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Acardiac-Acephalic Twin

(Marked Edema)
Normal lower extremities

Acephalus (Marked edema + cystic soft-tissue changes)

Acephalus (Marked edema + cystic soft-tissue changes)


Two fetuses with acardia-acephaly. Each was a monozygotic, monochorionic diamniotic twin gestation. Marked edema of the affected twin with cystic changes were seen. Movement as well as near normal growth of the lower extremities was present. Reversed flow through the umbilical arteries to the affected fetus was seen. There was no evidence of hydrops in the donor "pump" twin.


The acardius syndrome is a rare complication of multifetal gestation, occurring in 1% of monozygotic twin pregnancies or in one in 35,000 births. This anomaly is seen almost exclusively in monozygotic twinning, although acardius has also been reported in dizygotic twins. This condition almost certainly arises from vascular anastomoses in the placenta, artery to artery, with marked deformity in the recepient twin and the possibility of cardiac decompensation in the donor "pump" twin. The recepient twin develops severe anomalies, often referred to as acardia, that are incompatible with life. The donor twin, may develop cardiac failure with a reported mortality of 50-75%.

While all recepient twins in this syndrome will manifest acardius, four categories of this syndrome have been described in the literature based upon associated abnormalities. Acardius acephalus is the most common type in which there is absence of the head and upper torso and limbs with preservation of the lower limbs, genitalia and abdominal viscera. The rarest form of the syndrome is acardius acormus, in which the only developed structure is the fetal head with absence of all other structures. The third type is acardius amorphus in which there is an amorphous mass of bone, muscle, fat and connective tissue. The most developed type is acardius anceps, in which there are rudimentary cranial stuctures with otherwise persistent trunk, limbs and organs.

Classification of Acardiac Fetuses

Acardius amorphus The least differentiated form of acardia. Pathologically, only bones, cartilage, muscle, fat, blood vessels and stoma can be seen.. If rudimentary nerve tissue is present, it is called acardius myelantencephalus. It is theorized that some of the teratomas in newborns are remnants of this form of acardia.
Acardius acormus This form of acardia lacks a thorax. The umbilical cord insertion is into the head of the fetus. Pathologically, rudiments of thoracic structures are often found, calling into question this variation in the classification of acardia.
Acardius acephalus The most common form of acardia. This fetus lacks a head, thorax and upper extremities. Other morphological abnormalities may be seen.
Acardius anceps These fetuses have a head, thorax and abdominal organs but lack even a rudimentary heart. It is theorized that many of the other forms of acardia described begin as acardia anceps and evolve into one of the other forms due to poor oxygen supply to the remainder of the fetus.


From: Bonilla-Musoles F, Machado LE, Raga F, Osborne NG. Fetus Acardius: Two and Three dimensional ultrasonographic diagnoses. J Ultrasound Med 20:117-1127, 2001


Several theories exist to explain the abnormalites in this syndrome. One explanation is that the primary event is an abnormal development of the fetal heart with resulting agenesis or dysgenesis. The vascular anastomoses are felt to be secondary and allow continued perfusion of the recepient twin. A second explanation is a chromosomal abnormality in the acardiac twin with abnormal embryonic development. Reports of trisomies and mosaicism of the affected twin have appeared in the literature. The donor twin invaribly has a normal karyotype. The third and most common explanation for this syndrome is that the primary event is the development of arterial anastomoses with reversal of blood flow from the normal to the abnormal twin. The reduced oxygen tension would result in maldevelopment of the heart and upper torso and limbs. In cases of artery to artery anastomosis the returning blood goes in the direction towards the lower extremities (although deoxygenated). In arterio-venous anastomoses, agenesis of the lower extremities is likely to be more severe because the direction of blood perfusion is dorsocranial (presumably following the course of the primitive cardinal vein) Bonilla-Musoles et al. Documentation of reversal of flow using doppler velicometry as well as the first trimester identification of a normal heart in both twins with ultimate acardius in one twin, make this last explanation certainly the most likely. There are no maternal conditions which predispose to this condition and there have been no reports of recurrences of this syndrome within families.

Sonographically, these cases are often quite dramatic with the most recognizable feature being marked edema and dysmorphic features of the acardiac twin. No fetal heart or head are identified. The edematous soft tissue has almost a gelatin appearance with numerous cystic changes in the upper portion. A striking feature is the normal or accelerated growth of the lower extremities. In the report by Bonilla-Musoles et al they describe the following signs for the sonographic diagnosis: 1) monozygotic twin gestation, 2) biometric discordance between the twins, 3) diffuse subcutaneous edema or morphologic abnormalities of one of the twins or both, and 4) absence of cardiac activity (although hemicardia or pseudocardia may be present). A two vessel cord in the recipient twin is seen in over 50% of cases. Polyhydramnios is often seen and as been stated earlier, cardiac failure may develop in the donor "pump" twin. Reversal of flow in the umbilical arteries can be documented. The umbilical cord to the acardiac twin is often quite short and may be difficult to identify.

Management includes close scrutiny of the donor twin for the development of hydrops. A number of procedures have been tried to interrupt the vascular connections including: hysterotomy and removal of the acardiac twin, ligation of the umbilical cord and laser ablation of vessels.

Differential Diagnosis:

The most common differential diagnosis and mistake is to assume that the acardiac twin is a hydropic, recently demised twin. Documentation of blood flow to the twin and growth of the lower extremities will prevent this diagnosis. The other differential diagnostic consideration is that of a teratoma. Identification of an umbilical cord and well developed organs in the acardiac twin will allow one to make the correct diagnosis.


Gillim R, Hendricks CH. Holocardius. Obstet Gynecol 2:647-653, 1953

Van Allen MI, Smith DW, Shepard TH. Twin reversed arterial perfusion (TRAP) sequence: A study of 14 twin pregnancies with acardius. Semin Perinatol 7:285-293, 1983

Fries MH, Goldberg JD, Golbus MS. Treatment of acardiac-acephalus twin gestations by hysterotomy and selective delivery. Obstet Gynecol 79:601-604, 1992

Coulam CB. First trimester diagnosis of acardiac twins. Obstet Gynecol 88:729, 1996

Pretorius DH, Leopold GR, Moore TR, Bernischke K, Sivo JJ. Acardiac twin: Report of Doppler sonography. J Ultrasound Med 7:413-416, 1988

Moore TR, Gale S and Bernischke K. Perinatal outcome of forty-nine pregnancies complicated by acardiac twinning. Am J Obstet Gynecol 163:907-912, 1990

Bonilla-Musoles F, Machado LE, Raga F, Osborne NG. Fetus Acardius: Two and Three dimensional ultrasonographic diagnoses. J Ultrasound Med 20:117-1127, 2001

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Peter W. Callen, M.D.
Professor of Radiology, Obstetrics, Gynecology and Reproductive Science
University of California Medical Center, San Francisco, California