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I. DEFINITION

Thrombocytopenia is defined as a platelet count <150,000/µL and is further classified as mild (100,000–149,000/µL), moderate (50,000–99,000/µL), or severe (<50,000/µL).

II. INCIDENCE

Thrombocytopenia is more common in preterm (20%–30%) than term (<1%) infants. It is the most common hematologic abnormality among sick newborn infants in the neonatal intensive care unit (NICU). Pseudothrombocytopenia (lab error due to clumping) accounts for 15% of all isolated thrombocytopenia.

III. PATHOPHYSIOLOGY

  1. Normal platelets. Similar to older children and adults, the platelet life span in neonates is 7 to 10 days, and the mean platelet count is >200,000/µL.

  2. Etiology of thrombocytopenia. See Figure 128–1.

    1. Maternal disorders causing thrombocytopenia in infant

      1. Chronic intrauterine hypoxia is the most frequent cause of thrombocytopenia in preterm neonates in the first 72 hours of life. This is seen in cases of placenta insufficiency such as diabetes and pregnancy-induced hypertension.

      2. Preeclampsia (in particular with HELLP syndrome [hemolysis, elevated liver enzymes, low platelet count]). Thrombocytopenia is present at birth, is usually associated with neutropenia, and should recover by the second week of life.

      3. Maternal medications (eg, heparin, quinine, hydralazine, tolbutamide, and thiazide diuretics).

      4. Infections (eg, TORCH [toxoplasmosis, other infections, rubella, cytomegalovirus, and herpes simplex virus] infections, bacterial or viral infections). Not a component of perinatal Zika infection.

      5. Disseminated intravascular coagulation.

      6. Antiplatelet antibodies

        1. Antibodies against maternal and fetal platelets (autoimmune thrombocytopenia)

          1. Idiopathic thrombocytopenic purpura

          2. Drug-induced thrombocytopenia

          3. Systemic lupus erythematosus

        2. Antibodies against fetal platelets (isoimmune thrombocytopenia)

          1. Neonatal alloimmune thrombocytopenia is the most common cause of severe thrombocytopenia, seen mostly in term infants <72 hours of age. It is due to an incompatibility in human platelet antigen (HPA) between the newborn infant and its HPA-negative mother. HPA-1a is the most common incompatibility in Caucasians, whereas HPA-4b incompatibility is mostly seen in Asians. Only 10% of HPA-1a–negative women become sensitized after being exposed to HPA-1a because this immunologic response occurs only in the presence of specific human leukocyte antigens (HLAs) such as HLA-B8, HLA-DR3, and HLA-DR52a. HLA antibodies, although common, do not by themselves cause significant thrombocytopenia.

          2. Immune thrombocytopenia can be found in some cases of hemolytic disease of the newborn.

    2. Placental disorders causing thrombocytopenia in infant (rare): Chorioangioma, vascular thrombi, and placental abruption.

    3. Neonatal disorders causing thrombocytopenia

      1. Decreased platelet production

        1. Isolated. No identified cause.

        2. Thrombocytopenia and absent radius syndrome is characterized by normal neutrophil and red blood cell counts; absent radii, usually bilateral; and the presence of a normal thumb.

        3. Fanconi anemia is characterized by pancytopenia and the presence of an abnormal (hypoplastic or aplastic) thumb.

        4. Rubella syndrome.

        5. Congenital leukemia.

        6. Trisomies 13, 18, or 21 or Turner syndrome.

        7. Inherited metabolic disorders include methylmalonic, propionic, and isovaleric acidemia; ketotic glycinemia.

        8. Congenital amegakaryocytic thrombocytopenia.

      2. Increased platelet destruction

        1. Many “sick” newborns develop thrombocytopenia that is not associated with any specific pathologic state. About 20% of newborns admitted to the NICU have thrombocytopenia, and 20% of those counts are <50,000/µL. This form of thrombocytopenia generally improves after the primary sickness (respiratory distress syndrome, persistent pulmonary hypertension of the newborn) resolves.

        2. Pathologic states associated with thrombocytopenia

          1. Sepsis. Bacterial and Candida species.

          2. Congenital infections. TORCH infections, especially cytomegalovirus (CMV). Neonates with human immunodeficiency virus (HIV) and Enterovirus frequently have thrombocytopenia.

          3. Thrombosis (major blood vessels, intracardiac).

          4. Disseminated intravascular coagulation.

          5. Intrauterine growth restriction.

          6. Birth asphyxia.

          7. Necrotizing enterocolitis or bowel ischemia.

          8. Platelet destruction associated with giant hemangioma (Kasabach-Merritt syndrome).

  3. Platelet dysfunction

    1. Drug-induced platelet dysfunction

      1. Maternal use of aspirin

      2. Indomethacin

    2. Metabolic disorders

      1. Phototherapy-induced metabolic abnormalities

      2. Acidosis

      3. Fatty acid deficiency

      4. Maternal diabetes

    3. Inherited thrombasthenia (Glanzmann disease)

IV. RISK FACTORS

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