the outflow of the left ventricle, creating a functional obstruction In addition, anterior motion of the mitral valve into the left ventricle during systole further compromises outflow Patients may present with dyspnea, exercise intolerance, angina, syncope, or sudden death Adult congenital heart disease survivors are at risk for syncope because of the underlying cardiac condition and/or previous palliative/corrective surgeries These patients can present with heart failure, arrhythmias, or pulmonary hypertension It is important to exclude atrial arrhythmias in patients with syncope and congenital heart conditions or surgeries associated with a risk of atrial rhythm abnormalities (e.g., tetralogy of Fallot, Ebstein anomaly, and the Mustard, Senning, and Fontan procedures) Other Arrhythmias Supraventricular tachycardia is the most common symptomatic pediatric tachyarrhythmia, and syncope can theoretically result from compromised cardiac output, though such presentations are rarely seen First-degree heart block may be an incidental finding in patients with syncope However, second- and third-degree heart block need further investigation Search for evidence of myocarditis, cardiomyopathy, or congenital heart disease when such arrhythmias are observed Conduction disturbances are common after cardiac surgery Patients who have undergone correction of tetralogy of Fallot, aortic stenosis, and transposition of the great arteries may be particularly prone to syncope Ventricular arrhythmias may occur as a consequence of surgeries involving incision to the ventricles Rarely, direct blunt trauma to the chest (commotio cordis) may cause ventricular arrhythmias leading to syncope or sudden death OTHER CONDITIONS AND THOSE THAT MIMIC SYNCOPE There are several other conditions that may cause or mimic syncope The most frequent of these are seizures and migraine The rest are less frequent but still important conditions Hypoglycemia Low blood sugar is usually associated with feelings of weakness, diaphoresis, lightheadedness, and confusion that can mimic presyncope Infants may present with lethargy or jitteriness Diagnosis is rapidly established by obtaining a blood glucose level Epilepsy