Antihistamines and allergy

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I'm Andrew Boyden your host for this podcast. It's a pleasure to be speaking with Dr Carolyn Hawkins about antihistamines. Welcome Carolyn.

Hello.

Well I'm showing my age here, but when I went to medical school I think we only had sedating antihistamines to choose from but now we have the second and third-line less sedating products. How do these differ in their actions?

So there's very little difference in terms of the actions in managing symptoms associated with allergy but the sedating antihistamines have a number of very unfavourable effects that can be associated with them and so there are a number of studies now that have shown that children perform less well at school when they have allergic rhinitis treated with the sedating antihistamines and of more concern is that a number of aviation and car accidents have been found to be attributed to the use of sedating antihistamines.

So we clearly should generally be avoiding their use but are there any situations in which they are still indicated?

Yes there are some situations where a sedating antihistamine such as cyclizine or promethazine might still be used. For example they can be used specifically in the prevention of motion sickness and may sometimes be used where nausea might want to be controlled, for example in patients receiving chemotherapy.

I know the sedating antihistamines have been used for sedation in children say for instance with long-distance air travel. Is there a role in that area still?

No we don't believe that sedating antihistamines should be used in children and particularly those less than two years of age. There was a black box warning by the FDA in 2004 in America but there have been significant associations with both behavioural and significant sedation in children. We would also recommend against the use in pregnant women as well due to the risk of sedation.

Yes so it’s really a strong message we need to be sending out there through the community that really the use in this way is really historical and should be put to bed.

I would confirm that, yes.

With the newer antihistamines are some better than others or indicated in one situation over another?

So there are some situations where a particular non-sedating antihistamine might be preferred in particular clinical settings. Loratadine is metabolised in the liver, whilst the other non-sedating antihistamines cetirizine, desloratadine, fexofenadine aren't metabolised extensively and cetirizine for example is eliminated in the urine. For example, if you were using loratadine you would consider dose reduction in patients with severe liver dysfunction and for cetirizine that's excreted in the urine you would maybe reduce the dosing in kidney failure.

Okay. In general practice we're often turning to antihistamines and so for example, starting with allergic rhinitis, is this a straightforward diagnosis that can be made on history and examination alone or do you think there is a role for allergy testing?

I believe in the initial setting that there is a role for history and examination and making a diagnosis on the basis of those features but in terms of being able to identify specific triggers, for example determining whether a patient has seasonal rhinitis or whether they have rhinitis throughout the year, then there's a significant role for skin prick testing to identify specific allergens.

Yes and I suppose that's touching on treatment really for allergic rhinitis and of course the need to avoid trigger factors and of course this is where the role of medicines comes in as well. So apart from antihistamines are there are other medicines that you recommend?

Some other treatments that can be used in the setting of allergic rhinitis are topical corticosteroids but there are also topical nasal antihistamines for management of very mild disease alone. There are also other treatments such as intranasal ipratropium bromide or atrovent which can help in in reducing nasal secretions or rhinorrhea. Nasal irrigation with saline solution is also shown to improve some of the symptoms and the total amount of oral antihistamines that might be required.

So, just building on what you just said there about the various treatments available, is there a role for combination approaches?

Most definitely. It depends on the severity of the symptoms but if a patient had mild allergic rhinitis then you might decide to use either intranasal or oral antihistamines alone but often, once patients come to see us in the clinic, they've got significant symptoms that are poorly controlled with the use of antihistamines alone and often then that's when we would consider combining therapies to include both intranasal corticosteroids, maybe antihistamines and intranasal corticosteroid therapy.

Okay and these principles around treatment, for example avoidance of trigger factors and selection of appropriate medicines, of course also apply to another common condition seen in general practice, allergic conjunctivitis. What's your view and when may oral treatment be preferred over the use of topical agents?

In the setting that we see these patients they often have a combination of both allergic conjunctivitis and rhinitis and I think in that setting where the patients might find it difficult to avoid the allergen completely, particularly during spring, then there's a role for both the topical and a systemic antihistamine. Other treatments that might be considered are the mast cell stabilisers so sodium cromoglycate can be used but there're also topical eye drops, as you know – ketotifen, azelastine and olopatadine play a significant role both in their antihistamine and mast cell stabilising effects.

Can you please outline the role of antihistamines and the management of more acute allergic reactions and maybe which guidelines that you use to guide you in this area?

So, in a patient who looks like they might be having a generalised allergic reaction or anaphylaxis there is no role for the use of an antihistamine in the acute setting and adrenaline should be considered as the appropriate first-up treatment. So the way I would normally follow the approach to the management of an allergic reaction is to follow the ASCIA guidelines which breaks up the symptoms associated with a mild allergic reaction and distinguishes that from a generalised or severe allergic reaction such as anaphylaxis so if it looks like it's a mild to moderate allergic reaction with urticaria and abdominal pain with the exception of even venom anaphylaxis then I would consider using a non-sedating antihistamine in that setting first up. But if there's any evidence of signs or symptom that would be consistent with either a generalised allergic reaction or anaphylaxis then the administration of adrenaline would be my initial recommended therapy in this setting.

Absolutely and that's highlighted in the recently re-released Australian Prescriber anaphylaxis wall chart.

That's correct.

As we've discussed antihistamines are useful medications when used for the right indications. Are there any areas in which they're still being used but for which the evidence just isn't there?

Well we find that antihistamines are still used in the setting of cold and flu symptoms and there's actually no evidence to suggest that antihistamines are of benefit. As we've previously mentioned there's no evidence for the use of sedating antihistamines in children.

In summary, what would you say are take-home messages for pharmacists and GPs in relation to the appropriate use of antihistamines?

You could say that antihistamines are very effective in treating allergic rhinitis, allergic conjunctivitis and urticaria. I think the take-home message is that there are very few situations where a sedating antihistamine should be used.

Thanks Caroline.

That’s ok, no trouble.

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The full article on antihistamines is available online. The views of the host and the guests on this podcast are their own and may not represent Australian Prescriber or NPS MedicineWise.