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Abnormal murmurs are signs of a heart problem. In children, abnormal heart murmurs are usually caused by problems they are born with, such as a heart valve that doesn't work right or a hole in the wall between two heart chambers.
In adults, abnormal murmurs are most often caused by damaged heart valves. Heart valves operate like one-way gates, helping blood flow in one direction between heart chambers as well as into and out of the heart.
When disease or an infection damages a heart valve, it can cause scarring and can affect how well the valve works. The valve may not be able to close properly, so blood can leak through. Or the valve may become too narrow or stiff to let enough blood through. When a damaged heart valve cannot close properly, the problem is called regurgitation. When the valve can't let enough blood through, the problem is called stenosis.
Heart valves can be damaged by heart disease or by infections like rheumatic fever or endocarditis. The normal wear and tear that comes with aging can also cause some damage.
Some heart murmurs are caused by a thicker than normal heart. When the heart muscle grows too large, it can get in the way of normal blood flow and cause a murmur.