Bronchiolitis

An acute viral lower respiratory tract infection predominantly affecting infants under two years of age, characterised by inflammation and obstruction of the small airways (bronchioles), representing the commonest cause of hospital admission for infants in this age group during the winter respiratory virus season, with respiratory syncytial virus accounting for the majority of cases.

Pathophysiology and aetiology

Respiratory syncytial virus infects and directly damages the bronchiolar epithelium, causing necrosis, sloughing of dead cells into the airway lumen, submucosal oedema and mucus hypersecretion, producing small airway obstruction with air trapping, atelectasis and ventilation perfusion mismatch.

The small calibre of the infant airway means that even modest mucosal swelling produces disproportionate obstruction, explaining the characteristic vulnerability of this age group. Other viruses, including rhinovirus, human metapneumovirus, parainfluenza and adenovirus, account for a minority of cases and can produce a clinically similar illness.

A preceding one to three day prodrome of coryza and mild fever typically precedes the onset of lower respiratory tract symptoms, including cough, tachypnoea, wheeze and increased work of breathing (subcostal and intercostal recession, nasal flaring, head bobbing in infants), reflecting progressive small airway obstruction.

Widespread wheeze and fine crackles are characteristic auscultatory findings, and feeding difficulty, from a combination of nasal obstruction and increased work of breathing making coordinated sucking and swallowing difficult, is a common and clinically important feature given its relevance to hydration status and admission decisions.

Symptoms typically peak around day three to five of the illness before gradually improving over one to two weeks, though cough can persist for several weeks beyond this.

Risk factors for severe disease include prematurity, young age (particularly under twelve weeks), underlying chronic lung disease, congenital heart disease, immunodeficiency and neuromuscular disease, with these infants warranting a lower threshold for admission and closer monitoring.

Apnoea can be a presenting feature, particularly in young infants and those born prematurely, sometimes preceding or occurring without the more typical respiratory symptoms, an important and potentially serious atypical presentation to recognise.

Diagnosis is clinical, based on the characteristic age, preceding coryzal illness and examination findings of wheeze, crackles and increased work of breathing, without the routine need for chest radiography or blood tests in typical presentations.

Chest radiography is not routinely indicated and, where performed, frequently shows non specific hyperinflation and patchy atelectasis that can be misinterpreted as consolidation, potentially leading to inappropriate antibiotic treatment, and should be reserved for atypical presentations or where an alternative diagnosis or complication is suspected.

Viral testing (nasopharyngeal aspirate for respiratory syncytial virus and other respiratory viruses) may be performed for infection control and cohorting purposes in the inpatient setting, or where the result would influence management, rather than as a routine diagnostic requirement.

Pulse oximetry should be checked in every assessed infant given its central role in both diagnosis of severity and admission decision making, with the specific saturation threshold for concern and admission varying according to local guidance and the infant's baseline risk factors.

Assessment of feeding adequacy and hydration status is an essential component of the clinical evaluation, given the frequent contribution of feeding difficulty to the overall clinical picture and its relevance to the need for supplemental fluid or feeding support.

Supportive care is the cornerstone of management, since no specific antiviral treatment has consistent evidence of benefit in typical bronchiolitis, with management focused on maintaining oxygenation and hydration while the illness runs its natural course.

Nasal suctioning to clear secretions and relieve nasal obstruction can support feeding and comfort, particularly before feeds, though evidence for more aggressive or deep suctioning altering the overall clinical course is limited.

Supplemental oxygen is provided to maintain adequate saturation according to local thresholds, and infants with significant feeding difficulty or dehydration require supplemental fluid support, whether via nasogastric tube feeding or intravenous fluids, guided by the severity of respiratory distress and feeding tolerance.

Bronchodilators, inhaled corticosteroids and nebulised hypertonic saline have not shown consistent evidence of benefit in typical bronchiolitis and are not routinely recommended, reflecting the fact that the underlying pathophysiology (mucosal inflammation and sloughing, mucus plugging) differs from the smooth muscle bronchospasm targeted by bronchodilator therapy in asthma, a frequent source of inappropriate prescribing given the superficial similarity of wheeze in both conditions.

Antibiotics are not indicated for uncomplicated viral bronchiolitis and should be reserved for confirmed or strongly suspected secondary bacterial infection, which is uncommon.

High flow nasal cannula oxygen therapy or, for more severe respiratory failure, continuous positive airway pressure or invasive ventilation, are escalation options for infants with significant or worsening respiratory distress not responding to standard supportive care, managed within an appropriate inpatient or high dependency setting.

Palivizumab, a monoclonal antibody against respiratory syncytial virus, is available as passive immunoprophylaxis for selected high risk infants (significant prematurity, chronic lung disease, haemodynamically significant congenital heart disease) during the respiratory syncytial virus season, given as a monthly injection, though its use is generally restricted to specific high risk groups given cost considerations, with newer long acting monoclonal antibody products increasingly available with broader eligibility in some settings.

Admission is guided by the severity of respiratory distress, oxygen saturation, feeding tolerance and the presence of risk factors for severe disease, with clear safety netting advice provided for infants managed at home regarding signs of deterioration warranting urgent reassessment.