CausesBy Mayo Clinic staff
There are two types of heart murmurs: innocent murmurs and abnormal murmurs. A person with an innocent murmur has a normal heart. This type of heart murmur is common in newborns and children.
An abnormal heart murmur is more serious. In children, abnormal murmurs are usually caused by congenital heart disease. In adults, abnormal murmurs are most often due to acquired heart valve problems.
Innocent heart murmurs
An innocent murmur can occur when blood flows more rapidly through the heart. Conditions that may cause rapid blood flow through your heart, resulting in an innocent heart murmur, are:
- Physical activity or exercise
- Changes in your heart's structure, such as changes from heart surgery
- Not having enough healthy red blood cells to carry adequate oxygen to your body tissues (anemia)
- An excessive amount of thyroid hormone in your body (hyperthyroidism)
Changes to the heart due to aging or heart surgery may also cause an innocent heart murmur. Innocent heart murmurs may disappear over time, or they may last your entire life without ever causing further health problems.
Abnormal heart murmurs
Although most heart murmurs aren't serious, some may result from a heart problem. The most common cause of abnormal murmurs in children is when babies are born with structural problems of the heart (congenital heart defect). Common congenital defects that cause heart murmurs include:
- Holes in the heart or cardiac shunts. Many heart murmurs in children are the result of holes in the walls between heart chambers, known as septal defects. These may or may not be serious, depending on the size of the hole and its location. Shunts occur when there's an abnormal blood flow between the heart chambers or blood vessels, which may lead to a heart murmur.
- Heart valve abnormalities. Congenital heart valve abnormalities are present at birth, but sometimes aren't discovered until much later in life. Examples include valves that don't allow enough blood through them (stenosis) or those that don't close properly and leak (regurgitation).
Other causes of abnormal heart murmurs include infections and conditions that damage the structures of the heart and are more common in older children or adults. For example:
- Rheumatic fever. Although rare in the United States, rheumatic fever is a serious condition that can occur when you don't receive prompt or complete treatment for a strep throat infection. Untreated rheumatic fever can permanently affect the heart valves and interfere with normal blood flow through your heart.
- Endocarditis. This is an infection and inflammation of the inner lining of your heart and valves. Endocarditis typically occurs when bacteria or other germs from another part of your body, such as your mouth, spread through your bloodstream and lodge in your heart. Left untreated, endocarditis can damage or destroy your heart valves. This condition usually occurs in people who already have heart abnormalities.
- Valve calcification. This hardening or thickening of valves, called mitral or aortic valve stenosis, can occur as you age. These valves may become narrowed (stenotic), making it harder for blood to flow through your heart, resulting in murmurs.
- Mitral valve prolapse. In this condition, the valve between your heart's left upper chamber (left atrium) and the left lower chamber (left ventricle) doesn't close properly. When the left ventricle contracts, the valve's leaflets bulge (prolapse) upward or back into the atrium, which may cause a murmur.
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