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A small percentage of children with M-CM are at risk for dysrhythmia and possible sudden death.  The cause of this risk is not understood.  It is unclear if this may be due to brain stem compression from brain overgrowth (see brain section) which affects the vagal nerve, or abnormally formed heart tissue.  This risk seems greatest in younger children with M-CM and there appears to be less risk as they mature.  Also, there appears to be an increased risk of congenital heart defects.  Some reported abnormalities have included septal defects, Tetralogy of Fallot and coarctation of the aorta.  The incidence of structural heart defects in M-CM is not known but it appears to be relatively uncommon, perhaps seen in 5-10% of affected individuals.  Because the exact nature of the heart issues in M-CM remain unclear, a consultation with a pediatric cardiologist may be considered along with an electrocardiogram (abbreviated EKG). A physician may also consider performing an echocardiogram  specifically to look for structural defects, especially if a murmur is present.