A Study to Assess the Safety and Efficacy of Lacosamide Versus Placebo (a Pill Without Active Medication) in Patients With Idiopathic Generalised Epilepsy Who Are Already Taking Anti-epileptic Medications

Inclusion Criteria: * Subject with a confirmed diagnosis at least 24 weeks prior to Visit 1 and a disease onset prior to 30 years of age, consistent with idiopathic generalized epilepsy (IGE) experiencing primary generalized tonic-clonic (PGTC) seizures (Type IIE) that are classifiable according to the International League Against Epilepsy (ILAE) Classification of Epileptic Seizures (ILAE, 1981) * Subject has \>=3 PGTC seizures during the 16-week Combined Baseline (12-week Historical Baseline plus 4-week Prospective Baseline) * If a brain magnetic resonance imaging (MRI)/computed tomography (CT) scan has been performed, there must be no evidence of any progressive abnormality or any lesion likely to be associated with partial-onset seizures * Subject has been maintained on a stable dose regimen of 1 to 2 non-benzodiazepine marketed antiepileptic drugs (AEDs) with no benzodiazepine AEDs OR 1 benzodiazepine marketed AED with 1 to 2 non benzodiazepine marketed AEDs for at least 28 days prior to Visit 1 with or without additional concurrent stable vagus nerve stimulation (VNS) * Subjects are required to have had an electroencephalogram (EEG) report consistent with idiopathic generalized epilepsy (eg, generalized 3Hz epileptiform discharges and a normal EEG background) confirmed by a Central Reviewer Exclusion Criteria: * Subject is receiving any investigational drugs or using any experimental devices in addition to Lacosamide (LCM) * Subject meets the withdrawal criteria for SP0982 or is experiencing an ongoing serious adverse event (SAE) * Subject has an active suicidal ideation as indicated by a positive response ('Yes') to either Question 4 or Question 5 of the 'Since Last Visit' version of the Columbia-Suicide Severity Rating Scale (C-SSRS) * Subject has \>=2x upper limit of normal (ULN) of any of the following: alanine aminotransferase (ALT), aspartate aminotransferase (AST), alkaline phosphatase (ALP), or \>ULN total bilirubin (\>=1.5xULN total bilirubin if known Gilbert's syndrome). If subject has elevations only in total bilirubin that are \>ULN and \<1.5xULN, fractionate bilirubin to identify possible undiagnosed Gilbert's syndrome (ie, direct bilirubin \<35%) For randomized subjects with a Baseline result \>ULN for ALT, AST, ALP, or total bilirubin, a Baseline diagnosis and/or the cause of any clinically meaningful elevation must be understood and recorded in the electronic case report form (eCRF). If subject has \>ULN ALT, AST, or ALP that does not meet the exclusion limit at screening, repeat the tests, if possible, prior to dosing to ensure there is no further ongoing clinically relevant increase. In case of a clinically relevant increase, inclusion of the subject must be discussed with the Medical Monitor. Tests that result in ALT, AST, or ALP up to 25% above the exclusion limit may be repeated once for confirmation. This includes re-screening.