CD18: Getting to the bottom of things

Marianne de Brito and Portia Goldsmith

Dermatology Department, Royal London Hospital, London, UK

A 39-year-old woman presented with a 3-year history of an intensely itchy, nodular eruption in a concentric distribution on the buttocks and thighs. She had a background of psoriasis, well controlled on ustekinumab. This eruption was different to her usual psoriasis. There was no response to antibiotics and limited response to topical corticosteroids. Two courses of oral prednisolone improved the eruption but it recurred on cessation. Tinea incognito was considered but mycology was negative and there was no response to a prolonged course of oral itraconazole. Skin biopsies were taken of the upper thigh and revealed papillary dermal oedema and a superficial lymphocytic perivascular inflammatory infiltrate with occasional eosinophils. The rash gradually spread to the upper back, shoulders and arms. She required ultrapotent topical corticosteroids and repeated courses of oral steroids in order to control it. To limit the use of steroids, she was started on methotrexate 10 mg weekly in May 2021, to which she only had a partial response. Following discussion at a regional meeting, contact allergy to her toilet seat was considered and she was patch tested to the European Standard Battery, medicaments and to scrapings of her own ‘mahogany’ toilet seat. The patch testing was strongly positive to her 10-year-old toilet seat scrapings. There were no reactions to colophonium, formaldehyde, foramaldehyde releasers and acrylates. We did not test to hard woods. We diagnosed allergic contact dermatitis to her toilet seat. We postulated that the allergy was to the hard wood. We were unable to prove that the toilet seat was made of mahogany or that it was the wood and not another component of the seat that was the allergen. However, when she replaced her toilet seat with a plastic one her rash promptly resolved, allowing methotrexate to be stopped. Toilet seat dermatitis is reported in the literature to the various components of the toilet seat and to chemicals in cleaning agents. Patients may have had the toilet seat for years prior to reacting to it due to coating of varnish or leather wearing off over time. For wooden seats, the contact allergen may be the wood itself, to polypropylene for plastic seats and for leather seats to the chromate and the underlying foam. Severe cases have been linked to exotic hard woods like teak. This case stresses the importance of recognizing characteristic distribution of toilet seat dermatitis and of patch testing patients to their own toilet seats.