Decreased inhibitory neuronal activity in patients with frontal lobe brain tumors with seizure presentation: Preliminary study using magnetoencephalography

Background: Although 30-50 % of patients with brain tumors experience epileptic seizure as the presenting clinical symptom, and another 10-30 % are at risk for developing epilepsy in the later stages of the disease, the mechanisms of tumor-related epileptogenesis are poorly understood. We used magnetoencephalography (MEG) to investigate sensory evoked fields (SEFs) in patients with frontal lobe brain tumors as a means of evaluating the neuronal activity of peri-tumoral cortex. Methods: Twelve patients with frontal lobe brain tumors underwent MEG. We calculated the equivalent current dipole strength of two components of the primary sensory cortical response (N20m and P35m) and compared the P35m/N20m ratio in the tumor hemisphere vs. the normal hemisphere. There were two subsets of patients: group I, in which P35m/N20m was higher in the tumor hemisphere (n = 7), and group II, in which P35m/N20m was higher in the normal hemisphere (n = 5). We looked for associations between clinical factors and P35m/N20m within each group. Results: All patients with seizure presentation were in group I, whereas only two patients without seizure presentation were in group I (Fisher exact test, p = 0.028). No other clinical factors were related to P35m/N20m. The mean ratio of P35m/N20m equivalent current dipole strength in patients with seizure presentation was 4.07 ± 2.38 in the tumor hemisphere and 2.00 ± 0.55 in the normal hemisphere. This difference was statistically significant (Mann-Whitney test, p = 0.030). Conclusion: The paradoxical increase in P35m/N20m in patients with seizure presentation suggests that decreased inhibitory neuronal activity is a potential cause of tumor-related epilepsy.