Discussion

Acardiac twinis a very rare congenital anomaly occurring in monochorionic twin pregnancy. It was first described by Benedetti in 1533. It is also referred to as Chorioangiopagus parasiticus as the recipient twin is hemodynamically dependent on pump twin and thus acts as a parasite over it [1]. It receives blood from the pump twin via abnormal artery-to-artery or vein-to-vein communications on the surface of placenta. It is one of the types of twin to twin transfusion.

Normally umblical vein supplies oxygenated blood to the fetus and umblical artery takes back the deoxygenated blood away. But in TRAP sequence, there is an abnormal anastomosis between umblical arteries of the two monochorionic feti which causes the oxygenated blood to flow from pump twin to recipient twin and perfuses it and that is how the twin receives circulation even without a heart. There is a reversal of flow in umblical artery of acardiac twin, hence the name TRAP. This phenomenon can be demonstrated on Doppler study, which reveals reversed direction of spectral waveform in umblical artery of acardiac twin i.e. towards fetus (Figure 6). In 75% of cases, there can be associated single umblical artery in umblical cord of recipient twin [4].

The possible explanation behind the fetus having a better developed pelvis and lower extremities than head and thorax, as in our case, is that due to imbalanced circulation, the poor but relatively oxygenated blood from donor twin’s umblical artery flows in reversed direction into recipient’sumblical artery and from there-on into the lower body of fetus resulting into the lower body receivingrelatively more oxygen and nutrients than the upper torso. The de-saturated blood then flows in retrograde direction into the upper circulation i.e. the thorax, upper body and head resulting in mal-developed heart, head and upper torso, which are deformed or absent. This heterogeneous looking mass may simulate a teratoma on ultrasound scan [4].

There are two hypothesis regarding pathogenesis of TRAP sequence [5]

1. There is development of inherent deep placental anastomoses during early embryogenesis. Early pressure flow is more in one twin than that of the other, which leads to reversed perfusion in it. This causes malformation of acardiac twin [5].

2. Primary defect in embryogenesiswhich causes failure of heart development [5]. The perfusion of this twin is then maintained by artery-to-artery anastomoses with normal twin and the anastomoses not being responsible for cardiac anomaly.

Classification [5]

Schatz (1898) classified acardia into two groups:-

Hemiacardius- heart is formed imperfectly

Holoacardius- completely absent heart

Based on degree of cephalic and truncal mal-development, they can be classified as:-

Acardius acephalus: Cephalic structures (head and upper extremities) are absent. This is the most common form, accounting approximately two-thirds of the cases.

Acardius anceps: some cranial structure and neural tissue are present. Upper body and extremities are developed.

Acardius acormus: Cephalic structures present but truncal structures absent i.e Head without a body, cord is attached to head. It is the rarest form.

Acardius amorphous: No distinguishable cephalic or truncal structure noted. Only an amorphous mass is noted which simulates teratoma.

The present case was an Acardius Acephalus type wherein there was a malformed head and an absence of a bony cranium. Only a partially developed thorax, abdomen and lower body with flexed folded lower limbs were appreciated.

The pump twin is generally normal in structure; however in 10% of pump twins, congenital anomalies like cardiogenic defects, gastroschisis and skeletal abnormalities can develop [5]. There is also co-existence of VACTERL association found in surviving pump twin [6]. Polyhydramnios is generally associated since the pump twin is at high risk for developing anemia and heart failure as it is pumping for two. As a result there is an increased perfusion of fetal kidneys leading to overproduction of fetal urine and increased liquor consequently [7,8]. The perinatal mortality of pump twin is around 50-70%. It is either due to premature deliveryattributed to polyhydramnios or secondary to heart failure [4,7]. On Doppler study, difference in Resistive Index of umblical arteries of pump and acardiac twin more than 0.20 carries a good prognosis [4].

Since there is a high risk of perinatal complications, an early diagnosis and prompt intervention is needed to improve outcomes for pump twin. As a goal to protect the pump twin from bad outcome like prematurity and cardiac failure, various techniques have been proposed which are minimally invasive. The blood flow from pump twin to acardiac mass is interrupted by therapeutic interventions, thus reducing the hemodynamic overload on pump twin and eliminating the risk of cardiac failure and perinatal complications in it, resulting in better outcome. This is done under ultrasound guidance using a fetoscope [5]. Bipolar cord coagulation and Radiofrequency ablation are the two commonly used techniques [9]. The survival rate of pump twin after therapy with laser coagulation and RFA is 75-80% [10]. Others less commonly used techniques include cord occlusion by embolization, ligation, laser photocoagulation, alcohol injection, interstitial laser etc [5].

Although Invasive procedures are proposed to improve outcomes but they also carry an equal risk of procedure related pregnancy loss. Randomized control trials on timing to do interventional therapy are required [9]. Most recently, a novel non-invasive technique of High Frequency Ultrasound [HIFU] has been used which stops blood flow across vascular channels in TRAP but it also needs further development and research [9].