Staff Obstetrician and Gynecologyst, Fetal Medicine Unit, Assistant Professor, San Marcos National University,
Instituto Especializado Materno Perinatal - Lima - Perú

Synonyms: Acardiac monster, Holocardius, Chorioangiopagus parasiticus or Twin Reversal Arterial Perfusion Sequence (TRAP)

Definition: The acardiac monster is the most extreme manifestation of the feto-fetal transfusion syndrome. Most of the acardiac monsters have been reported in monochorionic twins (98%) but it is true that there are some cases reported in dichorionic and dizygotic pregnancies with fused placentas (2%). It is also frequent in triplets.

Case report: A 23 year-old primigravida went to her first antenatal care visit at the Institute Maternal Perinatal in Lima, at 32 weeks of gestation, as determined by the date of her last menstrual period. She was referred with abdominal pain and swelling of the lower limbs. She had no relevant medical or family history, but she brought the report of an obstetric ultrasound examination that was performed six weeks before, which revealed the presence of an active fetus in breech presentation with a left sided back. The biparietal diameter was 65 mm, the femur length 47 mm, the amniotic fluid was increased slightly and the placenta was anterior. Furthermore the presence of a hypoechoic solid mass of 204 x 117 mm, with hyperechoic areas that would correspond to osseous structure, was described. The ultrasound report concluded that this was a 26 weeks viable single pregnancy in breech presentation, with mild polyhydramnios. The mass in amniotic cavity was probably an in utero death of a second twin.

Physical examination of the patient revealed a blood pressure of 140/90 mmHg, fundal height of 44 cm and no audible fetal heart beat. A digital vaginal examination was postponed and the patient was admitted to the hospital with a diagnosis of a 32 weeks pregnancy with mild polyhydramnios, pregnancy- induced hypertension: mild pre-eclampsia and probable intrauterine fetal death. As an inpatient, an obstetric ultrasound was performed which confirmed the absence of fetal heart beat and the presence of a solid mass in the right half of uterus. A vaginal examination revealed 2 cm cervical dilation and gynecoid pelvis.
The medical team decided to continue with labor and ordered a coagulation profile which was normal. A macerated stillborn female fetus was delivered weighing 1,760 grams.

One hour and 30 minutes later, the acardiac twin was delivered. It weighed 1880 grams and had ambiguous genitalia. The placenta was delivered 25 minutes later. The placental diameter was 20 x 18 cm, it weighed 650 grams; and was described as monochorionic and biamniotic. The patient was sent to the operating theatre for curettage and was subsequently discharged 2 days later.

History:  The acardiac twin was first time described in 1553 by Benedetti and fully studied and classified by Geoffroy de Saint-Hilaire in 1836.

Prevalence:  It is reported to occur in about 1% of monochorionic pregnancies or 0.3 cases per 10,000 births (equivalent to 1 case per 35,000 births)

Etiology: It is most likely caused by placental anastomotic vascular connections between the twins leading to reversal of blood flow to one twin.

Pathogenesis: The pathophysiology of this problem involves an alteration in the arterial perfusion of one twin due to the existence of an arterio-arterial communication directly from the umbilical cord of the unaffected or pump fetus without a vascular connection to the placenta. Bloods enters the perfused fetus through its umbilical artery or arteries and exits through its umbilical vein, hence the term reversal arterial perfusion ⁷. Since the blood to the perfused fetus enters through the hypogastric arteries, the organs of the lower trunk are more likely to develop while those of the upper trunk are more likely to be absent or rudimentary. Those body parts that are present are usually abnormal secondary to perfusion with deoxygenated blood. 

Sonographic findings: The pump twin may show signs of hydrops, and the acardiac twin has no head or anencephaly with only the base of the brain present (rudimentary head). Sometimes the upper limbs are absent and club feet and absent toes are often seen. Although there are no cardiac pulsations, limbs movements are visible. Polyhydramnios is present and a single umbilical artery is seen in 50% of cases.

Implications for targeted examinations: Look in the normal fetus for signs of hydrops: cardiomegaly, hepatosplenomegaly, pleural effusion, ascites and abnormal Doppler.

Differential diagnosis: The acardiac twin may be mistaken for a dead anencephalic twin fetus.

Associated anomalies:  Omphalocele may be present in the acardiac twin.

Prognosis: All of the perfused twins die. The pump twin has a 50% mortality rate, with death most often caused by heart failure or death of the co-twin. 

Recurrence risk:  This anomaly has a sporadic inheritance pattern, and the recurrent risk is very small, although higher in diabetics.

Management: A number of therapeutic approaches have been attempted, including maternal digitalization, serial amniocentesis, maternal indomethacin therapy, and endoscopic ligation of the umbilical cord, thrombosis of the umbilical artery using the percutaneous placement of a thrombogenic coil or laser therapy and hysterotomy with selective delivery of the affected twin.
Serial ultrasound examinations are required every 1 to 2 weeks to assess the fetal status, and because of the high risk of prematurity and of delivering a compromised infant a consultation with a neonatologist is mandatory to discuss management options with the family. Delivery should occur in a tertiary center.

References:
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2. Napolitani FD, Schereiber Y: The acardiac monster. Am J Obstet Gynecol 1960; 80: 582.
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4. James WH: A note on the epidemiology of acardiac monsters. Teratology 1977; 16:211-216
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6. Healey MG: Acardia: predictive risk factors for the co-twin. Teratology 1994; 50:205-213.
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8. Simpson PC, Trudinger BJ, Walker A, Baird PJ: The intrauterine treatment of fetal cardiac failure in a twin pregnancy with an acardiac cephalic monster. Am J Obstet Gynecol 1983; 147, 842-845
9. Donnenfield AE, van de Woestijne J, Craparo F, Smiths CS, Ludomirsky A, Wiener S: The normal fetus of an acardiac twin pregnancy: perinatal management based on echocardiographic and sonographic evaluation. Prenat Diagn 1991; 11:235-244
10. Quintero RA, Reich H, Puder KS, Bardicef M, Evans MI, Cotton DB, Romero R: Brief report: umbilical-cord ligation of an acardiac twin by fetoscopy at 19 weeks of gestation. N Engl J Med 1994; 330:469-471.
11. Sepulveda W, Bower S, Hassani J, Fisk NM: Ablation of acardiac twin by alcohol injection into the intra-abdominal umbilical artery. Obstet Gynecol 1995; 147:842-845.
12. Ville Y, Hyett JA, Vandenbussche FPHA, Nicolaides KH: Endoscopic laser coagulation of umbilical cord vessels in twin reversal arterial perfusion sequence. Ultrasound Obstet Gynecol 1994; 4:396-398.