Wheezing, Bronchiolitis, and Bronchitis

Acute Bronchiolitis

Acute bronchiolitis is a diagnostic term used to describe the clinical picture produced by several different viral lower respiratory tract infections in infants and very young children. The respiratory findings observed in bronchiolitis include tachypnea, wheezing, crackles, and rhonchi which result from inflammation of the small airways ( Fig. 418.1 ). Despite its commonality, a universal set of diagnostic criteria for bronchiolitis does not exist, with significant disagreement about the upper age limit for appropriate use of the diagnosis. Some clinicians restrict the term to children younger than 1 yr, and others extend it to the age of 2 yr or beyond.

The pathophysiology of acute bronchiolitis is characterized by bronchiolar obstruction with edema, mucus, and cellular debris (see Fig. 418.1 ). Resistance in the small air passages is increased during both inspiration and exhalation, but because the radius of an airway is smaller during expiration, the resultant respiratory obstruction leads to expiratory wheezing, air trapping, and lung hyperinflation. If obstruction becomes complete, trapped distal air will be resorbed and the child will develop atelectasis. Hypoxemia is a consequence of ventilation-perfusion mismatch. With severe obstructive disease hypercapnia can develop.

Respiratory syncytial virus (RSV) is responsible for more than 50% of cases of bronchiolitis in most reports. Other agents include human metapneumovirus, rhinovirus, parainfluenza, influenza, bocavirus, and adenovirus . Viral coinfection is reported to impact severity and clinical manifestations, although its significance remains contested. Respiratory viruses can be identified in more than one third of asymptomatic patients younger than the age of 1 yr, calling into question the specificity of current tests for active infection. Although bacterial pneumonia is sometimes confused clinically with bronchiolitis, viral bronchiolitis is rarely followed by bacterial superinfection.

Well over 100,000 young children are hospitalized annually in the United States with the diagnosis of bronchiolitis, making it the most common diagnosis resulting in hospitalization for children younger than 1 yr of age in the United States over the past several decades. The increasing rates of hospitalization for bronchiolitis observed from 1980 to 1996 (thought to reflect increased attendance of infants in daycare centers, changes in criteria for hospital admission linked to pulse oximetry use, and/or improved survival of premature infants and other children at risk for severe disease) have not continued. Hospitalization rates have been stable in subsequent years despite introduction and routine use of RSV immunoprophylaxis in high-risk populations.

Bronchiolitis is more common in males, those exposed to second-hand tobacco smoke, those who have not been breastfed, and those living in crowded conditions. Risk is also higher for infants with mothers who smoked during pregnancy. Older family members, including older siblings, are a common source of infection; they might experience only minor upper respiratory symptoms (colds) given that bronchiolar edema may be less clinically apparent as airway size increases.

Asthma (see Chapter 169 ) is another important cause of wheezing, and the possibility of this diagnosis complicates the treatment of young children with bronchiolitis, although accurate diagnosis of asthma in the very young can be difficult. In prospective, longitudinal population cohort studies of infants, up to half of the cohort experienced a wheezing illness prior to school age, although when followed into adulthood only about 5–8% of patients prove to have asthma. In the largest U.S. cohort, 3 patterns of infant wheezing were proposed: transient early wheezing, comprising about 20% of the cohort, characterized by lower lung function at birth which improves with growth resulting in resolution of wheezing by age 3; persistent wheezing, comprising about 14% of the cohort, characterized by declining lung function and wheezing before and after age 3; and late-onset wheezing, comprising 15% of the cohort, characterized by relatively stable lung function and wheezing that does not begin until after age 3. The remaining 50% of the population did not suffer a wheezing illness. Following the cohort into adulthood revealed continued declines in the rates of persistent symptoms. Similar patterns are also seen in birth cohort studies in other countries.

Multiple studies attempting to predict which infants suffering from early wheezing illnesses will go on to have asthma in later life have failed to achieve discriminant validity. Interestingly, in both U.S. and U.K. prospective cohorts, wheezing with an onset after the first 18-36 mo of life is one of the strongest predictors of eventual asthma in both cohorts. Other proposed risk factors for persistent wheezing include parental history of asthma and allergies, maternal smoking, persistent rhinitis (apart from acute upper respiratory tract infections), allergen sensitization, eczema, and peripheral eosinophilia, although no single factor is strongly discriminative. Despite several randomized trials, there is no evidence that early administration of inhaled corticosteroids to high-risk populations can prevent the development of asthma.

History and Physical Examination

The initial history of a wheezing infant should describe the recent event including onset, duration, and associated factors ( Table 418.2 ). Birth history includes weeks of gestation, neonatal complications including history of intubation or oxygen requirement, maternal complications, and prenatal smoke exposure. Past medical history includes any comorbid conditions. Family history of cystic fibrosis, immunodeficiencies, asthma in a first-degree relative, or any other recurrent respiratory conditions in children should be obtained. Social history should include any second-hand tobacco or other smoke exposure, daycare exposure, number of siblings, pets, and concerns regarding home environment (e.g., dust mites, construction dust, heating and cooling techniques, mold, cockroaches). The patient's growth chart should be reviewed for signs of failure to thrive.

Acute bronchiolitis is usually preceded by exposure to contacts with a minor respiratory illness within the previous week (see Fig. 418.1 ). The infant first develops signs of upper respiratory tract infection with sneezing and clear rhinorrhea. This may be accompanied by diminished appetite and fever. Gradually, respiratory distress ensues, with paroxysmal cough, dyspnea, and irritability. The infant is often tachypneic, which can interfere with feeding. Apnea may precede lower respiratory signs early in the disease, particularly with very young infants. Term infants at a postconceptual age of <44 wk and preterm infants at postconceptual age <48 wk are at highest risk for apneic events.

On physical examination, evaluation of the patient's vital signs with special attention to the respiratory rate and oxygen saturation is an important initial step. The exam is often dominated by wheezing and crackles. Expiratory time may be prolonged. Work of breathing may be markedly increased, with nasal flaring and retractions. Complete obstruction to airflow can eliminate the turbulence that causes wheezing; thus the lack of audible wheezing is not reassuring if the infant shows other signs of respiratory distress. Poorly audible breath sounds suggest severe disease with nearly complete bronchiolar obstruction.