• Hereditary prolongation of the Q-T interval is associated with ventricular dysrhythmias. Children with hereditary prolongation of the Q-T interval may have symptoms that seem related to a neurologic abnormality. Two families had members who had been treated with phenytoin sodium for seizures and had died suddenly when the medication was discontinued. Children In both families were found to have prolongation of the Q-T interval. A history of seizures or of seizures in the family may be an indication of hereditary prolongation of the Q-T interval.