Food allergy is common in children, particularly in the preschool age group, in whom it has an incidence of 5–8%. Symptoms are generally mild and can include any of the following:
• skin—flushing, rashes, and itching;
• nose—sneezing, nasal congestion, and itching;
• eyes—tearing and itching;
• gut—vomiting, cramps and diarrhoea, colic;
• mouth—swelling of the lips and tongue, oral tingling.
A small proportion (much less than 1% of those with food allergy) suffers from more severe reactions. A practical definition of anaphylaxis (Box 44.1) includes one or both of:
• respiratory difficulties caused by laryngeal oedema and/or bronchospasm;
• hypotension (light-headedness or dizziness) caused by loss of vasomotor tone.
Fatal anaphylaxis is very uncommon in the UK; a survey identified only eight paediatric deaths over the 10-year period 1990–2000. Risk factors for anaphylaxis are:
• the presence of asthma, especially poorly controlled asthma;
• previous episodes of anaphylaxis;
• allergy to peanuts, tree nuts (walnuts and pecan nuts), and shellfish.
The contribution of food allergy to asthma symptoms is controversial, but up to 10% of children with asthma may have food-associated triggers to their usual wheeze.
A good history is the key to making a diagnosis of food allergy. Important features include the following.
• Timing: symptoms should occur within 2 h of ingestion.
• Amount: small amounts of food are usually sufficient to cause the reaction.
• Repetition: symptoms should occur on each occasion the food is consumed.
• Other atopic illness: eczema, hay fever, asthma. Most children with significant allergic reactions will be atopic.
• The severity and nature of the reactions should be documented.
Skin prick tests
Where there is a history suggesting an allergic reaction, SPTs can be helpful. Commercial extracts are available for most of the likely foods. Extracts from fruit and vegetables are less reliable, because the proteins in these foods are readily degraded. Although SPTs are generally very safe, even in those with a history of severe allergy, systemic reactions can occur, and resuscitation equipment and trained personnel should be available on site. Oral antihistamines need to be stopped at least 48 h prior to the test, and any topical emollients at the test site should be omitted on the day of the test.
• A negative SPT means that the child is very unlikely to react to that food.
• A positive SPT has a predictive value of around 60%.
• The severity of the reaction to an SPT does not correlate with the severity of clinical symptoms.
These blood tests (RAST and ELISA) are not superior to SPT but may be helpful when skin tests cannot be performed, either because of severe eczema or because antihistamines cannot be stopped. The interpretation of positive and negative results is the same as for SPTs. There is no added value in carrying out serum-specific IgE tests in children who have had SPTs.
• This remains the gold standard test.
• For most immediate allergic reactions, it is not required, particularly where the history is clear and there is a positive SPT.
• Where there is doubt, usually where the symptoms are more chronic and persistent (often gut symptoms), food challenge can be the best way to identify the causative food.
• The suspected foods need to be excluded from the diet for at least 2 weeks and the associated symptoms must have abated. A double-blind challenge can then be used to test individual foods.
• Once a food has been identified as being very likely to be, or definitely, associated with allergy, avoidance is the main aim of management. This is best achieved by discussion with a dietician.
• Simple treatment with oral antihistamines will reduce the duration of symptoms from mild allergic reactions.
• Children with a history of severe reactions (difficulty breathing, light-headedness, or dizziness) should be provided with an adrenaline auto-injector. These devices deliver an IM injection of adrenaline. Two strengths are available: 0.3 mg for those >30 kg, and 0.15 mg for those <30 kg. The dose should be repeated after 5 min, if there has been no improvement.
• Children with concomitant asthma are at increased risk of a severe reaction to foods to which they have an allergic sensitization, particularly if their asthma is poorly controlled. It is usually suggested that these children should carry an adrenaline auto-injector, even if previous allergic reactions to food were not classified as severe.
• The consequences of prescribing an adrenaline auto-injector should not be underestimated. The child and family must be told exactly how and when to administer the dose. The school will also need to be informed, and a member of staff identified who would be able to give the injection. The child will need to carry the adrenaline at all times when it is possible they will be exposed to the allergen. The auto-injector will need to be replaced every 6 months. The need to carry the auto-injector should be reviewed on an annual basis.
• Children with respiratory disease who have egg allergy may require the annual influenza vaccination. Egg-free influenza vaccines do exist and can be ordered through hospital pharmacy, and children with egg allergy cohorted to receive these in hospital
Food allergies tend to become less severe, as children get older. Eighty-five per cent of children with milk allergy will no longer react by the age of 3 years. Allergies to peanut, tree nuts, and shellfish are more likely to persist. Subsequent exposures do not tend to result in increasingly severe reactions.