Systemic lupus erythematosus (SLE or lupus) is a multisystem disease with a tendency to cause skin rashes. These often appear on light exposed skin after exposure to sunlight (for example face, neck, hands, and feet). These rashes are called photosensitive rashes and are one of the hallmarks of the disease  Some patients complain of feeling unwell after going out in the sun, even for relatively short periods of time (such as half an hour). For example, they may develop migraine, nausea (feeling sick) or joint pains. The joints may even become tender to touch and swollen. These are all manifestations of light sensitivity in lupus patients. Other aspects of the disease may deteriorate after sun exposure, including fever, pleurisy (chest pains on breathing in), kidney disease and more serious nervous system problems such as epilepsy (fits). Occasionally, patients are sensitive to fluorescent lighting but this is much less common than sun sensitivity.

What is it in sunlight that causes lupus to flare and causes rashes to develop in particular?

It is the ultraviolet (UV) waves in the sunlight. Ultraviolet light in the UVA and UVB wavebands is responsible. UV light damages cells in the skin (keratinocytes) causing them to die. In healthy people without lupus, these dead cells are cleared away quickly and any inflammation induced by the sun-induced skin damage is short-lived (sunburn).

However in lupus patients, the skin cells may be more sensitive to sun-induced damage and there is increasing evidence that the dying (apoptotic) cells are not cleared away efficiently. As a result the contents of the dying cells may be released and cause inflammation. Also, cell contents such as DNA (the genetic material) and other molecules including Ro which are never normally exposed to cells of the immune system are available to generate (start) an immune response. Immune responses and inflammation are the normal reactions of the body to infection, but here they are being generated inappropriately by the dying cells and the body mounts an immune response against its own cell constituents (autoimmunity). The end result of this process in susceptible people with certain types of lupus is the development of characteristic photosensitive rashes. Antibodies to Ro, in particular, are often found in people with these rashes. Why the rashes affect only some parts of the body at any one time and are not always sun-sensitive is still not understood.

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