Bacterial Pneumonia



Bacterial Pneumonia


John F. Modlin



In the United States, infection of the lower respiratory tract is recognized annually in 15 to 20 of 1,000 infants younger than 1 year of age and in 30 to 40 of 1,000 children 1 to 5 years old. Whereas respiratory viruses and Mycoplasma pneumoniae are the most common agents of lower respiratory tract disease in children and young adults, the weight of evidence suggests that pyogenic bacteria are responsible for 20% to 40% of cases. Bacterial pneumonia is observed most commonly in the winter and early spring and occurs almost twice as frequently in boys as in girls.

Diseases involving the airways, such as bronchopulmonary dysplasia, cystic fibrosis, and bronchiectasis, and such anatomic defects as cleft palate or tracheoesophageal fistula predispose affected individuals to the development of bacterial pneumonia. Pneumonia and lung abscesses are common infections in children with severe cognitive neurologic
disorders or diminished levels of consciousness. Children with hemoglobinopathies, especially sickle cell disease, demonstrate higher rates of bacterial pneumonia. Children who are immunodeficient on the basis of inherited or acquired disease or immunosuppressive therapy are at increased risk of developing pneumonia from a wide spectrum of bacteria and at additional risk from viruses, fungi, protozoa, and parasites. The diagnosis and management of opportunistic pulmonary infections that occur in immunocompromised children are covered in Chapter 232.

In practice, distinguishing bacterial pneumonia from other forms of pneumonia usually is difficult because infants and young children do not produce adequate sputum for Gram stain and culture and because routine chest radiographs do not commonly demonstrate abnormalities specific for bacterial infection. Furthermore, invasive procedures are warranted only in severely ill patients or in patients with underlying immunodeficiency.


ETIOLOGY

Age and the presence of underlying disease are the two most important patient characteristics determining the cause of bacterial pneumonia. Bacterial pneumonia presenting in the patient’s first 5 days of life, which generally is considered to be acquired in utero or intrapartum, is caused by the same organisms responsible for generalized neonatal sepsis (i.e., group B streptococci, Listeria monocytogenes, Haemophilus influenzae, and gram-negative enteric bacilli). Most cases of perinatally acquired pneumonia occur among low-birth-weight infants and infants who have peripartum complications. These infants may be at increased risk of developing nosocomial pneumonia caused by Pseudomonas aeruginosa, Escherichia coli, or other gram-negative bacilli if they require endotracheal intubation and mechanical ventilation. Chlamydia trachomatis, a pathogen acquired from the maternal genital tract at delivery, may not cause respiratory symptoms until the child’s second month of life.

After the neonatal period, Streptococcus pneumoniae, Moraxella catarrhalis, and group A streptococci are responsible for most cases of bacterial pneumonia in otherwise healthy children. Neisseria meningitidis seldom is reported. Staphylococcus aureus pneumonia now is seen rarely in infants and toddlers, and pneumonia caused by Bordetella pertussis and H. influenzae type b (Hib) has been controlled effectively in the United States and other developed countries by universal immunization. M. pneumoniae and Chlamydia pneumoniae infections are uncommon findings in children younger than age 3 years.

For school-aged children and adolescents, the etiologic spectrum for bacterial pneumonia narrows to M. pneumoniae, C. pneumoniae, and S. pneumonia. Many sources cite M. pneumoniae as the most common cause of bacterial pneumonia between 5 and 30 years of age.


PATHOGENESIS

The respiratory tract below the vocal cords normally is sterile; microorganisms are excluded from the tracheobronchial tree by nonspecific host defenses, including the blanket of mucus covering the mucosal epithelium, the ciliary transport activity, and the cough reflex. Secretory immunoglobulin A antibody present in mucosal secretions helps to protect against reinfection with specific organisms. Within the lung parenchyma, bacteria are cleared by lymphatic channels that drain to regional lymph nodes and by macrophages that line the terminal bronchioles and alveoli. The lung is protected also by systemic humoral and cell-mediated immune mechanisms, including passively acquired maternal antibody, which protects against pneumococcal and Hib infections in the child’s first 4 to 6 months of life. Alteration of any of these protective mechanisms is likely to predispose the child to the development of bacterial pneumonia.

Bacterial pathogens that cause pneumonia in children are transmitted from person to person by close personal contact or airborne spread. Colonization of young children’s upper respiratory tract with pathogenic bacteria is a common finding; the prevalence of carriage of pneumococci is 25% to 40% during the winter months and somewhat lower during other seasons. Pneumonia results from aspiration of pathogenic bacteria into the lower respiratory tract; often, the process is aided by concurrent viral infection, particularly with the influenza viruses and measles virus. Careful studies have documented the coexistence of a respiratory virus in 30% to 50% of children with bacterial pneumonia. Acute viral infection serves to disrupt the normal anatomic and physiologic barriers of the respiratory tract mucosa and briefly may suppress the activity of phagocytic leukocytes in the airways and lung.

Less commonly, bacteria spread to the lung hematogenously from a distant focus (e.g., bacterial endocarditis or septic jugular venous thrombosis). In such instances, pneumonia may occur in the setting of widespread pyogenic infection or other sites, including the meninges, bones, and joints.


CLINICAL MANIFESTATIONS

The signs and symptoms of bacterial pneumonia vary with affected children’s ages, with the organism, and with the presence or absence of underlying disease. Characteristically, older children and adolescents present with fever, chills, headache, dyspnea, productive cough, chest pain, abdominal pain, and nausea or vomiting. However, infants are likely to have mostly nonspecific symptoms of fever, lethargy, poor feeding, vomiting, or diarrhea. Apnea may be a prominent sign among infants younger than 2 months. Tachypnea may be overlooked by the parents, and cough, if present, often is not a prominent finding in very young infants. Similarly, the physical examination findings in young infants with pneumonia are less definitive; usually, percussion and auscultation do not elicit the characteristic dullness to percussion and decreased breath sounds found in older children and adults with pneumonia, and distinguishing rales from the sounds produced by a congested upper respiratory tract may be difficult.


DIAGNOSIS

In practice, the diagnosis of pneumonia is made with the demonstration of infiltrates on anteroposterior and lateral chest radiography. False-negative results sometimes are attributed to dehydration. Conversely, many noninfectious diseases of the lung, including malignancy, collagen-vascular disease, congestive heart failure, pulmonary embolus, allergic alveolitis, pulmonary hemorrhage, hemosiderosis and drug toxicity, may mimic the radiographic appearance of pneumonia. A pattern of peribronchial or “patchy” infiltrates (bronchopneumonia) does not distinguish viral or M. pneumoniae pneumonia from bacterial pneumonia, but demonstration of hyperinflation is most consistent with viral infection. The presence of lobar consolidation or pleural effusion suggests bacterial infection, whereas pneumatoceles and abscess cavities are virtually diagnostic (Fig. 233.1). Lobar consolidation, which is present in approximately one-half of children with bacterial pneumonia, correlates with the presence of bacteremia. Roentgenograms demonstrate bilateral disease in 20% to 25%
of pediatric patients. Spheric infiltrates, which seldom are seen early in the course of pneumococcal pneumonia, initially may be confused with a focal fungal, mycobacterial, or parasitic infection or with a metastatic neoplasm. The resolution of the pulmonary infiltrates lags behind clinical improvement of the patient. Routine follow-up radiographs contribute little to the management of the child with an uncomplicated course of pneumonia.

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Jul 24, 2016 | Posted by in ORTHOPEDIC | Comments Off on Bacterial Pneumonia
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