Guideline cutoffs
International consensus guidelines differ on the degree of QT prolongation required to diagnose LQTS. The European Society of Cardiology recommends that, with or without symptoms or other investigations, LQTS can be diagnosed if the corrected QT interval is longer than 480ms. They recommend that a diagnosis can be considered in the presence of a QTc of greater than 460 ms if unexplained syncope has occurred. The Heart Rhythm Society guidelines are more stringent, recommending QTc cutoff of greater than 500 ms in the absence of other factors that prolong the QT, or greater than 480 ms with syncope. Both sets of guidelines agree that LQTS can also be diagnosed if an individual has a Schwartz score of greater than 3 or if a pathogenic genetic variant associated with LQTS is identified, regardless of QT interval. Treatment
Those diagnosed with LQTS are usually advised to avoid drugs that can prolong the QT interval further or lower the threshold for TDP, lists of which can be found in public access online databases. In addition to this, two intervention options are known for individuals with LQTS: arrhythmia prevention and arrhythmia termination. Arrhythmia prevention
Arrhythmia suppression involves the use of medications or surgical procedures that attack the underlying cause of the arrhythmias associated with LQTS. Since the cause of arrhythmias in LQTS is early afterdepolarizations (EADs), and they are increased in states of adrenergic stimulation, steps can be taken to blunt adrenergic stimulation in these individuals. These include administration of beta receptor blocking agents, which decreases the risk of stress-induced arrhythmias.
LQTS is suggested if the QTc is longer than these cutoffs. However, as 5% of normal people also fall into this category, some suggest cutoffs of 470 and 480 ms for males and females respectively, corresponding with the 99th centiles of normal values.The major subtypes of inherited LQTS are associated with specific ECG features. LQT1 is typically associated with broad-based T-waves, whereas the T-waves in LQT2 are notched and of lower amplitude, whilst in LQT3 the T-waves are often late onset, being preceded by a long isoelectric segment. Schwartz score
The Schwartz score has been proposed as a method of combining clinical and ECG factors to assess how likely an individual is to have an inherited form of LQTS. The table below lists the criteria used to calculate the score. Other investigations
In cases of diagnostic uncertainty, other investigations may be helpful to unmask a prolonged QT. In addition to prolonging the resting QT interval, LQTS may affect how the QT changes in response to exercise and stimulation by catecholamines such as adrenaline. Provocation tests, in the form of exercise tolerance tests or direct infusion of adrenaline, can be used to detect these abnormal responses. These investigations are most useful for identifying those with concealed congenital Type 1 LQTS 1 (LQT1) who have a normal QT interval at rest. While in healthy persons the QT interval shortens during exercise, in those with concealed LQT1 exercise or adrenaline infusion may lead to paradoxical prolongation of the QT interval, revealing the underlying condition.