Herpes zoster sparing pityriasis versicolor – Anatopic response

A 62-year-old male with no known co-morbidities presented with painful vesicles on the right shoulder and front of the chest for a 3-day duration. On examination, there were multiple discrete grouped vesicles arising from erythematous skin and containing clear fluid, some vesicles showing umbilication distributed on the upper right pectoral region, lateral and right posterior shoulder in T2–4 dermatome suggestive of herpes zoster [Figure 1]. The patient also had multiple discrete and confluent hypopigmented macules with polycyclic border at the upper right pectoral region, right shoulder region, neck, and pre-sternal region suggestive of pityriasis versicolor [Figure 1]. Interestingly, the grouped vesicles of herpes zoster spared the areas affected by pityriasis versicolor [Figure 1] suggesting the classical anatopic response. Tzanck smear from vesicles showed multinucleate giant cells and the potassium hydroxide mount test showed spores and non-branching mycelia of the hypopigmented lesions. The patient was treated with acyclovir, fluconazole, and clotrimazole cream. Anatopic response is defined as the modulation of the inflammatory response of one dermatosis by another unrelated dermatosis at the same site sparing the areas affected by the initial dermatosis, here herpes zoster sparing pityriasis versicolor lesions. The postulated mechanism is that Malassezia species may form a physical barrier or cause local immunomodulation preventing another dermatosis from occurring at the same site.^[1]

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Figure 1:

Multiple grouped vesicles of herpes zoster sparing areas of pityriasis versicolor.

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