Long QT Syndrome is thought to be the cause when sudden death occurs from ventricular fibrillation- a fatal abnormal heart rhythm, or arrhythmia- and no other explanation is found. The syndrome can also become apparent when a young person with fainting or secondary seizures has an ECG as part of their evaluation. It is especially important to look for the syndrome when the fainting or seizures are excercise-related. However, the symptoms can occur during less strenuous activities and even on awakening from sleep... Prolongation of the QT interval on the ECG is the major marker for Long QT Syndrome. The majority of the QT interval is the time during a single heartbeat when the heart muscle is becoming "recharged" for the next heartbeat. The ECG records the electrical signals from the heart muscle and each wave is labeled as shown. 
 

Measuring the distance from the beginning of the Q-wave to the end of the T-wave is the QT interval... Since the syndrome was first described in the 1960s it has become obvious that it is congenital (present at birth) and genetic (inherited). The most common form of inheritance is the autosomal dominant form of Long QT syndrome... Individuals in these families will have symptoms of different severity and some family members with long QT interval will have no symptoms. This is often the case in autosomal dominant inherited conditions.

In autosomal dominant inheritance, an affected individual has a single altered gene which causes their condition. Each time an affected individual reproduces, he/she has a 1 in 2 or 50% chance of passing on the altered gene... It is estimated that only 40% of individuals with a long QT interval will have a history of fainting (also called syncope). The risk of sudden death is said to be about 1% per year in these people. In addition, there will be family members who must have the syndrome based on the inheritance pattern and who have normal QT intervals! This was one reason genetic studies of these families was undertaken. If the gene or genes producing Long QT Syndrome could be found it would be a direct way of identifying family members at increased risk of sudden death. This would have great benefit given the dire consequences of the condition and the fact that effective treatments are available...