Long QT Syndrome

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Clinical Features

• Episodes of syncope and asyncope may occur, possibly accompanied by seizures
• May be asymptomatic
• Patients with LQTS type 1 have symptoms with exercise and type 2 have symptoms with emotion triggers

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Risk Factors

• Most cases in the general population are acquired but in the case of athletes, the main risk factor is genetics
• 17 known genetic causes of Long QT syndrome (as of 2019), the specific etiologies are broad and involve ion channel defects
• Acquired long QT is more common in the general population and may co-exist with congenital long QT syndrome in the athlete population
  • Certain medications can prolong the QT interval as well as hypokalemia, hypomagnesemia and hypothermia.

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Evaluation

• ECG evaluation is the first step to assessing long QT syndrome
  • The QT interval is calculated into the QTc (to adjust for heart rate) with one of several formulas, of which Bazzet's is the most common
  • A normal QTc is <440ms in men and <460ms in women but the upper limit of normal can be up to 470ms or 480ms.

• The Schwartz criteria exists to thoroughly assess Long QT syndrome.
  • QTc greater than 480ms = 3 points
  • QTc 460 to 479mc = 2 points
  • QTc 450 to 459 (male) = 1 point
  • QTc greater than 480ms after 4th minute of exercise = 1 point
  • Torsades de pointes = 2 points
  • T wave alternans = 1 point
  • Notched T wave in 3 leads = 1 point
  • Clinical bradycardia = 1 point
  • Syncope with stress or exertion = 2 points
  • Syncope at rest = 1 point
  • Congenital deafness = 0.5 points
  • Family members with diagnosed Long QT syndrome = 1 point
  • Unexplained sudden death under age 30 among immediate family members = 0.5 points

• Result evaluation
  • A score of less than or equal to 1 is equal to low probability
  • A score of 2 to 3 is equal to intermediate probability
  • A score greater than 3.5 is equal to a high probability of congenital long QT syndrome.

• Depending on the score and clinical suspicion, then genetic testing may be considered.

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Classification

• 17 types of Long QT syndrome exist
  • The first 3 types make up over 90% of cases.

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Management

• A beta blocker is commonly prescribed to diagnosed cases of Long QT syndrome
• Other options include sodium channel blocking drugs and for severe cases, an ICD.
• Supplementation of potassium and magnesium is common
• Avoiding exercise is highly debatable and is a decision best made between the patient and physician.
• Drug induced cases are best treated with discontinuation of the offending medication.

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Complications

• Long QT syndrome can lead to torsades de pointes, a type of polymorphic ventricular tachycardia
  • This is an unstable and often pulseless rhythm which can lead to ventricular defibrillation and sudden death
  • Patients who are silent carriers and have QTc <500ms are less likely to have episodes of sudden cardiac death

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