number of different cortical patterns of activation. Using a pattern consisting of four cortical responses, the most frequent response pattern was an initial 20ms response in the anterior orientation with the three following responses in the posterior orientation and with no further phase reversals. A smaller subset of the sample showed the initial anterior orientation, but different patterns were seen in the remaining responses. A Chi square analysis revealed that the distribution of response patterns was highly significant. Conclusions: Patients with intractable epilepsy may show an atypical cortical pattern of response to median nerve stimulation. The most prevalent pattern of response in this study may be the expected response to median nerve stimulation, but the variability observed within the sample indicates that other factors may influence the pattern. This study was limited by the small sizes of the groups, and more participants are needed to improve statistical power within each group. The use of a control group would allow us to determine what patterns of response are more likely in participants who do not have epilepsy and what factors may influence the response pattern.