Guest blog by Hywel Williams
A new Cochrane Library Special Collection brings together recent Cochrane Reviews on allergy. In this guest blog, Hywel Williams, Co-ordinating Editor of the Cochrane Skin Group, looks in more detail at some of the reviews relating to skin disorders.
There is plenty of interesting information for those interested in skin allergies in this Special Collection. I shall confine myself to a quick comment on those reviews that are predominantly skin-related, although the skin is an important outcome for many of the other reviews on allergy prevention and anaphylaxis.
I'll start off with eczema, which as the Global Burden of Disease study suggests is the leading cause of disability life-adjusted years for a skin problem worldwide and which is especially common in children. Oral antihistamines are a very widely used treatment for eczema, often in conjunction with other treatments such as topical corticosteroids, yet a review by Apfelbacher and colleagues did not find a single randomised controlled trial (RCT) that had compared antihistamines versus placebo or no treatment for eczema. That’s scary, as it could imply that all those RCTs that do compare one antihistamines against another could be assessing regression to the mean, especially as it is now thought that histamine is not the cause of itch in eczema. The next review by Nankervis and colleagues evaluates house dust mite reduction for the treatment of eczema. I remember house dust mites being mentioned as a possible cause of eczema over 40 years ago, yet the review found only low-quality evidence from seven trials to guide us. No clear evidence exists to support the use a range of house dust mite reduction benefits at present, and none of the studies have evaluated patient-reported outcomes. Some large decent trials need to be done in this area, taking care to document whether study participants are allergic to house dust mite in the first place. Perhaps trying to reduce the effects of such a ubiquitous allergen reduction in such individuals is not the way forward, as the studies from peanut allergy suggest that inducing tolerance is a better approach.
Now let’s take a look at chronic urticaria, which although conveniently lumped under allergic disorders, is probably more of an autoimmune disease. Antihistamines are prescribed almost universally. The very comprehensive review by Sharma et al evaluated 73 studies of H1-antihistamines for chronic spontaneous urticaria, and this time found some evidence that they do have a modest symptomatic benefit compared with placebo. Despite several antihistamines being on the market, each claiming an advantage over the other, this review found no clear evidence that one antihistamine was better than another in standard doses. Adverse events were not a significant problem. So the best strategy for patients is to find the one that works for them and stick with it. I was always taught as a dermatology trainee that if a H1-antihistamine does not work alone, then a H2-antihistamines should be added. Yet Federowicz and colleagues found only four older studies with weak and unreliable evidence to support the use of H2-antihistamines for urticaria, so until such studies are done, I will not be using such a strategy for the patients that I see with this miserable and persistent condition.
And finally, Boyle and colleagues reviewed venom immunotherapy for preventing allergic reactions to people with insect sting allergy. They found reasonable RCT evidence to support the use of venom immunotherapy to prevent systemic and local sting reactions to a range of insects such as ants, bees and wasps in children and adults. The ‘sting in the tail’ is that such therapy carries a small risk of significant adverse reactions.