Clinical characteristics are highly variable, even within the same family. Many children develop early heart failure related to dilated cardiomyopathy (an enlarged heart with decreased contractility) that can be fatal. However, the use of digoxin seems to lead to an improvement in heart function. Children are usually small and can be hypotonic (“floppy”) and development is usually delayed. Most children can communicate but do not speak. Feeding issues and poor weight gain are very common and usually require use of a feeding tube. There are abnormalities in cardiac conduction with prolongation of the QT interval, but heart rhythm issues are rare.
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