Bronchiolitis

Overview

Bronchiolitis is a viral lower respiratory tract infection occurring in infants younger than two years, typically younger than twelve months, and is most commonly caused by respiratory syncytial virus (RSV); rhinovirus, human metapneumovirus, parainfluenza and influenza can also cause the illness. After a two- to three-day upper-respiratory prodrome of rhinorrhea, mild cough and low-grade fever, the disease spreads to the lower respiratory tract, where tachypnea, wheeze, fine crackles, chest-wall retractions and feeding difficulty develop.

In young infants, particularly those younger than three months or with a history of prematurity, apnea may be the first and sometimes the only manifestation; an apnea history must therefore be specifically sought in young infants presenting with apparently mild upper-respiratory symptoms. In the pathophysiology, the virus first infects the upper-airway epithelium and then the bronchiolar epithelium; epithelial necrosis, loss of ciliary function, submucosal edema and mucus hypersecretion combine to obstruct the small airways.

Because infant airways are proportionally narrower than adult airways, the same degree of mucosal edema produces much more pronounced obstruction. Diagnosis is clinical, based on history and physical examination; the American Academy of Pediatrics (AAP) 2014 guideline recommends against routine chest radiography and viral testing in a child aged one to twenty-three months with a typical clinical diagnosis of bronchiolitis, because these tests increase unnecessary antibiotic use and radiation exposure without changing management.

The illness typically reaches its worst point between the third and fifth days, and symptoms including cough may persist for two to three weeks; this prolonged course should be explained to parents because expecting early improvement can lead to unnecessary repeat visits. In Türkiye, RSV activity increases from late autumn through the winter months and creates a substantial burden in pediatric emergency departments during this period.

Key considerations

• Viral or bacterial pneumonia presents with toxic appearance and focal crackles or dullness on chest examination; unlike the diffuse bilateral wheeze of bronchiolitis, localized findings predominate.
• A first asthma episode or recurrent wheeze should be considered in a child older than one year, in a child with atopy, or after previous wheezing episodes; a marked response to a bronchodilator supports asthma rather than bronchiolitis.
• Foreign-body aspiration is distinguished by unilateral wheeze or reduced breath sounds after sudden-onset cough or a choking episode; a preceding upper-respiratory prodrome is usually absent.
• Congestive heart failure or underlying congenital heart disease may present with hepatomegaly, sweating during feeds, failure to thrive and a cardiac murmur, and may worsen respiratory distress independently of bronchiolitis.
• Pertussis is characterized by paroxysmal cough, post-tussive vomiting and apnea, particularly in young infants; fever is usually absent and the patient may look surprisingly well between coughing episodes.
• Structural airway anomalies such as tracheoesophageal fistula, vascular ring or laryngomalacia should be considered in recurrent or atypical, non-seasonal wheeze and cough associated with feeding.
• Cystic fibrosis has a progressive course with recurrent lower-respiratory infections, failure to thrive and steatorrhea, and must be distinguished from a single episode of bronchiolitis.
• Chlamydia trachomatis pneumonia occurs in an infant younger than three weeks with staccato cough, tachypnea and usually no fever; a history of transmission during vaginal delivery is important.
• Sepsis or serious bacterial infection should be considered, particularly in young infants, when there is toxic appearance, temperature instability and feeding refusal; respiratory findings may resemble bronchiolitis, but accompanying systemic toxicity is distinguishing.

Why it matters

The AAP 2014 guideline recommends against a bronchodilator (albuterol/salbutamol) or epinephrine trial because repeated randomized studies have not demonstrated clinically meaningful benefit; this is a strong recommendation with Evidence Quality B. Systemic or inhaled corticosteroids are likewise not recommended for the same reason.

Antibiotics should be given only for proven or strongly suspected bacterial co-infection, such as acute otitis media; routine use increases antibiotic resistance without benefit. Chest physiotherapy (vibration or percussion) is not recommended because studies have not shown improvement in clinical scores or length of stay.

Guidelines disagree substantially about nebulized three-percent hypertonic saline: AAP 2014 does not recommend routine emergency-department use but states that it may be considered in a patient expected to remain hospitalized for more than seventy-two hours; in contrast, the United Kingdom NICE NG9 guideline (2021 update) does not recommend hypertonic saline in hospitalized patients together with epinephrine, salbutamol, montelukast, ipratropium bromide or corticosteroids because the evidence is not consistent or of high quality.

This is a clear example of two major guidelines interpreting the same evidence base differently. Guidelines also differ in the threshold for starting oxygen:

AAP 2014 uses an SpO2 below ninety percent as the single threshold, whereas NICE NG9 (2021) recommends ninety percent in a child older than six weeks and ninety-two percent in an infant younger than six weeks or a child with underlying disease; this difference can be understood as the lower threshold prioritizing avoidance of unnecessary admission and the higher threshold providing a greater safety margin for vulnerable infants. According to a 2024 Cochrane review, high-flow nasal cannula (HFNC) may reduce the need for treatment escalation in selected hypoxic patients or patients with respiratory distress who do not respond to standard low-flow oxygen, but it should not be used as first-line treatment and is not the preferred option in routine low-risk illness.

Continuous pulse-oximetry, once widely used, is no longer routinely recommended in stable, low-risk patients because it can prolong unnecessary admission and contribute to alarm fatigue; the AAP 2014 guideline explicitly moved away from this practice.

Critical complications

• Apnea is recognized as witnessed cessation of breathing, particularly in an infant younger than three months or a premature infant; management consists of close observation with continuous cardiorespiratory monitoring and respiratory support when required.
• Respiratory failure is recognized by progressive tachypnea, increasing retractions, grunting and eventual paradoxical reduction in respiratory effort as fatigue develops; high-flow nasal cannula or non-invasive/invasive mechanical ventilation may be required, with consideration of intensive-care transfer.
• Dehydration is recognized by fewer wet diapers, a sunken fontanelle and loss of skin turgor; if oral intake is inadequate, it is corrected with nasogastric or intravenous fluids while avoiding excessively rapid replacement.
• Secondary bacterial infection most commonly presents as acute otitis media and is recognized by recurrent or rising fever, ear pain or irritability; when bacterial pneumonia is suspected, toxic appearance and focal examination findings should be sought and targeted antibiotics started if confirmed.
• Iatrogenic hyponatremia may result from excessive hypotonic intravenous fluids or inappropriate antidiuretic hormone secretion; isotonic solution is therefore preferred for maintenance fluids and electrolytes should be monitored closely.
• Pneumothorax or an air-leak syndrome during high-flow nasal cannula or non-invasive ventilation is uncommon but serious; it is recognized by sudden clinical deterioration, unilateral reduction in breath sounds or an abrupt increase in oxygen requirement and may require urgent chest imaging and chest-tube placement.
• The long-term risk of recurrent wheeze and asthma is increased; published follow-up studies have found recurrent wheeze through age six in a proportion of children hospitalized with RSV bronchiolitis, so these patients should be monitored and families informed about future wheezing episodes in primary care.

Special considerations

• The risk of apnea is substantially higher in infants younger than three months, and the threshold for hospital observation should be low even when the clinical illness appears mild to an adult observer; this age group is defined as a separate high-risk category in the AAP 2014 guideline.
• In an infant born prematurely, the risk of severe disease and mechanical ventilation is increased regardless of corrected age; seasonal RSV prevention with a single dose of the long-acting monoclonal antibody nirsevimab should be considered according to national indications.
• In an infant with cyanotic or hemodynamically significant congenital heart disease, RSV infection is more severe and oxygen should be started early using a lower hypoxemia threshold; NICE NG9 lists this group among those to whom the lower SpO2 threshold of ninety-two percent applies.
• In an infant with chronic lung disease, such as bronchopulmonary dysplasia, baseline oxygen requirement may already be high; assessment should use the patient's own baseline saturation as a reference because a single absolute threshold can be misleading.
• In an infant or child with immunodeficiency, including severe combined immunodeficiency, chemotherapy exposure or post-transplant immunosuppression, viral shedding may persist longer and the clinical course may be unpredictable; admission at a lower threshold and viral testing may be used more liberally because the result can affect management.
• In an infant with neuromuscular disease, secretion clearance is impaired and respiratory failure may develop more rapidly; respiratory effort should be monitored closely and the decision to provide respiratory support should be made early when necessary.
• Breastfeeding should be encouraged in a breastfed infant; when respiratory distress prevents direct breastfeeding, expressed breast milk can be given in small frequent amounts or by nasogastric tube, and breastfeeding does not need to be stopped completely.

Follow-up plan

In a discharged infant with mild disease, reassessment or a telephone check within twenty-four to forty-eight hours is recommended because the illness may worsen between the third and fifth days, and an infant who appears mildly affected at the first visit may deteriorate during this period. Before discharge, room-air saturation should remain stable above the accepted threshold, oral intake should have reached an adequate level (usually at least seventy-five percent of baseline intake), and respiratory distress should have clearly improved.

Families should be told about red flags including apnea, cyanosis, rapid or markedly labored breathing, reduced oral intake, fewer wet diapers and lethargy, and instructed to seek emergency care immediately if these develop. Because cough may last two to three weeks, persistent cough alone does not require a repeat visit; renewed respiratory distress or worsening feeding does.

In hospitalized patients, discharge should be based on a combined assessment of saturation, feeding and respiratory effort; some centers accept that brief desaturation episodes during sleep alone should not delay discharge. Because of the risk of recurrent wheeze, primary-care follow-up after severe bronchiolitis should include counseling about later wheezing episodes and possible asthma development.