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Twin Anemia-Polycythemia Sequence (TAPS) Twin anemia-polycythemia sequence (TAPS) is a novel form of chronic feto-fetal transfusion, characterized by large inter-twin hemoglobin differences, without signs of twin oligo-polyhydramnios sequence (TOPS). The pathogenesis of TAPS is based on the presence of few, minuscule arterio-venous (AV) placental anastomoses (diameter <1mm) allowing a slow transfusion of blood from the donor to the recipient and leading gradually to highly discordant Hb levels. TAPS may occur spontaneously or after laser surgery for TTTS (post-laser form). The spontaneous form complicates approximately 3 to 5% of monochorionic twin pregnancies, whereas the post-laser form occurs in 2 to 13% of TTTS cases. The optimal management in TAPS is not clear, and includes expectant management, induction of labor, intrauterine blood transfusion (intravenous and/or intraperitoneal, with or without partial exchange transfusion) , selective feticide or fetoscopic laser surgery. The outcome in TAPS is not well known. International cooperation between fetal centers using a web-based TAPS registry is needed to improve our knowledge on the on the short- and long-term outcome in TAPS cases and determine the optimal management.

Monochorionic twin pregnancy complication
Monochorionic (MC) twin pregnancies are at increased risk of serious complications. The higher rate of perinatal mortality and morbidity is related directly to the unique angio-architecture of the MC placenta. All MC placentas have vascular anastomoses connecting the circulations of both fetuses. Imbalanced inter-twin blood flow is the accepted etiology in twin-twin transfusion syndrome (TTTS), of one of the most severe disorders in MC pregnancies. Recently (2007), a new complication in MC twins has been discovered, termed twin anemia-polycythemia sequence (TAPS). TAPS is a form of chronic feto-fetal transfusion, characterized by large inter-twin hemoglobin (Hb) differences, without signs of twin oligo-polyhydramnios sequence (TOPS). TAPS may occur spontaneously or after laser surgery for TTTS (post-laser form). The spontaneous form complicates approximately 3 to 5% of monochorionic twin pregnancies, whereas the post-laser form occurs in 2 to 13% of TTTS cases. The pathogenesis of TAPS is based on the presence of few, minuscule arterio-venous (AV) placental anastomoses (diameter <1mm) allowing a slow transfusion of blood from the donor to the recipient and leading gradually to highly discordant Hb levels.

Diagnosis
The diagnosis can be reached either antenatally or postnatally. Antenatal diagnosis of TAPS is based on Doppler ultrasound abnormalities showing an increased middle cerebral artery peak systolic velocity (MCA-PSV) in the donor twin, suggestive of fetal anemia (>1.5 MoM), and a decreased MCA-PSV in the recipient twin, suggestive of polycythemia (<1.0 MoM), in the absence of signs of TOPS. Postnatal diagnosis of TAPS is based on the presence of (chronic) anemia in the donor (including reticulocytosis) and polycythemia in the recipient, in association with typical placental angioarchitecture as identified by injection with colored dye (see figure). Postnatal hematological criteria include an inter-twin Hb difference > 8g/dL and a reticulocyte count ratio >1.7.

Treatment
The optimal management in TAPS is not clear, and includes expectant management, induction of labor, intrauterine blood transfusion (intravenous and/or intraperitoneal, with or without partial exchange transfusion)  , selective feticide or fetoscopic laser surgery. Fetoscopic laser surgery is the only causative treatment for this disease (in analogy with TTTS) but is technically challenging due to the absence of polyhydramnios and the presence of only minuscule anastomoses.

Perinatal Outcome
Perinatal outcome in TAPS is not well known and appears to vary according to the severity of TAPS, reflecting the heterogeneous aspect of TAPS. Outcome may range from double intrauterine fetal demise to 2 healthy neonates without major morbidity at birth besides large intertwin Hb discordance. Knowledge on the neonatal and long-term morbidity in TAPS is scarce and based on case reports and small series. Neonatal morbidity in TAPS appears to be mainly limited to hematological problems at birth. Donor twins may be severely anemic and require blood transfusions, whereas recipient twins may be severely polycythemic and require partial exchange transfusion. However, recently, several cases of severe cerebral injury in TAPS have also been described.

Future research focus
More research, either through randomized multicenter trials (ideally) or international cooperation between fetal centers using a web-based TAPS registry is needed to increase awareness, gather information on the short- and long-term morbidity in TAPS cases and evaluate the best treatment for TAPS.