Complexity of ranolazine and phenytoin use in an infant with long QT syndrome type 3

Introduction Long QT syndrome type 3 (LQT3) results from gain-offunction mutations in the SCN5A gene, which encodes the major cardiac sodium channel, voltage-gated type V alpha subunit (NaV 1.5). Those mutations result in an increase in late sodium channel current, which leads to delayed ventricular repolarization, torsades de pointes (TdP), and sudden death. Traditionally, beta-blockers mexiletine and phenytoin have been the agents of choice in the management of LQT3. Ranolazine is a novel antianginal drug that has been shown to have multichannel blocking effects, including the late sodium channel current (INa,L) and the rapid delayedrectifier potassium current (IKr). The extended-release formulation of ranolazine has a half-life of 7 hours. It has been used in adults with LQT3 ; however, there are currently no published data on its use in the pediatric population. Data have shown that ranolazine is more effective in inhibiting the late vs the peak Naþ current in LQT3 caused by SCN5A mutations. We theorize that use of ranolazine in an infant with LQT3 and persistent TdP refractory to multiple medications would be effective. We report that ranolazine use in infants is very difficult and found that phenytoin was the most effective agent in our patient.