(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 red blood cell 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 hematocrit mimics polycythemia, but red blood cell 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 of hyperviscosity syndrome are those of heart failure, thrombosis (cerebral and renal vessels), and central nervous system 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.
Diagnosis of polycythemia is by arterial or venous (not capillary) hematocrit. Diagnosis of hyperviscosity syndrome is clinical. Capillary samples often overestimate hematocrit, so a venous or arterial hematocrit should be obtained before the diagnosis is made; most published studies of polycythemia use spun hematocrits, 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; red blood cell (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).
Asymptomatic infants should be treated with IV hydration (see treatment of dehydration in children). Symptomatic infants with hematocrit > 65 to 70% should undergo an isovolemic hemodilution (sometimes called partial exchange transfusion, although no blood products are given) to reduce the hematocrit 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 hematocrit 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.
Polycythemia in neonates is a venous hematocrit ≥ 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], central nervous system dysfunction) or mild (tremulousness, lethargy, or hyperbilirubinemia).
Treat with IV hydration and sometimes partial exchange transfusion.