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Background: Current frequency and risk factors for sensitization to methylisothiazolinone (MI), methylchloroisothiazolinone/methylisothiazolinone (MCI/MI), benzisothiazolinone (BIT) and octylisothiazolinone (OIT) in Spain are not well known. Objectives: To study the frequency of sensitization, risk factors and simultaneous sensitization between the four isothiazolinones. Materials and Methods: We analysed all 2019-2021 consecutive patients patch-tested with MI (0.2% aq.), MCI/MI (0.02% aq.), BIT (0.1% pet.) and OIT (0.1% pet) within the Spanish Contact Dermatitis Registry (REIDAC). Results: A total of 2511 patients were analysed. Frequencies of sensitization were: any isothiazolinone 15.7%, MI 6.8%, MCI/MI 4.8%, BIT 3.5% and OIT 0.5%. MI and MCI/MI sensitization was associated with being occupationally active, hand dermatitis, detergents and age over 40. BIT sensitization was associated with leg dermatitis and age over 40. About one in nine MI-positive patients were positive to BIT, whereas one in five BIT-positive patients were positive to MI. Conclusions: Sensitization to MI, MCI/MI and BIT is still common in Spain, while sensitization to OIT is rare. Currently, sensitization to MI and MCI/MI seems to be occupationally related. Although its origin is unknown, sensitization to BIT is more frequent in patients aged over 40 years. Simultaneous sensitization between MI and BIT is uncommon. The Spanish Registry of Contact Dermatitis (REIDAC) is promoted by the Fundación Piel Sana (Academia Española de Dermatología y Venereología), which has received financial support from the Spanish Medicines and Health Products Agency (Agencia Española de Medicamentos y Productos Sanitarios. https://www.boe.es/boe/dias/2022/04/11/pdfs/BOE-A-2022-5975.pdf) and Sanofi. The funders were not involved in the design and conduct of the study, collection, management, analysis and interpretation of data, preparation, review, approval of the manuscript, or decision to submit the manuscript for publication.

Alopecia induced by biological therapy is a rare side effect of this type of drugs. A total of 23 patients of psoriasiform eruptions with severe scalp involvement that induced alopecia during anti-tumor necrosis factor (anti-TNF) treatment of non-dermatological conditions have been previously reported. We present a 50-year-old man affected by plaque psoriasis that developed psoriasiform patches with alopecia over his scalp 10 months after initiating treatment with adalimumab. Punch biopsy of the alopecic area on the scalp revealed psoriasiform epidermal changes and alopecia areata-like dermal changes. Along with these findings, there was a dermal inflammatory infiltrate made up of eosinophils and plasma cells. In conclusion, scalp psoriasiform lesions with alopecia in patients treated with anti-TNF agents have been rarely reported. We describe a patient with anti-TNF therapy-related alopecia affected by psoriasis. Our patient has a peculiar histology with features of psoriasis and alopecia areata in addition to eosinophils and plasma cells. This entity may respond to topical treatment. However in patients of severe scalp involvement anti-TNF suspension should be considered.