Allergic Asthma

Allergic asthma is asthma driven by IgE-mediated sensitisation to inhaled allergens, with type 2 airway inflammation. It is the commonest asthma phenotype and usually begins in childhood.

Pathophysiology. Allergens (house dust mite, pollen, animal dander, fungi, cockroach) trigger IgE-mediated mast cell activation and type 2 (IL-4, IL-5, IL-13) inflammation, with eosinophils, airway hyperresponsiveness and remodelling.

Sensitisation is shown by positive skin prick tests or specific IgE with a matching clinical history. Atopic eczema, allergic rhinitis and food allergy often coexist.

Control is judged by symptoms, reliever use, exacerbations and lung function.

• Recurrent wheeze, cough, chest tightness and breathlessness
• Symptoms worse at night, early morning, with exercise, cold air or allergen exposure
• Seasonal or perennial pattern with allergen exposure
• Allergic rhinoconjunctivitis and eczema
• Exacerbations triggered by viral infection or allergen
• Variable symptoms with periods of being well

• Spirometry: FEV1/FVC under 0.70 (or under the lower limit of normal) with bronchodilator reversibility of 12% and 200 mL or more in FEV1.
• Peak flow variability: over 20% diurnal variation over 2 weeks.
• Fractional exhaled nitric oxide: over 40 ppb in adults (over 35 ppb in children) supports type 2 inflammation.
• Blood eosinophils: over 0.3 x 109/L supports type 2 inflammation.
• Skin prick tests or specific IgE to relevant aeroallergens, and total IgE.
• Bronchial challenge (methacholine or mannitol) if spirometry is normal.
• Chest X-ray if the diagnosis is unclear or there is an atypical feature.

Differential diagnosis.

• Chronic obstructive pulmonary disease and asthma-COPD overlap
• Vocal cord dysfunction and upper airway obstruction
• Bronchiectasis, cystic fibrosis and primary ciliary dyskinesia
• Gastro oesophageal reflux and chronic cough
• Heart failure and pulmonary embolism
• Allergic bronchopulmonary aspergillosis

• Reliever and preventer in adults: budesonide with formoterol 200/6 microgram, 1 inhalation as needed for symptoms (maximum 12 inhalations in 24 hours). For regular treatment, 1 to 2 inhalations twice daily with the same inhaler as reliever (maintenance and reliever therapy). Avoid reliever salbutamol alone.
• Step up for poor control: higher dose inhaled corticosteroid with formoterol, then add a long acting muscarinic antagonist (tiotropium 5 microgram, 2 puffs once daily) or montelukast 10 mg orally at night.
• Allergen avoidance where practical, and treatment of rhinitis with intranasal corticosteroid.
• Allergen immunotherapy (sublingual house dust mite tablet 12 SQ-HDM once daily, or subcutaneous) for stable mild to moderate asthma with proven mite sensitisation and rhinitis.
• Biologic therapy for severe disease: omalizumab by weight and baseline IgE (75 to 600 mg subcutaneously every 2 or 4 weeks).
• Acute exacerbation: salbutamol 5 mg nebulised (repeat every 20 minutes for 1 hour), prednisolone 40 to 50 mg orally daily for 5 days (children 1 to 2 mg/kg, maximum 40 mg), ipratropium 500 microgram nebulised, and magnesium sulfate 1.2 to 2 g intravenously over 20 minutes for severe cases.
• Inhaler technique, adherence review and a written action plan at every visit.
• Vaccination: yearly inactivated influenza vaccine and pneumococcal vaccine.

Special groups.

• Children: age appropriate inhaler with spacer, and dosing by age and weight.
• Pregnancy: continue controller therapy, since uncontrolled asthma is more harmful than treatment.

Monitoring. Symptom control score and reliever use at each visit. Spirometry yearly. Review every 3 months after a step change, and every 6 to 12 months when stable.

Referral and escalation. Refer to respiratory medicine or allergy for poor control on medium dose inhaled corticosteroid with a second controller, for suspected severe asthma, or for immunotherapy. Emergency for severe exacerbation, inability to speak, or oxygen saturation under 92%.