Pathergy Test

Pathergy phenomenon has been well known to dermatologists since it was first described in 1937 by Blobner as a state of altered tissue reactivity in response to minor trauma. The pathergy test is a nonspecific hypersensitivity skin reaction induced by needle prick that is performed to look for evidence of this phenomenon. Pathergy lesions are generally manifested clinically by erythematous induration at the location of skin trauma, which may remain as papules or progress to sterile pustules. Although the precise mechanism of pathergy has not yet been entirely elucidated, the skin injury by needle prick in patients exhibiting pathergy is thought to trigger a cutaneous inflammatory response that is exaggerated and more prominent than that seen in normal skin. An increased release of cytokines from cells in the dermis or epidermis is implicated in this aberrant reaction, which results in the perivascular infiltrates that are characteristically observed on histopathologic studies. While pathergy has been reported in numerous diseases, pathergy testing is primarily used in the diagnosis of Behcet Disease (BD). Hulusi Behcet, a Turkish dermatologist, first characterized the clinical entity now known as BD by a triad of recurrent aphthous stomatitis, relapsing uveitis, and genital ulceration. Since it was initially described in 1937, BD has come to be known as a multisystemic inflammatory disorder of unknown etiology with many additional cutaneous, gastrointestinal, articular, vascular, cardiopulmonary, and neurological manifestations. The mucocutaneous lesions of the disease often exhibit the pathergy reaction with the formation of new lesions or aggravation of previous ones following trivial trauma. However, pathergy is not restricted exclusively to the skin and can be more generally described as a state of disease hyperreactivity in any organ after injury. Examples of the pathergy reaction in extracutaneous tissue sites of BD patients include the exacerbation of synovitis after arthrocentesis, the onset of uveitis following intraocular injections, and the formation of aneurysms around vascular anastomoses. Along with BD, pathergy is also widely reported in various other disorders, including neutrophilic dermatoses such as pyoderma gangrenosum (PG) and Sweet syndrome. In these other conditions, the term pathergy refers to the occurrence of lesions following trauma that closely parallel the pathology of the primary disease. This has some resemblance to the Koebner phenomenon that has been reported in other skin conditions and most well known in psoriasis. However, this is in contrast to the needle prick induced pathergy lesions seen in BD, which are usually histologically and grossly distinct from the lesions that naturally occur in the disease. Similar to other features of BD, pathergy is often seen in a relapsing-remitting pattern and is not always present throughout the disease course. There are significant variations in the prevalence of pathergy among different populations, and its incidence has been decreasing over the past few decades. Nevertheless, pathergy testing is still one of the most vital components of the diagnostic criteria for BD.