Ectopic Contact Dermatitis — SCE Dermatology MCQ

Correct answer: C — Tosylamide/formaldehyde resin (toluenesulfonamide formaldehyde resin)

The correct answer is C, tosylamide/formaldehyde resin (toluenesulfonamide formaldehyde resin, TSFR). TSFR is a thermoplastic resin added to nail varnish to aid adhesion and film-forming, and it is the classic allergen responsible for so-called ectopic or connubial contact dermatitis, in which the periungual skin itself is often spared (having developed tolerance through repeated low-level exposure) while thin, sensitive skin transferred to by the fingers, particularly the eyelids, face and neck, becomes eczematous. UK patch test data confirm TSFR remains a well recognised and frequent cause of contact allergy to nail varnish in women presenting with facial or eyelid dermatitis. The clinical clue in this stem, eyelid dermatitis with no obvious cosmetic applied directly to the eyelid but a history of nail varnish use, is the classic pattern for TSFR allergy and should prompt patch testing to the cosmetic series including TSFR. Why the other options are wrong: A. Lanolin: this is a common allergen in moisturisers, emollients and topical medicaments (derived from wool wax), not a standard nail varnish ingredient. B. Colophony: found in adhesives, sticking plasters, mascara and some cosmetics, but not a characteristic nail varnish allergen. E. Formaldehyde: used as a preservative and nail hardener in some products, but it is a far less common cause of nail varnish related ectopic dermatitis than TSFR, which itself contains formaldehyde within its resin structure. D. Nickel: relevant to jewellery and metal contact dermatitis; it may coexist in patients using varnish to coat metal (eg earrings) but is not itself the nail varnish allergen. Key point: eyelid or facial dermatitis with spared, tolerant periungual skin in a nail varnish user points to tosylamide/formaldehyde resin as the causative ectopic contact allergen.

Reference: British Journal of Dermatology, Allergic contact dermatitis to non-methacrylate nail allergens in the UK, 2020 (183(4):754), https://academic.oup.com/bjd/article-abstract/183/4/754/6761562