Drug-induced photosensitisation

Photosensitisation reactions can be divided into two broad groups: those that are phototoxic and those that are photoallergic with the former being more common.

Drugs that may cause photosensitisation are listed in Box 13.6.

Box 13.6 Drugs that can cause photosensitisation   Amiodarone Tetracyclines   Chlorpromazine Thiazides   Nalidixic acid Quinine   Sulphonamides

Phototoxic sensitisation describes a direct damage to skin tissue caused by the light activating the photosensitising substance. The reaction can occur at virtually any time post-exposure to the photosensitising substance and light, from immediately to hours afterwards. Its appearance is usually that of sunburn (it may be accompanied by vesicles and blisters); however, amiodarone and sunlight turn the skin a blue-green colour in sensitive individuals. The reaction only occurs in the sun-exposed sites. In addition nail changes may be seen with the nails lifting from the nail bed (onycholysis). For those with darker skin colours, this may be the only sign of the drug-induced photosensitisation.

Photoallergic sensitisation is usually caused by topical applications and is a cell-mediated immune response in which the antigen is the light activated, photosensitising agent. It is characterised by an eczematous type, itchy reaction that can spread to anywhere on the body and will appear 24–72 hours after the exposure to the photosensitising substance and sunlight. Treatment involves avoiding where possible the medications that lead to photosensitisation. However, in many instances, the therapeutic value of the drugs will outweigh the photosensitisation risks and the patient should be given information and support to protect their skin from the sun using sunscreens, clothing and avoidance.