Twin anemia-polycythemia sequence in monochorionic twins: implications for diagnosis and treatment.

Twin anemia-polycythemia sequence (TAPS) is a recently described complication of monochorionic placentation characterized by discordance in hemoglobin (Hgb) levels in the absence of amniotic fluid abnormality characteristic of classical twin-twin transfusion syndrome (TTTS). The placental angioarchitecture that predisposes to TAPS consists of small diameter arteriovenous anastomoses and the absence of balancing arterioarterial anastomoses. This vascular pattern occurs sporadically in 3 to 5% of monochorionic twins or iatrogenically following 2 to 13% of selective fetoscopic laser surgeries for TTTS. The diagnosis is based on measurement of the middle cerebral artery peak systolic velocity (MCA-PSV) which is not part of the Quintero staging for TTTS. With mild disease increased MCA-PSV in the anemic donor twin and a decreased MCA-PSV in the recipient twin are characteristic while severe disease is associated with critical Doppler findings, hydrops or single twin demise as in TTTS. Treatment options include fetoscopic laser, fetal blood transfusion, conservative management, and often preterm delivery. The most promising approach to TAPS is its prevention since the iatrogenic form comprises the majority of cases. When the fetoscopic laser technique is modified by coagulating the chorionic plate along the vascular equator (equatorial dichorionization or "Solomon" technique) the incidence of postlaser TAPS and recurrent TTTS is significantly reduced, survival is improved, and there is no increase in complications.