Allergy is one of the most common childhood diseases, affecting more than 1 in 4 children at some point. The incidence seems to be increasing in many countries worldwide, and the reason is not clear. Exposure to pollutants may be one factor; over-cleanliness and lack of exposure to infections and allergens in early life may be another.
Allergy is caused when an individual develops IgE antibody against specific environmental allergens. Once sensitized, an atopic individual will trigger a type I (immediate) hypersensitivity response on exposure to the same allergen, leading to local or systemic inflammation. This inflammation is mediated by release of histamine and other cytokines from mast cells, and leads to:
- Acute inflammation (urticaria)
- Bronchospasm (asthma)
- Chronic inflammation (e.g. eczema).
Life-threatening airway obstruction (angioedema) or shock may occur if there is a massive systemic response to allergen exposure (anaphylaxis).
The age of onset is variable, but most atopic children develop symptoms by 5 or 6 years of age. Infants are likely to show eczema and milk or egg allergy. Preschool children tend to get asthma, initially triggered by viral infection and later by environmental allergens such as house dust mite. Allergic rhinitis and conjunctivitis are commoner in older children and young adults. A family history of atopy is often present. There is good evidence that prolonged exclusive breastfeeding reduces later allergy.
Eczema is discussed in detail in Chapter.
Acute asthma is discussed in Chapter. Chronic asthma management is discussed in Chapter 26.
Allergic rhinitis (hay fever) reaches a peak in adolescence. Sneezing, rhinorhoea, nasal congestion and itching are triggered by an IgE response to airborne allergens. Tree and grass pollens, mould spores and pet dander are common triggers. Pollens are particularly prevalent in early summer on dry, hot days. The child may exhibit the ‘allergic salute’ of rubbing their nose constantly with their hand. Nasal polyps can develop with chronic inflammation. Treatment involves antihistamines and nasal topical steroid.
Many children with allergic rhinitis will also have recurrent non-infective conjunctivitis; the eyes are red, feel gritty and itchy and tearful. Treatment involves topical antihistamines or topical mast cell stabilizers such as sodium cromoglicate.
Food allergy is IgE mediated and appears to be increasing, affecting 3–6% of preschoolers and 2–3% of school-age children. In young infants the symptoms are often cutaneous, with eczema, urticaria, and angioedema. Wheeze, diarrhoea or vomiting may be present. Colic occurs in babies. In infants and toddlers the commonest food allergens are cow’s milk protein, egg and peanuts. There is cross-reactivity (30%) between cow’s milk protein allergy and soya milk allergy. In older children reactions to citrus fruits, tree nuts or peanuts, fish or shellfish are more common.
The diagnosis is made on the basis of a clear history of exposure, the presence of significant specific IgE antibody or a positive skin prick test, and preferably confirmed by a standardized controlled food challenge. Treatment involves excluding the allergen from the diet, usually for a period of 2 years, and then a controlled food challenge. A dietician should advise on maintaining a balanced diet (e.g. calcium supplements if milk is excluded). Severe anaphylaxis is relatively rare and there is a danger of over-diagnosis leading to a very restricted diet and lifestyle. Children with concurrent asthma are at most risk and may need to carry adrenaline and wear a Medic-Alert bracelet. Very rarely there may be cross-reactivity between airborne allergens and food allergens (e.g. birch pollen and apples) leading to seasonal mucosal inflammation in response to certain foods (oral allergy syndrome).
Food sensitivity is not IgE mediated and causes predominantly gastrointestinal symptoms, such as abdominal pain, vomiting, diarrhoea and colitis.
Urticaria, Angioedema and Anaphylaxis
A variety of allergens including foods, insect stings and drugs may cause a severe acute allergic reaction. At its most extreme and life-threatening this is known as anaphylaxis. Many allergic reactions will start with an urticarial rash—raised, well demarcated itchy wheals with an erythematous border and a pale centre (see Chapter). In a few cases urticaria may non-allergic, triggered by mast cells releasing histamine in response to cold, pressure (the Koebner phenomenon), or other physical causes. Contact dermatitis (a delayed or type IV IgE-mediated reaction) also causes urticaria. Angioedema is acute tissue swelling around the eyes, lips or airway in response to an immediate type I IgE reaction. This may cause stridor and airway obstruction.
Anaphylaxis involves massive release of inflammatory mediators causing systemic inflammation and shock due to vasodilatation and capillary leak. Airway obstruction due to oedema and bronchospasm may occur. There is a very rapid onset of symptoms, often associated with flushing, tachycardia and a feeling of ‘impending doom’. Common triggers include drugs (e.g. penicillins, anaesthetic agents), foods (peanuts, shellfish), latex (in rubber gloves) and insect stings (wasps, bees). Treatment includes removal of the allergen, intramuscular adrenaline, oral antihistamines and intravenous hydrocortisone.
Patients with a history of anaphylaxis should be referred to an allergy clinic for specialist management. It may be appropriate to provide the child with an adrenaline auto-injector (e.g. Epipen) which can be used to administer a fixed dose of adrenaline intramuscularly at the onset of symptoms. Preventative advice to the child, their parents and school or nursery is critical, though should not over-restrict lifestyle.
- The incidence of atopy is increasing in industrialized countries.
- Eczema and milk allergy are common in infancy but normally resolve.
- Seasonal allergic rhinitis and conjunctivitis are common (up to 40% of teenagers)
- Testing for allergy is controversial as skin prick tests and IgE assays may be equivocal.
- Prevention by education and allergen avoidance is crucial for all atopic conditions.
- Severe anaphylaxis to foods is rare, but does cause a few preventable deaths each year.