Photo-toxic dermatitis to coal tar in chronic plaque psoriasis

A middle-aged woman presented with complaints of erythema, tenderness and burning sensation over lesions of plaque psoriasis since one week [Figure 1a]. On taking history in detail, she informed that she had been suffering from histopathologically proven plaque psoriasis for two years with episodes of exacerbation of disease during winter season. For the current episode of psoriasis, she consulted a local practitioner who advised her oral antihistamines and an ointment containing Coal tar (6.0%) with Salicylic acid (3.0%) [Figure 1b].

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Figure 1: A middle-aged woman presenting with irritation in pre-existing plaque psoriasis on lower legs. (a) Plaques of psoriasis with erythema, scaling, tenderness on lower legs. (b) Image of ointment containing coal tar (6.0%) with salicylic acid (3.0%). (c) On dermoscopy (DermLite 200 Hybrid, 10x) micropustules (white star) with white superficial scales (white arrow) and a red background could be observed, almost like pustular psoriasis.

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As the patient was unaware of any potential side-effects of the medicament and due to lack of proper guidance of using it, she applied it and went in sunlight every afternoon for getting some relief from the cold weather. For the first two days, she had immediate itching and burning in the lesions after sunexposure but no evident vesiculation. After two days, she noticed distinct erythema, burning and pustulation in the psoriatic plaques on lower legs.

On dermoscopy, micropustules with white superficial scales over red background with dotted 12 vessels could be observed almost like pustular psoriaisis [Figure 1c]. On correlation of history of coal tar application with photoexposure, there was initial confusion between phototoxicity and photoallergic contact dermatitis. She was advised potent topical corticosteroids and an oral antihistamine for management; after 1 week of this therapy, the erythema and irritation had subsided significantly over the lesions [Figure 2]. Then, a photopatch test was done that turned out to be negative, upholding our diagnosis of phototoxic reaction due to coal tar.

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Figure 2: Healed lesion on lower leg after one week of corticosteroid therapy.

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Coal tar is one of the by-products obtained by destructive distillation of coal, which has anti-inflammatory, antimicrobial, antipruritic, and cytostatic effects. Historically, it has been used as a therapeutic agent in psoriasis, but coal tar has a photosensitizing effect on skin (exacerbated skin sensitivity to sunlight) expressed in the form of a phototoxic reaction. This reaction mainly occurs in the ultraviolet A spectrum and the lowest wavelengths of the visible light. Coal tar-induced phototoxic reaction occurs in two ways: The first involves an oxygen-independent reaction with DNA (type I reaction), and the second involves cellular damage through an oxygen-dependent mechanism (Type II reaction). The therapeutic and photosensitizing actions of coal tar are due to (1) the photo-conjugation of the photoreactive ingredients of coal tar with DNA, causing interstrand cross-links; and (2) the production of singlet oxygen (¹O₂₎. As an efficient producer of ¹O₂, it causes cell-membrane damage and cellular edema, which leads to the skin photosensitization response, such as erythema, edema, and membrane damage. The phototoxic reaction leads to an immediate wheal formation associated with sharp burning, followed by a raised, red, infiltrated lesion that starts within hours and peaks at 24–48 h.

This case is just a representation of photo-contact dermatitis due to coal tar, which is rarely seen nowadays due to lack of regular usage of the agent for therapeutic purposes in dermatology practice. Clinicians need to be careful while prescribing agents like coal tar and enumerate the side effects and precautions beforehand to the patient.