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  • Bronchioles are small airways (< 2 mm in adults) that do not contain cartilage in their walls. Membranous and terminal bronchioles are the smallest conducting airways before the respiratory zone of the lung. Respiratory bronchioles (0.5 mm or less in adults) communicate directly with alveolar ducts.
  • In older children and adults, bronchioles contribute little to overall airway resistance, because their total cross sectional area is so much greater than that of the central airways. Whereas bronchioles contribute about 10% to total airway resistance in older children and adults, this relationship is much different in infants. Here, total airway resistance is distributed more evenly between peripheral and central airways, so that the bronchioles contribute about 50% to total airway resistance. For these reasons, small airway obstruction usually does not cause prominent symptoms in older children and adults, until it becomes widespread and severe. In contrast, symptoms from bronchiolar obstruction are quite common in infants



  • Bronchiolitis refers to inflammation of the bronchioles, of which there are many causes. The most common cause of bronchiolitis in infants and toddlers is infection. Usually, the course is short-lived and recovery is without sequelae. Many children, however, will develop post-bronchiolitis wheezing with subsequent respiratory illnesses, and asthma is seen more frequently in populations of children who were hospitalized with bronchiolitis in infancy. In contrast, bronchiolitis in adults is uncommon and often the cause cannot be determined. Adults with bronchiolitis are more likely to have an underlying disease like a connective tissue disorder, or a history of exposure to a toxic inhalant. Bronchiolitis in adults is usually a chronic, not an acute condition


Bronchiolitis in infants and toddlers

  • By far, the most common etiologic agent to cause acute infectious bronchiolitis in infants and toddlers is respiratory syncytial virus (RSV). Other viruses that cause bronchiolitis include influenza and parainfluenza viruses, and adenovirus. A recently described agent (though evidence of its role in bronchiolitis is present for >20 years) is Human Metapneumovirus (HMV), which is genetically related to RSV. In children up to 5 years of age, Mycoplasma pneumoniae can also cause bronchiolitis. The majority of children who develop bronchiolitis are < 2 years of age


Bronchiolitis in infants and toddlers

  • In the United States, RSV lower respiratory tract infection (LRTI, either bronchiolitis or pneumonia) is the leading cause of hospitalization of infants under 1 year of age. Virtually all children have been exposed to RSV by 2 years of age, and in the US, > 120,000 infants and toddlers are hospitalized because of an RSV LRTI annually. Among all hospitalized children, the mortality rate is approximately 0.2 – 0.5%. If those children with underlying conditions (chronic lung disease, congenital heart disease, prematurity, age < 6 weeks at time of infection) are considered, the mortality rate is approximately 3 - 4%. Globally, RSV infection directly or indirectly accounts for 600,000 to 1,000,000 deaths in children <5 years of age annually, making it the most important human pathogen after S. pneumoniae. A report from the CDC using estimates based on discharge data from the National Hospital Discharge Survey showed that over a 17 year period, while hospitalizations from other respiratory pathogens remained constant, bronchiolitis-related hospitalizations increased 239%. The majority of infants hospitalized for bronchiolitis in the US do not have underlying conditions


Bronchiolitis in infants and toddlers

  • In temperate climates, RSV infections occur primarily during annual outbreaks that occur between October and April, and which peak during the winter months. An “epidemic” exists when >10% of all specimens tested are positive, or when at least half of the laboratories that report results to the CDC note any RSV detected for 2 consecutive weeks. The onset and end of RSV epidemics vary regionally, with Southern states reporting the onset of the epidemic earliest (beginning at the end of August and concluding in mid-May), with later onset in the northeast (late November to early May), and the latest onset in the Midwest (early December to late May). Some states (Florida, Hawaii) report RSV at epidemic levels year-round


Bronchiolitis in infants and toddlers

  • RSV, a single-stranded RNA virus, infects epithelial cells lining the respiratory tract. The portal of entry is either through nasal or conjunctival mucosa. Infection in the upper airway results in production of secretory and circulating antibody. From there, infected secretions can be aspirated into the lower respiratory tract. RSV is trophic for epithelial cells of small airways, between 75 and 300 microns in diameter. After entering the epithelial cell, it replicates and is released when the cell lyses and dies. Epithelial cell lysis causes release of inflammatory mediators and chemoattractants. Peribronchiolar edema and mononuclear and lymphocytic infiltration occur, with stimulation of mucus production from mucus glands and goblet cells. The bronchiolar lumen becomes filled with infected secretions and cellular debris from sloughed epithelial cells. In some infants and young children, airway smooth muscle contraction contributes to airway obstruction


Bronchiolitis in infants and toddlers

  • The clinical and physiological results of this process are well described. A hallmark of RSV infection in infants is profuse rhinorrhea, with thin clear secretions. Cough is prominent, and if the bronchiolar obstruction is significant, the infant will also be tachypneic and may have intercostal, sternal and subcostal retractions. Crackles and wheezing may be present on auscultation of the chest. In some young infants, central apnea may be the presenting sign of an RSV infection. Hypoxemia is the most common pulmonary function abnormality, resulting from ventilation-perfusion mismatching. When measured, pulmonary compliance is low and resistance is high, so that the infant develops a rapid-shallow breathing pattern to conserve work. The small airway obstruction results in air trapping (subcostal retractions on examination). The chest radiograph typically demonstrates increased interstitial markings, reflecting the peribronchiolar inflammation, and hyperinflation. Often, areas of atelectasis are present


Bronchiolitis in infants and toddlers

  • While the clinical presentation is stereotypical, there are instances in which a confirmation of the infecting agent is desirable (epidemiological data, cohorting hospitalized patients, infants < 2 months old with fever, immunocompromised hosts). The best samples to enhance the yield of the test in children are nasopharyngeal washes or aspirates. Although viral culture is always considered to be the “gold standard” for identification of a pathogen, the yield is extremely dependent upon not only the quality of the sample, but also upon how quickly the sample is set up for culture and how carefully it is transported from bedside to the laboratory. Additionally, detection by culture takes several days. In contrast, viral identification by antigen detection (ELISA) fluorescent antibody, or polymerase chain reaction (PCR) are rapid (minutes to hours), sensitive and specific. “Rapid respiratory panels” are available to detect RSV, influenza A and B, parainfluenza, and adenovirus


Bronchiolitis in infants and toddlers

  • As many as 25% of all infants who become infected with RSV will develop bronchiolitis, but only 1 – 2% require hospitalization. Infants are hospitalized when they have significant oxyhemoglobin saturation or frank cyanosis. Alternately, if an infant is so tachypneic that he or she is unable to drink, or dehydration has already developed, the infant should be hospitalized to receive intravenous fluids. Alternately, if the infant appears distressed, is very young or at increased risk because of underlying conditions, or the parents are distressed because of the degree of the infant’s illness, hospitalization should be considered
  • The mainstays of therapy for this self-limited illness are judicious fluid replacement therapy, and administration of supplemental oxygen. Overhydration can flood the pulmonary interstitium and worsen airway obstruction, and must be avoided. Bronchiolitis in infants causes acute respiratory compromise, so supplemental oxygen should not be withheld because of a concern that an hypoxic drive to breathe will be blunted


Bronchiolitis in infants and toddlers

  • A number of therapies have been used to try to overcome the various causes of airway obstruction in infants with bronchiolitis. Since the disease is self-limited, many of the therapies used impact minimally on the duration of symptoms or hospitalization. Nevertheless, approximately one third of infants will demonstrate some response to bronchodilators, and those with more severe disease will have a more noticeable response to steroids. One reason that anti-inflammatory and anti-viral therapies do not alter the course of RSV bronchiolitis to the degree expected is can be found in the relationship of viral replication to the clinical course of the illness. By the time an infant is brought to medical attention, viral replication has nearly peaked and the inflammatory cascade has been well-established


Bronchiolitis in infants and toddlers

  • There are simple yet effective and inexpensive interventions that can decrease the risk of acquiring an RSV illness that should be discussed especially with families of high-risk infants. Transmission of RSV illness is from hand (touching infected secretions or surfaces) to nose, so handwashing is a highly effective way to prevent spread of infection. Avoidance of crowded locations or people with obvious upper respiratory illness, day care and second hand tobacco smoke are also important measures that diminish the risk of acquiring an RSV infection. For infants at high risk for severe RSV disease, passive immunoprophylaxis with an engineered humanized monoclonal antibody against the F (fusion) protein of RSV is available


Bronchopulmonary dysplasia (BPD)

  • Bronchopulmonary dysplasia (BPD) is a lung disease of premature infants. It was initially described by Northway et al. in 1967 in survivors of respiratory distress syndrome (RDS) who had received mechanical ventilation and still required supplemental oxygen at one month of age. The initial description was largely radiographic and described 4 stages, from diffuse “ground glass” changes typical of RDS to cystic changes. A definition was proposed by Bancalari et al which included assisted ventilation for at least 72 hours, persistent respiratory symptoms and need for supplemental oxygen at 28 days of age


Bronchopulmonary dysplasia (BPD)

  • More recently, as neonatal care has improved dramatically, it is very common for younger premature infants to require supplemental oxygen at 1 month of chronologic age. Newer definitions evolved which took into account gestational age (i.e. need for supplemental oxygen at 36 weeks post-conceptual age) and degree of prematurity. It is recognized that these definitions have inherent ambiguity as “need” for supplemental oxygen may be interpreted variably. Some have chosen to utilize the term “chronic lung disease of infancy” to reflect that infants cared for more recently may have a wider range of pathology that more historical cases. The incidence of BPD varies depending on many factors, including the definition used. It is uncommon in larger premature infants (>1500 grams), and quite common in small prematures (e.g. ~60% of infants <750 grams require supplemental O2 at 36 weeks post conceptual age)


Bronchopulmonary dysplasia (BPD)

  • The pathogenesis of BPD is multifactorial. In older reports (sometimes now referred to as “classical BPD”), much lung injury was caused by ventilator-induced volutrauma and barotrauma, as well as oxidant injury from high inspired oxygen concentrations. In these cases, surviving infants were relatively larger or more mature, and ventilator strategies were more aggressive resulting in significant airway and parenchymal damage. In “new BPD” very small infants are surviving with arrest of alveolar/lung development at earlier stages. That is, infants born at 24-28 weeks gestation (who would not have survived at all many years ago) now are born with lungs in the canalicular or early saccular stage of development with few alveoli. Very premature infants are treated with glucocorticoids antenatally, in an attempt to mature surfactant production, although this may also impair alveolarization


Bronchopulmonary dysplasia (BPD)

  • Clinically, infants with BPD/CLDI are born with respiratory distress syndrome as a consequence of surfactant deficiency and lung immaturity. Newer ventilation strategies aim to minimize ventilator-induced injury by limiting pressures. Non-respiratory complications of prematurity (e.g. necrotizing enterocolitis, intraventricular hemorrhage, etc.) can often times prolong the course of mechanical ventilation, further increasing changes for chronic lung disease. Pulmonary complications of RDS can also include air leak (pneumothorax, pneumomediastinum, and pulmonary interstitial emphysema), airway damage resulting in malacia, and infections


Bronchopulmonary dysplasia (BPD)

  • BPD/CLDI should improve with time, as alveolarization continues post-natally and lung and airway sizes increase with further somatic growth. Early in infancy, however, infants with BPD have a high risk of re-hospitalization, especially with viral lower respiratory tract infections. Increased metabolic expenditure for breathing can also result in poor growth, and higher than usual caloric intakes are required. This can require caloric supplements, and use of nasogastric or gastrostomy tube feedings. Hypoxemia can inhibit growth, and use of supplemental oxygen after discharge is quite common. It can enhance growth and prevent cardiac sequelae of hypoxemia (i.e. right ventricular hypertrophy or failure). Other treatments for infants with BPD have included bronchodilators (especially for intercurrent illnesses with wheezing) and diuretics to decrease interstitial edema and improve lung compliance. The mechanism of action of diuretics is likely more complex; complications can include metabolic alkalosis (due to loss of chloride) and hypercalciuria/nephrolithiasis). Anti-inflammatory therapies such as corticosteroids have been utilized for many years and, indeed, have beneficial effects on lung mechanics. However, side effects are commonplace (growth failure, developmental delay, GI bleeding, hypertension, etc.) and significantly limit their utility


Bronchopulmonary dysplasia (BPD)

  • Routine preventative care, especially at that directed to prevent viral infections, is important. Viral bronchiolitis is the most common reason for rehospitalization in infants with BPD. Good hand hygiene can be utilized without cost and obvious benefit. Immunizations to prevent influenza are safe and effective in infants as young as 6 months. Passive immunization against RSV has been available since the late 1990s, although cost remains a significant factor limiting more widespread use.
  • Over many years, as lungs and airways grow, children with BPD/CLDI tend to have fewer pulmonary symptoms. Lung function may remain mildly impaired, and they may have evidence of air trapping and small airway obstruction. An asthma-like phenotype may be more common in children with BPD

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