Variations in the major histocompatibility complex (MHC) are the most important genetic risk factor for the development of SLE, and a number of non-MHC genetic variants also play a part. Exposure to sunlight and artificial ultraviolet radiation (UVR) may precipitate the disease or lead to flare ups, probably by exposing previously hidden nuclear or cytoplasmic antigens to which autoantibodies are formed. Such autoantibodies to DNA, nuclear proteins and other normal antigens are typical of LE, and immune complexes formed from these are deposited in the tissues or found in the serum. Hydralazine and procainamide are the most likely drugs to trigger SLE, while isoniazid, methyldopa, quinidine, minocycline, chlorpromazine and anti-tumour necrosis factor (anti-TNF) agents precipitate the disease just occasionally.

The skin changes may be transient, continuous or recurrent; they correlate well with the activity of the systemic disease. Acute SLE may be associated with fever, arthritis, nephritis, polyarteritis, pleurisy, pneumonitis, pericarditis, myocarditis and involvement of the central nervous system. Internal involvement can be fatal, but about three-quarters of patients survive for 15 years. Renal involvement suggests a poorer prognosis.

The skin disease may cause scarring or hyperpigmentation, but the main dangers lie with damage to other organs and the side effects of treatment, especially systemic steroids.

Systemic steroids are helpful in gaining control of acute exacerbations, but should not be used for routine use because of the risk of adverse effects. Large doses of prednisolone (Formulary 2, p. 419) are often needed to achieve control, as assessed by symptoms, signs, erythrocyte sedimentation rate (ESR), total complement level and tests of organ function. Having gained control with systemic steroids,slower acting immunosuppressive agents, such as methotrexate, azathioprine (Formulary 2, p. 418), cyclophosphamide and other support drugs (e.g. antihypertensive therapy or anticonvulsants) can be introduced. Antimalarial drugs may help some patients with marked photosensitivity, but the effectiveness of these is reduced in smokers. Sunscreen and appropriate clothing should reduce UV-induced flares. Long-term and regular follow up is necessary and the disease should be managed in conjunction with a rheumatologist.

Patients with subacute cutaneous LE are often photosensitive. The skin lesions are sharply marginated psoriasiform plaques, sometimes annular, lying on the forehead, nose, cheeks, chest, hands and sun-exposed surfaces of the arms and forearms. They tend to be symmetrical and are hard to tell from discoid LE, or SLE with widespread discoid lesions.

As in SLE, the course is prolonged. The skin lesions are slow to clear but, in contrast to discoid LE, do so with little or no scarring.

Systemic disease is frequent, but not usually serious. SSA(Ro) can cross the placenta and children born to mothers who have, or have had, this condition are liable to neonatal LE with transient annular skin lesions and permanent heart block.

The morphology is characteristic, but lesions can be mistaken for psoriasis or widespread discoid LE. Annular lesions may resemble tinea corporis (p. 240) or figurate erythemas (p. 141).

Patients with subacute cutaneous LE should be evaluated in the same way as those with acute SLE, although deposits of immunoglobulins in the skin and antinuclear antibodies in serum are present less often. Many have antibodies to the cytoplasmic antigen SS-A(Ro) and SS-B(La).

Subacute cutaneous LE does better with antimalarials, such as hydroxychloroquine (Formulary 2, p. 424), than acute SLE. Moderate potency topical corticosteroid creams help, and oral retinoids (Formulary 2, p. 420) are also effective in some cases. Systemic steroids may be needed too, especially if there are signs of internal disease.

Psoriasis is hard to tell from discoid LE when its plaques first arise but psoriasis has larger thicker scales, and later it is usually symmetrical and affects different sites from those of discoid LE. Discoid LE is common on the face and ears, and on sun-exposed areas, whereas psoriasis favours the elbows, knees, scalp and sacrum. Discoid LE is far more prone than psoriasis to scar and cause hair loss.