SIADH in Systemic JIA Resolving After Treatment With an IL-6 Inhibitor.

Interleukin-6 (IL-6) is implicated in the pathogenesis of both systemic juvenile idiopathic arthritis (SJIA) and syndrome of inappropriate secretion of antidiuretic hormone (SIADH), but the 2 have not been previously described as occurring together. We report a case of a 6-year-old girl with symptoms of arthralgia, daily fevers, evanescent rash, lymphadenopathy, and laboratory evaluation showing elevated inflammatory markers, consistent with SJIA. At presentation, the patient had hyponatremia with a sodium level of 128 mEq/L. She had low serum osmolality with elevated urine osmolality, consistent with SIADH. Hyponatremia improved temporarily during times of fluid restriction as expected in SIADH, but did not resolve until SJIA was treated successfully with tocilizumab, an IL-6 receptor antibody that inhibits IL-6 activity. The positive response to treatment with tocilizumab supports the role of IL-6 in the pathogenesis of both SJIA and SIADH. Patients with SJIA should be monitored for SIADH to avoid complications of untreated hyponatremia.