Bronchiolitis

Definition

Bronchiolitis is an acute viral infection of the bronchioles — the smallest airways of the lung — occurring mainly in infants under 12 months. It is one of the commonest reasons for infant hospital admission in the cooler months, and it is also one of the conditions where the evidence most clearly favours doing less: almost every intervention once used routinely has been shown not to help.

Pathophysiology

What happens in the small airways

The virus infects and inflames the bronchiolar epithelium. Infected cells die and slough into the lumen, mucus production increases, and the airway wall becomes oedematous. The obstruction is therefore mechanical — debris, mucus and swelling — rather than bronchospasm, which is the single most useful fact about the condition and explains why bronchodilators do not work.

Why infants are so vulnerable

Airway resistance rises with the fourth power of the reduction in radius, so in an airway one to two millimetres across, a fraction of a millimetre of swelling has an enormous effect. Infants also have compliant chest walls, horizontally set ribs, fewer collateral ventilation channels and diaphragms that fatigue quickly — so they compensate poorly and tire fast.

Air trapping and feeding failure

Partial obstruction allows air in but impedes it out, producing hyperinflation, patchy collapse and a flattened diaphragm working at a mechanical disadvantage. Meanwhile the infant cannot coordinate sucking, swallowing and breathing at a respiratory rate of 60–70, so feeding fails before oxygenation does. Poor feeding is often the earliest reliable sign of deterioration.

Co-morbidities

Most affected infants are otherwise well. Severe disease is more likely in those born premature, those under about three months, and infants with chronic lung disease of prematurity (bronchopulmonary dysplasia), congenital heart disease, neuromuscular conditions, Down syndrome, cystic fibrosis or immunodeficiency. Household tobacco smoke exposure increases severity independently and is one of the few modifiable factors.

Prevalence

Bronchiolitis is very common: most children have had at least one episode by the age of two, and around 2–3% of all infants are hospitalised with it in their first year. It is highly seasonal, peaking in the cooler months, and respiratory syncytial virus causes the majority of community cases and hospital admissions in this age group. Seasonality was disrupted by pandemic-era public health measures and out-of-season surges have since occurred.

Causes and risk factors

• Respiratory syncytial virus (RSV) — the leading cause, accounting for most hospitalised cases.
• Other viruses — rhinovirus, human metapneumovirus, parainfluenza, adenovirus, influenza and SARS-CoV-2; co-infection is common.
• Age under three months and prematurity — the two strongest predictors of severe disease.
• Chronic lung, cardiac, neuromuscular or immune conditions.
• Tobacco smoke exposure, crowding, childcare attendance and school-aged siblings.
• Not breastfed — breastfeeding is modestly protective against severe disease.

Symptoms

The typical course

It starts like a cold, with runny nose and mild cough for two to three days. It then moves to the chest: persistent cough, wheeze, crackles, fast and laboured breathing, and reduced feeding. Symptoms are usually worst around days three to five and settle over seven to fourteen days, although cough commonly lingers for three to four weeks.

Signs of increased work of breathing

Raised respiratory rate, intercostal and subcostal recession, tracheal tug, nasal flaring, head bobbing and grunting — grunting being a late and serious sign. Apnoea can occur in very young or premature infants, sometimes before any other sign.

Feeding and hydration

Taking less than about half of normal feeds, fewer wet nappies, a dry mouth or a sunken fontanelle. Parents are usually the first to notice feeding change, and asking about it specifically is more informative than any single examination finding.

Warning signs

Diagnosis

Why diagnosis matters

Bronchiolitis is a clinical diagnosis, and making it confidently is what prevents harm: it avoids unnecessary antibiotics, chest X-rays, blood tests and bronchodilators, and redirects care to feeding, hydration and breathing support. Both major guidelines emphasise this restraint explicitly.^1,2

How is it diagnosed?

From the age, the season and the characteristic pattern — a coryzal prodrome followed by cough, tachypnoea, wheeze and widespread fine crackles in an infant under 12 months. Investigations are usually unnecessary. Oxygen saturation is measured, and viral swabs or imaging are reserved for atypical, severe or uncertain presentations, or for infection-control cohorting.

Why routine imaging is avoided

Chest X-ray in bronchiolitis commonly shows hyperinflation and patchy atelectasis that is readily misread as consolidation, leading directly to antibiotic prescribing that does not help. Imaging is used when pneumonia, an inhaled foreign body or a cardiac cause is genuinely suspected, not to confirm the diagnosis.

Assessing severity

Severity is graded clinically on feeding, respiratory rate, work of breathing, oxygen saturation and behaviour. Trend matters more than any single reading, and an infant who was distressed and becomes quiet may be tiring rather than improving.

Investigations for related conditions

In severe, recurrent or atypical presentations, clinicians consider congenital heart disease, an inhaled foreign body, gastro-oesophageal reflux with aspiration, cystic fibrosis, immunodeficiency or a vascular ring. Recurrent wheeze beyond infancy shifts the differential towards preschool viral wheeze and asthma.

Management

Management and goals

The goals are to maintain hydration and oxygenation, support feeding, recognise deterioration, and avoid interventions that do not help. Management is supportive throughout, and most infants are cared for at home with good safety-netting.

Supportive care

Small, frequent feeds; clearing the nose with saline before feeds; minimal handling; upright positioning when settled; and a smoke-free environment. In hospital, nasogastric or intravenous fluids are used where feeding is inadequate, and oxygen where saturations fall below the locally agreed threshold.

Oxygen and respiratory support

Standard low-flow oxygen is first line. High-flow nasal cannula therapy is used where standard oxygen fails: in a large randomised trial it significantly reduced escalation of care, but did not shorten oxygen therapy, hospital stay or intensive care admission — so it is a rescue therapy for the deteriorating infant rather than an early routine intervention.³ CPAP and intubation are reserved for respiratory failure.

What does not work

Restraint here is active management. Nebulised hypertonic saline has at most a marginal effect on length of stay and is not routinely recommended.⁴ Bronchodilators do not improve oxygen saturation, admission rate or duration of illness, because the obstruction is not bronchospasm.⁵ Systemic or inhaled corticosteroids do not reduce admissions or length of stay.⁶ Antibiotics have no role in typical bronchiolitis, and adrenaline offers no sustained benefit. Explaining to parents why nothing is being prescribed takes longer than prescribing, and is the better clinical act.

Identifying deterioration

Escalating work of breathing, rising oxygen requirement, apnoea, feeding failure, dehydration or an infant becoming quiet and exhausted all indicate deterioration. Days three to five are the expected peak, so worsening after day five, or a second fever, prompts reassessment for secondary bacterial infection.

Medications

Medications for bronchiolitis

There is no specific antiviral in routine use, and as above, bronchodilators, corticosteroids and antibiotics are not indicated in typical disease. Antipyretics may be used for comfort. Where an infant has a genuine coexisting condition — established asthma in an older child, chronic lung disease of prematurity — that condition’s own treatment continues.

Prevention is the real pharmacology

The substantive advance is preventive. Nirsevimab, a single-dose long-acting monoclonal antibody, significantly reduced medically attended RSV lower-respiratory infection in healthy late-preterm and term infants,⁷ and maternal RSV vaccination in late pregnancy reduced severe RSV illness in infants in a large international trial.⁸ Eligibility and funding are changing — verify current ATAGI advice and National Immunisation Program status. Alongside these: hand hygiene, keeping unwell contacts away from young babies, breastfeeding, and eliminating tobacco smoke exposure.

Living with bronchiolitis

Caring for a baby at home

Feed little and often, clear the nose before feeds, keep the baby upright when settled and handle minimally when they are working hard to breathe. Parents should know that days three to five are usually the worst, so they can distinguish an expected course from genuine deterioration.

Knowing when to seek help

Every family should leave with the warning signs above, a clear instruction to return rather than wait, and a plan for after-hours care. Written or illustrated safety-netting is more reliable than verbal advice given to an exhausted parent.

The cough outlasts the illness

Cough commonly persists for three to four weeks after everything else has settled, and this is expected rather than a sign of failed treatment. Telling parents in advance prevents repeat presentations and unnecessary antibiotic courses.

Parental exhaustion

Nights of broken sleep, feeding difficulty and fear are a substantial and under-acknowledged part of the illness. Practical support, honest information about duration, and permission to seek review again all matter.

Recurrent wheeze afterwards

A proportion of infants go on to have recurrent viral wheeze in the preschool years. Families should know this is common, usually outgrown, and not the same as having been diagnosed with asthma.

Prognosis

The outlook is excellent for the great majority: full recovery within one to two weeks, with cough lingering longer. A minority, mostly young or high-risk infants, need hospital support, and a small number require intensive care. Long-term problems are uncommon. Recurrent viral wheeze in early childhood follows in a proportion, and an association between severe infant RSV bronchiolitis and later asthma is well described, though whether the illness causes the asthma or reveals a pre-existing predisposition remains unresolved. Mortality in high-income settings is very low.

Role of the physiotherapist

The evidence here is unusually clear, and it says do not treat. A Cochrane review of chest physiotherapy in infants under 24 months with acute bronchiolitis found that neither conventional techniques (percussion, vibration, postural drainage) nor slow passive expiratory techniques improved disease severity, respiratory parameters, oxygen requirement or length of stay — and the interventions cause distress, vomiting and transient desaturation.⁹ Australian and international guidelines accordingly recommend against it.^1,2

Acting on that is a real clinical skill. Referrals for “chest physio” in bronchiolitis remain common, and the correct response is to review the infant, explain why treatment is not indicated, and redirect the conversation to feeding, positioning, minimal handling and safety-netting — which is genuinely useful advice that families value. Declining to treat is not declining to be involved.

Selected infants are the exception: those with significant neuromuscular disease, cystic fibrosis, primary ciliary dyskinesia, an artificial airway or a demonstrably ineffective cough, in whom secretion retention is a real and separate problem. These children are assessed individually by a specialist paediatric team, and any treatment is directed at their underlying condition rather than at the bronchiolitis. Physiotherapists also contribute to family education on smoke exposure and immunisation, and to the developmental follow-up of infants who required prolonged respiratory support.

Part 1 · References

1. National Institute for Health and Care Excellence. Bronchiolitis in children: diagnosis and management (NG9). London: NICE; 2021.
2. Ralston SL, Lieberthal AS, Meissner HC, et al. Clinical practice guideline: the diagnosis, management, and prevention of bronchiolitis. Pediatrics 2014;134(5):e1474–e1502.
3. Franklin D, Babl FE, Schlapbach LJ, et al. A randomized trial of high-flow oxygen therapy in infants with bronchiolitis. N Engl J Med 2018;378(12):1121–1131.
4. Zhang L, Mendoza-Sassi RA, Wainwright C, Klassen TP. Nebulised hypertonic saline solution for acute bronchiolitis in infants. Cochrane Database Syst Rev 2017;(12):CD006458.
5. Gadomski AM, Scribani MB. Bronchodilators for bronchiolitis. Cochrane Database Syst Rev 2014;(6):CD001266.
6. Fernandes RM, Bialy LM, Vandermeer B, et al. Glucocorticoids for acute viral bronchiolitis in infants and young children. Cochrane Database Syst Rev 2013;(6):CD004878.
7. Hammitt LL, Dagan R, Yuan Y, et al. Nirsevimab for prevention of RSV in healthy late-preterm and term infants. N Engl J Med 2022;386(9):837–846.
8. Kampmann B, Madhi SA, Munjal I, et al. Bivalent prefusion F vaccine in pregnancy to prevent RSV illness in infants. N Engl J Med 2023;388(16):1451–1464.
9. Roqué-Figuls M, Giné-Garriga M, Granados Rugeles C, Perrotta C, Vilaró J. Chest physiotherapy for acute bronchiolitis in paediatric patients between 0 and 24 months old. Cochrane Database Syst Rev 2023;(4):CD004873.

References are numbered in citation order (Vancouver/BMJ style) and were current at the time of writing. Guidelines are living documents — verify against the latest version before clinical use.