Abstract Body: Background
Congenital long QT syndrome (LQTS) encompasses a heterogenous group of conditions with two-thirds of cases due to LQT1 and LQT2, while a small minority are due to LQT3. While variations exist in baseline characteristics amongst patients with LQT3, young age and increased QTc length remain critical predictors of first cardiac event.

Case
A 69-year-old male with history of hypertension and premature ventricular complexes (PVCs) presented after a syncopal event during a work argument. Review of outpatient cardiology records revealed a recent electrocardiogram with normal sinus rhythm (NSR) and QTc of 458 ms. Recent holter monitor showed 1.84% PVC burden without sustained arrythmias. He was previously treated with sotalol for PVCs but stopped this 6 months prior due to limited effectiveness and was switched to metoprolol. In the ED, laboratory studies were notable for normal potassium of 4.6 mEq/L, calcium 8.8 mg/dL, and magnesium 1.7 mg/dL. An EKG showed NSR with QTc of 523 ms. He suddenly developed substernal chest pain, and EKG showed polymorphic ventricular tachycardia (VT). He was given four grams of magnesium with resolution of VT, but QTc remained > 500 ms. Transthoracic echocardiogram, left heart catheterization, and cardiac MRI were without abnormalities.

Decision-Making
This patient with a syncopal event and polymorphic VT had no identifiable electrolyte disturbances on labs and no evidence of ischemia or cardiomyopathies on further testing. Though he had previously taken sotalol, he was not currently on any QT prolonging agents. Despite the lack of prolonged QTc intervals on outpatient EKGs, the presence of persistent QTc prolongation before and after polymorphic VT without clear provoking factors raised suspicion for LQTS. He was started on mexiletine, an ICD was placed, and genetic testing was sent which revealed a mutation in SCN5A consistent with LQT3.

Conclusion
This case highlights an atypical presentation of LQT3 characterized by a history of normal QTc duration on EKGs and age of first cardiac event later in life. This patient’s presentation underscores the variability in phenotypic expression and limitations of relying on baseline QTc duration, which can lead to diagnostic delays in care. Genetic testing is essential in confirming the diagnosis in patients with atypical presentations.