Acardia is a rare and serious malformation that occurs exclusively in monizygous twins - twins developing from a single egg. Acardia represents one of the most severe and rare congenital anomalies. It is characterized by the absence of functioning heart. Acardia results from the artery to artery connections in the placenta, thereby causing a physically normal fetus to circulate blood for itself as well as a severely malformed fetus suffering from heart regression. In other words, fetus acardius is a parasite and it receives blood supply from the donor twin. Because the pump twin heart has to pump for two, there is a high risk of going into heart failure and this would lead to the death of the normal twin.
The most common variety is the acardius acephalus where the head is lacking and so are the upper extremities. Other types are acardius anceps, acardius acormus and acardius amorphous. While in acardius anceps, the most highly developed form, a partly developed head with remnants of cranial bones and brain tissue are present with developed body and extremities, acardius acormus is the rarest form of acardia. The monster is a head without a body. Acardia amorphous is the least developed monster not recognizable as a human form, with minimally developed visceral organs. Since there is no gross human form, the name acardius amorphous.
As to the cause of acardia, the etiology of acardiac monster is still unknown. Genetic defects have been reported to be the cause. Some researchers suggest chromosomal abnormalities to be the reason. Krause and Bejdl suggest that compression of the cephalic pole of the embryo prohibiting curving and fusion of the primitive heart tube to be the basic cause of this anomaly. As a result, the dependant entodermal organs like thyroid, esophagus, trachea, lung, liver and others are also not formed.
A pregnant woman carrying an acardiac twin is unlikely to have any unusual symptoms. An acardiac twin is often found incidentally on prenatal ultrasound. As no two acardiac twins are formed exactly alike, they may present differently. Several improved imaging techniques like 2D ultrasonography, 3D ultrasonography and transvaginal Doppler ultrasonography have made diagnosis of acardia possible even in the first trimester of pregnancy. Such early diagnosis helps to reduce the risk of complications. Fetal echocardiography is also recommended to assist in early detection of heart failure in the normal twin. Chromosome studies are also done on both fetuses.
One line of treatment is watching for the earliest signs of heart failure in the pump twin with frequent ultrasounds. If heart failure is identified and the pregnancy is also far enough, then the pump twin should simply be delivered. Physicians recommend prenatal interruption of the blood vessel connections before heart failure develops in the pump twin, thus sacrificing the acardiac twin.
Specialists use laser, electrical cauterization and electrodes, serial amniocentesis, medications and other treatments successfully. If the acardiac twin is large enough and the amount of blood flow to it can cause heart failure in the healthy twin, then blood flow is stopped with Fetal Image-Guided Surgery. The acardiac or parasitic twin never survives, as it is severely malformed and does not have a functioning heart. The normal twin is at risk for heart failure and complications associated with premature birth. The normal twin is expected to have about 10% risk for malformations.
Bibliography / Reference
Collection of Pages - Last revised Date: February 18, 2018