An acardiac twin, also known a recipient twin, refers to the haemodynamically disadvantaged twin of a twin-pair in the setting of twin reversed arterial perfusion (TRAP) sequence. The acardiac twin undergoes secondary atrophy and is non-viable.
Acardiac twinning is thought to affect 1 in 100 monozygotic twin pregnancies and 1 in 35,000 pregnancies overall 2,3. There is no recognised familial recurrence.
The acardiac twin undergoes secondary atrophy of the heart and dependent organs (brain) and often develops a characteristic set of anomalies including acardia and acephalus. This twin exerts abnormal strain on the opposite (pump) twin by stealing its circulation.
There are two schools of thought in the pathogenesis of acardiac twins:
- some propose that the primary defect is one of cardiac embryogenesis (dysmorphogenesis)
- others consider the primary cause being an abnormal vascular communication between embryos in the placenta with arterial to arterial communication leading to reversed flow of blood to the haemodynamically disadvantaged or recipient twin, with resulting secondary atrophy of the heart and dependent organs 5
An acardiac twin can present with four morphological types:
- most common type
- there is an absence of the head and upper torso and limbs in the acardiac twin with preservation of the lower limbs, genitalia and abdominal viscera
- most developed type
- rudimentary cranial structures present with otherwise persistent trunk, limbs and organs
- however lacks even a rudimentary heart
- least differentiated type
- comprises of an amorphous mass of bone, muscle, fat and connective tissue
- if rudimentary nerve tissue is present, it is then called acardius myelantencephalus
- rarest type
- the only developed structure is the fetal head
- all other structures are essentially absent
- the umbilical cord insertion is directly into the fetal head
- pathologically, rudiments of thoracic structures may be present
While features can vary with each subtype, general features include
- fetal biometric discordance
- marked edema (fetal anasarca) of the affected twin with cystic changes
- normal or accelerated growth of the lower extremities due to an imbalance in the interfetal circulation.
- retrograde supply of the desaturated blood to the upper body and head leads to maldevelopment of the heart, head, and upper torso, which are completely absent or severely deficient
- the umbilical cord to the acardiac twin is often quite short and may be difficult to identify
Shows reversed flow through the umbilical arteries to the affected fetus.
The other donor (pump) twin may develop cardiac failure (hydrops) with a reported mortality of 50-75%.
Treatment and prognosis
The acardiac twin is non-viable and management is aimed at maintaining the viability of the other (donor/pump) twin, including close surveillance for development of hydrops. Interrupting blood flow to the acardiac twin may be performed by various methods which include hysterotomy and removal of the acardiac twin, ligation of the umbilical cord and/or laser ablation of vessels.
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