Dermatology: Practical and Conceptual Image Letter | Dermatol Pract Concept. 2024;14(1):e2024055 1 Reverse Isotopic Phenomenon in Acute Drug Eruption Carlos Manuel Martorell Moreau1, Luis Javier Del Pozo Hernando1 1 Department of Dermatology, Hospital Universitari Son Espases, Palma, Spain Citation: Martorell Moreau CM, Del Pozo Hernando LJ. Reverse Isotopic Phenomenon in Acute Drug Eruption. Dermatol Pract Concept. 2024;14(1):e2024055. DOI: https://doi.org/10.5826/dpc.1401a55 Accepted: July 9, 2023; Published: January 2024 Copyright: ©2024 Martorell Moreau et al. This is an open-access article distributed under the terms of the Creative Commons Attribution- NonCommercial License (BY-NC-4.0), https://creativecommons.org/licenses/by-nc/4.0/, which permits unrestricted noncommercial use, distribution, and reproduction in any medium, provided the original authors and source are credited. Funding: None. Competing Interests: None. Authorship: Both the authors have contributed significantly to this publication. Corresponding Author: Carlos Manuel Martorell Moreau, Department of Dermatology, Hospital Universitari Son Espases, Valldemossa, 79, 07120 Palma, Illes Balears, Palma, Spain. E-mail: cmartorell15@gmail.com Case Presentation A 90-year-old patient was brought to the emergency depart- ment with a 3-day history of a progressively worsening er- ythematous rash on the trunk and abdomen. She had been taking valacyclovir and metamizol for 4 days after develop- ing herpes zoster on the upper left flank prior to onset of symptoms. Physical examination revealed a maculopapular erythematous rash over the trunk, abdomen and proximal extremities. The rash spared crusts and erosions that fol- lowed a dermatomal distribution suggestive of herpes zos- ter infection (Figure 1A). On dermoscopy, individual lesions were composed of a central reddish crust surrounded by a white halo with shiny white lines, and perilesional erythema with linear and dotted vessels (Figure 1B). A skin biopsy was performed, including a crust, rash, and unaffected skin. The biopsy revealed three histopatho- logically distinct zones. In the center of the biopsy, there was an unspecific erosion that tested negative for herpes virus 1 and 2. The periphery of the biopsy showed a moderate dermal inflammatory infiltrate with eosinophils (Figures 1, C and D). In between those areas, there was a zone with minimal changes corresponding to the spared skin. A diag- nosis of acute drug eruption with reverse isotopic phenom- enon over healing herpes zoster lesions was made. The rash disappeared shortly after discontinuing all medication and receiving a short course of systemic corticosteroids. The pa- tient declined further testing, including drug patch testing. Teaching Point Reverse isotopic phenomenon or isotopic non-response de- scribes the absence of a skin disease in an area previously affected by another dermatosis. This sparing effect is thought to be related to dysfunction of antigen-presenting cells, par- ticularly Langerhans cells [1,2]. 2 Image Letter | Dermatol Pract Concept. 2024;14(1):e2024055 References 1. Kannangara AP, Fleischer AB Jr, Yosipovitch G, Ragunathan RW. Herpes zoster virus associated ‘sparing phenomenon’: is it an innate possess of HZV or keratinocyte cytokine(s) mediated or combination? J Eur Acad Dermatol Venereol. 2008;22(11): 1373-1375. DOI: 10.1111/j.1468-3083.2008.02627.x. PMID: 18422536. 2. Jaka-Moreno A, López-Pestaña A, López-Núñez M, et al. Wolf’s isotopic response: a series of 9 cases. Actas Dermosifiliogr. 2012;103(9):798-805. DOI: 10.1016/j.ad.2012.02.007. PMID: 22681715 Figure 1. (A) Maculopapular erythematous rash over the trunk, which spared the herpes zoster lesions. (B) Central reddish crust surrounded by a white halo with shiny white lines, and perilesional erythema with linear and dotted vessels. (C) On the right side of the skin biopsy, a denser dermal inflammatory infiltrate corresponding to the affected skin as opposed to the spared skin on the left. (D) Dermal edema with perivascular and interstitial infiltrate of lymphocytes, plasma cells, and eosinophils.