Identifying oral allergy syndrome

In a new series, Dr Michael Radcliffe explains why hay fever patients can suffer a form of food allergy.

Rachel, 25 years old, had hay fever that was appearing earlier each year. Last year her symptoms started on Easter Sunday.

In the course of a visit to the surgery to pick up a prescription for hay fever, she revealed that she was terrified of food allergy and had now stopped eating fruit and was avoiding all nuts.

She had previously complained that eating apples and peaches gave her itchy lips but blood tests for allergies to these foods were negative.

She said that her food allergy problem was worsening. Cherries, nectarines, tomatoes, carrots, mango, apricots, nuts and melon made her mouth itch, and her tongue and lips sometimes felt swollen.

When she ate raw celery it caused her throat and ears to itch and gave her streaming nose and eyes. When she ate a peach her throat felt tingly and tight. Peeling potatoes made her hands, lips and eyes itch, and although she had no symptoms after eating them, she was worried that this was the next food she would need to exclude.

Oral allergy syndrome
Rachel's history indicates that she had oral allergy syndrome (OAS), a localised form of true food allergy.

In this condition, swelling or itching of lips, mouth, tongue or throat follow immediately after contact with certain foods. Many patients do not realise that they have a mild form of food allergy and the condition is significantly under-diagnosed.

OAS occurs in patients with seasonal rhinitis as a result of certain hay fever pollen allergens cross-reacting with homologous proteins present in range of fresh fruits, nuts and raw vegetables (see box). Luckily, these proteins are heat-labile so the same foods when cooked are unlikely to cause symptoms.

There are several allergens that can be responsible for the condition. The most common in the UK is birch pollen. It has been estimated that as many as 75 per cent of birch-allergic patients experience a sensation in the lips or tongue after eating raw apples, which is a mild form of OAS.

Allergy to grass and mugwort pollens can also cause OAS and a similar condition occurs in some patients with rubber latex allergy. A significant proportion of these patients experience oral allergy symptoms when they eat foods including banana, avocado and kiwi fruits.

Children and teenagers occasionally develop OAS but it is more common in adults. It is the most common presentation of true food allergy in patients aged over 16 years.

Symptoms
Redness, swelling and itching in the mouth are the main symptoms of OAS. In addition, there may be a blotchy, pimply or even blistering rash on the lips, tongue, the inside of the mouth and soft palate.

Symptoms in the oesophagus or stomach occur infrequently. These cause discomfort, heartburn, nausea or even vomiting.

General symptoms such as urticaria, rhinitis and asthma are relatively unusual, but when they do occur it is normally after a period of minutes or hours. These symptoms are more likely if the patient ignores the initial oral symptoms and eats all of the food causing the reaction.

Children who present with food allergy are more likely to have widespread symptoms such as rash, vomiting, wheeze and rhinitis in response to foods including egg, nuts and seafood.

Diagnosis
Patient history is more important than tests in the diagnosis of OAS.

Faced with a patient presenting with oral allergy symptoms, an astute GP may be able to predict that the patient is an April hay fever sufferer.

When the GP can also recite foods that cause the response, the patient is left in no doubt that the correct diagnosis has been reached.

In OAS the standard skin prick and specific IgE (RAST) blood tests for food allergy are unreliable.

Food allergens causing OAS are highly susceptible to heat and processing, and so are likely to be destroyed during the process of making the extracts used in these tests. False negatives are therefore common.

When it is essential to identify exactly which fresh fruits or raw vegetables are causing OAS, allergy skin prick testing using the juices from the fresh fruits and raw vegetables themselves can be tried. This is a more sensitive test and is a surprisingly effective method of diagnosis.

A skin prick test for birch pollen is the most useful of the standard allergy tests. Together with a patient history this can be used to make a diagnosis of OAS.

Standard allergy tests covering peanuts and all the tree nuts are often helpful. Almond, hazelnut and walnut are the nuts normally involved in OAS.

Occasionally a patient has both ordinary (systemic) nut allergy and fruit/nut associated OAS. In these cases, allergy tests will clarify if any other nuts need to be avoided.

Treatment
Once the diagnosis is clear, the patient needs to be given a clear explanation of the reason for the condition, and be reassured about its relatively benign nature. This usually comes as a considerable relief.

They should be given a list of the foods that potentially cause a reaction, to help them identify those that are causing their symptoms.

Anaphylaxis
Anaphylaxis and asphyxia due to laryngeal oedema are rare complications of OAS. In most cases a prescription of self- administered adrenaline by injection is not necessary.

There are exceptions to this rule. Patients with both OAS and an ordinary food allergy, as well as those with OAS causing mild throat narrowing together with significant risk factors, such as severe asthma, may require self-administered adrenaline. Each case therefore needs to be considered on its merits.

Many patients will ask about desensitisation treatments. At present these do not exist, but interesting developments in pollen immunotherapy offer hope that this form of treatment may soon be available.

Several studies of specific immunotherapy using birch pollen have demonstrated at least a temporary restoration of apple tolerance among birch pollen allergic OAS patients.

FOODS INVOLVED IN ORAL ALLERGY SYNDROME
Allergen
Foods that cross react
Birch pollen Almond, apple, apricot, raw carrot, raw celery,
cherry, coriander, fennel, hazelnut, kiwi,
nectarine, parsley, parsnip, peach, pear,
peppers, plum, raw potato, prune, tomato,
walnut.
Rye-grass pollen Melon, peanut, tomato, watermelon.
Rubber latex Almond, apple, apricot, avocado, banana,
chestnut, cherry, dill, fig, ginger, kiwi,
mango, melon, oregano, papaya, passion fruit,
peach, pear, plum, raw potato, sage, raw tomato.

Dr Radcliffe is a consultant in allergy medicine at the Royal Free NHS Trust, London

Next week: ficus fruit allergy

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