Abstract
Ultrasound evaluation of middle cerebral artery peak systolic velocity (MCA-PSV) is the standard investigation for the diagnosis of fetal anemia. It has replaced serial amniocentesis for surveillance in red cell alloimmunization but has also proved effective in the detection of fetal anemia due to other disease states. MCA-PSV greater than 1.5 multiples of the median (MoM) for gestational age is an indication for diagnostic cordocentesis. Critically, the seminal work on this screening tool reported a sensitivity of 100% for the detection of moderate-to -severe fetal anemia. We present a case of significant fetal anemia associated with normal indices on repeated interrogation of the middle cerebral artery. A 25-year-old woman presented for a routine structural sonogram at 20 weeks’ gestation. Fetal hydrops was identified with severe ascites, scalp edema and a pericardial effusion. Maternal serology confirmed recent parvovirus seroconversion with positive IgG and IgM. MCA-PSV was 31 cm/s, corresponding to between 1.0 MoM and 1.29 MoM for this gestation. Despite the normal MCA-PSV, a diagnostic cordocentesis was performed which confirmed significant fetal anemia with haematocrit of 23%. Intrauterine transfusion (IUT) of 35cc packed red cells was performed. Repeat sonographic evaluation 4 days later showed persistent hydrops and MCA-PSV 23 cm/s (normal). Two weeks following IUT, fetal hydrops had resolved. MCA-PSV remained within normal limits. The remainder of the pregnancy was uncomplicated. This case illustrates the possibility of false negative MCA-PSV in the setting of fetal anemia. In the absence of another cause for fetal hydrops definitive diagnostic testing with fetal cordocentesis should be performed.
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