Morphologic and Electrophoretic Features of Hemoglobin Bart's Hydrops Fetalis

A healthy couple of Southeast Asian ethnicity was noted to have microcytosis during the first-trimester assessment. Hemoglobin (Hb) electrophoresis ruled out beta-thalassemia trait and Hb variants, but an α-globin gap-PCR assay confirmed that both parents were heterozygous for the - -SEA deletion (α⁰-thalassemia). Fetal ultrasound (US) at 21 weeks gestation showed no abnormalities, with biometry consistent with gestational age. However, a follow-up US at 27 weeks gestation revealed fetal growth restriction, an increased cardiothoracic index, pericardial effusion, hepatomegaly, and placentomegaly, suggestive of hydrops. Middle cerebral artery peak systolic velocity (MCA-PSV) was elevated, consistent with fetal anemia. After extensive consultation, the couple opted to proceed with intrauterine transfusion (IUT). Fetal complete blood count (CBC) showed microcytic hypochromic anemia for gestational age (Hb 92 g/L, MCV 104 fL, MCH 24 pg), thrombocytopenia (platelet count 108 × 10⁹/L), reticulocytosis (371 × 109/L), and significant erythroblastosis (nucleated red blood cells 104.7 × 109/L). A fetal blood smear (Figure 1 Panels A and B) further showed striking presence of Howell–Jolly bodies (arrows) and Pappenheimer bodies (arrowheads) compatible with splenic immaturity expected for age. The diagnosis of Hb Bart's hydrops fetalis (BHFS) was confirmed by Hb high-performance liquid chromatography (Bio-Rad Variant II, Bio-Rad Laboratories, Montreal, Canada) showing lack of Hb F and Hb A, with the presence of Hb Bart's (Figure 1 Panel C). Capillary electrophoresis (Minicap, Sebia, Montreal, Canada) further showed the presence of Hb Portland 1, Hb Gower 1, and likely epsilon tetramers (Figure 1 Panel D). α-Globin gap-PCR assay was compatible with - -/- - genotype. A repeat fetal US 1-week post-IUT showed improvement in hydrops markers and a reduction in MCA-PSV to normal range. After confirmation of the diagnosis and further consideration, the parents elected to terminate the pregnancy. BHFS is the most common cause of fetal hydrops in Southeast Asia. Previously considered universally fatal, advances in IUT and perinatal care have enabled increasing numbers of patients to survive, and stem cell transplantation may offer a curative option for a select few. The pathophysiology of this condition is unique, driven by the production of non-functional Hb Bart's (γ4) instead of normal fetal Hb (Hb F, α2γ2) due to the absence of α-globin production. This process begins early in pregnancy after the initiation of definitive erythropoiesis. The severity of hydrops often exceeds that is predicted by the degree of anemia observed in other causes of fetal anemia. This discrepancy is attributed to the high proportion of Hb Bart's, which has an extremely high oxygen affinity and does not participate in tissue oxygen delivery. Consequently, the degree of anemia estimated by MCA-PSV may not correspond with the severity of fetal hydrops. In affected fetuses, residual expression of ζ-globin results in the production of small amounts of Hb Portland 1 (ζ2γ2) typically <20% of total fetal Hb, which provides limited oxygen transport, allowing survival into the second or third trimester. Higher ζ-globin expression is thought to correlate with a milder phenotype. Early diagnosis and consideration of IUT for those who choose to continue their pregnancy after counseling, even before the appearance of hydrops features, are recommended. In the absence of rapid access to genetic testing, Hb analysis showing the absence of Hb F and Hb A, along with the presence of a large amount of Hb Bart's is diagnostic, and quantification of Hb Portland 1 (a functional oxygen transporter) on Hb electrophoresis may have prognostic value [1-3]. The authors declare no conflict of interest. Additional anonymized data are available on request from the authors.