Subjects will repeat the lactate infusion procedure as described in Visit 2. At the conclusion of the second lactated infusion, subjects who have responded to treatment with levetiracetam will be offered up to 3 months continued treatment with levetiracetam and office visits at no charge until subjects are appropriately referred for continued follow-up treatment. Subjects not responding to levetiracetam treatment will be offered treatment with an FDA-approved medication for panic disorder.