ORGANUM
Bronchiolitis is a disease of small bronchioles with increased mucus production and occasional bronchospasm, sometimes leading to airway obstruction.
It is most commonly caused by a viral lower respiratory tract infection. It is most commonly seen in infants and young children, with most severe cases occurring among infants. Bronchiolitis is potentially life-threatening.
Respiratory syncytial virus (RSV) is a primary cause of bronchiolitis, followed in frequency by human metapneumovirus, parainfluenza viruses, influenza viruses, adenoviruses, rhinoviruses, coronaviruses, and infrequently, Mycoplasma pneumoniae. Viral bronchiolitis is extremely contagious and is spread by contact with infected respiratory secretions. Although coughing produces aerosols, hand carriage of contaimined secretions is the most frequent mode of transmission.
Risk factors for severe disease include:
History of prematurity (less than 32 to 34 weeks gestational age)
Age younger than 3 months
Neuromuscular disease
Congenital heart disease
Chronic lung illness
Immunodeficiency
Bronchiolitis is a leading cause of hospitalisation of infants. Bronchiolitis occurs almost exclusively during the first 2 years of life, with a peak age at 2 to 6 months. Many healthy children with bronchiolitis can be managed as outpatients; however, premature infants and children with chronic lung disease of prematurity, hemodynamically significant heart disease, neuromuscular weakness or immunodeficiency are at increased risk of severe, potentially fatal disease. Children acquire infection after exposure to infected family members, who typically have symptoms of an upper respiratory tract infection, or from infected children in daycare. In the US, annual peaks are usually in the late winter months from December through March.
The clinical features of bronchiolitis are primarily due to airway obstruction and diminished lung compliance. The virus infects the epithelial cells in the airways and induces an inflammatory reaction that leads to ciliary dysfunction and cell death. The accumulated debris, edema of the airways, narrowing of the airways due to the release of cytokines eventually lead to symptoms and lowered lung compliance. The patient then tries to overcome the decreased compliance by breathing harder. Typical features include:
Air trapping
Increased mucus production
Atelectasis
Labored breathing
Decreased ventilation
Bronchiolitis caused by RSV has an incubation period of 4 to 6 days. Bronchiolitis classically presents as a progressive respiratory illness similar to the common cold in its early phase with cough and rhinorrhea. It progresses over 3 to 7 days to noisy, raspy breathing audible wheezing. There is usually a low-grade fever accompanied by irritability, which may reflect the increased work of breathing. In contrast to the classic progression of disease, young infants infected with RSV may not have a prodrome and may have apnea as the first sign of infection. Most infants will improve with good hydration.
Physical signs of bronchiolar obstruction include prolongation of the expiratory phase of breathing, nasal flaring, intercostal retractions, suprasternal retractions, and air trapping with hyperexpansion of the lungs. During the wheezing phase, percussion of the chest usually reveals only hyperresonance, but auscultation usually reveals diffuse wheezes and crackles throughout the breathing cycle. With more severe diseases, grunting and cyanosis may be present.
Routine laboratory tests are not required to confirm the diagnosis and may only be needed to rule out other causes. It is important to assess oxygenation in severe cases of bronchiolitis. Pulse oximetry is adequate for monitoring oxygen saturations. Frequent, regular assessments of cardiorespiratory monitoring of infants are necessary because respiratory failure may develop precipitously in very tired infants even though blood gas values taken before rapid decompensation are reassuring.
Urine cultures may be obtained in children who have no other source of infection and continue to spike temperatures - concomitant urinary tract infections are known to occur in about 5% to 10% of cases.
Antigen test (usually by immunofluorescence or enzyme-linked immunosorbent assay [also referred to as ELISA]) of nasopharyngeal secretions for RSV, parainfluenza viruses, and adenoviruses are sensitive tests to confirm the infection. Rapid viral diagnostic via PCR can also be used. Identifying the viral agent is helpful for cohorting children with the same infection but not necessary to make the diagnosis of bronchiolitis - it is mostly academic.
Chest radiographs frequently show signs of lung hyperinflation, including increased lung lucency or depressed diaphragms, Areas of increased density may represent either viral pneumonia or localised atelectasis. A CXR should only be ordered if there is clinical suspicion of a complication such as pneumothorax or bacterial pneumonia.
Bronchiolitis treatment consists of supportive therapy, including respiratory monitoring, control of fever, hydration, upper airway suctioning, and if needed, oxygen administration. Indications for hospitalization include:
Moderate to severe respiratory distress
Hypoxemia
Apnea
Inability to tolerate oral feeding
Lack of appropriate care available at home
Hospitalisation of high-risk children with bronchiolitis should be considered. Among hospitalised infants, supplemental oxygen by nasal cannula is often necessary, but intubation and ventilatory assistance for respiratory failure or apnea are required in fewer than 10% of these infants. Bronchodilators and corticosteroids are seldom effective and are not generally recommended. Instead, these children should be provided with humidified oxygen and nebulised hypertonic saline. Ensuring that the infant is well hydrated is key, especially for those who cannot eat.
It is possible for bronchiolitis to be prevented via monthly infections of palivizumab, an RSV-specific monoclonal antibody, which is initiated just before the onset of the RSV season. Palivizumab is indicated for some infants under 2 years old with chronic lung disease, very low birth weight, neuromuscular disorders that make it difficult to clear the airway, and those with hemodynamically significant cyanotic and acyanotic congenital heart disease. Immunisation with influenza vaccination is recommended for all children older than 6 months and may prevent influenza associated disease.
Most hospitalised children show marked improvement in 2 to 5 days with supportive treatment alone. The course of the wheezing phase varies, however. Tachypnea and hypoxia may progress to respiratory failure requiring assisted ventilation. Apnea is a major concern for very young infants with bronchiolitis.
Most cases of bronchiolitis resolve completely, although minor abnormalities of pulmonary function may persist for several years. Recurrence is treated similarly to the first episode. The incidence of asthma seems to be higher for children hospitalized for bronchiolitis in infants, but is is unclear whether this is casual or whether children prone to asthma are more likely to be hospitalized with bronchiolitis. There is a 1% to 2% mortality rate, higher among infants with pre-exisiting cardiopulmonary or immunological impairment.
The difficulty in the diagnosis of bronchiolitis lies in differentiating it with other disease associated with wheezing:
Asthma : Absence of fever, age of presentation (bronchiolitis typically occurs in the first year of life while asthma presents in older children with previous wheezing episodes), presence of personal or family history of asthma
Airway foreign body : Focal area on radiography that does not inflate or deflate
Congenital airway obstructive lesion
Cystic fibrosis : Associated with poor growth, chronic diarrhea, and a positive family history
Exacerbation of chronic lung disease
Viral or bacterial pneumonia
Other LRTIs
Cardiogenic asthma : Associated with pulmonary congestion secondary to left-sided heart failure
Wheezing associated with gastroesophageal reflux