Viral bronchiolitis (Part one)
Viral bronchiolitis: Pathophysiology, Epidemiology, Risk Factors.
Viral bronchiolitis is a common worldwide disease of infants and young children. It is a significant cause of hospitalization in infancy. In the year 2002–3, 0.1% of all hospital bed days in England were for acute bronchiolitis with a mean length of stay of 2.7 days, and in a study in one UK region, the incidence of bronchiolitis-related admission was 30.8 per 1000 infants.
The underlying pathophysiology is inflammation of the small airways (bronchioles). Infection of the bronchiolar and ciliated epithelial cells produces increased mucous secretion, cell death and sloughing, followed by a peribronchiolar lymphocytic infiltrate and submucosal oedema. This combination of debris and oedema results in distal airway obstruction. During expiration, the additional dynamic narrowing produces disproportionate airflow decrease and air trapping. The effort of breathing is increased due to increased end-expiratory lung volume and decreased lung compliance. Recovery of pulmonary epithelial cells occurs after 3–4 days, but cilia do not regenerate for approximately two weeks.3 The debris is cleared by macrophages.
Fifty to ninety percent of bronchiolitis is caused by respiratory syncytial virus (RSV) infection. RSV is a negative-sense, enveloped RNA virus that is unstable in the environment, surviving only a few hours on environmental surfaces. RSV is spread from respiratory secretions through close contact with infected persons or contact with contaminated surfaces or objects. Infection can occur when infectious material contacts mucous membranes of the eyes, mouth, or nose, and possibly through the inhalation of droplets generated by a sneeze or cough. RSV infects virtually all infants and young children in the first 3 years of life with a peak incidence of hospitalized patients between 2–6 months of life.6 During their first RSV infection, between 25–40% of infants and young children have signs or symptoms of bronchiolitis or pneumonia, and 0.5–2.5% require hospitalisation.2 5 In the USA it is estimated that there are 90 000 hospitalizations and 4500 deaths annually with RSV bronchiolitis, and in the UK there are 20 000 admissions annually with this condition. Mortality runs as high as 0.5–1.5% in hospitalized patients, increasing to 3–4% for patients with underlying cardiac or pulmonary disease. RSV is the only respiratory virus to produce predictably a sizeable outbreak of infection each year. There are two main antigenic groups, A and B, and RSV A and RSV B may both be present in an epidemic, the proportion of the two groups varying each year and by location. Epidemics occur in the winter months in temperate climates, and in tropical climates during the hottest months and the rainy season. In the UK, epidemics run from mid-November to late March with annual variation in the severity of the epidemics. Because RSV infection only confers partial protection from subsequent infection, re-infection with RSV is frequent and occurs throughout life, but after 3 years of age, infections are generally milder and confined to the upper respiratory
tract. Other organisms that cause bronchiolitis include parainfluenza, rhinovirus, adenovirus, influenza, Mycoplasma pneumonia, and metapneumovirus.
There are host and environmental factors related to disease severity (Table 1). Host factors that are associated with increased severity of disease include prematurity, infection before 6 months of age, congenital heart disease, bronchopulmonary dysplasia, cystic fibrosis, and immune
deficiency. There are some indicators that there may be a genetic predisposition to severe infection.
Environmental factors that are associated with increased severity of disease include poverty, crowding, exposure to tobacco smoke, and malnutrition.
Factors that increase the incidence of infection include young age, multiple gestations, family history of atopy, lack of parental education, household crowding, lack of breastfeeding, older school-age siblings, daycare attendance, passive smoke exposure, and discharge from a neonatal intensive care unit between September and December (northern hemisphere). Boys are 1.25 times more likely to be admitted than girls.