Acardia is a very rare, serious malformation that occurs almost exclusively in monozygous twins (twins developing from a single egg). This condition results from artery to artery connections in the placenta causing a physically normal fetus to circulate blood for both itself and a severely malformed fetus whose heart regresses or is overtaken by the pump twin's heart.
Description
Acardia was first described in the sixteenth century. Early references refer to acardia as chorioangiopagus parasiticus. It is now also called twin reversed arterial perfusion sequence, or TRAP sequence.
Mechanism
Acardia is the most extreme form of twin-twin transfusion syndrome. Twin-twin transfusion syndrome is a pregnancy complication in which twins abnormally share blood flow from the umbilical artery of one twin to the umbilical vein of the other. This abnormal connection can cause serious complications including loss of the pregnancy.
In acardiac twin pregnancies, blood vessels abnormally connect between the twins in the placenta. The placenta is the important interface of blood vessels between a mother and baby through which babies receive nutrients and oxygen. This abnormal connection forces the twin with stronger blood flow to pump blood for both, straining the heart of this "pump" twin. This abnormal connection causes the malformed twin to receive blood directly from the pump twin before this blood gathers new oxygen. The poorly deoxygenated blood from the normal twin as well as the pressure deficiency as a result of trying to serve both infants may be the cause of the other twin's malformations.
The acardiac twin
The acardiac twin is severely malformed and may be incorrectly referred to as a tumor. In 1902, a physician named Das established four categories of acardiac twins based on their physical appearance. There is controversy surrounding the use of these traditional four categories because some cases are complex and do not fit neatly into one of Das's four categories. These four traditional categories include acardius acephalus, amorphus, anceps, and acormus.
Acardius acephalus is the most common type of acardiac twin. These twins do not develop a head, but may have an underdeveloped skull base. They have legs, but do not have arms. On autopsy they are generally found to lack chest and upper abdominal organs.
Acardius amorphus appears as a disorganized mass of tissues containing skin, bone, cartilage, muscle, fat, and blood vessels. This type of acardiac twin is not recognizable as a human fetus and contains no recognizable human organs.
Acardius anceps is the most developed form of acardiac twin. This form has arms, legs, and a partially developed head with brain tissues and facial structures. This type of acardiac twin is associated with a high risk for complications in the normal twin.
Acardius acormus is the rarest type of acardiac twin. This type of acardiac twin presents as an isolated head with no body development.
Genetic profile
There is no single known genetic cause for acardia. In most cases, the physically normal twin is genetically identical to the acardiac twin. In these cases, physical differences are believed to be due to abnormal blood circulation.
Aneuploidy, or an abnormal number of chromosomes, has been seen in several acardiac twins, but is rare in the normal twins. Trisomy 2, the presence of three copies of human chromosome 2 instead of the normal two copies, has been reported in the abnormal twin of two pregnancies complicated by TRAP sequence in different women. For both of these pregnancies the pump twin had normal chromosome numbers. Since monozygotic twins are formed from a single zygote, scientists theorize that an error occurs early in cell division in only one of the two groups of cells formed during this process.
Demographics
TRAP is a rare complication of twinning, occurring only once in about every 35,000 births. Acardia is believed to complicate 1% of monozygotic twin pregnancies. Risks in triplet, quadruplet, and other higher order pregnancies are even higher. Monozygotic twinning in higher order pregnancies are more common in pregnancies conceived with in vitro fertilization (IVF), hence increased risk for TRAP sequence is also associated with IVF.
This condition has been documented over five centuries occurring in many countries and in different races. As of 2001, specific rates for recurrence are unknown. However, a mother who has had a pregnancy complicated by TRAP sequence is very unlikely to have another pregnancy with the same complication.
Two cases of acardia have been associated with maternal epilepsy and the use of anticonvusants. One report, in 1996, describes an acardiac twin pregnancy in an epileptic mother who took primidone, a seizure medication, in the first trimester of her pregnancy. Another report, in 2000, describes an acardiac twin pregnancy in an epileptic mother who took a different seizure medication, oxcarbazepin.
Diagnosis
A mother carrying an acardiac twin pregnancy is not likely to have any unusual symptoms. An acardiac twin is most often found incidentally on prenatal ultrasound. No two acardiac twins are formed exactly alike, so they may present differently. During ultrasound, an acardiac twin may appear as tissue mass or it may appear to be a twin who has died in the womb. Acardia is always suspected when, on ultrasound, a twin once considered to be dead begins to move or grow, or there is visible blood flow through that twin's umbilical cord. In 50% of cases the acardiac twin has only two, instead of the normal three, vessels in the umbilical cord. A two vessel umbilical cord may also be found in some normal pregnancies.
Ultrasound diagnostic criteria for the acardiac twin usually include:
absence of fetal activity
no heart beat
continued growth
increasing soft tissue mass
undergrowth of the upper torso
normal growth of the lower trunk
An acardiac fetus may also be missed on prenatal ultrasound. A 1991 report describes an acardiac twin who was missed on ultrasound and only detected at delivery. In rare cases a diagnosis of acardia is not possible until autopsy.
Treatment and management
As of 2001, there is no consensus on which therapy is best for pregnancies complicated by TRAP sequence. No treatment can save the acardiac twin, so the goal of prenatal therapy is to help the normal twin. The normal twin is not always saved by prenatal treatment.
Specialists have used laser and electrical cauterization, electrodes, serial amniocentesis, medications, and other treatments successfully. Physicians often recommend prenatal interruption of the blood vessel connections (thus sacrificing the acardiac twin) before heart failure develops in the pump twin.
Cutting off blood circulation to the acardiac twin can be accomplished by cauterizing or burning the blood vessel connections. In a 1998 study of seven pregnancies treated with laser therapy the rate of death in the normal twin was 13.6%, a vast improvement over the expected 50% death rate. Medications like digoxin may be used to treat congestive heart failure in the normal twin. Current studies examining the success and failure rates of these treatments will be helpful in determining which therapy is the best option.
Fetal echocardiography is recommended to assist with early detection of heart failure in the normal twin. Chromosome studies are recommended for both fetuses in all pregnancies complicated by TRAP sequence.
Prognosis
The acardiac or parasitic twin never survives as it is severely malformed and does not have a functioning heart. Complications associated with having an acardiac twin cause 50–70% of normal twins to die. The normal twin is at risk for heart failure and complications associated with premature birth. Heart failure in the normal twin is common. The normal twin of an acardiac twin pregnancy has about a 10% risk for malformations. Therapy is thought to decrease the normal twin's risk for heart failure and premature birth. Improvement of therapies will undoubtedly lead to a better outlook for pregnancies complicated by TRAP sequence.
PERIODICALS
Arias, Fernando, et al. "Treatment of acardiac twinning." Obstetrics & Gynecology (May 1998): 818-21.
Brassard, Myriam, et al. "Prognostic markers in twin pregnancies with an acardiac fetus." Obstetrics and Gynecology (September 1999): 409-14.
Rodeck, C., et al. "Thermocoagulation for the early treatment of pregnancy with an acardiac twin." New England Journal of Medicine 339 (1998): 1293-95.
ORGANIZATIONS
Twin Hope, Inc. 2592 West 14th St., Cleveland, OH 44113. (502) 243-2110. <http://www.twinhope.com>.
