Given her clinical presentation, medical staff suspect that Margaret may have inherited Long QT Syndrome (LQTS). QT is a measure of the time between the start of the Q wave and the end of the T wave in the electrical cyle of the heart. LQTS is diagnosed when the QT measures more than/= 480mn in repeated 12 lead ECGs. (In a patient with unexplained syncope, if the QT measures more than/=460 in repeated 12 lead ECGS, then LQTS should be considered.)
LQTS causes an abnormality of the heart’s electrophysiology. While the mechanical function of the heart is entirely normal, there is an electricophysiological problem due to defects in the ion channels of the heart muscle cells. Ion Channels are important for the electrical activation (depolarisation) of the heart that results in the cardiac contraction and electrical recovery (repolarisation).
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These ion channel defects cause QT prolongation and predispose to a very fast and malignant heart rhythm called Torsades de Pointes which, if sustained, may lead to sudden loss of consciousness (syncope) and sudden cardiac death.
Inherited LQTS is the most common channelopathy and occurs in approximately 1:2000 people.
In most cases of LQTS (LQTS 1, LQTS 2), the potassium channels that regulate the movement of potassium ions from the inside to the outside of the cells are affected. In a smaller proportion of cases (LQTS 3), a sodium channel that regulates the flow of sodium from the outside to the inside of cells is affected.
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LQTS can be either inherited or acquired.
Inherited LQTS is caused by faults in the genes that control the production of certain types of ion channels within the heart. There are 7 different types of LQTS but the most common are LQTS 1, 2 and 3. In LQTS 1 and LQTS 2 there is an abnormal flow of potassium ions through the ion channels of the heart. Problems can occur when an individual with one of these types of LQTS exercises or feels strong emotions. In LQTS 3 there is an abnormal flow of sodium through the ion channels of the heart. Problems can arise when the heart is beating at a slower rate then normal, for example, when an individual is asleep. All three types of LQTS can trigger a rapid, uncontrollable heart rhythm.
Acquired LQTS is caused by medicines, both cardiac drugs and non-cardiac drugs. (A link to information on those drugs known to cause LQTS can be found later in the case study.)
An individual with inherited LQTS can be asymptomatic until they are prescribed a drug which induces the condition.
