A 21-year-old woman develops a rash on her forearms shortly after spending the afternoon washing up at a restaurant where she has just started working. The rash is very itchy, but she has no other symptoms and feels well otherwise. The rash is shown below:

1. What is the most likely diagnosis?
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The rash shown here is a large number of wheals coalescing on the forearm, and the presentation is highly consistent with urticaria, possibly secondary to exposure to a chemical irritant at her new workplace.

Urticaria is a superficial swelling of the skin (epidermis and mucous membranes) resulting in a red, raised, itchy rash that can be localised or widespread. It is a common condition, affecting approximately 15% of people at some point during their lives. It can be acute or chronic, with the acute form being much more common. 

The typical skin lesion seen in urticaria is the wheal (or weal). Wheals typically consist of three features:

  • A central swelling that is red or white in colour and usually surrounded by an area of redness (the flare).
  • They are generally very itchy, and this is sometimes accompanied by a burning sensation.
  • They are usually fleeting, with the skin returning to its normal appearance within 1–24 hours.

 

Wheals can vary significantly in size from just a few millimetres to lesions that can be as large as 10 cm in diameter. There may be a single lesion, or numerous lesions and lesions may coalesce to form large patches. They may also be associated with swelling of the soft tissues of the eyelids, lips and tongue (angio-oedema).

 

 
Image sourced from Wikipedia
Courtesy of Dr. James Heilman CC BY-SA 3.0

2. What can cause this condition?
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An identifiable trigger is only found in around 50% of cases of acute urticaria. Common triggers include:

  • Allergies, e.g. foods, bites, stings, drugs
  • Skin contact with irritants, e.g. chemicals, nettles, latex
  • Physical stimuli, e.g. firm rubbing (dermatographism), pressure, extremes of temperature
  • Viral infections
3. How should you manage this condition?
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The current NICE guidelines advise that people requiring treatment should be offered a non-sedating (second-generation) antihistamine. Examples of second-generation antihistamines include cetirizine, loratadine, fexofenadine, desloratadine and levocetirizine.

Conventional first-generation antihistamines (e.g. promethazine and chlorpheniramine) are no longer recommended for urticaria because they are short-lasting, have sedative and anticholinergic side effects, can impair sleep, learning and performance and can interact with alcohol and other medications. Lethal overdoses with first-generation antihistamines have also been reported. Terfenadine and astemizole should also not be used, as they are cardiotoxic in combination with certain drugs, e.g. erythromycin and ketoconazole.

If symptoms are severe, a short course of an oral corticosteroid can be given (e.g. prednisolone 40 mg for up to seven days), in addition to the second-generation antihistamine.

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