Koebner's sheep in Wolf's clothing: does the isotopic response exist as a distinct phenomenon?

Until 1995, a case of psoriasis developing within the dermatome of a healed herpes zoster was taken as a Koebner phenomenon. In this year, however, the term 'isotopic response' was introduced by Wolf et al. to describe 'the occurrence of a new skin disorder at the site of another, unrelated and already healed skin disease', thus appearing 'on apparently unaffected and healthy skin'. Initially, the term was mainly related to herpes zoster, but today the name 'Wolf's isotopic response' is used to include a plethora of other triggering factors such as healed cutaneous leishmaniasis, tinea or varicella. For obvious reasons, such triggering factors cannot be taken as examples of 'unaffected and healthy skin'. Notably, the authors themselves have categorized the dermatome of a healed herpes zoster as a 'vulnerable area'. In a recent commentary, Wolf et al. have expanded the definition of healed skin diseases triggering an 'isotopic response'. They now included 'scars, pigment changes, color changes or various other minimal changes by the first disease'. Hence, there is no clear-cut criterion to distinguish the isotopic response from a Koebner reaction. Wolf et al. even argue that, if the triggered disorder precedes the appearance of generalized skin lesions, then it is not a Koebner reaction but 'Wolf's isotopic response'. In our view, such definition is unacceptable. All reactions of this kind represent examples of a Koebner phenomenon. Accordingly, the 'isotopic response' should today be taken as a historical error.