Persistent PhytoPhotoDermatitis Without Repeated Phytotoxin Exposure

Phytophotodermatitis is a phototoxic reaction that occurs with cutaneous exposure to phototoxic plant chemicals activated by subsequent ultraviolet A (UVA) exposure [1]. We report three cases of recurrent phytophotodermatitis following light exposure, after only an initial exposure to the phototoxic plant Giant Hogweed (Heracleum mantegazzianum).

Case 1 is a 35‐year‐old female with a 5‐year history of intermittent episodes of an itchy, erythematous, blistering eruption on her right forearm. It first developed several hours after a fall into Giant Hogweed on a bright sunny day. Subsequent episodes occurred without any further exposure to Giant Hogweed or other triggers. The eruption occurred annually at the same site in May/June and lasted for several weeks before self‐resolving. Initial clinical assessment revealed a 20cm × 10cm erythematous, well‐demarcated vesicular rash on the right forearm (Figure 1a). Her skin appeared normal between episodes, with no scarring or post‐inflammatory change. There were no episodes documented after May 2017.

FIGURE 1.

Cutaneous eruptions (a) vesicular rash in case 1, (b) bulla formation in case 2, (c) ulceration in case 3.

Case 2 is a 66‐year‐old male with a history of blistering on both forearms (Figure 1b). It first developed 24 h after falling into an area of Giant Hogweed on a sunny day in July 2017. He developed further similar blistering eruptions at the same site on sunny days despite using sun protection, and with no further trigger, between 2018 and 2021. The episodes became less severe with time and the last documented episode in 2021 resulted in erythema only.

Case 3 is an 18‐year‐old female with a recurrent vesiculobullous ulcerating eruption on her left wrist (Figure 1c). It first occurred on a sunny day in August 2020 after brushing past Giant Hogweed, resulting in sap on her left wrist extensor surface, which bleached the blue T‐shirt she used to wipe it. One day later, she developed a vesiculobullous eruption at the same site on the wrist, which ulcerated and eventually healed with cribriform scarring after several weeks. Over the summers of 2021–2023, despite careful photoprotection, she developed recurrence of the eruption at the same site on sun exposure only. The episodes worsened in the summers of 2021 and 2022 but improved in the summer of 2023, and during the summer of 2024, she did not have any reactions. She is left with an area of significant scarring.

Case 1 histology showed subtle surface hyperparakeratosis, epidermal spongiosis, occasional ‘sunburn cells’ in the epidermis, mild basal layer vacuolation and a mild chronic dermal inflammatory cell infiltrate. Direct immunofluorescence showed diffuse staining with IgA, IgG, IgM and C3, consistent with non‐specific changes. Case 3 histology showed minor acanthosis and orthokeratosis, vacuolar interface change, and a band‐like lymphoplasmacytic inflammatory infiltrate in the mid to deeper dermis. The initial photodiagnostic and other investigations findings for all 3 cases are presented in Table 1.

TABLE 1.

Investigation findings.

	Case 1	Case 2	Case 3
Monochromator phototesting on back	Negative	Negative	Negative
UVA provocation testing at affected site	Negative (dose: 25 J/cm2) a	Positive with bulla formation with 25 J/cm2	Positive with oedema and erythema with 10 J/cm2 (5 J/cm2 over two consecutive days)
UVA provocation testing at non‐affected site	Negative	Negative	Negative
Porphyrin screen	Negative	Negative	Negative
Lupus serology screen	Negative	Negative	Negative
Photopatch testing	Negative	Negative	Negative
Histology		NA
Other	No auto‐antibodies on indirect immunofluorescence	NA	Wood's light examination negative Borrelia burgdorferi serology negative Patch testing showed + reaction to hydroperoxides of limonene 0.3% (not considered relevant)

^a Photoprovocation testing done at time when the condition had resolved.

Case 3 underwent subsequent UVA provocation testing given the improvement in 2023. The testing, undertaken Spring 2024, produced a non‐oedematous erythema to 10 Jcm⁻² (5 Jcm⁻² over 2 consecutive days). In Autumn 2024, UVA provocation testing with the same dosing produced a negative response.

All three cases share an initial Giant Hogweed exposure with repeated cutaneous eruptions in the presence of sunlight, but without any further phototoxin exposure. All cases improved over years. We have termed this pattern, Persistent PhytoPhotoDermatitis (PPPD). Case 1's negative UVA provocation result is likely due to the condition self‐resolving before testing. The mechanism to explain the persistent reaction is unknown. We suggest that it is likely to have an immunological basis, given the recurrent nature of the eruptions at the exact sites of the plant phototoxin exposure. Giant Hogweed sap contains high furanocoumarin content, and these linear organic molecules bind to cellular DNA in exposed skin. We hypothesise that at a cellular level there could have been a chemical alteration during the first reaction resulting in the induction of an endogenous persistent phototoxin or photoallergen, leading on to ongoing localised cutaneous photosensitivity following further UVA exposure.

Localised photosensitivity without a trigger has been recognised. The term Fixed Sunlight Eruption is used to describe a rare idiopathic photodermatosis characterised by skin lesions occurring at the same anatomical site upon repeated exposures to sunlight [2]. Broad‐spectrum Abnormal Localized Photosensitivity Syndrome (BALPS) [3], shares features with Fixed Sunlight Eruption, with recurrent episodes on light exposure at specific sites. Therefore, there is a possibility that the initial phototoxin contact triggered the development of local photosensitivity.

The reaction pattern we report could be considered akin to Persistent Light Reaction, a term coined in the 1960s following initial topical photoallergy to a halogenated salicylanilide or fragrance, with ongoing photosensitisation and an eczematous eruption after the initiating event [4, 5, 6]. However, our patients did not have evidence of an initial photoallergic episode as there was no history to suggest prior sensitisation, the eruptions were not eczematous, and photopatch testing was negative.

A hypothesis of a persistent cutaneous psoralen depot is thought unlikely given the superficial and brief furanocoumarin exposure on the epidermis.

To our knowledge these are the first reported cases of Persistent PhytoPhotoDermatitis manifesting as blistering eruptions over several years following an initial phytophototoxic reaction to Giant Hogweed. Two cases demonstrated positive UVA provocation testing, and we demonstrated for one case as the condition improved and resolved that the UVA provocation reactions reduced and resolved. We have considered possible explanations and reviewed similar photosensitivity conditions, which help provide insights into the potential underlying mechanism.

Author Contributions

All authors meet the authorship contribution criteria.

Ethics Statement

The authors have nothing to report.

Consent

Written informed consent has been given by the patients for publication of their cases.

Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

Research data are not shared.