Congenital long-QT syndrome concealed by hypercalcemia in Williams Syndrome.

We report a case of gene-positive long-QT syndrome (KCNH2) in a patient with concomitant Williams Syndrome. The hypercalcemia that developed in association with Williams Syndrome pseudo-normalized the QTc interval on surface ECG, concealing the clinical and electrocardiographic manifestations of the disease. Initiation of medical therapy for hypercalcemia unmasked the prolonged QT interval, allowing for the diagnosis of long-QT syndrome to be made.