Long QT Syndrome is one of the unrecognized killers diagnosable on ECG. It is one of the causes of sudden cardiac death, and can present as a family history or personal history of syncope, seizure or arrhythmia, or can present incidentally on ECG. There are multiple acquired precipitants, including many medications.
This disorder, along with hypertrophic cardiomyopathy and Brugada syndrome are sometimes recognized on ECG in asymptomatic patients.