Twin reversed arterial perfusion (TRAP) sequence is a rare complication of monochorionic pregnancies. It develops when the following conditions are present:
1. lack of a well formed heart in one of the twins (so-called acardiac twin), and
2. a superficial artery to artery placental anastomosis providing perfusion of the acardiac twin by the donor (pump) twin. This can be shown on Doppler ultrasound by reversal of the acardiac twin umbilical arterial blood (i.e. flow towards the fetus).
It is thought to occur in ~0.5% (range 0.3-1 % 1,4) of monozygotic pregnancies with an estimated incidence of 1:35,000 births overall 1,10.
It can rarely occur with higher order multifetal pregnancies 3.
TRAP is considered to primarily result from an abnormal placental arterial-to-arterial anastomosis. Venovenous anastomoses may also be present 11.
Classically this results in one normal and one abnormal twin:
- viable twin (pump twin)
- usually normal, but 9% risk of abnormal karyotype
- non-viable twin (recipient/acardiac twin)
The acardiac twin is haemodynamically disadvantaged receiving deoxygenated blood from donor twin which then supplies iliac arteries first. This will result in more perfusion compromise in the upper part of the body so atrophy of the heart and dependent organs (brain) are more pronounced. Eventually, a characteristic set of anomalies including acardia and acephalus develop.
- severely abnormal, with variable degrees of morphologic development (see morphologic types above).
- the heart is absent or structurally abnormal, with <20% of fetuses having identifiable cardiac tissue15
- most commonly the head and upper extremities are absent or severely underdeveloped, while the lower body is better developed
- may be amorphous with no distinguishable anatomic structures
- severe subcutaneous edema and cystic hygromas common
- single umbilical artery common
- often, but not invariably is structurally normal.
- multiple anomalies have been described, including
- at risk high output cardiac failure
- on Doppler interrogation, flow in the acardiac twin umbilical arteries seen entering the fetus and blood leaving via the umbilical vein 13
Fetal MRI may be considered as an adjunct to ultrasound, to evaluate non-cardiac related abnormalities in the pump twin 15.
TRAP sequence pump twin is at risk of:
- cardiac failure
- cerebral ischemic sequelae
- preterm birth
- fetal demise in utero
The risk of adverse outcomes in the pump twin is closely related to its size ratio to the acardiac twin.
Treatment and prognosis
As the acardiac twin is non-viable, the majority of efforts in management are focused or maintaining the viability of the other donor (pump) twin. The perinatal mortality for the pump twin can be as high as 50% 10.
Treatment is around the surgical destruction of the inter-twin anastomosis and includes:
- endoscopic laser coagulation/radio-frequency ablation 5
- surgical (fetoscopic) ligation of acardiac twin umbilical cord
- selective delivery of acardiac twin 7
- 1. Chandramouly M, Namitha . Case series: TRAP sequence. Indian J Radiol Imaging. 2009;19 (1): 81-3. doi:10.4103/0971-3026.45352 - Free text at pubmed - Pubmed citation
- 2. Rizzuto MI, Cottam R, Bradley RJ. Twin reversed arterial perfusion (TRAP) sequence: a rare late finding in a monochorionic twin pregnancy. J Obstet Gynaecol. 2009;29 (6): 549-50. doi:10.1080/01443610902780831 - Pubmed citation
- 3. Cavoretto P, Serafini A, Valsecchi L et-al. Early diagnosis, follow-up, and prenatal treatment of a case of TRAP sequence occurring in a dichorionic triamniotic triplet pregnancy. J Clin Ultrasound. 37 (6): 350-3. doi:10.1002/jcu.20583 - Pubmed citation
- 4. Bornstein E, Monteagudo A, Dong R et-al. Detection of twin reversed arterial perfusion sequence at the time of first-trimester screening: the added value of 3-dimensional volume and color Doppler sonography. J Ultrasound Med. 2008;27 (7): 1105-9. J Ultrasound Med (full text) - Pubmed citation
- 5. Livingston JC, Lim FY, Polzin W et-al. Intrafetal radiofrequency ablation for twin reversed arterial perfusion (TRAP): a single-center experience. Am. J. Obstet. Gynecol. 2007;197 (4): 399.e1-3. doi:10.1016/j.ajog.2007.07.051 - Pubmed citation
- 6. Coulam CB, Wright G. First trimester diagnosis of acardiac twins. Early Pregnancy. 2000;4 (4): 261-70. - Pubmed citation
- 7. Hanafy A, Peterson CM. Twin-reversed arterial perfusion (TRAP) sequence: case reports and review of literature. Aust N Z J Obstet Gynaecol. 1997;37 (2): 187-91. - Pubmed citation
- 8. Malhotra N, Sinha A, Deka D et-al. Twin reversed arterial perfusion: report of four cases. J Clin Ultrasound. 2004;32 (8): 411-4. doi:10.1002/jcu.20056 - Pubmed citation
- 9. Rodeck CH, Whittle MJ. Fetal medicine, basic science and clinical practice. Elsevier Health Sciences. (2008) ISBN:0443104085. Read it at Google Books - Find it at Amazon
- 10. Sepúlveda WH, Quiroz VH, Giuliano A et-al. Prenatal ultrasonographic diagnosis of acardiac twin. J Perinat Med. 1993;21 (3): 241-6. - Pubmed citation
- 11. Pezzati M, Cianciulli D, Danesi G. Acardiac twins. Two case reports. J Perinat Med. 1997;25 (1): 119-24. - Pubmed citation
- 12. Entezami M, Albig M, Knoll U et-al. Ultrasound Diagnosis of Fetal Anomalies. Thieme. (2003) ISBN:1588902129. Read it at Google Books - Find it at Amazon
- 13. Salamipour H, Weissleder R, Wittenberg J et-al. Primer of diagnostic imaging. Mosby. ISBN:032302422X. Read it at Google Books - Find it at Amazon
- 14. Joseph C. Sommerfeldt, Rita E. Putnins, Karen Fung Kee Fung, David Grynspan, Khaldoun Koujok. AIRP Best Cases in Radiologic-Pathologic Correlation: Twin Reversed Arterial Perfusion Sequence. (2014) RadioGraphics. 34 (5): 1385-90. doi:10.1148/rg.345130043 - Pubmed
- 15. Beth M. Kline-Fath, Ray Bahado-Singh, Dorothy Bulas. Fundamental and Advanced Fetal Imaging. (2019) ISBN: 9781451175837