October 2019 | Case of the Month

The eventual diagnosis (after investigations) was a dermal hypersensitivity reaction, which is a recognised histopathological finding that clinically is usually associated with very itchy scattered red papules. The condition is poorly understood and when no underlying cause is identified (as in this case) it is sometimes referred to as the red itchy bump disease.

Many aetiological factors have been proposed including insect bites, drugs, and vaccines. There have been occasional reports related to internal malignancies including lymphoma. In cases secondary to insect bites the diagnosis is referred to as papular urticaria, in such cases ​lesions tend to be grouped on exposed areas and can blister, non-exposed sites can be affected.

Except in papular urticaria other cases of red itchy papules require a thorough history (including systematic enquiry), examination and routine blood screen (haematology and biochemistry), and a biopsy of one of the lesions to look for an alternative diagnosis as dermal hypersensitivity is a diagnosis of exclusion. Histology usually shows a mild superficial perivascular lymphocytic infiltrate with no or minimal deep dermal extension, associated with some perivascular and interstitial eosinophils and mild dermal oedema. Usually the epidermis is normal or only mildly spongiotic. Sometimes, lymphocytic vasculitis is associated.

Where a cause cannot be identified the condition can be difficult to treat. Response to super-potent topical steroids, topical preparations containing menthol, and the periodic use of sedating antihistamines can be tried but response is often poor. Patients with persistent symptoms not responding to treatment should be referred for further evaluation and treatment, which in the first instance is likely to be a trial of phototherapy. For more information refer to the relevant chapter.