Background: Lennox-Gastaut syndrome (LGS) is a severe childhood epileptic syndrome with high pharmacoresistance. The treatment outcomes are still unsatisfied. The investigator previous study of cathodal transcranial direct current stimulation (tDCS) in children with focal epilepsy showed significant reduction in epileptiform discharges. The investigator hypothesized that cathodal tDCS when applied over the primary motor cortex (M1) combined with pharmacologic treatment will be more effective for reducing seizure frequency in participants with LGS than pharmacologic treatment alone.