Lower respiratory tract infections (LRTIs) remain one of the most frequent reasons for health care visits and hospitalizations in otherwise healthy, young children living in the United States. Familiarity with the causes of LRTIs and accurate clinical diagnosis remain the cornerstones of appropriate management. Currently available vaccines and annual immune-protectants such as palivizumab are important for reducing the burden of disease.
How can the cause of LRTI best be determined?
In most cases, the cause of LRTI can be determined on the basis of epidemiologic features, the child’s age and clinical presentation, and judicious use of ancillary laboratory testing and radiographs (eg, to confirm diagnostic suspicion or assess clinical severity). Making a clinical diagnosis first helps prevent the pitfall of overreliance on chest radiograph findings, which is known to increase unnecessary antibiotic use in children whose clinical syndrome is consistent with viral infection.
Once a child presents with symptoms of LRTI—fever, cough, reduced energy or oral intake, and some degree of respiratory distress and/or hypoxemia—the first step toward diagnosis is a history and physical examination to localize the infection and determine the clinical syndrome. Useful characteristics for distinguishing between tracheobronchitis, bronchiolitis, and community-acquired pneumonia (CAP) are described in the Table (see page 3). (Age and epidemiologic risk factors are fundamentally important to diagnosis and are the basis of empiric treatment decisions in CAP.)
Diagnosing common types of LRTIs
Acute tracheobronchitis is often diagnosed when a patient presents with cough, gagging, and substernal chest pain accompanied by low-grade fever, rhinitis and, occasionally, conjunctivitis. The lung examination is typically clear. This, along with absence of tachypnea and hypoxemia, distinguishes tracheobronchitis from viral or atypical pneumonia. Nearly all cases of acute tracheobronchitis in children are related to viral nasopharyngitis. The infection resolves spontaneously.
Children and adults who present with persistent (>2 weeks) unabating cough may be infected with Bordetella pertussis. This organism causes epidemic pertussis and is an increasingly common cause of tracheobronchitis in some areas (most recently, California and Washington). In infants, B pertussis can cause apnea, recurrent choking, or an apparent life-threatening event. The pertussis cough alone, however, is indistinguishable from a viral cough.
Bronchiolitis, an inflammation of the airways down into the bronchioles, is characterized by significant nasal congestion and profuse rhinorrhea that occurs simultaneously with lower respiratory tract signs. This pattern is in contrast to that of pneumonia, which is characterized by an upper respiratory tract prodrome followed by sequential onset of lower respiratory tract involvement. Infants with bronchiolitis typically are fussy and have prominent nasal symptoms, nasal flaring, retractions, tachypnea, diffuse inspiratory crackles and expiratory wheezes, and hypoxemia.
Bacterial pneumonia causes infants to appear listless, drowsy, or anxious. They have high fever, tachypnea, and tachycardia, but a surprisingly quiet chest.
In both infants and children, careful examination may reveal focally decreased breath sounds or dullness to percussion over an area of consolidation. Older children typically have dry cough and may complain of vomiting, diarrhea, chest pain, or abdominal pain. In a cooperative child with a larger chest, the examiner may discover splinting, egophony, bronchial breath sounds, fine end-inspiratory rales, and a reduction in fremitus.
Viral and atypical pneumonia are more common than bacterial pneumonia. Both are typically characterized by diffuse or multifocal crackles or wheezes accompanied by fever, tachypnea, and hypoxemia. Wheezing alone, without fever or hypoxemia, is unlikely to be related to CAP (2% vs 20% with fever and oxygen saturation < 92%).3
Evidence indicates a significant rate of viral-viral and viral-bacterial coinfection in patients with CAP. One recent study found Streptococcus pneumoniae in 20% to 40% of patients who had severe CAP associated with positive Mycoplasma pneumoniae serology or a positive viral antigen (respiratory syncytial virus [RSV], influenza) test.4 This reminds us to be vigilant when following and reevaluating patients whose initial diagnosis is a viral syndrome.
When should ancillary testing be performed?
Ancillary testing is most useful to:
• Establish the onset of a seasonal epidemic (as with influenza and RSV rapid antigen testing)
• Confirm the diagnosis of an infection that may require specific treatment (eg, influenza, B pertussis, M pneumoniae)
• Distinguish between viral and bacterial pneumonia, and
• Assess illness severity and check for complications in infants or children hospitalized with CAP.
Infants and children with clinically diagnosed viral bronchiolitis require no further diagnostic testing. The exception is if they do not improve as expected after symptoms peak at 3 to 4 days of illness.5