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Perinatal Polycythemia and Hyperviscosity Syndrome

By Andrew W. Walter, MS MD, Associate Professor of Pediatrics; Attending Physician, Sidney Kimmel Medical College at Thomas Jefferson University; A. I. duPont Hospital for Children

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Polycythemia is an abnormal increase in RBC mass, defined in neonates as a venous Hct 65%; this increase can lead to hyperviscosity with sludging of blood within vessels and sometimes thrombosis. The main symptoms and signs of neonatal polycythemia are nonspecific and include ruddy complexion, feeding difficulties, lethargy, hypoglycemia, hyperbilirubinemia, cyanosis, respiratory distress, and seizures. Diagnosis is made clinically and with an arterial or venous Hct measurement. Treatment is with partial exchange transfusion.

(Prenatal and perinatal changes in erythropoiesis are discussed in Perinatal Physiology.)

The terms polycythemia and hyperviscosity are often used interchangeably but are not equivalent. Polycythemia is significant only because it increases risk of hyperviscosity syndrome. Hyperviscosity is a clinical syndrome caused by sludging of blood within vessels. Sludging occurs because increased RBC mass causes a relative decrease in plasma volume and a relative increase in proteins and platelets.

Incidence of polycythemia is about 3 to 4% (range 0.4 to 12%), and about half of infants with polycythemia have hyperviscosity.


Dehydration causing relative hemoconcentration and an elevated Hct mimics polycythemia, but RBC mass is not increased.

Causes of true polycythemia include intrauterine hypoxia, perinatal asphyxia, placental transfusion (including twin-to-twin transfusion), some congenital abnormalities (eg, cyanotic congenital heart disease, renovascular malformations, congenital adrenal hyperplasia), certain delivery procedures (eg, excessively delayed cord clamping, holding neonate below the level of the mother before cord clamping, stripping the cord toward the neonate at delivery), maternal insulin-dependent diabetes, Down syndrome or other trisomies, Beckwith-Wiedemann syndrome, and intrauterine growth restriction. Polycythemia is also more common when the mother resides at a high altitude.

Premature infants rarely develop hyperviscosity syndrome.

Symptoms and Signs

Symptoms and signs of hyperviscosity syndrome are those of heart failure, thrombosis (cerebral and renal vessels), and CNS dysfunction, including tachypnea, respiratory distress, cyanosis, plethora, apnea, lethargy, irritability, hypotonia, tremulousness, seizures, and feeding problems. Renal vein thrombosis may also cause renal tubular damage, proteinuria, or both.


  • Hct

  • Clinical evaluation

Diagnosis of polycythemia is by arterial or venous (not capillary) Hct. Diagnosis of hyperviscosity syndrome is clinical. Capillary samples often overestimate Hct, so a venous or arterial Hct should be obtained before the diagnosis is made; most published studies of polycythemia use spun Hcts, which are no longer routinely done and are generally higher than those done on automated counters. Laboratory measure of viscosity is not readily available.

Other laboratory abnormalities may include low blood glucose and calcium ion levels, maternal diabetes, or both; RBC lysis; thrombocytopenia (secondary to consumption with thrombosis); hyperbilirubinemia (caused by turnover of a higher number of RBCs); and reticulocytosis and increased peripheral nucleated RBCs (caused by increased erythropoiesis secondary to fetal hypoxia).


  • IV hydration

  • Sometimes phlebotomy plus saline replacement (partial exchange transfusion)

Asymptomatic infants should be treated with IV hydration (see treatment of dehydration in children). Symptomatic infants with Hct > 65 to 70% should undergo an isovolemic hemodilution (sometimes called partial exchange transfusion, although no blood products are given) to reduce the Hct to 55% and thereby decrease blood viscosity. Partial exchange is done by removing blood in aliquots of 5 mL/kg and immediately replacing it with an equal volume of 0.9% saline. Asymptomatic infants whose Hct remains persistently > 70% despite hydration may also benefit from this procedure.

Although many studies show immediate measurable effects of partial exchange, the long-term benefits remain in question. Most studies have failed to document differences in long-term growth or neurodevelopment between children who have received a partial exchange transfusion in the neonatal period and those who have not.

Key Points

  • Polycythemia in neonates is a venous Hct 65%.

  • Hyperviscosity is a clinical syndrome involving sludging of blood within vessels and sometimes thrombosis.

  • Manifestations are varied and can be severe (heart failure, thrombosis [cerebral and renal vessels], CNS dysfunction) or mild (tremulousness, lethargy, or hyperbilirubinemia).

  • Treat with IV hydration and sometimes partial exchange transfusion.