Phytophotodermatitis occurs when furanocoumarin (psoralen)-containing plants contact skin and there is ultraviolet A radiation exposure. Persistent phytophotodermatitis in the absence of repeated plant exposure is not well documented.
We present a 19-year-old female who brushed past giant hogweed (Heracleum mantegazzianum) with her left extensor wrist in August 2020. Redness and discomfort developed at the site within a few hours. The next day it became vesiculo-bullous, followed by ulceration and resolution with scarring weeks later. She developed recurrence with worsening eruptions in the two subsequent summers, but had a less severe episode in 2023. On presentation to Dermatology, a necrotic ulcer suggestive of pyoderma gangrenosum was noted. Diagnostic biopsy showed inflammation with prominent deep lymphoplasmacytic infiltrate and lichenoid features. UVA provocation testing at a cumulative dose of 10Jcm2 to the affected site over two consecutive days produced an oedematous erythemal response, whilst a single provocation test at 10Jcm2 to the right wrist over one day was negative. Monochromator testing, lupus serology, porphyrins and Borrelia burgdoferi serology were negative. Patch and photopatch testing indicated a non-relevant contact allergy to limonene. Management has included topical corticosteroids, topical tacrolimus, dressings and photoprotection.
The UVA photoprovocation result supported the diagnosis of persistent phytophotodermatitis without repeated plant exposure. We are aware of two other similar cases.1 Our case adds to the literature and is the second case to show positive UVA provocation testing. The mechanism is unclear. We postulate that there could be a cellular chemical alteration by the initial psoralen-UVA effect, resulting in localised persistent light sensitivity or, less likely, a depot of cutaneous psoralen. We wish to highlight this rare entity, which in this case mimicked pyoderma gangrenosum.