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Acardiac twin

Acardiac twins (or recipient twins) are haemodynamically disadvantaged non-viable twins that undergo secondary atrophy in association with a twin reversed arterial perfusion sequence.

Epidemiology

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.

Pathology

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 the acardiac twin. 

  • 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
Sub types

An acardiac twin can present with four morphological types:

  • acardius anephus:
    • most common type 
    • there is absence of the head and upper torso and limbs in the acardiac twin with preservation of the lower limbs, genitalia and abdominal viscera. 
  • acardius anceps:
    • most developed type
    • rudimentary cranial structures present with otherwise persistent trunk, limbs and organs
    • however lacks even a rudimentary heart 
  • acardius amorphus:
    • 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
  • acardius acormus:
    • 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
Associations

Radiographic features

Ultrasound

While features can vary with each sub types, 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
  • umbilical cord to the acardiac twin is often quite short and may be difficult to identify
Doppler assessment

Shows reversed flow through the umbilical arteries to the affected fetus.

Complications

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 the majority of efforts in management are focused or maintaining 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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