Definition

• Polycythemia is defined as an increase in hematocrit levels greater than two standard deviations above the mean for gestation and postnatal age.
• It is diagnosed when the peripheral venous hematocrit is 65% or more.
• Alternatively, it is diagnosed when the venous hemoglobin concentration is greater than 22 g/dL.
• Hyperviscosity is defined as a viscosity greater than 14.6 centipoise at a shear rate of 11.5 per second.
• The incidence of polycythemia varies from 1.5% to 4% of all live births.

Pathophysiology And Hyperviscosity

• The viscosity of blood is directly proportional to hematocrit and plasma viscosity. It is inversely proportional to the deformability of red blood cells.
• The relationship between viscosity and hematocrit is nearly linear up to a hematocrit of 60% to 65%.
• Blood viscosity increases exponentially at a hematocrit of 70% or greater.
• Increased viscosity impairs tissue oxygenation and decreases blood flow. This leads to an increased risk of microthrombus formation.
• Microthrombi can cause significant damage if they occur in the cerebral cortex, kidneys, and adrenal glands.
• In the early postnatal period, hematocrit peaks around 2 hours of age. It may normally reach up to 71% due to transudation of fluid out of the intravascular space.
• Hematocrit gradually declines to 68% by 6 hours and stabilizes by 12 to 24 hours of life.

Etiology And Risk Factors

• The causes of polycythemia can be categorized into increased erythropoiesis and secondary transfusions.

Clinical Features

• Most infants with polycythemia are asymptomatic. Approximately 50% of neonates develop one or more symptoms.
• The symptoms are often non-specific and may be related to underlying conditions.

Screening Protocol

• Routine screening of asymptomatic newborns is not recommended.
• Screening is indicated for high-risk neonates. These include SGA, large for gestational age (LGA), and infants of diabetic mothers.
• Mono-chorionic twins (especially the larger twin) and infants with morphological features of intrauterine growth restriction require screening.
• The initial screening is performed at 2 hours of life. If levels are high, it is repeated at 6, 12, 24, and 48 hours.
• A peripheral venous sample is preferred. Capillary samples overestimate the hematocrit by 5% to 15%.
• High capillary hematocrit values must always be confirmed with a venous sample.

Management

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graph TD
    %% Node Definitions
    A([Venous hematocrit: 65% or more]) --> B(Exclude dehydration<br>Check weight loss)
    
    B --> C(Symptomatic)
    B --> D(Asymptomatic)
    
    C --> H(Partial exchange<br>transfusion PET)
    
    D --> E(Hematocrit:<br>75% or more)
    D --> F(Hematocrit:<br>70-74%)
    D --> G(Hematocrit:<br>65-69%)
    
    E --> H
    F --> I(Consider<br>hydration)
    G --> J(Monitor for<br>symptoms)

Initial Assessment

• Exclude dehydration before diagnosing polycythemia. Check for excessive weight loss greater than 10% to 15%.
• If dehydration is present, correct it by increasing fluid or feed intake. Re-evaluate the hematocrit after correcting dehydration.
• Exclude associated metabolic problems like hypoglycemia.

Management Based On Symptoms And Hematocrit

• Management depends on the presence of symptoms and the absolute venous hematocrit value.

Symptomatic Neonates

• If the venous hematocrit is 65% or more and the neonate is symptomatic, partial exchange transfusion (PET) is indicated.
• Symptoms like jitteriness may persist for 1 to 2 days following PET despite lowering the hematocrit.

Asymptomatic Neonates

• Hematocrit 65% to 69%: Merely observe the infant. Monitor for symptoms and repeat the hematocrit in 4 to 6 hours.
• Hematocrit 70% to 74%: Manage conservatively with hydration. Add extra fluids or feeds of 20 mL/kg/day to achieve hemodilution.
• Hematocrit 75% or more: These infants are usually managed with PET.

Partial Exchange Transfusion (PET) Procedure

• PET involves removing a calculated volume of blood and replacing it with normal saline. The target hematocrit is 55%.
• Crystalloids like 0.9% normal saline are preferred over colloids. They are less expensive, equally effective, and carry no risk of transfusion-associated infections.
• The peripheral venous route is considered safer than the umbilical venous route. Umbilical vein use for PET may be associated with an increased incidence of NEC.
• The volume of exchange is calculated using the following formula: $$Volume (mL) = \frac{\text{Blood volume} \times (Observed \ Hematocrit - Desired \ Hematocrit)}{ Observed \ Hematocrit.}$$
• The assumed blood volume is approximately 80 to 90 mL/kg for term babies. It is 90 to 100 mL/kg for preterm babies.
• As a general rule of thumb, the volume of blood exchanged is usually 15 to 20 mL/kg of body weight.

Outcomes Of Therapy

• PET rapidly reverses physiological abnormalities associated with hyperviscosity. It improves cerebral blood flow, cardiac function, and capillary perfusion.
• There are no proven long-term neurodevelopmental benefits of PET in asymptomatic infants or those with minor symptoms.