Chapter 7: Diseases of Infancy and Childhood

It is difficult to precisely identify the diseases of infancy and childhood because many disorders that begin in childhood become clinically evident in adulthood if a long period of time is required to generate a pathologic lesion. The topics chosen for inclusion in this chapter are disorders presenting almost exclusively in childhood. However, they obviously represent only a small fraction of “childhood disorders.” Many disorders in other sections of this book, such as hemophilia and numerous infections, occur or are diagnosed primarily in childhood. The chapter begins with an overview of prenatal and neonatal laboratory testing.

Prenatal Testing and Screening

The disorders that can be diagnosed before birth number in the thousands. In families in which there is a history of a particular disorder, it is not uncommon to test prenatally for that particular disorder, often with DNA-based diagnostic tests. However, for the vast majority of families without a history of a specific illness, prenatal screening may also be undertaken. A screen differs from a test in that it does not provide a definitive diagnosis but rather an assessment of the risk of a diagnosis. For most of these families, screening is preferred as an initial step because it is less invasive; for example, there are several maternal serum screening assays (see below) for fetal Down syndrome (also known as trisomy 21), but testing for fetal Down syndrome requires an invasive procedure such as chorionic villus sampling or collection of amniotic fluid. The decision to screen is a personal one for families and includes considerations such as parental age and desire to avoid having a diseased child.

Neonatal Screening

Neonatal screening was originally developed to detect diseases such as phenylketonuria (PKU) and congenital hypothyroidism, for which early detection and intervention could prevent catastrophic consequences such as intellectual disability (mental retardation). Improvements in assay methodology, such as decreases in cost and the availability of tandem mass spectrometry for single assay detection of many abnormalities, have expanded the neonatal screening menu to include assays for diseases that are not preventable but are often treatable. All 50 US states screen for over 26 disorders, including amino acidurias (such as PKU and maple syrup urine disease), organic acidemias (such as isovaleric acidemia), fatty acid disorders (such as medium-chain acyl-CoA dehydrogenase [MCAD] deficiency), hemoglobinopathies associated with hemoglobin S, congenital hypothyroidism, congenital adrenal hyperplasia, cystic fibrosis, and classical galactosemia, among others. As new methods and new assays are developed, some variation between states' neonatal screening menus will inevitably continue to exist; these variations are tracked on a Web site maintained by the National Newborn Screening & Global Resource Center.

As with any screening program, positive results require follow-up confirmation testing; false-positives do occur. Suggested actions and algorithms for neonatal screen positives are published on the Web by the American College of Medical Genetics and Genomics (ACMG). Urgent intervention is required for some of these disorders, to preserve life or prevent intellectual disability.

Neonatal Testing

The laboratory evaluation of an infant who appears clinically well in the first 24 hours of life but develops signs of illness on the second or third day may include:

• Blood gases to detect metabolic acidosis/alkalosis

• Urinalysis to detect ketonuria

• Complete blood count to detect abnormalities in blood cells

• A blood glucose test to detect hypoglycemia

• A blood ammonia test to detect elevated ammonia

• Liver function tests to detect hepatic dysfunction

• Prothrombin time and partial thromboplastin time to detect coagulopathies

• Blood lactate to detect lactic acidosis

Table 7–1 lists a number of screening laboratory tests that are typically ordered when there is suspicion that a neonate (or older child) is suffering from an inborn error of metabolism.

The results of these tests only suggest specific disorders, with additional testing required to identify a specific metabolic defect. Definitive tests to make a conclusive diagnosis of a metabolic disorder often involve the measurement of specific enzyme activities or various metabolites in a pathway. Because sepsis is often suspected, it must be ruled out in the sick infant if there are any signs or symptoms of infection.

Description

A major cause of neonatal mortality and morbidity is prematurity, defined as birth prior to 37 weeks of gestation. When preterm labor or premature rupture of membranes causes prematurity, the underlying etiology is not often apparent, although it is believed to be commonly associated with infection or inflammation. Maternal correlates of prematurity include diabetes, obesity, intervention for infertility, genital or urinary infection, periodontal disease, low socioeconomic status, and other factors. Conflicting information exists about the value of intervention such as use of antibiotics for infection or infection risk.

Another cause of prematurity is iatrogenic, when the medical condition of the mother and/or fetus compels intervention to produce early delivery. The timing of such elective intervention for early delivery is influenced by the risk for fetal organ immaturity. Principal among these concerns is lung immaturity that is associated with the development of respiratory distress syndrome in the newborn.

Diagnosis

Risk of preterm delivery can be assessed by measurement of fetal fibronectin in cervical or vaginal fluid. This glycoprotein is produced by fetal membranes and appears in the cervix and vagina early in pregnancy as implantation develops, but normally disappears by week 20. Its reappearance in the third trimester often precedes labor and delivery. Its chief clinical value lies in its negative predictive value, that is, patients thought to be at risk for preterm labor who are negative for fetal fibronectin in their cervicovaginal fluid are very unlikely to deliver within 1 week of the laboratory result. The major barrier to the widespread use of the fetal fibronectin test, when positive, is that clinical interventions to end preterm labor are only partially successful.

In those instances when fetal or maternal health dictates early delivery, there are several tests available to assess fetal lung maturity. A simple and inexpensive test is to count lamellar bodies in amniotic fluid, using the platelet channel in a conventional hematology automated analyzer. These lamellar bodies are surfactant-containing products of Type II pneumocytes. The finding of greater than 50,000 lamellar bodies per microliter of amniotic fluid predicts lung maturity. If fewer bodies are present, further testing on the amniotic fluid sample is warranted. Other tests include identification of the presence of phosphatidylglycerol (PG), and determination of the ratio of lecithin to sphingomyelin (L/S ratio). (See Chapter 14 for additional information.)

Description

Down syndrome is the most commonly encountered, clinically significant autosomal chromosome aberration affecting individuals beyond infancy. This genetic defect, which can be detected by cytogenetic analysis, is trisomy 21. More than 90% of Down cases occur as a result of meiotic nondisjunction. Down syndrome is characterized by intellectual disability, cardiac malformations, malformations of the digestive tract, eyes, and ears, and the development of an Alzheimer-like disease process in later life.

The overall birth prevalence of Down syndrome is approximately 1 in 1000 births. However, a woman's individual risk to deliver an infant with Down syndrome depends substantially on her age. The risk increases significantly past age 35 years, with an incidence in the range of 1:270 to 1:100 by age 40 years.

Screening and Diagnosis

The neonatal diagnosis of Down syndrome is clinical, with metaphase chromosome analysis on peripheral blood serving merely to confirm the diagnosis.

Noninvasive fetal screening for Down syndrome involves many more tests (Table 7–2) that are used in combination to develop a risk assessment of Down syndrome during pregnancy. Definitive diagnosis of fetal Down syndrome during pregnancy is established by an invasive test, namely, metaphase analysis of cells from either chorionic villus sampling (typically limited to first trimester) or amniotic fluid collection. The decision to engage in fetal screening for Down syndrome or to move from screening tests to invasive diagnostic testing once a risk assessment is completed depends on patient preference. The invasive tests to assess for Down syndrome during pregnancy do carry a risk of miscarriage.

First-trimester screening typically consists of measurement of 2 analytes in maternal serum: pregnancy-associated plasma protein A (PAPP-A) and the free beta subunit of human chorionic gonadotropin (fβhCG); the former is low and the latter high in mothers carrying a Down syndrome fetus. A third part of first-trimester screening is ultrasound assessment for nuchal translucency, which is increased as a result of fluid accumulation in the neck of a Down syndrome fetus. This first-trimester screening is associated with a sensitivity of approximately 85%, with a 5% false-positive rate. Nuchal translucency alone is not recommended in singleton pregnancy because its sensitivity is only about 70%. However, in multiple gestation pregnancies, the interpretation of maternal serum markers can be problematic, while the nuchal translucency test permits evaluation of each fetus. Determination of nuchal translucency is highly operator-dependent and requires specific training.

Second-trimester screening typically consists of the so-called quadruple screen of maternal serum, consisting of measurement of the following analytes: alpha-fetoprotein, unconjugated estriol (both decreased in mothers carrying a Down syndrome fetus), and total hCG and inhibin A (both increased in such mothers). The quadruple screen has a detection rate of approximately 81% with a 5% false-positive rate. An older test, the “triple screen” includes all of these second-trimester markers except inhibin A. It is characterized by higher false-positive rates and lower detection rates. Maternal serum results are typically described in the form of “multiples of the median” (or MoM); the normal range is highly dependent on several factors including gestational age, number of gestations, maternal weight, and race.

Combining first- and second-trimester screens can provide an even higher level of detection. One approach is to sequentially conduct the tests; that is, inform the patient of the results of the first-trimester screen as soon as they are available (this permits her to choose a more definitive diagnostic method if indicated), and later perform the quadruple screen in the second trimester if appropriate. A noninvasive test that rivals the sensitivity and specificity of invasive testing is DNA sequencing of circulating cell-free fetal DNA (ccffDNA) in maternal serum. This test could also identify other chromosome aneuploidy syndromes such as trisomy 18 and trisomy 13. Current recommendations for follow-up of positive ccffDNA sequencing results are to confirm the abnormality by invasive testing (metaphase analysis on chorionic villus or amniotic fluid samples), but a negative ccffDNA sequencing test in a patient whose first- or second-trimester screen is positive for Down syndrome may provide an option to forgo invasive testing.

A final point about maternal serum screening is that the alpha-fetoprotein assay in the quadruple screen, if elevated, provides a measure of increased risk for neural tube defects such as spina bifida. These cases can be further studied by amniotic fluid collection, with assessment of acetylcholinesterase as well as alpha-fetoprotein (both elevated with neural tube defects) and high-resolution ultrasound examination.

Description

A number of maternal infections affect the fetus and newborn. Bacterial vaginosis, sexually transmitted diseases, and others increase the risk of preterm labor. Rubella and syphilis are associated with congenital anomalies. Neonatal death has been linked to a number of infections, including cytomegalovirus (CMV), group B streptococcus, herpes simplex, Listeria, parvovirus, among others. Postnatal disease in the offspring occurs with many infections, such as hepatitis B and C, human immunodeficiency disease (HIV), CMV, rubella, toxoplasmosis, and syphilis.

Screening and Diagnosis

Routine prenatal care is designed to screen for several of these infections, for the purpose of identifying pregnant women who need intervention or identifying susceptibility for a poor outcome in the mother. The results of maternal screening tests may have implications for the fetus, especially when they indicate maternal infection during pregnancy. An example of the latter is rubella serology screening of maternal serum. Routine testing includes serologic testing for syphilis, hepatitis B and C, and HIV. Routine rectovaginal culture for group B streptococcus is performed late in the third trimester. Detection through nucleic acid testing and/or culture is carried out for Chlamydia and for gonorrhea in high-risk mothers, and for herpes simplex in mothers with genital lesions.

Description

Hemolytic disease of the newborn (HDN), also known as erythroblastosis fetalis, is a syndrome in which the newborn becomes anemic from the destruction of his/her RBCs in utero. This RBC destruction is a result of maternal IgG antibodies formed against a red cell antigen, most commonly the Rh antigen (also known as D antigen), which are then delivered into the fetal circulation across the placenta. Antibody production by a mother who is Rh-negative results from exposure to Rh-positive fetal cells during pregnancy and, to a much greater extent, at delivery. Therefore, the women at greatest risk for delivering infants with HDN are Rh-negative mothers who conceive Rh-positive babies, and are in the second or subsequent pregnancies. It is general practice to identify risk of sensitization during pregnancy and treat the mother prophylactically by rapidly removing Rh-positive fetal cells through passive immunization with Rh immune globulin. Such immunizations are almost always effective in preventing the mother's immune system from developing these alloantibodies. Treatment is employed not only following delivery, when fetal to maternal bleeding is expected, but also during pregnancy itself.

Other red cell alloantibodies may be involved, although much less commonly. A form of HDN, usually mild, results from transplacental passage of IgG-class antibodies against A or B red cell antigens, to a Type A, B, or AB fetus. This disorder may occur with the first pregnancy, as it involves maternal antibodies that normally arise without the requirement of a previous pregnancy or incompatible blood transfusion.

Neonatal disease related to maternal antibody may occasionally involve targets other than RBC antigens; examples include neonatal alloimmune thrombocytopenic purpura (NAIT) with maternal antiplatelet antibodies.

Screening and Diagnosis

ABO/Rh typing is used in routine prenatal care to identify the mother's blood type. An antibody screen against a standard panel of red cells, employing the indirect antiglobulin method (see Chapter 2), is also routinely performed to determine if there are maternal alloantibodies (such as anti-Rh) that might be a threat to the fetus.

Testing for fetal blood type (which establishes presence or absence of fetal susceptibility in that pregnancy), as well as monitoring for fetal anemia and hyperbilirubinemia (the latter a consequence of the RBC destruction), typically requires invasive procedures such as amniocentesis (withdrawal of amniotic fluid) or cordocentesis (withdrawal of blood from the cord, in utero). Both of these carry some risk of pregnancy loss or fetal damage. A new, noninvasive approach is possible with genetic testing for the antigen genes. Genotyping of the father that reveals homozygosity of the antigen gene implies fetal antigen positivity. Heterozygosity in the father implies a 50% chance of fetal antigen negativity, in which case there would be absence of risk for HDN. Confirmation of fetal antigen negativity can be accomplished by genotyping of fetal DNA that is present in maternal plasma. Titering the quantity of maternal antibody as a disease predictor in the fetus has been used, but the amount of antibody does not always correlate with the severity of the disease. Testing of amniotic fluid for bilirubin by spectrophotometric analysis can be performed to help manage the disease; fetuses at high risk of severe anemia, based on the amniotic fluid bilirubin, would be delivered if tests of fetal lung maturity on the amniotic fluid sample (see above) reveal a low risk of respiratory distress syndrome. Otherwise, intrauterine blood transfusion and exchange could be performed. A noninvasive ultrasound test can detect fetal middle cerebral artery blood flow rates that correlate well with the presence of fetal anemia. This test has been shown to be more sensitive, specific, and accurate than the invasive amniotic fluid bilirubin measurement.

Laboratory evaluation of newborns for HDN includes complete blood count and direct antiglobulin test (DAT) on cord blood. The latter test detects the presence of immunoglobulin and/or complement deposited on the surface of red blood cells. See Table 7–3 for a list of tests that are helpful in the laboratory evaluation of HDN.

Description

Cystic fibrosis is an autosomal recessive disease that results from a mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) gene on the long arm of chromosome 7. The clinical presentation is dysfunction of exocrine glands, from abnormal chloride conduction across the apical membrane of epithelial cells, and subsequently chronic obstructive lung disease and exocrine pancreatic insufficiency. The most commonly found mutation in the CFTR gene is δF508 (loss of a phenylalanine codon at position 508), and it is present in approximately 70% of cases. However, more than 1800 mutations in this gene have been described, most of which are rare and some of which are equivocally associated with disease.

The incidence of cystic fibrosis is approximately 1 in 2500 to 3400 Caucasian births, 1 in 17,000 births from individuals of African descent, and 1 in 90,000 Asian births. As many as 1 in 29 Caucasians may be a carrier for cystic fibrosis.

Screening and Diagnosis

Extending an offer to screen the carrier status of couples planning for pregnancy is now recommended, especially for Caucasians (who are at highest risk) and those with a family history of disease. The carrier screen is a molecular genetic test designed to detect the most common mutations, typically around 23 in number.

Screening newborns for the disease is increasingly common. The usual approach is to screen blood spots for immunoreactive trypsinogen (IRT) and then follow up elevated values with either a multiple-mutation DNA test or repeat IRT on a new sample.

Laboratory evaluation for screen-positive patients includes the same test used in the assessment of patients clinically suspected of having the disease: quantitative sweat chloride. This analysis involves testing to determine whether the patient exhibits elevated levels of chloride in sweat samples; these samples are typically elicited with the use of pilocarpine and iontophoresis.

Genetic testing for disease detection (as opposed to screening) involves a larger number of mutation investigations, typically more than 75. If necessary, nucleotide sequencing of the gene can be performed; this procedure would typically be limited to the proband within a family; if an unusual mutation is found, other family members can be tested with sequencing of DNA from the involved exon rather than the entire gene.

Five classes of gene mutations, each specific for a different type of CFTR malfunction, have been described. The development of CFTR-modulating drugs, specific for a particular class of mutations, makes the genetic characterization of CF all the more important. Table 7–4 presents the laboratory evaluation of cystic fibrosis.

Description and Diagnosis

Amino acidurias are defects in the metabolism of amino acids that lead to their accumulation. The primary amino acidurias are a result of an inherited enzyme defect within a degradative pathway for a specific amino acid, or in a transporter in the renal tubules, that alters the reabsorption of the amino acid. Selected primary amino acidurias associated with impaired amino acid degradation are shown in Table 7–5. The defective enzymes and the amino acid or other compound in elevated concentration are listed for the individual disorders. In contrast to primary amino acidurias, secondary amino acidurias are accumulations of amino acids that arise when the organs responsible for their elimination or degradation, notably the liver and kidney, are functionally impaired. The diagnosis of a primary amino aciduria may be made by demonstrating a decrease in a specific enzyme activity required for the metabolism of the amino acid or by finding the associated abnormality in the gene coding for the enzyme. The presence of characteristic clinical signs and symptoms for the individual disorders is highly contributory toward establishing a diagnosis. The different amino acidurias vary from essentially benign, with no apparent disease, to lethal disorders. Furthermore, the clinical features can vary widely within a single disease, because some of the amino acidurias have multiple forms and because some enzyme deficiencies can result in an elevation of the same amino acid.

Description and Diagnosis

Lysosomal storage diseases, like the amino acidurias, are inborn errors of metabolism. The lysosomal storage diseases result from the lysosomal accumulation of compounds that should otherwise be degraded by the enzymes in the lysosome. The diagnosis of a lysosomal storage disease is made by identifying the products stored in the tissues or excreted in the urine. Demonstration of an enzyme deficiency is usually sufficient for diagnosis of a specific lysosomal storage disease. With some disorders, DNA analysis for the mutation producing the enzyme deficiency also can be performed. As with the amino acidurias, a number of lysosomal storage disorders have subtypes, because different enzyme deficiencies may result in the accumulation of similar or identical compounds. Table 7–6 provides a list of selected lysosomal storage disorders grouped into those associated with impaired degradation of sphingolipids (the sphingolipidoses), mucopolysaccharides (the mucopolysaccharidoses—the more recently invoked term for mucopolysaccharide is glycosaminoglycan), and glycogen. Among the lysosomal storage diseases, Tay–Sachs, one of the sphingolipi–doses, has been the most thoroughly studied. As a class of disorders, the lysosomal storage disorders are rare. The one with the highest prevalence is Gaucher disease, which affects 1 in 600 in the Ashkenazi Jewish population. Each lysosomal storage disorder has its own characteristic clinical features, and most signs and symptoms of these diseases are expressed early in life. Many of these disorders can even be diagnosed in utero or in the period immediately after birth. There is often an urgency to establish the diagnosis so that appropriate treatment can be instituted as soon as possible.

Description

Neuroblastoma is a solid tumor that affects infants and toddlers. It is the most common malignancy under 1 year of age, and most patients are diagnosed by age 2 years. It presents either as a mass, often in the abdomen or neck, or with signs and symptoms of tumor spread, including bone pain or spinal cord dysfunction. Neuroblastoma is one of the “small round cell” tumors of childhood, and it must be distinguished from other tumors of that type, which include lymphoma, rhabdomyosarcoma, Ewing sarcoma, and primitive neuroectodermal tumor (PNET).

Diagnosis

Measurement of urinary catecholamines can be used to alert clinicians to the likelihood that an infant's tumor mass is a neuroblastoma, as the other “round cell tumors” listed above do not excrete catecholamines. Unlike pheochromocytoma (see Chapter 22), urinary metanephrines are not helpful, as neuroblastomas do not produce an abundance of epinephrine. For the diagnosis and monitoring of neuroblastoma, vanillylmandelic acid (VMA) and homovanillic acid (HVA) are typically measured in urine by high-performance liquid chromatography (HPLC) methodology.

The treatment of neuroblastoma patients can be followed with quantitation of VMA and HVA in their urine. Detection of metastases in bone marrow samples of these patients is challenging with histologic methods as there are often very few tumor cells, but molecular studies are now being used to detect minimal residual disease. Examples of such assays being used for this purpose include reverse transcriptase-polymerase chain reaction (RT-PCR) for tyrosine hydroxylase (an enzyme involved in catecholamine synthesis) and for the proto-oncogene MYCN. The latter gene is of interest because its copy number in neuroblastoma tumor samples has prognostic significance. Amplification of MYCN is associated with a poorer prognosis in these patients. There is interest in developing other biomarkers for neuroblastoma; chromogranin A and neuron-specific enolase are 2, among several, in use or in development.