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Perinatal Polycythemia and Hyperviscosity Syndrome


Andrew W. Walter

, MS, MD, Sidney Kimmel Medical College at Thomas Jefferson University

Reviewed/Revised Dec 2022

Polycythemia is an abnormal increase in red blood cell mass, defined in neonates as a venous hematocrit 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 hematocrit measurement. Treatment is with partial exchange transfusion.

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.


Causes of true polycythemia include intrauterine hypoxia, perinatal asphyxia, placental transfusion (including twin-to-twin transfusion), some congenital abnormalities (eg, cyanotic congenital heart disease Overview of Congenital Cardiovascular Anomalies Congenital heart disease is the most common congenital anomaly, occurring in almost 1% of live births ( 1). Among birth defects, congenital heart disease is the leading cause of infant mortality... read more Overview of Congenital Cardiovascular Anomalies , renovascular malformations Renal Anomalies The urinary tract is a common location for congenital anomalies of varying significance. Many anomalies are asymptomatic and diagnosed via prenatal ultrasonography or part of a routine evaluation... read more , congenital adrenal hyperplasia Overview of Congenital Adrenal Hyperplasia Congenital adrenal hyperplasia is a group of genetic disorders, each characterized by inadequate synthesis of cortisol, aldosterone, or both. In the most common forms, accumulated hormone precursors... read more ), 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 Diabetes Mellitus in Pregnancy Pregnancy makes glycemic control more difficult in preexisting type 1 (insulin-dependent) and type 2 (non–insulin-dependent) diabetes but does not appear to exacerbate diabetic retinopathy,... read more , Down syndrome Down Syndrome (Trisomy 21) Down syndrome is an abnormality of chromosome 21 that can cause intellectual disability, microcephaly, short stature, and characteristic facies. Diagnosis is suggested by physical anomalies... read more Down Syndrome (Trisomy 21) or other trisomies, Beckwith-Wiedemann syndrome, and intrauterine growth restriction Small-for-Gestational-Age (SGA) Infant Infants whose weight is < the 10th percentile for gestational age are classified as small for gestational age. Complications include perinatal asphyxia, meconium aspiration, polycythemia... read more . Polycythemia is also more common when the mother resides at a high altitude.

Symptoms and Signs

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.


  • Hematocrit

  • Clinical evaluation

Diagnosis of polycythemia is by arterial or venous (not capillary) hematocrit because capillary samples often overestimate hematocrit. Most published studies of polycythemia use spun hematocrits, which are no longer routinely done and are generally higher than those done on automated counters.

Diagnosis of hyperviscosity syndrome is clinical. Laboratory measure of viscosity is not readily available.

Other laboratory abnormalities may include low blood glucose and calcium ion levels, red blood cell (RBC) lysis, thrombocytopenia (secondary to consumption with thrombosis), hyperbilirubinemia Neonatal Hyperbilirubinemia Jaundice is a yellow discoloration of the skin and eyes caused by hyperbilirubinemia (elevated serum bilirubin concentration). The serum bilirubin level required to cause jaundice varies with... read more (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)

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.

Key Points

  • 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.

NOTE: This is the Professional Version. CONSUMERS: View Consumer Version
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