An Open-Label Extension of the Study XEN496 (Ezogabine) in Children With KCNQ2-DEE

Inclusion Criteria: * Subject completed participation in the primary study, XPF-009-301. A subject who withdraws from the primary study due to meeting protocol-specified worsening criteria will be considered as having completed participation in the primary study. * The caregiver is willing and able to be compliant with diary completion, visit schedule, and study drug administration. * Subject's caregiver achieved a minimum of 85% compliance with daily diary completion during both baseline and the double-blind period of the primary study. Exclusion Criteria: * Any adverse event(s) or serious adverse event(s) during the primary study XPF-009-301, which in the opinion of the investigator and sponsor's medical monitor, would preclude the subject's entry into the OLE study. * A clinically significant condition or illness, or symptoms other than those resulting from KCNQ2-DEE, present at screening/baseline that, in the opinion of the investigator, would pose a risk to the subject if s/he were to enter the study. * Any conditions that were specified as exclusion criteria in the primary study, XPF-009-301. * It is anticipated that the subject will require treatment with at least 1 of the disallowed medications during the study. * Any change in cardiac rhythm or atrioventricular conduction in the primary study that, in the investigator's opinion, is a significant risk to subject safety.