INTRODUCTION
Food allergy can be complicated and can be a difficult consultation with patients (and sometimes particularly parents)
Ifan.ie is a useful resource
An adverse food reaction can be any adverse reaction to food
This can be immune related (food allergy) or not realted to the immune system (food intolerance)
Examples of food intolerance:
Metabolic - lactose intolerance
Pharmacologic - Vasoactive amines, salicylates, histamine, caffeine, tyramine
Toxic -
Food poisoning
Scromboid poisoning:
Occurs due to ingestion of spoiled fresh, canned or smoked fish with high histamine levels due to improper processing or storage
Occurs with fish from scrombidae family (eg tuna and mackerel) and other dark fleshed fish (sardines, anchovies)
The cooking destroys the bacteria but not the histamine produced by the fish
Unlike food allergy many people can potentially get scromboid poisoning from same dish
Symptoms include:
Flushing, rash, urticaria, palpitations, headache, sweating, burning mouth/throat
Can get GI symptoms - nausea, vomiting, abdominal cramps, diarrohea
Bronchospasm and respiratory distress also described
Symptoms usually begins within 90 minutes of ingestion and symptoms usually disapear within 3-36 hours
FOOD ALLERGY
Food allergy is an immune related reaction to a food allergen
You are more likely to develop food allergy if have atopic eczema
Particularly if atopic eczema presents at an early age or is severe
Food allergy is classified based on whether it involves specific IgE antibodies for that food
The two broad categories of food allergy affecting the skin are the immediate Type 1 IgE mediated allergic reaction and delayed Type 4 non-IgE meadiated allergic reactions
Sometimes you can get a presentation where the picture appears mixed
TYPE 1 HYPERSENSITIVITY REACTION (IgE MEDIATED)
This is the classic type of food allergy that people would know about where you get a very quick reaction
Very small amounts of food can elicit a significant reaction (usually through ingestion but occasionally can be airborne exposure)
Very rapid onset of symptoms usually within 20 mins, but often within 1-2 mins (definitely within 2 hours)
Get a spectrum of lip swelling, urticaria (commonly face/chest) anaphylaxis and can get immediate vomiting after ing
Presentation:
Commonest type of food allergy
Spectrum of urticaria (commonly face and chest but can be widespread) and angioedema (commonly lip swelling)
On their own urticaria and angioedema are ‘minor’ symptoms
Cough/hoarseness (indicating upper airway obstruction) are under appreciated and should be treated as 'severe' symptoms
Wheeze and feeling faint (severe symptoms)
Can also get immediate vomiting after ingestion of the food
Affects about 5-6% of young children in Ireland
Often have other atopic conditions (eg eczema, asthma, allergic rhinitis)
Children with both food allergy and asthma are at increased risk of severe food allergic reaction
Most commonly implicated foods in type 1 allergies:
Egg
Milk
Wheat
Soya
Peanut
Treenut
Fish
Shellfish
[account for >90% cases]
First few years life: milk, egg, peanut
>3 years: peanut, treenuts, fish, shellfish
Most children outgrow allergy to milk or egg
Most children don’t outgrow peanut, treenut, fish, shellfish allergy
DIGANOSIS OF IgE MEDIATED FOOD ALLERGY
A good clinical history is the cornerstone of diagnosis of food allergy
Based on the history and experience of the physician you may then draw up a ‘pretest probability’ on whether you think they have an allergy to a certain food
Then you may order either skin prick testing or RAST testing to appropriate allergens to try to confirm or refute this diagnosis
After these tests you then have a ‘post test probability’ which combines the clinical information and the tests performed. You can gt three results essentially from this:
Patient has a definite food allergy
Patient definitely doesn’t have food allergy
Not conclusive one way or the other
If not conclusive one way or the other you may need to refer on for further testing like an oral food challenge
Other tests sometimes mentioned in literature:
Basophil activation test - this test is used in research but generally not in clinical practice yet
IgG4 testing - not recommended
RAST testing (blood test):
Looks at specific IgE antibodies to certain foods and other possible allergens
If a test is high/positive this only confirms the child is sensitised to a given food
The majority of sensitised children are actually ont food allergic
Essentially this means the patient may have a ‘positive result’ in the lab but actually doesn’t have any clinical symptoms or signs of allergy to this food
A positive test might increase index of suspicion but the only way of confirming the allergy is by trial of dietary exclusion to see if symptoms abate or doing an oral food challenge in an appropriate setting
It is essential that you do not order RAST testing to a whole range of allergens blindly as may result in restricting foods that the child is not allergic to which can have consequences for the childs wellbeing
Skin prick testing:
Looks at masst cell reactivity in skin after an allergen is introduced
A tiny amount of allergen is introduced in to the skin to see if this elicits a small allergic response with a wheal and flare at the site
The allergen can be an extract of the food or can be a specific protein of the food (eg Ara h2 protein in peanut)
Patient needs to avoid taking any anti-histamines 48 hours beforehand as may result in a false negative
RAST testing is measured as a level of IgE
Skin prick testing is measured as a diameter
Depending on the level of IgE or the size of the wheal it can aid in assessing if a patient has a food allergy
For example SPT to peanut:
5mm - sensitivity 92.9%, Specificity 94%
8mm - sensitivity 25.4%, specifficity 98.5%
But low level positive tests have a relatively low positive predictive value (50%) for food allergy. As a result ordering such tests should be done with caution and should only be tested for 2 or 3 suspected foods
Negative IgE results are highly specific and have a negative predictive value of 95% for milk, egg and peanut
Above tests should only be ordered in appropriate clinical setting eg
-definite history of a reaction to a particular food group
-severe, refractory atopic dermatitis
- exudative unrelenting facial involvement in an infant
Oral allergy syndrome:
Also known as food pollen syndrome
Is another type of IgE mediated food allergy
Often seen in patients with hay fever who are sensitised to birch pollen
When they put things like apple, pineapple, apricots or other fruits and even some nuts they get a tingling sensation in the mouth
This occurs when the food is raw but doesn’t occur when the food is cooked
Delayed non IgE mediated food allergy:
More delayed presentation than type 1 reaction so can be more difficult to elicit from history
As it is more tricky for parents to spot the child may get continued exposure to an allergen and have more chronic symptoms
Usually have cutaneous and/or GI symptoms:
Flare of eczema
Patient may have symptoms of reflux, colic, persistent cryging,diarrhoea with blood or mucus (less common constipation) and food aversion
Key points:
Most children with mild-moderate eczema that respond well to treatment do not have a food allergy
Atopic children with food allergy often present early in life with severe eczema not responding or poorly responding to potent steroids
In infants under 2, food allergy can exacerbate existing eczema, but there is no justification for manipulating diet until skin care with emollients and topical steroids has been optimised
Children over 2 years with eczema should not have dietary manipulation without expert medical assessment
Early introduction of certain foods (eg peanut) may actually decrease incidence of food allergy
LEAP studyNEJM, 2015:
· Infants with high risk atopic disease getting sustained peanut consumption in the first 11 months of life compared to peanut avoidance resulted in a significantly smaller proportion of children with peanut allergy at 60 months
· Principle is that introducing allergen to GI immune system creates appropriate immune response to allergen however if allergen is introduced through the skin in the environment the infant will develop an abnormal immune response to allergen
There is no evidence for delaying introduction of allergenic foods
Only time where may consider delaying introduction of allergenic foods is in the case of a peanut allergy in a sibling
Elimination diets can result in loss of tolerance to a food, nutritional deficiencies and negative emotional and social effects
Type 4 hypersensitivity reaction (non IgE mediated food reaction)
To further complicate matters there are also delayed types of food allergies
Symptoms may not appear for more than 24 hours after exposure
Small dose may not be tolerated but incremental doses are not tolerated
Symptoms include enteropathies (ask about GI symptoms- loose stools, blood/mucus, abdo pain) and eczema
There is no diagnostic lab tests for non IgE mediated food reactions
Dietirican supervised exclusion and reintroduction is the only supportable diagnostic test
This should be defined (4-6 weeks duration) and exlude no more than 4 foods
Consider non IgE mediated food allergy in children with:
Ø Refusal to feed
Ø Severe aversive feeding behaviour
Ø Problems progressing weaning diet
Ø Growth faltering
Ø Stools with mucus/blood in them
Especially if they also have eczema that is difficult to control
Reactions that look like allergy but aren’t:
Sometimes get red rash around mouth when eat or drink tomato/orange (or other citrus fruits). This is due to skin irritation from the natural acids in the foods and is not an allergy.
Milk allergy:
Human breast milk is best for the baby
Majority of infant formulae made using modified cow’s milk and contain cow’s milk protin
Cow’s milk protein allergy varies from 2-7.5%.
Resolution excpected in between 75 and 90% of cases before 5-6 years of age
Rates may be lower in severe cases
Can be IgE mediated or non IgE mediated:
IgE mediated- rapid onset urticarial, swerlling, airway symptoms
Non IgE mediated- delayed gastrointestinal and skin symptoms
Most children present before 6-7 months of age (during this time milk is major fodd so other food allergies usually don’t need to be considered)
Exclusively breast fed infants with cow’s milk allergy may respond to time defined complete maternal exclusion of cow’s milk (should be supervised by dietician)
Infant formula:
Usually contains cow’s milk protein
Some contain extensively hydrolysed protein- more hypoallergenic
Amino acid based formulas- even more hypoallergenic
So amino acid formulas may be indicated in those presenting younger with severe and persistent allergy
No place for lactose free formula
Other mammalian milk (goat, sheep, camel, donkey, horse etc) and/or plant based milk (soy, rice, oat, almond, coconut) not indicated
-some are not nutritionally complete
-some cross react with cow’s milk and cause reactions