Key Facts

Viral-induced wheeze is the most common form of wheeze in preschool children. It is pathophysiologically distinct from atopic asthma and generally has an excellent prognosis with most children outgrowing symptoms by school age.

Epidemiology

Approximately 30% of children wheeze in the first 3 years of life. ~60% of these are 'transient early wheezers' who stop wheezing by age 6. Boys are more commonly affected. Peak incidence is in the winter viral season. Only ~30-40% of wheezy preschool children go on to develop persistent asthma.

Aetiology

• Viral triggers: Rhinovirus (most common), RSV (especially bronchiolitis), parainfluenza, adenovirus, human metapneumovirus
• Risk factors: Small airway calibre (male, preterm), parental smoking, older siblings (viral exposure), nursery attendance
• NOT driven by atopy, allergens, or exercise (unlike atopic asthma)

Pathophysiology

Viral infection causes airway inflammation, epithelial damage, and mucosal oedema in small-calibre airways. In young children, the airways are relatively narrow (resistance proportional to radius⁴), so even small amounts of mucosal oedema and mucus cause significant airflow obstruction. This is a predominantly neutrophilic (not eosinophilic) inflammatory response, explaining the relatively poor response to corticosteroids compared with atopic asthma.

Typical Features

• Wheeze only during viral URTI episodes
• Cough, rhinorrhoea, low-grade fever preceding wheeze
• No symptoms between episodes (no interval wheeze)
• Usually <3 years of age
• No atopic features (no eczema, no food allergy, normal IgE)
• Family history of atopy often ABSENT

Red Flags

• Wheeze between viral episodes — consider asthma
• Persistent wheeze not responding to bronchodilators — consider foreign body, CF, anatomical abnormality
• Failure to thrive with wheeze — consider CF, immunodeficiency
• Fixed wheeze or stridor — structural airway abnormality (tracheomalacia, vascular ring)

Bedside

• SpO2: In acute episode
• Clinical assessment: Work of breathing, feeding ability, hydration
• Objective tests (spirometry, FeNO) NOT possible in <5 years — diagnosis is clinical

Bloods

• Not routinely needed
• Total IgE, specific IgE: If atopic asthma suspected (interval symptoms, atopic features)

Imaging

• CXR: Only if diagnostic uncertainty (exclude foreign body, congenital anomaly)

Special Tests

• NPA (nasopharyngeal aspirate): If viral aetiology needs confirmation (e.g., RSV in bronchiolitis)
• Sweat test: If recurrent wheeze with failure to thrive or recurrent chest infections

Acute Episode

• Salbutamol via spacer: 2-10 puffs (trial of treatment); response may be less marked than in asthma, especially <1 year
• Oxygen: If SpO2 <92%
• Prednisolone: 1-2mg/kg for 3 days — less effective than in asthma; consider only for moderate-severe episodes
• Supportive: Adequate hydration, antipyretics

Prevention/Maintenance

• Intermittent montelukast: At onset of viral symptoms — may reduce severity and duration
• Regular low-dose ICS: Consider trial if frequent episodes (>3/year) or severe episodes; assess response at 8 weeks
• Parental smoking cessation: Single most important modifiable risk factor

Non-pharmacological

• Parental education: Explain difference from asthma; provide reassurance about prognosis
• Written management plan: When to use inhaler, when to seek medical help
• Follow-up: Reassess at 8 weeks if ICS trial started; discontinue if no benefit

Referral Criteria

• Recurrent severe episodes requiring hospitalisation — respiratory paediatrician
• Symptoms persisting beyond age 5 with atopic features — investigate for asthma
• Failure to thrive, chronic cough, recurrent infections — investigate for CF, immunodeficiency

• ~60% stop wheezing by age 6 (transient early wheezers)
• ~20% develop persistent atopic asthma
• ~20% have episodic wheeze that gradually improves through childhood
• Risk factors for progression to asthma: Atopic eczema, parental asthma, persistent wheeze between episodes, blood eosinophilia, positive aeroallergen sensitisation
• Lung function: Generally normal in adulthood for those with isolated viral-induced wheeze

Viral-Induced Wheeze vs Atopic Asthma