Cystinosis and lupus erythematosus: coincidence or causation.

A 14-year-old boy with known stable cystinosis, treated with cysteamine since infancy, presented with a deterioration of renal function with haematuria in conjunction with a nodular rash, arthralgia, leucopenia, hypocomplementaemia and raised antinuclear antibodies. He was diagnosed with spontaneous onset of systemic lupus erythematosus (SLE), and his renal biopsy was consistent with lupus nephritis. It is unusual for patients with one severe disease to develop another disease process completely unrelated to their original condition, but it can occur. However, other distinct variants of lupus have been described, including drug-induced lupus (DIL), which have features that over-lap with SLE. The potential differential diagnosis of the SLE as a form of DIL in association with cysteamine is discussed.