Blood cysts are commonly seen in fetuses and neonates. They are most likely on the atrioventricular valve leaflets, especially the tricuspid. The incidence is increased in chromosomal disorders, especially trisomy 18.
The lesion consists of non-organized blood in a sac lined by flat endothelium.
Blood cysts are thought to represent invaginations of endothelium into the valve leaflet.
J. G. Begg found blood-filled cysts in heart valve cusps in 56% of 112 fetuses and infants (J Path Bact 1964;87:177-8). Histology revealed endothelial-lined cysts containing blood. Some cysts were multilocular. The favored theory regarding the origin of these cysts is that they represent dilation of invaginations into the valve leaflets. They have been noted to be more frequent in cases of asphyxia but others refute this opinion. Frequent locations include the atrioventricular valves, especially on the atrial surfaces. Some have reported evidence of endothelial-lined channels from the cyst through the surface endocardium. Cysts can vary in size from microscopic to 3 mm. Sometimes cysts persist and enlarge. There are case reports of giant cysts causing problems in adults, occasionally requiring surgery.