eMedicine Specialties > Pediatrics: General Medicine > Infectious Disease

Pneumonia

Author: Nicholas John Bennett, MB, BCh, PhD, Fellow in Pediatric Infectious Disease, Department of Pediatrics, State University of New York Upstate Medical University
Coauthor(s): Joseph Domachowske, MD, Professor of Pediatrics, Microbiology and Immunology, Department of Pediatrics, Division of Infectious Diseases, State University of New York-Upstate Medical University; Isabel Virella-Lowell, MD, Department of Pediatrics, Division of Pulmonary Diseases, Pediatric Pulmonology, Allergy and Immunology
Contributor Information and Disclosures

Updated: Jan 12, 2009

Introduction

Background

Pneumonia is characterized by inflammation of the alveoli and terminal airspaces in response to invasion by an infectious agent introduced into the lungs through hematogenous spread or inhalation. The inflammatory cascade triggers the leakage of plasma and the loss of surfactant, resulting in air loss and consolidation. This is in contrast to pneumonitis, which is caused by noninfectious agents such as radiation or chemicals.

An inhaled infectious organism must bypass the host's normal nonimmune and immune defense mechanisms in order to cause pneumonia. The nonimmune mechanisms include aerodynamic filtering of inhaled particles based on size, shape, and electrostatic charges; the cough reflex; mucociliary clearance; and several secreted substances (eg, lysozymes, complement, defensins). Macrophages, neutrophils, lymphocytes, and eosinophils carry out the immune-mediated host defense. 

Conditions that allow pneumonia-causing infectious organisms to circumvent the upper airway defense mechanisms include the following:

• Intubation, tracheostomy, impaired cough reflex, and aspiration: These conditions provide infectious organisms with easier access to the alveoli and terminal airspaces.
• Ciliary dyskinesia, bronchial obstruction, viral infection, cigarette smoke, and certain chemical agents: These conditions create disruption in the mucociliary blanket.
• Anatomic abnormalities (eg, sequestrations), gastric fluid aspiration or other causes of noninfectious inflammation, altered pulmonary blood flow, and pulmonary edema: These conditions increase the predisposition for pneumonia.
• Immunodeficiency and immunosuppression: These conditions increase predisposition for pneumonia.

Pathophysiology

Inoculation of the respiratory tract by infectious organisms leads to an acute inflammatory response in the host that typically lasts 1-2 weeks. This inflammatory response differs according to the type of infectious agent.

• Viral infections
  • These infections are characterized by the accumulation of mononuclear cells in the submucosa and perivascular space, resulting in partial obstruction of the airway. Patients with these infections present with wheezing and crackles.
  • Disease progresses when the alveolar type II cells lose their structural integrity and surfactant production is diminished, a hyaline membrane forms, and pulmonary edema develops.
• Bacterial infections
  • The alveoli fill with proteinaceous fluid, which triggers a brisk influx of RBCs and polymorphonuclear cells (red hepatization) followed by the deposition of fibrin and the degradation of inflammatory cells (gray hepatization).
  • During resolution, intra-alveolar debris is ingested and removed by the alveolar macrophages. This consolidation leads to decreased air entry and dullness to percussion. Inflammation in the small airways leads to crackles. Wheezing is less common than in viral infections.
  • The inflammation and pulmonary edema that result from these infections cause the lungs to become stiff and less distensible, thereby decreasing tidal volume. The patient must increase his or her respiratory rate to maintain adequate ventilation.
  • Poorly ventilated areas of the lung may remain well perfused, resulting in ventilation/perfusion (V/Q) mismatch and hypoxemia. Tachypnea and hypoxia are common.
• Fungal infections
  • Fungal infections are unusual and are typically found in patients with inadequate immune function (eg, patients with acquired immunodeficiency syndrome [AIDS], patients who have undergone chemotherapy, newborn infants).
  • The pathology may be a diffuse infiltrate of organisms or focal areas of fungal growth.
  • Patients often appear ill and may have more subtle physical findings than their overall clinical appearance may suggest.

Frequency

United States

Pneumonia accounts for 13% of all infectious illnesses in infants younger than 2 years. In a large community-based study conducted by Denny and Clyde, the annual incidence rate of pneumonia was 4 cases per 100 children in the preschool-aged group, 2 cases per 100 children aged 5-9 years, and 1 case per 100 children aged 9-15 years.¹  

Mortality/Morbidity

The United Nations Children's Fund (UNICEF) estimates that 3 million children die worldwide from pneumonia each year. Although most fatalities occur in developing countries, pneumonia remains a significant cause of morbidity in industrialized nations.

Age

Pneumonia can occur at any age, a lthough it is more common in younger children. Different age groups tend to be infected by different pathogens, which affects diagnostic and therapeutic decisions. See Causes for specific details.

Clinical

Physical

Because pneumonia is common and is associated with significant morbidity and mortality, properly diagnosing pneumonia, correctly recognizing any complications or underlying conditions, and appropriately treating patients is important.

The signs and symptoms of pneumonia are often nonspecific and widely vary based on the patient’s age and the infectious organisms involved.

• Newborns
  • Newborns with pneumonia rarely cough; they more commonly present with tachypnea, retractions, grunting, and hypoxemia.
  • Grunting in a newborn is due to vocal cord approximation as they try to provide increased positive end-expiratory pressure (PEEP) and keep their lower airways open. Grunting suggests a lower respiratory tract disease. Retractions result from the effort to increase intrathoracic pressure to compensate for decreased compliance.
• Older infants: Grunting may be less common; however, tachypnea, retractions, and hypoxemia are common and may be accompanied by a persistent cough, congestion, fever, irritability, and decreased feeding.
• Toddlers and preschoolers: These children most often present with fever, cough (productive or nonproductive), tachypnea, and congestion. They may have some posttussive emesis.
• Older children and adolescents
  • This group may also present with fever, cough (productive or nonproductive), congestion, chest pain, dehydration, and lethargy.
  • Extrapulmonary signs and symptoms include (1) abdominal pain or an ileus accompanied by emesis in patients with lower lobe pneumonia, (2) nuchal rigidity in patients with right upper lobe pneumonia, or (3) a rub caused by pericardial effusion in patients with lower lobe pneumonia due to Haemophilus influenzae infection.
• All children
  • Many children present with nasal flaring, which increases airflow to respiratory surfaces.
  • Auscultation of the lung fields may yield rales, wheezing, diminished breath sounds, tubular breath sounds, or pleural friction rub. The affected lung field may be dull to percussion. Decreased tactile and vocal fremitus, as well as egophony, may be appreciated over the area of pneumonia.

Causes

Various organisms cause pneumonia. Bacterial, viral, mycoplasmal, chlamydial, fungal, and mycobacterial infections are relatively common and have similar presentations, complicating clinical diagnosis.

To complicate matters, basic laboratory and radiologic testing is often not helpful in determining the etiology of pneumonia, and the treatments widely vary. However, certain age trends in the etiology of pneumonia can aid in decision-making, even before testing is complete. 

• Newborns (age 0-30 d)
• Infections with group B Streptococcus, Listeria monocytogenes, or gram-negative rods (eg, Escherichia coli, Klebsiella pneumoniae) are a common cause of bacterial pneumonia. These pathogens can be acquired in utero, via aspiration of organisms present in the birth canal, or by postnatal contact with other people or contaminated equipment.
• Some organisms acquired perinatally may not cause illness until later in infancy, including Chlamydia pneumoniae, Ureaplasma urealyticum, Mycoplasma hominis, cytomegalovirus, and Pneumocystis carinii. Infants infected with these organisms present between age 4-11 weeks with an afebrile pneumonia characterized by a staccato cough, tachypnea, and, occasionally, hypoxia.
• Community-acquired viral infections occur in newborns, although less commonly than in older infants. The most commonly isolated virus is respiratory syncytial virus (RSV). The transfer of maternal antibodies is important in protecting newborns and young infants from such infections, making premature infants (who may not have benefited from sufficient transfer of transplacental immunoglobulin G [IgG]) especially vulnerable to lower-tract disease. In addition, premature infants may have chronic lung disease of prematurity, with associated hyperreactive airways, fewer functional alveoli, and baseline increased oxygen requirements. 
• Infants and toddlers
• Viruses are the most common cause of pneumonia, accounting for approximately 90% of all lower respiratory infections.
• RSV is the most common viral pathogen, followed by parainfluenza types 1, 2, and 3 and influenza A or B. RSV infection occurs in the winter and early spring. Parainfluenza type 3 infection occurs in the spring, and types 1 and 2 occur in the fall. Influenza occurs in the winter.
• Other viruses that cause pneumonia less frequently in infants include adenovirus, enterovirus, rhinovirus, coronavirus, herpesvirus, and cytomegalovirus. A recent addition to this list is human metapneumovirus, which causes an illness similar to RSV and may be responsible for one third to one half of non-RSV bronchiolitis.
• Bacterial infections in this age group are uncommon and are attributable to Streptococcus pneumoniae, H influenzae type B (less common in immunized children), or Staphylococcus aureus. Infants or toddlers with bacterial pneumonia may present with lethargy, irritability, acidosis, hypotonia, or hypoxia that is out of proportion to ausculatory findings.
• Children younger than 5 years, children enrolled in daycare, or those with frequent ear infections are at increased risk for invasive pneumococcal disease and infection with resistant pneumococcal strains. They are often treated with an antibiotic within a month of contracting pneumonia.
• Evidence suggests that breastfeeding has a protective effect against invasive pneumococcus. 
• Children aged 5 years (ready to start school)
• Mycoplasma pneumoniae is the most common cause of community-acquired pneumonia and accounts for 20% of pneumonia cases in the general population, 9-16% of cases in early-school–aged children, 16-21% of cases in older children, and 30-50% of cases in college students and military recruits.
• Mycoplasma infections are indolent, with gradual onset of malaise, low-grade fever, headache, and cough. C pneumoniae is also fairly common in this age group and presents in a similar fashion.
• School-aged children and adolescents: Bacterial pneumonia (10%) is common, and these children are often febrile and appear ill.
• Tuberculosis (TB) pneumonia in children warrants special mention.
• Children with TB usually do not present with symptoms until 1-6 months after primary infection.
• Infants and postpubertal adolescents are at increased risk of disease progression. These children may present with fever, night sweats, chills, cough (which may include hemoptysis), and weight loss.
• Chest radiography findings may include hilar or mediastinal lymphadenopathy, atelectasis, or consolidation of a segment or lobe (usually right upper lobe), pleural effusion, cavitary lesions (in adolescents and adults only), or miliary disease. 
• A history of exposure to possible sources should be obtained (eg, immigrants from Africa, certain parts of Asia, and Eastern Europe; contacts with persons in the penal system; close contact with known individuals with TB).
• If TB is not treated during the early stages of infection, approximately 25% of children younger than 15 years develop extrapulmonary disease.

Bordetella pertussis also causes pneumonia, although predominantly in infants who have not completed their vaccinations or in children who did not receive vaccinations. Bronchopneumonia occurs in 0.8-2% of all pertussis cases and 16-20% of hospitalized cases. The survival rate with this complication is much lower than in pneumonia attributed to other causes. A study conducted in the United Kingdom showed that 59% of deaths from pertussis are associated with pneumonia. Clinical presentation includes coryza, malaise, fever, paroxysms of cough occasionally accompanied by emesis, apnea, poor feeding, and cyanosis.

Viral pneumonias are common in this age group and are usually mild and self-limited. However, as in adults, viral pneumonias are occasionally severe and can rapidly progress to respiratory failure, either as a primary manifestation of viral infection or as a consequence of subsequent bacterial infection. Group A streptococcal, pneumococcal, and staphylococcal secondary infections are all relatively common.

Aspiration pneumonia is more common in children with neurological impairment and swallowing abnormalities. Oral anaerobic flora, with or without aerobes, is the most common etiologic agent.

In immunosuppressed individuals, opportunistic infections with organisms such as Aspergillus species, Candida species, Pneumocystis species, and cytomegalovirus can occur.

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