Twin reversed arterial perfusion (TRAP) sequence, also known as acardia, is a rare anomaly unique to multiple gestation in which one twin has an absent, rudimentary, or non-functioning heart. Schatz (1898) classified acardia into two main groups: hemiacardius (imperfectly formed heart) and holoacardius (absence of heart). Das (1902) subdivided acar-dia into four groups: acardius acephalus, acardius amorphus or anideus, acardius acormus, and acardius anceps or para-cephalus (Table 120-1). Simonds and Gowen (1925) added a further subgroup: acardius myelacephalus.

Contemporary authors have considered these classifications meaningless because the pathogenesis is probably similar for all cases (Benirschke and Kim, 1973; Van Allen et al., 1983). Van Allen et al. recommended twin reversed arterial perfusion sequence to describe all acardiac fetuses. The TRAP sequence denotes a common pathophysiology for all forms and leads to an explanation of how a gradation of abnormalities can be produced (Van Allen et al., 1983). The fundamental requirement for the TRAP sequence is the development of arterial-to-arterial vascular anastomoses between the umbilical arteries of twins early in embryogenesis. The importance of vascular arterial anastomosis as the pathophysiology for acardia was first elucidated in 1879 by Ahlfeld. The embryo with the hemodynamic advantage becomes the pump twin. The pump twin retrogradely perfuses the other twin with deoxygenated blood along the umbilical artery/arteries to the iliac artery/arteries to the abdominal aorta. The lower limbs and abdominal organs supplied by the iliac arteries and abdominal aorta preferentially receive a better blood supply and usually develop better than the upper part of the body (Van Allen et al., 1983). TRAP occurs in monochorionic pregnancies, and previous theories of polar body fertilization to explain acardius in sex discordant twins have been discounted (Bieber et al., 1981; Fisk et al., 1996). In addition, the TRAP sequence is more common in monozygotic triplets than in monozygotic twins (James, 1977; Healey, 1994). Some authors have suggested a slight female preponderance in acar-diac twins while other studies have not supported this idea (James, 1977; Healey, 1994).

The members of a TRAP sequence are known as the perfused twin and the pump twin. The perfused twin in a TRAP sequence is an example of the impact of vascular disruption on morphogenesis. Multisystem malformations, as well as unusual body form, are found in the perfused twin. Figure 120-1 illustrates the gradation of loss of normal body form ranging from an amorphous appearance to an individual with more severe abnormalities found in the upper part of the body. The malformations found in cases of acardia include growth abnormalities, partial or complete absence of the cranial vault, anencephaly, holoprosencephaly, absent or rudimentary facial features, absent or rudimentary upper and/or lower limbs, absent lungs and heart, gastrointestinal atre-sia, omphalocele, gastroschisis, and absent liver, pancreas, spleen, and kidneys (Van Allen et al., 1983). Of 33 acar-diac fetuses with a known karyotype, 11 (33%) were abnormal (Healey, 1994). The karyotypic abnormalities included monosomy, trisomy, deletions, mosaicism, and polyploidy. The pattern of structural abnormalities found in the perfused twins with abnormal karyotypes is not appreciably different from those with normal karyotypes (Van Allen et al., 1983). Van Allen et al. suggested that the abnormal karyotype is not responsible for the malformation complex, but rather that it contributes to the discordant development between twins, increasing the likelihood of reversal of arterial blood flow if an anastomosis occurs.

The presence of an acardiac twin requires a “pump” twin to provide circulation for itself as well as its acardiac co-twin. In many cases the acardiac twin is almost equal in size to the normal twin (Figure 120-2). The pump twin is usually morphologically and genetically normal. In a review of 34 pump fetuses with a known karyotype, 3 (8.8%) were abnormal as a result of trisomy (Healey, 1994). The pump twin may show evidence of the physiologic consequence of fetal cardiac overload and congestive heart failure with hepatosplenomegaly. The principal perinatal problems associated with acardiac twinning are pump twin congestive heart failure, polyhydramnios, and preterm delivery (Moore et al., 1990).

The reported fetal/neonatal mortality in the pump twin is substantial ranging from 50% to 75% (Gillim and Hendricks, 1953; Napolitani and Schreiber, 1960; Van Allen et al., 1983; Moore et al., 1990; Sogaard et al., 1999). One factor thought to be contributing to the high perinatal mortality rate is the increased cardiac demands placed on the pump twin to perfuse the acardiac twin (Sullivan et al., 2003).

Premature delivery is another important factor determining the prognosis for the pump twin (Figure 120-2) (Van Allen et al., 1983; Moore et al., 1990; Healey, 1994). In one study where approximately 55% of acardiac pregnancies resulted in fetal or neonatal death, approximately one quarter of the pregnancies delivered after 36 weeks. Preterm delivery and the attendant long-term morbidities complicated the remaining quarter (Moore et al., 1990).

The incidence of the TRAP sequence is estimated as 1% of monozygotic twins, with birth estimates ranging from 1/35,000 to 1/50,000 births (Gillim and Hendricks, 1953; Napolitani and Schreiber, 1960; D’Alton and Simpson, 1995). Acardia was observed in 1 of 606 twin pregnancies, and the rate of twins was calculated at 1 in 86.5 births in the United States (Gillim and Hendricks, 1953). Van Allen et al. (1983) have suggested these figures to be a gross underestimate of the true frequency of the TRAP sequence because most cases go unrecognized due to early pregnancy loss. In contrast, an analysis of data from the Eurocat Network (European Registration of Congenital Anomalies and Twins) gave a prevalence of acardia of 0.064 in 10,000 births, which is much lower than were previous estimates in the literature (Haring et al., 1993).

Antenatal diagnoses of the TRAP sequence have been reported in the literature since 1980. Ultrasonographic features useful in the diagnosis of acardia include absence of normal cardiac structure and cardiac movement and variable structural abnormalities. Common structural abnormalities identified in the acardiac fetus include anencephaly, omphalocele, and absence of upper limbs. Most cases have edematous soft tissue, and large cystic hygromalike spaces are commonly identified in the skin (Mack et al., 1982).

The placentation is most commonly monochorionic diamniotic (74%), in which a thin membrane will be seen dividing the sac of the acardiac fetus from the pump fetus (Healey, 1994). Monoamnionicity is present in approximately 24% of cases (Healey, 1994). In exceptional cases, dichorion-icity may be diagnosed (Healey, 1994). Polyhydramnios is common as are abnormalities in the umbilical cord or in its insertion (Dashe et al., 2001). The umbilical cord will demonstrate a single umbilical artery in approximately two thirds of cases, and in one-third the number of cord vessels will be normal (Healey, 1994). A velamentous insertion of the cord or a conjoined cord insertion may be present (Dashe et al., 2001).

Measurement of the acardiac twin should be performed, because the ratio of the weight of the acardiac twin to that of the pump twin is useful to predict pregnancy outcome. Because of the structural abnormalities, the biometric parameters of biparietal diameter, abdominal circumference, and femur length may not be available or reliable in an ac-ardiac fetus. This problem of the antenatal determination of the acardiac twin’s weight has been addressed by Moore et al. (1990). The dimensions and weights of 23 acardiac twins were used for the analysis. A second-order regression equation (weight [g] = -1.66 x length + 1.21 x length²) was computed and was predictive of acardiac weight with the use of its longest linear measurement (r = 0.79; P < 0.001; SEE = 326 g). When the actual and equation-predicted weights were compared, the mean error (±SE) in prediction was 240 ± 156 g. Careful Doppler examination of the acardiac fetus may also demonstrate reversal of flow in the umbilical artery of the acardiac fetus, with flow going from the placenta toward the acardiac fetus (Figure 120-3) (Benson et al., 1989; Malone and D’Alton, 2000).

The pump twin should have a detailed structural survey performed because trisomy has been reported in up to 9% of cases (Healey, 1994) and sonographic features typical of a trisomic fetus may be identified. Fetal echocardiography is helpful in detecting early signs ofin utero congestive heart failure in the pump twin. Atrial and ventricular enlargement can be an initial feature of impending cardiac decompensation and can be measured using M-mode by obtaining a transverse view through the cardiac chambers (Allan, 1986; DeVore, 1987). The ventricular fractional shortening capacity can also be calculated using M-mode with the formula (D –S) D x 100, where D is the diastolic and S is the systolic ventricular size. A low value is indicative of poor cardiac contractility. A pericardial effusion may be present and is a sign of congestive heart failure. Tricuspid regurgitation, demonstrated by Doppler studies of the tricuspid valve, is also a sign of congestive heart failure (Silverman et al., 1985; Shenker et al., 1988). Combined ventricular output (CVO) can be measured to determine if the pump twin is in a high output state. In TRAP sequence at a gestational age too early to determine CVO, Kinsel-Ziter et al. have demonstrated a good correlation of increased CVO with increased cardiothoracic ratio (Kinsel-Ziter et al., 2009).