• Percent reduction from baseline in primary generalized tonic-clonic seizure rates and percent responders (>=50% reduction in PGTC seizure rate from baseline), during the double-blind phase. Subjects' global evaluation of improvement in seizure severity.
  

• Percent reduction from baseline of all seizure types and percent of treatment responders in all seizure types, during the double-blind phase. Safety evaluations conducted throughout the study.
  

Epilepsy is a disease characterized by seizures, which are abnormal electrical discharges in the brain that temporarily disrupt normal brain function. Seizures are classified as "generalized," involving all or most of the brain at the same time, or "partial onset," starting in one area of the brain. Generalized tonic-clonic seizures are also referred to as grand mal seizures and are common in people with generalized epilepsy in which the cause is not known. In a tonic-clonic seizure, the person loses consciousness, the body stiffens (tonic phase), and then the individual falls to the ground. This is followed by jerking movements in which the muscles contract and relax quickly (clonic phase). After a minute or two, the jerking movements usually stop, and the person regains consciousness. Antiepileptic medications, such as topiramate, are selected based on a patient's seizure type. Topiramate is a drug that is currently widely used for the treatment of seizures in adults and pediatric patients (2 to 16 years of age). This is a randomized, double-blind, parallel-group, placebo-controlled study to evaluate the effectiveness and safety of topiramate as an add-on therapy in patients with Primary Generalized Tonic-Clonic (PGTC) seizures. The study is in two phases: baseline (8 weeks) and double-blind treatment (20 weeks). Patients are given diaries to record information about their seizures during the phases of the study. During the baseline phase, the patient continues to receive a constant dosage of one or two antiepileptic drugs they have been taking. In the double-blind phase, patients are randomly assigned to either topiramate or placebo. The double-blind phase is divided into two periods: titration, in which the topiramate dose is gradually increased (8 weeks) (patient's antiepileptic medication continues; this dose remains the same) and stabilization (12 weeks). The dose of both topiramate and the patient's antiepileptic drug remain constant during the stabilization period. Based on the investigator's judgment, patients completing the double-blind treatment could enter a long-term extension phase of the study to continue treatment. The primary assessment of effectiveness is the percent reduction in primary generalized tonic-clonic seizure rates from baseline to the double-blind phase. Safety assessments include the frequency of adverse events during the study, results of clinical laboratory tests (hematology, biochemistry, and urinalysis), measurements of vital signs and body weight, physical examination and electrocardiogram (ECG) findings, plasma levels of topiramate and other study antiepileptic drugs, and neurological examinations. The study hypothesis is that topiramate as an add-on is superior to placebo in reducing the seizure rate from baseline to the double-blind phase of the study. Topiramate (25 mg or 100 mg tablets) or placebo, taken by mouth, starting at a dose of 25 or 50mg/day, gradually increasing to a maximum daily dose of 175 mg to 400 mg (based on body weight) or to a maximum tolerated dose (whichever dose is less). Maximum dosage continues for 12 weeks.