Background
The term polycythemia means an increased cell number. However, it is mostly used to refer specifically to increased circulating red blood cell (RBC) mass. RBC mass is estimated using the hematocrit (Hct) measurement, which is defined as the percentage of RBCs in a given volume of blood. Term newborns usually have a higher Hct (51 ± 7%) compared to older children and adults. This increased Hct is a normal compensatory mechanism in these infants for the relative tissue-level hypoxia that is prevalent in the intrauterine environment, and it is exacerbated by the high affinity of fetal hemoglobin for oxygen. [1, 2] Polycythemia in the newborn is defined as a central venous Hct over 65% or a hemoglobin value above 22 g/dL.
Pathophysiology
Most of the clinical significance of polycythemia is related to the associated increase in blood viscosity. [3] Blood viscosity increases linearly with increased Hct, but it can increase exponentially when Hct is greater than 42%. In addition to RBC mass several other factors also determine blood viscosity. [4] These include blood pH, RBC deformability, plasma protein concentration, platelet and white blood cell (WBC) volume as well as endothelial factors. Most of these factors are not significantly different between infants who are otherwise well. Therefore, RBC mass (or Hct) is the primary determinant of neonatal blood viscosity, and up to 50% of infants with polycythemia have hyperviscosity, although only 24% of infants with hyperviscosity have a diagnosis of polycythemia. [5]
Because the increased viscosity of a fluid and a smaller radius of the conduit through which it flows can increase the internal resistance of the fluid according to Poiseuille's law, polycythemia-induced hyperviscosity increases the resistance of blood to flow, especially in the microcirculation. The resultant decreased microcirculatory perfusion and increased risk for thrombosis and ischemia of end organs are the factors responsible for most of the complications associated with polycythemia. Increased blood viscosity can cause hypoperfusion directly by decreasing blood flow (as is seen in the lungs and the renal system), whereas other organs can become hypoperfused due to changes in arterial oxygen content (as noted in the brain). [6]
Etiology
Increased circulating RBC mass in the newborn could be secondary to actively increased RBC production by the fetus or the newborn, or due to passive transfusion of RBCs into the fetal or neonatal circulation.
Increased fetal erythropoiesis
Increased fetal erythropoiesis is usually a fetal response to intrauterine stress and fetal hypoxia associated with increased fetal oxygen consumption resulting in fetal hypoxia that could be related to several primary etiologic factors. Most of these conditions are also associated with intrauterine growth restriction (IUGR). Underlying causes include the factors outlined below.
Placental insufficiency
Placental insufficiency could be secondary to the following:
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Preeclampsia
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Primary renovascular disease
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Chronic or recurrent abruptio placenta
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Maternal cyanotic congenital heart disease
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Postdate pregnancy
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Maternal smoking
Endocrine abnormalities
These include congenital thyrotoxicosis and maternal diabetes with poor glycemic control.
Genetic disorders
Genetic conditions that increase fetal erythropoiesis include the following:
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Trisomy 13
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Trisomy 18
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Trisomy 21
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Beckwith-Wiedemann syndrome
Hypertransfusion
Polycythemia-hyperviscosity could also be secondary to increased blood volume secondary to transfusion of blood either from maternal or sibling fetal sources.
Placental-fetal transfusion
Animal studies have suggested that acute fetal hypoxia can lead to increased fetal blood volume before birth, but this is unlikely to lead to significant polycythemia. [7] Delayed cord clamping (DCC) allows for an increased blood volume to be delivered to the infant. When cord clamping is delayed more than 3 minutes after birth, blood volume increases 30%. However, potential complications of DCC include polycythemia and hyperbilirubinemia. [8] Gravity, because of the position of the delivered infant in relation to the maternal introitus, and oxytocin release could also be causative factors that increase the volume of blood that is transfused into the newborn infant's circulation during DCC.
Several studies have examined the incidence of polycythemia as a potential complication when DCC is practiced. A study of 242 newborns whose cords were clamped at less than 60 seconds, between 1 minute and just under 2 minutes, or between 2 and 3 minutes following birth found that their Hct values at 48 hours after birth were 53%, 58% and 59% respectively. [9] Ferritin and hemoglobin levels also increased in association with later cord clamping. In addition, the number of infants with polycythemia was significantly higher in the group that was clamped at 2-3 minutes, but none of the infants from any of these groups required treatment for symptoms related to polycythemia-hyperviscosity. [9]
A more recent study of 73 infants showed that DCC at 5 minutes after birth did not lead to an increased incidence of polycythemia when compared to early cord clamping. [10] Another study that compared early cord clamping before 10 seconds after delivery with DCC at 3 minutes or later found no differences in the incidence of polycythemia at age 4 months in these infants. [11] Thus, although DCC increases Hct levels, currently available evidence indicates that there is minimal risk for symptomatic polycythemia that requires management.
Twin-to-twin transfusion syndrome
Twin-to-twin transfusion syndrome (TTTS) due to a vascular communication occurs in approximately 10% of monozygotic twin pregnancies. In intrapartum asphyxia, blood volume is shifted from the placenta to the fetus.
Monochorionic diamniotic (MCDA) twin pregnancies with amniotic fluid discordance appear to increase the risk of development of twin anemia-polycythemia sequence (TAPS), a form of TTTS, by nearly two-fold. [12]
Epidemiology
Polycythemia is a relatively common disorder, occurring in 1-5%% of neonates. [13] It is more common in infants who are small for their gestational age (SGA) and in infants who are large for their gestational age (LGA). Infants born at higher elevations also have a higher incidence. However, most infants with polycythemia are of appropriate size or weight for their gestational age (AGA). Infants of mothers with diabetes have a polycythemia incidence of 10-30%.
Age-related demographics
The central venous Hct level peaks 6-12 hours after birth and then declines until the infant is aged 24 hours, at which time it equals the Hct level in cord blood. Fewer than 40% of infants with a Hct level above 64% at 2 hours still have a high value at 12 hours or later.
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