Heart murmurs in children are very common. A heart murmur just means a sound. Some heart murmurs are sounds produced by actual defects or abnormalities with the heart. For example, a ventricular septal defect (a hole in the wall separating the lower 2 chambers of the heart) makes a very specific noise as blood travels through the hole. Abnormalities with heart valves like aortic valve stenosis can also produce heart murmurs as blood flows past the defective valve. On the other hand, many heart murmurs are what we call innocent heart murmurs. With an innocent heart murmur, the heart is perfectly normal. The murmur in this case is simply the normal sound that blood is making as it flows through the heart.
What is the most common innocent heart murmur? Undoubtedly it is the Still's murmur. Named for English pediatrician George Frederic Still, the Still's murmur is extremely common in children. It's often described as a vibratory type noise. I'm not sure myself if this is completely accurate, but at least that's the way it is frequently described. Certainly it is a very unique noise. Once you've heard a Still's murmur several times, it becomes very difficult to confuse it with anything else.
I hear Still's murmurs at every age. I've heard it in babies a few hours old all the way up to full grown adults. I really think that this murmur can be present at pretty much any age. Classically it is most common in school aged children. It's a very distinct noise that is best heard along the left sternal border. Some experts feel that it might be more prominent in the flat position as opposed to sitting or standing, but I have not always found this to be the case.
The cause of the Still's murmur is not entirely clear. Some studies have demonstrated that children with a Still's murmur are more likely to have an accessory mitral valve chord attaching to the ventricular septum, what is termed a false tendon. The chordae tendineae anchor the papillary muscles to the mitral valve leaflets. Occasionally there may be a stray chord that extends from a papillary muscle directly to the ventricular septum. Some experts think that vibrations of these anomalous chords are what produce a Still's murmur.
In my opinion the Still's murmur is very unique. When a classic Still's murmur is present, it's hard to confuse it with anything else. Having said this, many children may have slight variations in the sound or quality of the murmur which may suggest more worrisome causes. Rarely subaortic stenosis may be confused with a somewhat harsher Still's murmur. Subaortic stenosis can be caused by an isolated subaortic membrane, or can found in association with heart problems like hypertrophic cardiomyopathy. In these cases an echocardiogram can easily distinguish normal from abnormal. However, in many instances a Still's murmur can easily be identified just by listening; in these cases additional testing is often unnecessary.