Re: Review of the correlation between blood flow velocity and polycythemia in the fetus, neonate and adult: appropriate diagnostic levels need to be determined for twin anemia-polycythemia sequence. A. Lucewicz, K. Fisher, A.Henry and A. W. Welsh. Ultrasound Obstet Gynecol 2016; 47: 152-157.

Abstract

Linked Comment: Ultrasound Obstet Gynecol 2016; 47: 152–157 Lucewicz and colleagues review the fetal, neonatal and adult literature on the use of a decreased middle cerebral artery peak systolic velocity (MCA-PSV) to diagnose fetal polycythemia. They conclude there is too little evidence to assume reduced MCA-PSV in polycythemic fetuses and therefore a decreased MCA-PSV should not be part of the definition of twin anemia–polycythemia sequence (TAPS). An accurate diagnosis of TAPS is important because this transfusion imbalance occurs in about 5% of monochorionic twin pregnancies and, if undiagnosed, may lead to late intrauterine demise or long-term morbidity1. In TAPS, one twin becomes progressively anemic and the other becomes increasingly polycythemic due to a chronic transfer of red blood cells via minuscule and usually unidirectional anastomoses. Whereas twin–twin transfusion syndrome (TTTS) is essentially a severe amniotic fluid discordance prior to fetal viability, spontaneous TAPS is characterized by severe hemoglobin discordance after fetal viability. TAPS can also occur as a complication of incomplete laser surgery in which minuscule, usually unidirectional, anastomoses are missed. This iatrogenic form of TAPS generally manifests itself within 1 to 5 weeks after the procedure and complicates up to 13% of surviving twin pairs. TAPS is diagnosed antenatally based on the presence of an elevated MCA-PSV (>1.5 multiples of the median (MoM)) in the anemic donor in combination with a decreased MCA-PSV (≤ 1MoM) in the polycythemic recipient. Although an MCA-PSV > 1.5 MoM has been validated to diagnose fetal anemia, data are lacking on a decreased MCA-PSV to predict fetal polycythemia. Whereas fetal anemia occurs in different scenarios, such as alloimmunization, parvovirus infection, fetomaternal hemorrhage and the anemic twin of a TAPS pair, polycythemia is a problem only in the setting of TAPS and thus in about 1 in 10 000 pregnancies. The strength of the report by Lucewicz and colleagues is that it exposes this knowledge gap; although both fetal and neonatal data seem to support the concept that polycythemia reduces blood flow velocity in the brain. A limitation of the report is that it does not bring forward any new data to fill the gap. However, in June last year, a joint publication by three European tertiary care centers2 showed that an MCA-PSV ≤ 1 MoM (according to the reference ranges by Klaritsch et al.3) accurately predicts polycythemia in the setting of TAPS with a sensitivity of 94% (95% CI, 71–99%) and a specificity of 100% (95% CI, 91–100%). Therefore, an MCA-PSV > 1.5 MoM in one twin coinciding with an MCA-PSV ≤ 1 MoM in the other remains the best antenatal definition for TAPS – until a better alternative is found.