[Valvular cyst and atrioventricular canal in a child with trisomy 21].

A child with Down's syndrome with an atrioventricular canal of ostium primum type had also a blood cyst of the mitral valve. Aside more frequent complex heterotopic cysts due to migration anomalies, simple cysts are rare with only 27 cases described in medical literature. They are often symptomatic and are cured by surgical removal. Our hypothesis, in accord with recent embryologic observations, is that simple cysts situated close to the valves and fibrous ring of the heart may result from heteroplastic change of tissue coming from primitive pericardial mesothelium. This primitive mesothelium participates to the formation of the fibrous skeleton of the heart.