Erythema Multiforme Sparing Regressing Herpes Zoster Lesion: Reverse Isotopic Phenomenon?

The document describes a case of erythema multiforme (EM) that notably spared an area of regressing herpes zoster in a 67-year-old Korean female patient. This unusual clinical presentation, where the EM eruption did not affect the area previously involved by herpes zoster, is proposed to be termed "reverse isotopic phenomenon." The patient had developed herpes zoster ten days before the onset of EM and had been treated with oral famciclovir. The hypothesis presented in the document suggests that the decrease in Langerhans cells (LC) in the resolving herpes zoster lesion led to a reduced expression of CD8+ or CD4+ cells in the area, resulting in the sparing of the lesion by EM. This case is significant as it is a rare instance of EM triggered by herpes zoster and demonstrates a rare reverse isotopic response. No commercial support was identified for this case report.