A pretty rash

Discussion

Allergic contact dermatitis is a type IV hypersensitivity reaction.1 Henna is a natural product derived from the plant Lawsonia inermis. Lawsone (2-hydroxy-1,4-naphthoquinone), also known as hennotannic acid, is the natural colouring ingredient that produces a light brown colour.2 However, black henna is not natural and is produced by the addition of para-phenylenediamine (PPD) to the 2-hydroxy-1,4-naphthoquinone molecule. Black henna is a well known cause of allergic hypersensitivity reactions, which start 48 hours to 10 days after exposure. They vary from eczematous eruptions to blistering, as well as permanent scarring including keloid formation.1

The patient is now sensitised to PPD, which is found mainly in hair dyes, dark textile dyes, and printer ink. To prevent any further severe reactions, she should be advised to avoid coming into contact with any of these PPD containing dyes. She was initially treated with topical hydrocortisone. This was ineffective, so she consulted a dermatologist who treated her with high potency topical steroid for one week. The inflammation quickly resolved but she was left with post-inflammatory hyperpigmentation, which faded over several months. It is essential to treat these reactions with potent topical steroids at an early stage to avoid long term pigmentary changes and scarring.