Evaluating efficacy & safety of lacosamide versus Placebo in a blinded fashion as add-on Therapy for Primary Generalized Tonic-clonic (PGTC) seizures in subject 4 years of age with idiopathic generalized epilepsy currently taking 1 to 3 antiepileptic drugs. Maximum duration of study drug administration is 28 weeks. Eligible subjects may choose to enter the open-label extension study after completion.