![]() ![]() Oral antihistamines also have the benefit of treating associated conjunctival symptoms. They are particularly useful for nasal itchiness, sneezing and rhinorrhoea, but are less effective for nasal obstruction. Drugs can help and oral antihistamines are one of the mainstays of treatment. It is important to ask the patient if they also have respiratory symptoms as a worsening in allergic rhinitis can lead to increased asthma symptoms.Īvoiding trigger factors is the first step in the management of allergic rhinitis but some triggers can be difficult to avoid. Allergic rhinitis may be seasonal (usually due to grass, tree or weed pollens) or perennial (due to triggers such as pet hair, house dust mite or mould). Other conditions may cause similar symptoms, but they can be distinguished from allergic rhinitis by allergy testing to confirm positive allergen-specific IgE to specific triggers. The newer antihistamines are likely to be as safe in pregnancy but have not been used by as many women, so they do not have the same evidence of safety.Īllergic rhinitis refers to nasal inflammation due to the release of histamine and other mediators from IgE-mediated mast cell degranulation in the nose. However, pregnant women must be warned about the other aspects of safety such as sedation and consider whether they should not drive while taking these drugs. An exception is promethazine for which adverse events have been reported in animal studies (at very high doses). They have been taken by a large number of pregnant women and women of childbearing age without any proven increase in malformations or harm to the fetus. The main role for sedating antihistamines is in pregnancy, where they can be used for any of the common indications for antihistamines, as they have the strongest evidence of safety. 7 The risk is higher for intravenous use and led to an FDA warning. However, for promethazine there is a risk of severe tissue injury, including gangrene, with both intramuscular and intravenous administration. Sedating antihistamines are still favoured by some, as parenteral formulations are available. Sedating antihistamines can also have anticholinergic effects that can be particularly problematic in older patients who are more susceptible to adverse effects such as dry mouth, urinary retention and delirium. 3 This led to a black box warning by the US Food and Drug Administration (FDA) in 2004. There is also concern about the use of promethazine in children less than two years old as serious behavioural and other adverse effects can occur. 3 An audit of media reports found a number of car accidents attributed to sedating antihistamines, but none attributed to less sedating antihistamines. 5 Sedating antihistamines have been found to be a cause of aviation accidents. 3, 4 Studies have shown poorer school performance in children with allergic rhinitis treated with sedating antihistamines, compared to children treated with non-sedating antihistamines and healthy children. 3 The main concerns are their sedative properties and interference with rapid eye movement sleep. Their unfavourable adverse effect profile has prompted the Global Allergy and Asthma European Network to recommend making these antihistamines prescription-only, rather than over-the-counter, drugs. The sedating, first generation antihistamines now have little role in therapeutics. Dose reduction should be considered in patients with severe liver or kidney dysfunction. Cetirizine is eliminated in the urine, while fexofenadine is excreted in the faeces. Loratadine is metabolised in the liver, while cetirizine, desloratadine and fexofenadine are not metabolised extensively. The H 1 antihistamine drugs therefore act as inverse agonists. 1 The presence of histamine stabilises the receptor in its active form while antihistamines stabilise the inactive form of the receptor. ![]() There is a balance between the active and inactive forms of the receptor. 2 Although the receptors bind histamine, they can also signal constitutively without histamine binding to the cell surface. H 1 histamine receptors are found on a variety of cells including airway and vascular smooth muscle cells, endothelial cells, epithelial cells, eosinophils and neutrophils. There are four types of histamine receptors in the body (H 1-H 4), with H 1 and H 2 being most widely expressed. Antihistamines bind to histamine receptors on the surface of cells. ![]()
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