First trimester diagnosis of acardiac twins.
OBJECTIVE
To describe ultrasonographic findings from 5 to 17 weeks of gestation in monozygotic twin pregnancy resulting in acardia of one twin.


METHODS
Transvaginal ultrasonographic examinations were performed using a 7.5 MHz probe at weekly intervals from week 5-11. Uterine artery PI were measured using color Doppler flow. Embryonic heart rates were determined with M-mode.


RESULTS
A monochorionic gestational sac containing 2 embryos demonstrated embryonic cardiac activity between 5 and 7 weeks. One twin lost cardiac activity at 7 weeks. A diagnosis of acardiac twins was made at 1 weeks when growth of the lower extremities and no development above the thorax as well as reversal of blood flow through the umbilical cord of the acardiac twin was noted by 14 weeks.


CONCLUSIONS
Ultrasonographic demonstration of independent embryonic heart rates from 5-6 weeks and reversal of blood flow through the umbilical cord of the acardiac twin suggest that the primary pathogenesis in acardia is cardiac dysmorphogenesis secondary to reversal of blood flow rather than primary cardiac agenesis. Acardiac twins can spontaneously ligate connection with their viable twin. Acardiac twinning is a rare complication of multifetal gestation occurring in 1% of monozygotic twin pregnancies or 1 in 35000 births (1). Acardius, as the name implies, is anomaly in which the heart is absent. Acardius can only occur in multiple gestations since its circulation must be maintained by vascular anastomosis with an accompanying fetus (2). Although little doubt exists that intrauterine growth of the acardiac twin is achieved by the perfusion afforded from the normal co-twin via vessel anastomosis in the placenta, the pathogenesis of acardia has been disputed. Some authors (3-5) have presented evidence supporting the concept that the primary defect is one of cardiac embryogenesis (dysmorphogenesis) and that the vascular anastomosis between the vessels, although necessary for the intrauterine survival of the acardiac twin, is not responsible for the cardiac findings. Others (6-8) believe that the primary cause is an abnormal vascular communication between embryos leading to reversed flow of blood to the hemodynamically disadvantaged or recipient twin, with the resulting secondary atrophy of the heart and dependent organs. Inadequate perfusion of the recipient twin is responsible for the development of characteristic set of anomalies including acardia and acephalus. The hemodynamically advantaged or pump twin is structurally normal, but is at risk for in utero cardiac failure and without treatment dies on 50% to 75% of cases (9). Ultrasonography has assumed increasing importance in prenatal diagnosis of congenital anomalies (10). Using this technique, intrauterine diagnosis of acardiac twins has been made as early as 18 weeks of gestation allowing umbilical cord ligation of the acardiac twin at 19 weeks of gestation and uncomplicated birth of abnormal twin at 36 weeks of gestation (11). We now report intrauterine diagnosis of acardiac twins during the first trimester of pregnancy. Unique aspects of the case suggest the pathogenesis of acardia involves arterioarterial anastomosis of placental vessels with retrogade blood flow into the recipient twin and not primary arrest in cardiac development. Further, spontaneous ligation of the vascular connection is documented followed by an uncomplicated delivery of a normal infant at term.
Coulam
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CB
The Center for Human Reproduction, 750 N. Orleans St., Chicago, IL 60610, USA.
Wright
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G
D004200:Diseases in Twins / Q000175:diagnosis*; D005260:Female; D006333:Heart Failure / Q000097:blood / Q000175:diagnosis* / Q000000981:diagnostic imaging; D006801:Humans; D007231:Infant, Newborn; D008297:Male; D011247:Pregnancy; D011261:Pregnancy Trimester, First / Q000097:blood; D011296:Prenatal Diagnosis / Q000379:methods*; D014430:Twins, Monozygotic; D016216:Ultrasonography, Prenatal / Q000379:methods