The main purpose of this study is to evaluate the effectiveness of the study drug lacosamide (200-600 mg/day) when added to a stable dose of levetiracetam (1000-3000 mg/day) with withdrawal of the concomitant sodium channel blocking-antiepileptic drug (AEDs) in subjects not well controlled on their current regimen.

Inclusion Criteria:

• Subject is male or female, at least 18 years of age
• Subject has a diagnosis of epilepsy with partial-onset seizures according to the International Classification of Epileptic Seizures (1981)
• Subject is taking levetiracetam (LEV) in combination with 1 sodium channel blocking antiepileptic drug (defined as carbamazepine, lamotrigine, oxcarbazepine, phenytoin, or eslicarbazepine) as adjunctive treatment for epilepsy
• The minimum required seizure frequency during the 8-week Retrospective Seizure Baseline is on average ≥ 2 partial-onset seizures per 28 days with at least 1 seizure per 4 week period within the 8-week Retrospective Seizure Baseline. Additionally, subjects must experience at least 1 seizure during the 4-week Prospective Seizure Baseline
• Subject has been maintained on a stable dose of LEV and a sodium channel blocking antiepileptic drug (SCB-AED) for at least 4 weeks prior to the Screening Visit (Visit 1) and during the 4-week Prospective Seizure Baseline
• The minimum required seizure frequency during the 8-week Retrospective Seizure Baseline is on average ≥ 2 partial-onset seizures per 28 days (based on investigator assessment of subject report) with at least 1 seizure per 4 week period within the 8-week Retrospective Seizure Baseline
• Subject has been maintained on a stable dose of levetiracetam (LEV) and a sodium channel blocking antiepileptic drug (SCB-AED) for at least 4 weeks prior to the Screening Visit (Visit 1) and during the 4-week Prospective Seizure Baseline, with or without additional concurrent stable vagal nerve stimulation (VNS). The VNS must have been in place for at least 6 months prior to the Screening Visit (Visit 1) with constant settings for at least 4-weeks prior to the Screening Visit (Visit 1) and throughout the duration of the study

Exclusion Criteria:

• Previous use of lacosamide
• History of alcohol or drug abuse
• History of seizure disorder characterized primarily by isolated auras
• History of primary generalized seizures
• History of status epilepticus within the 12-months
• History of clustering seizures
• Nonepileptic events, including pseudoseizures that could be confused with seizures
• History of any medical or psychiatric condition that, in the opinion of the investigator, could jeopardize the subject's health or would compromise the subject's ability to participate in this study
• Lifetime history of suicide attempt, or suicidal ideation in the past 6 months
• Hypersensitivity to any component of lacosamide (LCM)
• History of acute or sub-acute progressive central nervous system disease
• History of severe anaphylactic reaction or serious blood dyscrasias
• Impaired renal function (ie, Creatinine Clearance (CLcr) is lower than 30 mL/min) at Visit 1
• History of sick sinus syndrome without a pacemaker, or atrioventricular (AV) block, or subject has any other clinically significant electrocardiogram (ECG) abnormalities
• History sodium channelopathy, such as Brugada syndrome
• History of myocardial infarction in the last 3 months