Twin reversed arterial perfusion (TRAP) sequence: A case of an acardiac parabiotic twin

Abstract

Introduction

Twin Reversed Arterial Perfusion (TRAP) sequence is an uncommon and severe complication of monochorionic twin pregnancies, characterized by an acardiac twin lacking a functional heart and a pump twin that maintains circulation for both.

Presentation of case

We report a case involving a 17-year-old primigravida diagnosed with a monochorionic diamniotic twin pregnancy at 8 weeks gestation. At 35 weeks, ultrasound revealed a teratogenous mass measuring 11 cm × 9.4 cm in the right fundal area, with bone-like particles, suggestive of an acardiac twin. An anomaly scan confirmed one live fetus and one edematous, non-viable fetus. At 36 weeks, an elective lower segment cesarean section was performed, delivering a healthy neonate followed by a dysmorphic, non-viable fetus weighing 626 g. The acardiac twin lacked a distinguishable head, body, or limbs, except for a rudimentary limb at the caudal end. Histological examination revealed skin-covered tissue with appendages, fibrous tissue, nerve bundles, blood vessels, and mature adipocytes, but no developed organs.

Discussion

TRAP sequence results from abnormal vascular connections in the placenta, leading to reversed arterial perfusion of the acardiac twin. The pump twin is at risk of high-output cardiac failure, polyhydramnios, and preterm labour. Management strategies depend on the size ratio between twins and the pump twin's condition, ranging from expectant management to invasive interventions like radiofrequency ablation.

Conclusion

This case underscores the importance of early diagnosis and monitoring in monochorionic twin pregnancies to manage TRAP sequence effectively and improve outcomes for the pump twin.

Highlights

• •TRAP sequence is a rare complication in monochorionic twin pregnancies.
• •Early Doppler diagnosis improves pump twin survival in TRAP cases.
• •Large acardiac twin increases risk; intervention may be necessary.

1. Introduction

Twin Reversed Arterial Perfusion (TRAP) sequence is a rare and severe complication unique to monochorionic twin pregnancies, occurring in approximately 1 in 35,000 births [1]. It involves the development of an acardiac twin, which lacks a functional heart and relies on the pump twin for circulation through abnormal vascular anastomoses [2,3]. This reversed perfusion leads to significant risks for the pump twin, including high-output cardiac failure, hydrops fetalis, and increased perinatal mortality [1]. Early diagnosis through Doppler ultrasound is crucial for appropriate management and improving survival rates of the pump twin [4]. The work has been reported in line with the SCARE criteria [5].

2. Case presentation

A 17-year-old primigravida was confirmed to have a monochorionic diamniotic twin pregnancy at 8 weeks gestation. The antenatal period was largely uneventful, aside from moderate anemia detected in blood investigations. An anomaly scan confirmed one live fetus and one edematous, non-viable fetus at 22 weeks. No intrauterine intervention was performed due to limited resources; however, antenatal fetal echocardiography revealed no major cardiac anomalies, and diagnostic accuracy was reinforced by the observation of specific flow dynamics, including reversed flow in the umbilical artery. Regular counselling sessions arranged for parents. Again, findings on 35 weeks, a transabdominal ultrasound revealed a heterogeneous mass in the right fundal area measuring 11 cm × 9.4 cm, containing bone-like particles. An urgent anomaly scan indicated one live fetus and another with an edematous appearance, suggestive of an acardiac twin. At 36 weeks, an elective lower segment cesarean section was performed, delivering a healthy neonate (2.45 Kg) followed by a dysmorphic, non-viable fetus weighing 626 g. The weight ratio between the acardiac twin and the pump twin was 25.55 %, a relevant factor in assessing prognosis and guiding management.

The acardiac twin exhibited no distinguishable head, body, or limbs, except for a rudimentary limb at the caudal end. The umbilical cord was attached just below the head region. The pump twin was delivered without complications and is developing normally, both physically and mentally. (See Fig. 1, Fig. 2, Fig. 3, Fig. 4.) (See Table 1.)

Fig. 1.

External appearances.

Fig. 2.

Placenta.

Fig. 3.

Acardiac twin internal appearance.

Fig. 4.

Histology pictures a) Umbilical artery, b Skin, c) Lymphatic malformation.

Table 1.

Morphologic classification of acardiac twins with relative frequencies.

Subtype	Morphologic features	Frequency (%)
Acardiac acephalic	Pelvis and lower extremities are present without development of cranio-thoracic and upper extremity structures	60–75
Acardiac anceps	The trunk and extremities are developed with partial development of cranial structures	20
Acardiac acormus	Development of cranial structures only	10
Acardiac amorphous	Amorphous tissue mass	5

Histological examination revealed skin-covered tissue with appendages, fibrous tissue, nerve bundles, blood vessels, and mature adipocytes, but no developed organs such as the heart, lungs, liver, spleen, or kidneys. The placenta weighed 414 g and was monochorionic diamniotic without any retroplacental clots.

3. Discussion

An acardiac monster, also known as an acardiac twin or Twin Reversed Arterial Perfusion (TRAP) sequence, is a rare and severe complication of monochorionic twin pregnancies characterized by abnormal fetal development of one twin—the acardiac twin—which lacks a functioning heart and often other vital structures [4]. The condition arises due to arterio-arterial and veno-venous anastomoses in the shared placenta, where the pump twin supplies circulation to the malformed acardiac twin in a reversed flow pattern. This reversed perfusion causes the acardiac twin to receive deoxygenated blood, leading to poor development of cephalic structures and vital organs [1].

Based on morphology, acardiac twins are classified into four types: acardius acephalus (no head, most common), acardius anceps (partially formed head), acardius amorphous (no recognizable form), and acardius myelacephalus (presence of head and lower limbs) [4]. Ultrasound findings typically reveal a monochorionic diamniotic pregnancy with one normal fetus (the pump twin) and another mass or fetus with absent cardiac activity, reversed blood flow in the umbilical artery of the acardiac twin (confirmed via Doppler), and poorly developed or absent upper body structures [3]. The condition can lead to serious complications such as high-output cardiac failure in the pump twin, polyhydramnios, preterm labour, and a perinatal mortality rate of approximately 50 % for the pump twin [1]. But in this case, there were no signs of pump twin compromise such as cardiac strain or polyhydramnios throughout the pregnancy.

Recent literature emphasizes the importance of early detection and size ratio assessment between the twins in guiding management decisions. According to a study in their systematic review of 859 TRAP cases over 35 years, intervention significantly improved survival outcomes, with pump twin survival reaching 82.6 % among those receiving therapy compared to 64.5 % with expectant management. The most effective techniques included monopolar coagulation (89 %), laser ablation (80 %), and radiofrequency ablation (RFA) (78 %). The gestational age at intervention—preferably <16 weeks—was found to be crucial in determining outcomes [7].

A recent case report introduced microwave ablation (MWA) as a novel and promising alternative to RFA. This minimally invasive procedure led to the successful delivery of a healthy infant at 33 weeks, suggesting that MWA may be particularly useful where conventional methods are inaccessible [8].

Furthermore, another study reported a rare late-diagnosed case at 24 weeks that was successfully managed with combined RFA and intrauterine transfusion (IUT) due to developing hydrops in the pump twin. This dual approach helped stabilize the fetus, resulting in a healthy delivery at 36 weeks [9].

In addition to procedural advances, genetic and autopsy studies are shedding light on the underlying pathology of TRAP. A case reported by another author revealed previously undocumented structural malformations and chromosomal aberrations, such as abnormalities at 6q11.1 and 15q25.1, potentially offering insights into the etiology and developmental arrest seen in acardiac twins [10].

In one of the most unique presentations, another author described TRAP in conjoined monochorionic-monoamniotic twins, managed successfully with individualized vascular mapping and radiofrequency ablation. The pump twin survived to 35 + 1 weeks, underlining the importance of tailored, image-guided intervention in complex anatomical variants [11].

Management depends on the severity and may involve expectant monitoring if the acardiac twin is small and the pump twin is stable or intervention—such as radiofrequency ablation, laser coagulation, or bipolar cord occlusion—if the acardiac twin is large (weighing more than 70 % of the pump twin), if there are signs of cardiac strain in the pump twin, or if there is rapid polyhydramnios or growth of the acardiac twin. Prognosis largely depends on early detection, the size ratio between the twins, and the cardiovascular status of the pump twin, with timely intervention significantly improving survival outcomes [1,6].

4. Conclusion

This case highlights the critical importance of early diagnosis and monitoring in monochorionic twin pregnancies to identify and manage TRAP sequence effectively. Timely intervention can significantly improve outcomes for the pump twin, emphasizing the need for heightened awareness and surveillance in such high-risk pregnancies.

Consent

Written informed consent was obtained from the patient's parents/legal guardian for publication and any accompanying images. A copy of the written consent is available for review by the Editor-in-Chief of this journal on request.

Ethical approval

Hence our Institutional Review Board does not require ethical approval for reporting individual cases, ethical clearance is not necessary for this study.

Funding

No funding was received for this study.

Author contribution

All authors contributed to the conception, design, and writing of this case report. All authors have read and approved the final manuscript.

Guarantor

Dr. Sayanthan B.

Research registration number

N/A.

Conflict of interest statement

The authors declare no conflicts of interest.