|Home > Medicine > Cardiovascular Disorders|
|Long QT Syndrome||Symptoms||Causes||Treatment|
Inherited Long QT Syndrome
Inherited LQTS is caused by the mutations of genes that encode a special class of proteins called ion channels, which are embedded on the surface (membrane) of muscle cells in the heart. Each ion channel has a tiny pore that can be open or closed. When the pore is open, some ions (sodium, calcium, or potassium, depending on the type of ion channels) may flow into and out of the cell. Because ions carry charges, their movement across cell membrane changes the electric potential of the heart cell membrane. This change is reflected in the EKG recording.
If an ion channel gene is mutated, the produced ion channel may not work properly. This may result in abnormal heart rhythm. At least nine genes are associated with LQTS. Their mutations result in different types of LQTS, referred to as LQT-1 to LQT-9 (see Appendix).
If you have LQT-1 and LQT-2, the flow of potassium ions through the ion channels in your heart cells isn't normal, which may cause problems when you exercise or when you're surprised or experience emotional stress. During these situations, you may develop a rapid and uncontrollable heart rhythm that may prevent your heart from pumping blood. This will result in death if it's not quickly brought under control. If you have LQT-3, the flow of sodium ions through ion channels in your heart cells isn't normal, which also may trigger a rapid and uncontrollable heart rhythm that can result in death. In LQT-3, problems usually develop when your heart beats slower than normal, such as during sleep.
Noninherited Long QT Syndrome
Acquired, or noninherited, LQTS may be brought on by certain medicines and other medical conditions. Presently, more than 50 medicines have been recognized to cause LQTS. Some of the commonly used medicines that may cause LQTS include:
Some people with medication-induced LQTS may also have an inherited form of the syndrome that doesn't cause symptoms unless they take certain medicines that lengthen the QT interval or lower potassium levels in the blood. This is sometimes called silent LQTS.
Other Causes of Acquired Long QT Syndrome
Excessive diarrhea or vomiting that causes a major loss of potassium or sodium ions from the bloodstream may cause LQTS. The syndrome lasts until the levels of these ions in the blood return to normal. The eating disorder anorexia nervosa and certain thyroid disorders may cause a drop in potassium ion levels in the blood, causing LQTS.
The prolongation of the QT interval is due to slow repolarization of the membrane potential. Mutations in these K+ channels cause the loss of function, thereby reducing outward K+ current and consequently slowing repolarization. Mutations of the Na+ channel, SCN5A, cause incomplete inactivation, thereby increasing inward Na+ current which also slows repolarization (reference).
Caveolin-3 is the major component of a specific membrane domain (called caveolae) in the striated muscle. The SCN5A-encoded cardiac sodium channel localizes in caveolae. Mutations in the CAV3 gene are implicated in sudden infant death syndrome.