The skin pathergy test: innately useful?

Pathergy is the term used to describe hyper-reactivity of the skin that occurs in response to minimal trauma. A positive skin pathergy test (SPT), characterised by erythematous induration at the site of the needle stick with a small pustule containing sterile pus at its centre, is among the criteria required for a diagnosis of Behçet's disease (BD) and in certain population has been shown to be highly specific for this condition. Problems with standardising the induction manoeuvre for the SPT as well as the method of assessment of the response have limited the usefulness of the SPT in the clinical setting. Extensive investigation into histopathological and immunological aspects of pathergy has led to a number of hypotheses relating to the aetiology of the skin pathergy reaction and the disease itself, but the cause is considered to be unknown. Pathergy lesions, the development of new skin lesions or the aggravation of existing ones following trivial trauma, are also reported in pyoderma gangrenosum and has been noted in other neutrophilic dermatoses such as Sweet's syndrome. The response of such patient groups to the systematic application of the SPT has not been described. We propose that a new way of considering the pathergy reaction is to see it as an aberration of the skin's innate reactivity from a homeostatic reactive mode closely coupled to tissue healing to an abnormal destructive/inflammatory mode. Our understanding of BD and other similar conditions would profit by more detailed mechanistic knowledge of skin homeostasis to minimal trauma in both health and disease through a more structured and systematic use of the SPT.