The neutrophil is a myeloid-derived white blood cell important in combating bacterial and fungal infections. For a more detailed discussion of neutrophil development and immunologic function, refer to chapters 49 and 51. Most neutrophil abnormalities in the neonate involve quantitative disorders of the neutrophil, with the vast majority of clinical issues involving decreased neutrophil counts or neutropenia. Neutropenia is defined as an absolute neutrophil count (ANC) less than 1500/mm3. In a newborn, there can be many causes, some acquired with expected resolution, such as increased utilization of neutrophils with low marrow reserve or failures of production or release from the bone marrow, or there can be congenital causes, which are expected to be lifelong. The patient may have different clinical presentations depending on the etiology. Because of the low neutrophil reserve (capacity to produce neutrophils) in the bone marrow, the neonate is especially prone to development of neutropenia. Differentiating the etiology of neutropenia in an infant is therefore dependent on thorough knowledge of the maternal (both pregnancy and delivery) and neonatal history, as well as the physical examination of the infant, in conjunction with appropriate laboratory tests.
INITIAL CLINICAL APPROACH
Because the severity and the cause of neutropenia may often be fatal in this patient population and because of the limitations of blood volume available for laboratory testing, a prompt but logical approach to the workup is essential. A thorough history can indicate if immediate assessment is necessary. Information, such as appropriate prenatal care, maternal medications, difficulties during pregnancy, fetal vital signs, complications during delivery, and Apgar scores should be included in the initial assessment. The past medical histories of parents and family members, in particular infections, early childhood deaths, or specific diagnoses, may suggest a congenital cause of neutropenia. In addition to assessment of the infant for possible sepsis, particular attention should be made to specific physical findings related to the different causes of neutropenia, such as splenomegaly, skin lesions, and physical anomalies. For example, infants with neonatal lupus may have neutropenia but often will have a rash and splenomegaly. Patients with congenital bone marrow failure syndromes may exhibit abnormal thumbs, be small for gestational age or have a low birth weight, or exhibit a classical facies/congenital abnormalities. Thus, the family history and the appearance of the infant, toxic vs nontoxic, will help narrow the differential diagnosis and thus guide the diagnostic tests and management.
Initial baseline diagnostic tests recommended include a complete blood cell count (CBC) with differential, calculation of the ANC (see Figure 110-1), chemistry, blood cultures, and chest x-ray. Abnormalities detected during physical examination, such as skeletal anomalies and enlarged spleen, warrant further imaging. If the degree of acuity has been thoroughly considered, management may occur depending on the differential diagnosis.